electrical power quality and utilisation

6th International Conference

ELECTRICAL POWER QUALITY AND September 19-21, 2001, Cracow, Poland UTILISATION

Organised by: Technical University of Lodz Institute of Electrical Power Engineering Lodz, Poland University of Mining and Metallurgy Institute of Electrical Drive and Industrial Equipment Control Cracow, Poland

in collaboration with: Pryazovsky State Technical University of Mariupol Mariupol, Ukraine Ukrainian National Academy of Sciences Institute of Electrodynamics Kiev, Ukraine

PROCEEDINGS

Sponsored by: Polish Power Grid Company 00-496 Warsaw, 2 Mysia Str.

SEMICON Ltd. 04-761 Warsaw, 43 Zwoleska Str.

LEM c/o SEMICON Ltd. 04-761 Warsaw, 43 Zwoleska Str. Lodz Power Distibution Company 90-950 ód, 58 Tuwima Str.

Kraków Power Distibution Company 30-960 Kraków, 27 Dajbór Str. Zakad Dowiadczalny Aparatury Naukowej i Automatyki 30-059 Kraków, 30 Mickiewicza Av.

Edited by:

Ryszard Paweek (Technical University of Lodz) and Ryszard Klempka (University of Mining and Metallurgy)

Copyright © 2001 by: x Technical University of Lodz Institute of Electrical Power Engineering, Poland x University of Mining and Metallurgy Institute of Electrical Drive and Industrial Equipment Control, Poland All Rights Reserved

ISBN-83-914296-1-X Nakad 200 egz. Printed with the help of Polish Power Grid Company Printed in Poland by "Hossa DRUK", 93-460 ód, 27 Chocianowicka Str.

Under the scientific auspices of: x

The Electrotechnics Committee of the Polish Academy of Sciences: Electrical Power System Branch Power Electronics and Electrical Drive Branch

x

State Committee of Scientific Research

x

The Institute of Electrical and Electronics Engineers, Poland Section

x

The Institution of Electrical Engineers, Polish Center

Co-Chairmen of the Conference Zbigniew Kowalski

Technical University of Lodz, Poland

Maciej Tondos

University of Mining and Metallurgy, Cracow, Poland

Organising Committee Irena Wasiak

Technical University of Lodz, Lodz — chairperson

Zbigniew Biernat

University of Mining and Metallurgy, Cracow

Zbigniew Gabryjelski

Technical University of Lodz, Lodz

Ryszard Klempka


Marek Litewka


Piotr Macko


Ryszard Paweek


Roman Sikora


INTERNATIONAL SCIENTIFIC COMMITTEE CO-CHAIRMEN: Z. Kowalski

Technical University of Lodz, Poland

M. Tondos

University of Mining and Metallurgy, Cracow, Poland

MEMBERS: N. Al-Khayat Y. Baghzouz H.P. Beck L.S. Czarnecki L. Frckowiak T. Glinka Z. Hanzelka M. Hartman M. Hering R. Janiczek J. Järvik W. Koczara J. Kulczycki Z. Kumierek V.G. Kuznetsov L.C. Markel S. Massucco W. Mielczarski R. Mieski M. Pawlik A. Piatowicz M. Sakulin M. Samotyj M. Sobierajski D. Stade R. Strzelecki H. Tunia G. Winkler D. Zaninelli I.V. Zhezhelenko

Newage International, United Kingdom University of Nevada, USA Technical University of Clausthal, Germany Louisiana State University, USA Poznan University of Technology, Poland Silesian Technical University, Poland University of Mining and Metallurgy, Cracow, Poland Gdansk Branch of the Electrotechnical Institute, Poland Technical University of Warsaw, Poland Polish Power Grid Company, Poland Technical University of Tallin, Estonia Technical University of Warsaw, Poland University of Mining and Metallurgy, Cracow, Poland Technical University of Lodz, Poland Institute of Electrodynamics of UNAS, Ukraine Santech, Inc, USA University of Genoa, Italy Technical University of Lodz, Poland Technical University of Lodz, Poland Technical University of Lodz, Poland Institute of Power Engineering, Poland Technical University of Gratz, Austria Electric Power Research Institute, USA Wroclaw University of Technology, Poland Technical University of Ilmenau, Germany Technical University of Zielona Gora, Poland Swietokrzyska Technical University in Kielce, Poland Technical University of Dresden, Germany Technical University of Milan, Italy Pryazovsky State Technical University of Mariupol, Ukraine

CONTENS Foreword .........................................................................................................................................................11

Plenary session .............................................................................................................................................13 1. KOWALSKI Z.: Current Problems of Electrical Power Quality and Utilisation Presented in the Papers of the 6-th International Conference on “Electrical Power Quality and Utilisation” (EPQU’01) (Poland) ..............................................................................................................................15 2. GELLINGS C.W.: CEIDS (Consortium for Electric Infrastructure to Support a Digital Society) (USA) .....................................................................................................................................................25 3. LOBOS T.: Advanced Spectrum Estimation Methods for Signal Analysis in Power Electronics and Systems (Poland) .............................................................................................................................35

Section 1. Power Quality Parameters: Evaluation and Standardisation ......................................49 1.1. LO SCHIAVO L., MALAMAN R.: The Regulation of Quality of Supply in Italy (Italy).....................51 1.2. McEACHERN A., MONCRIEF W.A.: Revenue and Harmonics: An Evaluation of Some Proposed Rate Structures (USA) ...........................................................................................................63 1.3. WINKLER G., MEYER J.: Advanced Power Quality Rating Using Statistical Tolerance Intervals (Germany)...............................................................................................................................71 1.4. ZHEZHELENKO I.V., SAYENKO Y.L.: Some Aspects of Calculation of Power Quality in Accordance with European Standards (Ukraine) ..................................................................................79 1.5. VYSKOIL V., ŠPAEK Z., HRKOVÁ J.: Power Quality on the Boundaries Between Transmission and Distribution Network, Observations to the Standardization and Evaluation of PQ, Distortion of Currents in Characteristic Points of the Network (Czech Republic)........................85

Section 2. Methods of Power Quality Analysis: Modelling and Simulation .................................93 2.1. WILKOSZ K., SOBIERAJSKI M., KWASNICKI W.T.: Fourier Analysis of Voltage at the Terminal Node of STATCOM for Different Parameters of the Frequency-Dependent Line Model (Poland, Canada)....................................................................................................................................95 2.2. MIENSKI R., PAWELEK R., WASIAK I.: Compensation of Time-Varying Loads by Means of SVC – Modelling and Simulation (Poland) ..........................................................................................103 2.3. OZDEMIR S., OZDEMIR E., ERTUNC H.M.: The Simulation of Fuzzy Logic Based Advanced Static Var Compensator in Matlab (Turkey) .......................................................................................113 2.4. ZHEZHELENKO I.V., SAYENKO Y.L., NESTEROVICH V.V.: Using Wavelet Transform for Identification of Electric Circuits Parameters (Ukraine) ....................................................................119 2.5. KOVERNIKOVA L.I., SMIRNOV S.S.: Equivalenting of Electric Network when Calculating Modes of Harmonics (Russia)..............................................................................................................123 2.6. LEZHNYUK P., LUKIANENKO Y., YARNYH L., VYDMYSH V.: Simulation of the Higher Harmonics Spreading Process in Electrical Circuits With the Use of the Nodal Equation (Ukraine) ..............................................................................................................................................129 2.7. OLARU D., TRUSCA V.: Numerical Simulation of the Non-Linear Resonance Phenomena Using Spline Approximated Characteristic (Romania) .......................................................................133

2.8. RENNER H., FICKERT L., SAKULIN M.: Calculation of Voltage Dips in a Meshed 110-kVNetwork (Austria) .................................................................................................................................139 2.9. LEVA S., MORANDO A.P., ZANINELLI D.: The Influence of the Zero-Sequence Component on the Line Voltage Drop (Italy) ..........................................................................................................145 2.10. BANKO S., SABARNO L., SEVASTUK I., TRACH I.: Optimum of Rapidly Resorting of Voltage in Autonomy Electrical Power Systems (Ukraine) .................................................................153 2.11. KUZNETSOV V., TUGAY Y., DMITRIEV E.: Suppression of Overvoltages Caused by OpenPhase Idle Operating Conditions in Subtransmission Network (Ukraine)...........................................159 2.12. LEVA S., MORANDO A.P.: Power Properties of Single-Phase Non-Sinusoidal Systems: The Use of Generalised Rotating Vector Concept (Italy) ...........................................................................165 2.13. SAVINA N.V.: Modelling of Parameters of the Electric Power Quality Characterizing NonSinusoidal and Asymmetry of Voltage (Russia)....................................................................................173 2.14. KOZLOV A., LOGINOV V.: Conditions of Steady-State Stability of Controllable AC Transmission Lines (Russia) ................................................................................................................179 2.15. PLOUTENKO A.D.: The Conception of a System of Analysis of Models of Access to Distributed Databases (Russia)...............................................................................................................................187

Section 3. Power Quality Measurements: Techniques, Instruments, Results ............................193 3.1. SANTARIUS P., GAVLAS J., BIOVSKÁ B.: Evaluation of Power Quality in Regional Distribution Networks (Czech Republic) .............................................................................................195 3.2. PROCHÁZKA K., KYSNAR F., VYSKOIL V., HRKOVÁ J.: Properties of Instruments for the Measurement of Power Quality on the Boundary Between the Transmission System and the Distribution Systems and the Results of Their Testing (Czech Republic)............................................203 3.3. WACLAWIAK M., McGRANAGHAN M.: Substation Power Quality Performance Monitoring and the Internet (USA).........................................................................................................................211 3.4. KOPONEN P., VEHVILÄINEN S.: Improved Power Quality Monitoring kWh-Meter (Finland) .....217 3.5. CAROLSFELD R., SEMCZUK M.: Pass or Fail: The Future of Power Quality Monitoring (Canada, Poland) ..................................................................................................................................225 3.6. HASHAD M., HARTMAN M., HANZELKA Z., BIEN A.: The Hypothesis for the Wrong Measurements Results Obtained During the Flickermeter Comparative Test (Poland) ......................231 3.7. BIEN A., ROZKRUT A.: A Measurement Scale for the Light Flickering Phenomenon (Poland) ......237 3.8. BARKAN J., LESCHENKO S., VASILJEV A.: Factors Having Influence on the Accuracy of Flicker Determination (Latvia) ............................................................................................................241 3.9. KUMIEREK Z., KORCZYSKI M.J.: Subharmonics in Electrical Power System Identification Problems (Poland) .........................................................................................................245 3.10. ZHEZHELENKO I.V., SAYENKO Y.L., BARANENKO T.K.: Calculation of Interharmonic Voltage Spectrum in the Points of Industrial Power Supply Systems (Ukraine) ..................................253 3.11. PACHOLSKI K.: The High-Speed Overloading of Measuring Transducers of Distorted Electrical Signals (Poland)...................................................................................................................259

Section 4. Methods of Power Quality Improvement: Filters, Power Compensation, Phase Balancing, etc. ............................................................................................................263 4.1. BRENNA M., FARANDA R., VALADÈ I., BALLOCCHI G.: Flicker Reduction by Using Distributed Generation and Active Power Compensators (Italy) ........................................................265 4.2. BAJSZCZAK G.: Voltage Distortion Compensation in Transmission Networks by Series Converter Filter (Poland).....................................................................................................................273 4.3. RUSISKI J., KOT E., BENYSEK G.: Active Power Filter’s Behaviour in Non-Periodic Conditions (Poland) .............................................................................................................................279 4.4. DZIEA J.A.: Input-Output Decoupling in Active Power Filter Control (Poland).............................285 4.5. MIENSKI R., PAWEEK R., WASIAK I.: Application of SVC for Load Balancing (Poland).........291 4.6. VARETSKY Y.: Exploitative Characteristics of SVC Filter Circuits (Ukraine)................................297 4.7. KLIMASH V.: Transformer and Thyristor Based Compensator of Voltage Deviations and VAR with Four-Quadrant Control (Russia) .................................................................................................303 4.8. TIRONI E., VALADÈ I., LOPES G., UBEZIO G.: Voltage Quality Improvement Using Superconducting Magnet Energy Storage (SMES) Devices in LV System With Neutral Conductor (Italy)..................................................................................................................................309 4.9. FARANDA R., TIRONI E., VALADÈ I., UBEZIO G.: Comparison Between UPS Line Interactive Devices Designed to Solve Power Quality Problems (Italy) .............................................317 4.10. POSTOLATI V.M., BICOVA E.V., KUZNETSOV V.G.: Characteristics of Controlled Electrical Transmission Lines of an Alternating Current of the Increased Capability and of Their Application for Improvement of Quality of Parameters Modes Power System and Increase of Reliability of Electro Supply (Ukraine, Moldavia) ..........................................................................327 4.11. SAKKOS T., SARV V., JÄRVIK J.: Power Quality Improvement in Diode Rectifiers Using Ripple-Power Re-Rectification and Multifunctional Filter Elements (Estonia) ..................................333 4.12. ABREU J.P.G., BERNARDES D.F.: Converter Transformers Phase-Shift - A Useful Approach - (Brazil) .............................................................................................................................339 4.13. RUSINARU D., MIRCEA I.: Aspects Regarding Load Current Symmetrization in Unbalanced Power Systems (Romania) ...................................................................................................................347 4.14. WALCZAK J., GRABOWSKI D.: Neural Networks for Optimization of Power Systems (Poland)................................................................................................................................................353

Section 5. Power Quality in Competitive Electricity Markets. Economic Aspects of Power Quality and Costs of Supply ..................................................................................361 5.1. MIELCZARSKI W., WASILUK-HASSA M., SAMOTYJ M.J.: Power Quality in Electricity Markets (Poland, USA)........................................................................................................................363 5.2. GOMES R.J.R., BRASIL D.O.C., MEDEIROS J.R.: Power Quality Management as a Goal of ONS (Operador Nacional do Sistema Eletrico) the Brazilian Transmission ISO (Brazil) ..................369 5.3. EGUIA P., TORRES E., FERNANDEZ E., SAENZ J.R.: The New Quality of Supply Regulatory Framework in Spain. Will it Benefit the Network User? (Spain) ......................................377 5.4. HOWARD M.W.: Life Cycle Cost Analysis for End Use Power Quality Mitigation with Advanced Energy Storage Technologies: A Case Study (USA) ..........................................................383 5.5. BAJSZCZAK G.: Cost Evaluation of Non-Active Power Compensation in Transmission and Distribution Networks (Poland) ...........................................................................................................393 5.6. ARANGO H., ABREU J.P.G., DOMINIGUES E.G., PAULILLO G.: Using the Pricing Theory of Financial Derivatives to Predict Payments of Electric Energy Revenues (Brazil)..........................401 5.7. SZKUTNIK J.: The Quality of Distribution as a Factor Decreasing Costs of Energy (Poland).........407 5.8. GRINKRUG M., TKACHEVA Y.: Improvement in Power Voltage Quality and Reduction in Power Losses at City Low Voltage Transformer Substations (Russia)................................................411

Section 6. EMC in Electrical and Electrical Power Engineering. Electrical and Exploitative Characteristic of Loads and Electrical Energy Converters .................417 6.1. BECK H.P., RÖSNER J.: Wind Energy Converter with Asynchrounous Machines and 12-pulse AC Controller in Generator Mode (Germany).....................................................................................419 6.2. PAULILLO G., ABREU J.P.G.: Analysis of 12-Pulse Power Converters Under Unbalanced Voltage Supply A Novel Transformer Topology (Brazil) .....................................................................425 6.3. KOCZARA W., DZIUBA R., LEONARSKI J., AL.-KHAYAT N.: Variable Speed Set for Embedded Power Generation (Poland, UK) ........................................................................................433 6.4. STRZELECKI R., SMOLESKI R., KEMPSKI A.: Reduction of the Bearing Current in PWM Motor Drives by Means of Common Mode Voltage Cancellation (Poland).........................................439 6.5. JANSON K., JÄRVIK J., VINNAL T.: Installed Capacities of Reactive Components and Transformer in Line Frequency Resonant Converters (Estonia) .........................................................445 6.6. MYASOEDOV Y.V., SAVINA N.V.: Evaluation of Influence of Low of the Electric Power Quality on the Account of Electric Power Consumption in Networks with the Tractive Load (Russia).................................................................................................................................................451 6.7. FUSTIK V., ILIEV A., WEBER H., PRILLWITZ F.: Dynamic Characteristics of the Unit A in HPP Vrutok in Islanded Operation (Macedonia, Germany)................................................................459 6.8. TOROPCHINA L.V., TOROPCHINA S.V.: Quality Parameters of Electric Power and Their Influence on the Work of Electric Receivers (Russia) ..........................................................................467 6.9. BARABOI A., ADAM M., PANCU C.: The influence of Supply Voltage Quality to the Actuators Behavior (Romania).............................................................................................................471 6.10. ..WIDLOK H.: Energy Quality Aspects of Modernisation of a Large-Power Rolling Mill Drive (Poland) ........................................................................................................................................... 477

Section 7. Reliability and Continuity of Supply ..................................................................................485 7.1. PASKA J., MOMOT A., BARGIEL J., GOC W.: Application of TRELSS and Implementation of Value Based Transmission Reliability Approach at Polish Power Grid Company (Poland)..............487 7.2. BARGIEL J., GOC W., PASKA J., SOWA P., SZEWC B., TEICHMAN B.: Reliability in Contracts for Electric Energy Supply and Settlements (Poland)..........................................................495 7.3. RUSEK S.: The Relation Between Classical and Global Indices of Reliability (Czech Republic) .....503 7.4. HRADÍLEK Z.: Reliability and Continuity of Towns Electic Power Supply (Czech Republic)..........509 7.5. MIENSKI R., PAWELEK R., PAWLIK M., WASIAK I.: Supply Reliability Improvement by Means of Unconventional Energy Sources (Poland)............................................................................513

Index of Authors ........................................................................................................................................519

Foreword We are greatly honoured to present you – all the participants of the 6th International Conference “Electrical Power Quality and Utilisation” (EPQU’01) and all specialists concerned – a set of section conference papers. The present conference is the continuation and development of prior conferences on the same or similar subject-matter, which have been organised by/or together with the Institute of Electrical Power Engineering of the Technical University in Lodz. The first one entitled “Electrical Power Quality in the National Electrical Power System” took place in Lodz in 1987. The next one entitled “Electrical Energy Quality” was organised in Spala in 1991, and was followed by the third one named “Efficiency and Quality of Electrical Power Supply for Industrial Plants”, which was held in Mariupol (Ukraine) in 1994. Since then the conferences under the name “Electrical Power Quality and Utilisation” (EPQU) have been organised jointly by the Technical University of Lodz and University of Mining and Metallurgy in Cracow. The 4th EPQU conference in September 1997 and the 5th one were held at the Polonia Institute of the Jagiellonian University in Cracow-Przegorzay. A venue for the present 6th conference will be the Wawel Castle. These Proceedings include 67 papers which have been divided into the following subject sections: 1. Power Quality Parameters: Evaluation and Standardization – 5 papers 2. Methods of Power Quality Analysis. Modeling and Simulation – 15 papers 3. Power Quality Measurements.: Techniques, Results – 11 papers 4. Methods of Power Quality Improvement – 14 papers 5. Power Quality in Competitive Electricity Markets. Economic Aspects of Power Quality and Costs of Supply – 7 papers 6. EMC in Electrical Engineering. Electrical and Exploitative Characteristics of Loads and Electrical Energy Converters – 10 papers 7. Reliability and Continuity of Supply – 5 papers The section names reflect the main subjects-matters of our conference and result from topics of the papers submitted and accepted by the Scientific Committee. We regret that some valuable papers of the authors who will not be able to participate in our conference could not be published. They would possibly enrich our conference. The papers include much interesting and creative, in both the knowledge and utility respect, information that refers to electrical power quality and utilisation. They are due to be the basis of creative discussion during the conference meetings. We believe that all section papers as well as the discussion during the conference will contribute to the development of electrical power quality and its utilisation in general and in free market standards. We also hope that the results of this conference may be directly applied to design process and to utilising electrical power network and equipment as well as electrical energy receivers and converters. We wish all the participants to have a nice and unforgettable stay in Cracow. Co-chairmen of the EPQU Conference

Prof. Zbigniew Kowalski

Prof. Maciej Tondos

Lodz-Cracow, August, 2001

Plenary session

1. KOWALSKI Z.: Current Problems of Electrical Power Quality and Utilisation Presented in the Papers of the 6-th International Conference on “Electrical Power Quality and Utilisation” (EPQU’01) (Poland) ..................................................................................................................................15

2. GELLINGS C.W.: CEIDS (Consortium for Electric Infrastructure to Support a Digital Society) (USA).........................................................................................................................................................25

3. LOBOS T.: Advanced Spectrum Estimation Methods for Signal Analysis in Power Electronics and Systems (Poland).................................................................................................................................35

Plenary Session

13

14

Electrical Power Quality and Utilisation


ELECTRICAL POWER QUALITY AND UTILISATION September 19-21, 2001, Cracow, Poland

CURRENT PROBLEMS OF POWER QUALITY AND UTILIZATION IN THE PAPERS PRESENTED ON THE 6th INTERNATIONAL CONFERENCE „ELECTRICAL POWER QUALITY AND UTILIZATION” (EPQU’01) IN CRACOW, SEPTEMBER 19 – 21, 2001 Zbigniew KOWALSKI Technical University of Lodz Lodz (Poland)

1. INTRODUCTION 67 papers were qualified to press and to presentation on the Conference. The criteria for the acceptance of papers were opinions of the members of the Conference Scientific Committee, and author’s or co-author’s declaration of participation in the Conference. The papers are divided into the following thematic sections: Section 1 - Power quality parameters: Evaluation and Standardisation (5 papers).

Many interesting current problems connected with electrical power quality and utilization are presented in the papers. Most of the papers are of significant cognitive and application value. This paper is the short review of most important, subjectively selected problems considered in the published conference papers. The problems are characterized mainly on the ground of summaries and eventually final conclusions of the authors.

Section 2 - Methods of Power Quality Analysis. Modelling and Simulation (15 papers).

2. SECTION 1: POWER QUALITY PARAMETERS – DETERMINATION AND STANDARDIZATION

Section 3 - Measurements of power quality – measuring methods and instruments (11 papers).

Papers in this section deal mainly with problems of standardization and monitoring of power quality parameters. The papers report already the results of practical application of Standard EN 50160, which determines requirements for the values of power quality parameters in public networks, and the controversy surrounding the application of these requirements. Moreover, power quality as a factor in calculation of electricity prices was discussed.

Section 4 - Methods for power quality improvement – filters, compensators, and balancing (14 papers). Section 5 - Power quality in competitive electricity markets – economic aspects of power quality and costs of supply (7 papers). Section 6 - EMC in electrical engineering – electrical and operating characteristics of energy receivers and converters (10 papers). Section 7 - Reliability and continuity of power supply (5 papers). The papers are prepared by the authors from 19 countries.

Plenary Session

The power quality legal regulation in Italy is thorougly presented in paper 1.1. It gives the main reasons for introduction of power quality regulation in distribution and supplying networks and the analysis of Italian experience in that field before and after establishing the special independent legislative commission, which is engaged in problems of the power quality legal regulation. Standards and other rules in the USA specify in detail limits on voltage and current harmonics in

15

supplying networks, especially in the point of common coupling (PCC). The authors of paper 1.2 inform that utilities in the USA not only encourage and convince their consumers (and vice-versa) to reduce harmonics in supplying networks but also apply appropriate economic incentives. The authors present some approaches to such incentives, for instance, that increased costs resulting from distortion of voltage and current waveforms might be fairly shared among all rate payers by suitable modification of the rates, in a way agreed with consumers. As it can be seen from paper 1.3, in Germany there many informations have been collected about the application of requirements for power quality in MV and LV public networks specified in Standard EN 50160. Some irreversible damages in devices are reported, although requirements of this Standard for power quality parameters were satisfied in appropriate point of common coupling. The authors propose to modify some requirements of EN 50160 for better assessment of power quality. They point out that electricity utilities more often use long-term measurement systems to monitor power quality. They describe the new algorithm for power quality parameter analysis using long-term measurements. The algorithm makes it possible to standarize power quality more accurately than EN 50160 it does. This algorithm is integrated into the IMEDA system’s computer programm enabling measurements, statistical analysis, archiving and presentation of power quality parameters. Problems of the practical application of EN 50160 are reported also by the authors of paper 1.4 from Ukraine. These problems are connected with the evaluation of voltage variation in the networks supplying the rolling mill, and the selection of parameters for devices limiting that variation. The authors present also the original method for determination of negative-phase-sequence impedance of the system having a thyristor converter. The method enables better evaluation of unbalance in current in that system. An interesting analysis of power quality parameters, which have been determined for selected networks in the Czech Republic on the ground of EN 50160, is presented by the authors of paper 1.5. The authors published the results of experimental investigations of power quality parameters in supplying networks, mainly in 110 kV networks powered from 400/110 kV and 220/110 kV substations. Values of some of above parameters are significant in networks under investigation and may be taken into account when 16

closing contracts for delivery of electric energy to distribution networks In the paper there is also discussed the change in active power losses caused by the harmonic distortion of currents in electric power networks. On the ground of a few papers in Section 1, it must be stated that informations about practical application of EN 50160 are not numerous. Some papers suggest that modification of this Standard is required taking into account the present measuring and analytical possibilities.

3. SECTION 2: METHODS FOR POWER QUALITY ANALYSIS – MODELLING AND SIMULATION This section contains most papers (15), which deal with many problems of modelling, simulation and analysis of electrical circuits, the current and voltage waveforms of which are distorted. Moreover, power quality components are discussed. The main topics covered in Section 2 are the following: 1. Overall problems of the modelling and analysis of circuits, the currents and voltages of which are distorted. 2. Determination of losses, voltage dips, voltage levels and overvoltages in electric power systems. 3. Determination of distribution of current higher harmonics in electric power systems. Section 2 contains also two papers (14 and 15) that deal with problems not strictly connected with the range of topics of this section. In the area of analysis of non-linear circuits or systems, the current and voltage waveforms of which are distorted, a number of new solutions and methods for calculation and simulation are presented. For instance: 1.

In paper 2.12, the original method for investigation of linear and nonlinear single-phase AC circuits is given and illustrated with examples of computation. This method uses the Fourier transform and the concept of so called generalized rotating vector. The main topic is determination of the power and power balance in these circuits.

2.

Use of the vavelet transform and the adequate example of the simulation model for identification of electric circuit parameters is presented (paper 2.6). The authors of the Electrical Power Quality and Utilisation

concept, which is a continuation of the approach given in the EPQU’99 Conference, state that the vavelet transform has advantages over the Fourier transform, especially when circuit parameters are variable with time. 3.

4.

The authors of paper 2.1 have presented the interesting method for testing the frequencydependent electrical parameters of electric power systems by means of the STATCOM harmonic generator. Many examples of calculations illustrate the investigation carried out by means of the proposed method for computer simulation. Paper 2.2 presents the computer simulator for investigation of operating conditions in electric power systems having nonlinear and time-varying loads. By means of that simulator, power quality parameters in the point of common coupling may be calculated acc. to Standard EN 50160. The example given shows the application of that simulator for a system using the static compensator of quick variable loads. The authors noticed that such compensator reduces voltage fluctuation but increases the distortion of voltages at the same time. In their opinion, to evaluate the compensation properly, all power quality parameters shall be tested simultaneously. It may be supposed that the results of computer simulation of the fuzzy logic based, static VAR compensator, which are presented in paper 2.3, may be also used to suppress voltage fluctuation in electric power systems.

5.

The authoress of paper 2.13 has presented the concept of strategy for testing and inspection of power quality parameters in electric utility companies, in particular the non-sinusoidal character and assymetry of voltages. That concept is based on the probabilistic-spectral presentation of quality parameters in electric power systems.

The original method for voltage drop calculation in non-symmetrical and non-sinusoidal current loaded three-phase four-wire lines is presented in paper 2.9. Some examples of calculations illustrate this method, which uses socalled Park transformation and the zero-sequence component of current in the three -phase systems analysed. Problems of voltage dips in the nodes of electric power systems are dealt in two papers: 1.

paper 2.8. Those methods are based on the statistical analysis of long-term failures and on the algorithm for calculation of voltages under various short-circuit conditions. 2.

The concept of the decrease in voltage dips in autonomous electric power systems by use of fast acting voltage regulators is presented in paper 2.10.

Problems connected with overvoltages caused by ferroresonance in electric power systems are presented in papers 2.7 and 2.11. Paper 2.7 points out difficulties with resonance phenomena simulation, caused by nonlinearity of some devices in the systems analysed. Nevertheless, the authors proposed the original method for calculation of voltages under resonant conditions (illustrated with results of simulation calculations). This method uses differential equations, in which one of the variables is the magnetic flux of a nonlinear inductive component. The authors of paper 2.11 inform about damages to transformers caused by ferroresonance, and their heavy consequences. The facts described shall deserve more attention to the resonance phenomenon when planning and designing of distribution systems. The authors propose some methods for prevention and supression of resonance overvoltages and suggest using of special devices for detection of ferroresonance in electric power systems. No doubt papers 2.5 and 2.6, which deal with determination of higher harmonic distribution in electric power systems, will arouse interest. Paper 2.6 presents the computer method for equivalenting the electric power networks, which is used to calculate the distribution of current higher harmonics. The method is also given for calculation of voltage higher harmonics at network nodes resulting from distribution of current higher harmonics. The author of paper 2.7 has described the algorithm for calculation of distribution of current higher harmonics in electric power systems, based on the system of nodal equations using so called Hauss’ method. On the ground of the above review of problems that are considered in the papers in Section 2, it may be supposed that many papers present very useful original methods for calculation and simulation in the field of power quality.

Methods for calculation of voltage dips in nodes of the 110 kV network are presented in

Plenary Session

17

4. SECTION 3: MEASUREMENTS OF POWER QUALITY – MEASURING METHODS AND INSTRUMENTS

read and analysed in the place of their reception. The authors made suggestions about how to set a sampling time and minimalize errors.

Papers in Section 3 show three fundamental topics:

Overall measurement problems and monitoring of power quality in the future are presented in paper 3.5. The authors (from Canada and Poland) consider that starting electricity markets need suitable standards for power quality, which will be used to define obligations between energy suppliers and consumers. Application of standards will require the wide use of power quality monitoring that help make optimal business decisions in the field of the reliable delivery and the utilization of energy. The authors formulate thesis that reliable delivery of energy to consumers, in particular in industry, results in overall development, and that suitable power quality monitoring will be very helpful in proper assessment of power quality.

1.

overall measuring problems connected with and monitoring of power quality parameters,

2.

problems connected with measurements of flickers (voltage fluctuation severity factors)

3.

identification of subharmonics.

Paper 3.1 analyses the complex investigations of power quality parameters in regional distribution networks in the Czech Republic in aspect of Standard CSN EN 50160 specifications. Voltage higher harmonics, fluctuations and unbalance have been measured. The investigations were inspired by degradation of power quality noticed in distribution networks, caused mainly by nonlinear receivers like TV sets, computers and compact fluorescent lamps. Authors of that paper pointed out the importance of power quality monitoring, in particular as from the year 2002 when the new Energy Law will come into effect in the Czech Republic. The Czech authors presented in their second paper (3.2) test results of four types of power quality analysers installed in the points of delivery between the transmission and the distribution system. Measurement data were transmitted by telephone lines. The test results revealed that small values of the flicker and of the voltage unbalance are the least reliable. Authors of paper 3.3 inform that in the USA, in the system of the United Illuminating Company, the new power quality monitoring system has been established. That system is integrated into the general distribution monitoring system that accomplishes also monitoring for energy management, operation of protective devices, and energy distribution automatic functions. That system uses intranet fully accessible to energy consumers. It is utilised for calculation of electric energy costs and takes into account sensitivity of consumers to power quality. Authors of paper 3.4 present problems connected with measurements of power energy parameters in Finland and propose the new method for measurement thereof. This method is based on the adaptive observer of a voltage vector state and uses so called intelligent kWh-meter. Results of measurements, which are carried out by means of that system, may be transmitted to long distances, 18

Problems connected with measurements of the flicker (Pst and Plt) are the subject of papers 3.6, 3.7, and 3.8. In the year 2000, the authors of paper 3.6 carried out comparative tests of the flicker by means of ten usual voltage fluctuation meters, commonly called flickermeters, made by several producers. The flickermeters were constructed in accordance with recommendations of Standard IEC 61000-4-15. The authors stated that results of measurements carried out with particular flickermeters were different under the same conditions. They put forward a hypothesis that different results may be caused by a human factor or by faults in construction or workmanship of flickermeters. They state that the parameters of flickermeters shall be correctly set up. On the ground of the test results, the authors proposed the new calibration procedure for flicker-meters to limit differences in results of measurements carried out by means of flickermeters of various producers. The authors of paper 3.7 presented their experience gained by measurement of the flicker Pst carried out by means of flickermeters. They placed their remarks and suggestions for normative determination of the flicker, and pointed out that tesults of flicker measurement are not very useful when the change or modernization of an electric power system is planned in a voltage fluctuation area. Paper 3.8 contains many aspects of the measurement and calculation of the flicker. A number of questions connected with construction and operation of flickermeters are discussed, for instance, the method for demodulation of control signals, the construction of A/D converter, the Electrical Power Quality and Utilisation

frequency of discretization, the block for statistic conversion of instantaneous value of voltage deviation, and the effect of the above components and factors on the total error of measurements and calculation. On the ground of analyses and tests, the authors proposed to simplify functioning of microprocessor units of the flickermeter resulting in lower costs. Paper 3.11 presents the results of testing of measuring transducers for distorted electrical signals. The results are concerned with the construction of flickermeters. The author analyses the effect of power trasnducer high-speed overloading that causes a transducer error called overload error. The author made suggestions about how to avoid the effect of high-speed overloads to measurement results. Problems connected with identification of subharmonics in electric power systems are the subject of papers 3.9 and 3.10. The authors of paper 3.9 proposed the new method for determination of interharmonics in distorted current and voltage waveforms. They pointed out the substantial problems with the digital conversion of higher harmonics, interharmonics and subharmonics. They reported that various current distortions, which are caused by nonlinear receivers, especially interharmonics, are determined with an insufficient accuracy. The method of calculation of interharmonic voltage spectrum in the points of industrial power supply systems are presented also by the authors of paper 3.10. This method is based on the spectralcorrelation theory of random functions. From the papers in Section 3 result the following overall conclusions: 1.

Integrated systems for power quality parameter monitoring have been developed and are and will be increasingly used in power engineering.

2.

Structural improvements in standard flickermeters are necessary in order that indications of flickermeters might be uniform, their errors comparable, service faciliated and costs lower.

3.

It is desirable to develop methods for calculation of accuracy in determination of higher harmonics, interharmonics and subharmonics of currents drawn by nonlinear receivers.

Plenary Session

5. SECTION 4: METHODS FOR POWER QUALITY IMPROVEMENT – FILTERS, COMPENSATORS, AND BALANCING Papers in this section contain problems connected with the reduction of various power quality parameters: ¾ voltage fluctuations, ¾ voltage and current higher harmonics, ¾ assymetry of currents and voltages, ¾ voltage deviations and the description of the new electric power system that improves the main parameters of electrical energy delivered. Basically, only two papers deal with the reduction of voltage fluctuations in electric power systems. Paper 4.1 analyses the effect of configuration and parameters of a distribution network supplying a time-varying receiver on a voltage fluctuation level in that network. Two types of networks are considered: radial network and meshed network. When a receiver was supplied from the meshed network, the amplitudes of voltage fluctuations were lower than in case of the radial network, because the short-circuit power at receiving busbars was lower in the first case. On the ground of investigations, the authors confirm the known fact of reducing the amplitudes of voltage fluctuations by decrease of the short-circuit power in the network supplying a time-varying receiver. The supply system equivalent impedance, which determines the short-circuit power, may be reduced by the series compensation of the supply system equivalent reactance. As it is known, voltage fluctuations in electric power systems are reduced, for instance, by means of static VAR compensators. In the case of arc furnaces supplied in-parallel with such compensator, filters of current higher harmonics are connected. A number of operating problems connected with co-operation of HH filters with a static compensator, particularly in the case of an earth fault and other transient states, are discussed by the author of paper 4.6. The interesting simulation results of transient phenomena in the supply system for an arc furnace are also given in that paper. Problems of reducing the current and voltage harmonics in electric power systems are the content of papers 4.2, 4.8, 4.3, 4.9, and 4.11.

19

Those papers present in general novel solutions for improvement of non-sinusoidal current and voltage waveforms in electric power systems, i.e for the reduction of current and voltage harmonics. ¾ Paper 4.2 presents the new method for voltage distortion compensation in transmission networks by means of a series converter filter. ¾ The authors of paper 4.8 presented the original method for the improvement of distorted voltage waveforms and power parameters in LV networks, using a superconducting magnet as an energy storage device. ¾ In paper 4.3, identification and compensation of current non-periodic components is proposed. This compensation results in the reduction of current harmonics in three-phase circuits by means of the Kalman filter. ¾ Paper 4.11 present the new efficient threephase diode rectifier system that includes the series-resonance filter of third harmonic and the auxiliary AC/DC converter. This system reduces or eliminates the third harmonic and provides high power factor in the wide range of changes in the supply voltage and load parameters. Reduction of current and voltage assymetry in electric power systems is the subject of papers 4.5, 4.13, and 4.14. The authors of paper 4.5 presented the model and results of testing (by means of EMTP program) of the FC/TCR type static VAR compensator (SVC) used for digital simulation of transient states in electric power systems having this compensator, in particular for load balancing. They stressed the importance of modelling for a control system performing the balancing algorithm. Various solutions of current balancing in three-phase systems by means of static VAR compensators are proposed in paper 4.13. As one of the criteria for optimal balancing, the minimum power losses in a supply system are taken. The simulation calculation of current balancing in the supply system for an unbalanced traction receiver illustrate results of theoretical considerations.

the proposed balancing system having a suitable impedance balancing device. Paper 4.7 proposes the new system for reduction of voltage deviations in electric power systems by use of the longitudinal component of a boosting voltage. The system responds automatically to changes in voltage deviation at the compensator installation point. Paper 4.9 presents interesting problems connected with co-operation (through power converters) between electric energy storage devices in uninterruptible supply systems (UPS) and the electric power system in aspect of power quality. The authors proposed the new solution of UPS device that will improve a supply voltage for a non-linear, time-varying assymetric receiver. Moreover, that device ensures continuity of supply. The useful paper 4.10 of the authors from the Ukraine and the Moldavia, added to Section 4 of the Conference, presents theoretical basis, guidelines for designing, and informations concerning the new controlled AC transmission systems FACTS, or the CFATS for short. The main characteristics of CFACTS 10, 35, and 110 kV systems in operation are given, as well as the design parameters of CFATS 220, and 1150 kV systems (at present under construction). The presented characteristics of CFATS systems are compared with those of traditional systems having double circuit lines and two single circuit lines. Natural power transmitted by CFATS systems is 20 to 50 % higher, current density in conductors is from 2 to 3 times higher, drift and power flow in a system are controllable, environmental effect is lower because of the maximum electric field intensity under a line is decreased by 15 to 40 %, investment expenditure is 10 to 30 % lower, etc. Paper 4.4 is also worthy of notice and presents the differential geometry method for analysis of the power active filter control as well as the results of theoretical considerations for linearization of input-output characteristics of such filters. To sum up, it may be stated that many new solutions improving power quality parameters in suplying networks and distribution networks are presented in the papers in Section 4.

The novel solution for balancing the currents and power in phases of electric power systems are presented by the authors of paper 4.14. This solution consists in using two neural networks in

20


6. SECTION 5: POWER QUALITY IN COMPETITIVE ELECTRICITY MARKETS – ECONOMIC ASPECTS OF POWER QUALITY AND COSTS OF SUPPLY Problems of power quality in competitive electricity markets is presented in the current EPQU Conference for the second time. The papers in this section discuss the following topics: ¾ overall problems connected with power quality in electricity markets, ¾ power quality in power systems in Brasil and Spain, ¾ special aspects of energy and its quality in energy management. Paper 5.1 shall be considered as fundamental in this section. It is the result of co-operation between Polish and American specialists. The paper evaluates thoroughly international and national standards on the subject of power quality existing in the world, in particular in Europe and in Poland. The authors presented their opinion about the legal power quality regulations. Those regulations should be obeyed by both sides, i.e. energy suppliers and users, and should be helpful to raise standards of power supply services. Adequate power quality is one of the important factors in the assessment of such services. Costs of power quality improvement are important and should be shared between suppliers and users. Paper 5.2 deals with problems of power quality in Brazilian power engineering in connection with the new legal regulations in the electric power sector in Brazil. Energy quality management in electric power systems is one of the important tasks of the national Independent System Operator (ISO). The authors of the paper present the main rules and procedures and their own opinions connected with providing the adequate quality of power supplied to customers under the new conditions of the Brazilian electric power sector.

Interesting informations about problems connected with power quality in electricity markets in Spain are given in paper 5.3. In Spain, from the year 1998, the new regulatory framework in electricity sector is obligatory. Quality of power supplied to customers is of great importance in legal acts of that framework. There is a fundamental problem connected with measurements of power quality parameters. The authors suppose that several years are needed to solve this problem. It is worth mentioning that the authors are of the opinion that power quality shall be assessed not only on the ground of reliability and continuity of supply and quality parameters (measurable factors) but also on the ground of a human factor, i.e. satisfaction of consumers. Other papers in this section present some aspects of power quality in electric power systems: ¾ Paper 5.4 discuss costs of devices for improving power quality (for instance, devices using advanced technologies for energy storage), in particular taking into account the total operating time of the devices. ¾ In paper 5.5, the comparison of power losses and costs is presented for devices compensating reactive power and improving voltage conditions. Two versions of devices are compared: a single device and a hybrid device comprised of a thyristor compensator and filters for 5th and 7th harmonics. ¾ The authors of paper 5.7 proposes to widen the present concept of power quality in electric power systems by addition of energy loss factor defined as the ratio of the justified technical losses to the real technical losses in the network of a supply company. On the ground of the above review it may be stated that problems connected with power quality in existing and starting electricity markets are represented in the Conference on a rather small scale.

The authors propose that standarized requirements for power quality in their large country become regionalized (connected with particular regions), and that social and economic aspects of development in regions were taken into account.

7. SECTION 6: EMC IN ELECTRICAL POWER ENGINEERING – ELECTRICAL AND OPERATING CHARACTERISTICS OF ENERGY RECEIVERS AND CONVERTERS

Strategy for calculation of energy prices in Brazil is proposed in paper 5.6.

In papers in Section 6, utilization and conversion of electric energy are presented, in particular: 1.

Plenary Session

Original systems for energy conversion, especially electric energy, their characteristics and test results.

21

2.

Effect of inappropriate supply voltage parameters on the operation of electric energy receivers and converters and on the cost of energy.

In the area of adverse effects of inappropriate voltage quality parameters on electric energy receivers, results of investigations presented in papers 6.8 and 6.9 are worth mentioning.

3.

Evaluation of drive system modernisation on the basis of power quality improvement.

Paper 6.8 describes the trial of global approach to investigating the effect of inappropriate voltage quality parameters on characteristics of various groups of electric energy consumers.

No doubt the new and modernised solutions of electric energy converter systems presented in papers 6.1, 6.2, 6.3, and 6.4 will arouse the interest of delegates to the Conference. ¾ Paper 6.1 describes the wind energy converter having six-phase asynchronous machine and 12-pulse AC converter, its characteristics and the results of experimental investigations. The authors inform that the converter system decreases the amplitudes of current harmonics which it generates. ¾ The novel topology for supply of AC/DC thyristor converters from a three-phase network with unbalanced voltages is presented by the authors of paper 6.2. They proposed the system including a 12-phase converter that is supplied from a special three-winding transformer called T-ADZ transformer. Results of investigations show that the system enables the odd current harmonics generated by the AC/DC converter to be significantly decreased, and voltage asymmetry in the network to be reduced. ¾ The new system for generation variablefrequency three-phase voltages supplying an autonomous three-phase four-wire network in the system with a Diesel engine is presented by the authors of paper 6.3. That system is an integrated variable speed generator including the main drive device, the special magnetic generator and the power electronics threephase AC/DC converter. The paper presents interesting results of experimental investigations of that generating system. On the ground of satisfactory results, the system can be used to supply both linear and nonlinear receivers with a stable voltage. The voltage distortion in the system does not exceed the permissible value. ¾ The authors of paper 6.4 presented the investigation results of the experimental original system for supply of squirrel-cage motors from an three-phase AC inverter in EDM-type drives (Electric Discharge Machine) using a special active filter. The system reduces both distortion of voltage waveform and power losses.

22

Paper 6.9 presents the simulation investigations of the effect of a supply voltage on the dynamic electromechanical characteristics of DC actuators, and their results. It has been evidenced that the dynamic characteristics of such actuators may be tested by means of EMTP program in ATP version of the year 1999. Experimental investigations of power quality parameters before modernisation, and simulation investigations after modernisation of the drive system in a high-capacity rolling mill, and their interesting results are presented by the author of paper 6.10. Emission of current harmonics in the drive system, distortion of voltage supplying the drive system in two cases of the converter control, and other power quality parameters were investigated. As the result of modernisation, the parameters under consideration do not exceed their permissible values. It may be supposed that the method presented in paper 6.6 will arouse the interest of power engineers. It is the method for calculation of costs of supplied electric energy, taking into account inappropriate power quality parameters. The authors also pointed out that low power quality results in errors of energy measurements. Papers 6.5 and 6.7 are worth noting. Paper 6.5 presents results of investigations concerned with a resonance, linear frequency converter having a special capacitor and transformer in its supply circuit. The authors of paper 6.7 describe experimental investigations of dynamic characteristics of a power unit in one of hydro power stations, and their results. The investigations were carried out in aspect of power quality requirements. On the ground of the reviewed papers in Section 6, it may be stated that problems connected with conversion and utilisation of electric energy are represented in the EPQU’01 Conference on a rather small scale. Like in the previous EPQU Conference, the investigation results, which are


concerned with the effect of inappropriate power quality parameters on the operation of electric energy receivers and network equipment, and with importance of those parameters for electric energy users, are insufficient to a certain extent.

time between failures and mean failure duration per year. The author proposed above thesis on the ground of analysis of power failures in the Czech Republic. 4.

8. SECTION 7: RELIABILITY AND CONTINUITY OF POWER SUPPLY Problems of papers in Section 7 are connected with the range of topics of the EPQU’01 Conference but not with the main topics. Reliability and continuity of supply are components determining the quality of supply, and power quality is also such component. Five papers in this section present a number of important current problems connected with reliability and continuity of supply, and interesting investigation results, useful in practical applications. Major results and conclusions are the following: 1.

2.

As given in paper 7.1, the main reliability factors of the Poland’s power system, particularly in 400 kV, 220 kV, and 100 kV networks, were calculated for the year 2020, and compared with those for the year 1998. Those calculations and comparisons are of great importance for the evaluation of above system and the strategy of its development. The results are a fruit of the co-operation between the Electric Power Research Institute (EPRI) in the USA and the Polish Power Grid Company, and confirm practical usefullness of the TRELLS programm for investigation of reliability of transmission lines in large electrical power systems. In many countries, also in Poland, the electrical market requires to determine reliability of energy supply in the points of settlements between the suppier and the user. Each energy supplier should be familiar with supply reliability parameters required. So suppliers are increasingly interested in problems connected with calculation of the accepted reliability factors, and for this reason, statistical data on unreliability of various electrical power devices must be known, as the authors of paper 7.2 have pointed out.

3.

When evaluating reliability of supply to users, in opinion of the author of paper 7.3, it is possible to assess at present, by means of the author’s method, three global reliability factors: number of failures per year, mean

Plenary Session

To improve reliability of supply to users in urban electric power systems, the author of paper 7.4 proposed the separate operation of transformers in 110/220 kV substations, i.e. each of the transformers supplies a separate group of consumers: one transformer – public utility comsumers (using electric heating durably), the second – industrial and other consumers. Those transformers may also operate more economically, because the daily load patterns of a.m. two groups of consumers are different.

5.

The authors of paper 7.5 have confirmed that in case of users, that are sensitive to supply interruptions, the reliability of supply may be improved by means of unconventional energy souces such as synchronous generators driven by a gas turbine, and Diesel generators. In paper 7.5, the computer simulation method (using SYMEL program) is presented and illustrated by the example of supply to some receivers in a sewage treatment plant. The method may be used to select stand-by electric energy sources, and to evaluate their operation.

9. CONCLUSION The papers published as the materials of the EPQU’01 Conference have contributed to significant cognitive and application development in the field of electrical power quality and utilization. It applies especially to problems connected with power quality. The number of papers concerning the use of electric energy as well as the range of topics are rather small, though the presented investigation results are very useful. The novel valuable papers on power quality in supplying networks and distribution networks deal especially with the following: ¾ standardization and problems of power quality in existing and starting electricity markets, ¾ results of simulation investigations of power systems in aspect of power quality, ¾ measurements, determination and analysis of power quality parameters in supplying networks and distribution networks, and their results,

23

¾ concepts of systems for improvement of power quality parameters; investigation results. The conference materials contain also useful papers concerning reliability of power supply and describing new solutions of electric energy conversion. Effect of inappropriate quality parameters on the operation of electric energy receivers and electrical power devices and systems is presented only in a small number of papers. The papers submitted to the organizers of the Conference are rather different in the scientific and linguistic level. The papers were qualified by the members of the Conference Scientific Committee. In respect of contents, about 30 % of the submitted papers were evaluated as very good and ca. 40 % as good. No doubt the contents of the papers will be widened during the Conference, and the discussion will contribute to the development of the Conference topics.

24




CONSORTIUM FOR ELECTRIC INFRASTRUCTURE TO SUPPORT A DIGITAL SOCIETY Clark W. GELLINGS Electric Power Research Institute Palo Alto, California (USA) 1. INTRODUCTION The public profile of electricity and those who provide it is likely to dramatically increase. Unlike the industrial analog economy of the 20th century, the networked digital economy only runs on electricity—that is, a perfect stream of electrons seamlessly linked with a real-time flow of information. The nation's productivity and competitive advantage depend on rapidly facilitating this transformation. Organizations that fail in this endeavor will likely be bypassed or eliminated. Among the most critical issues facing our increasingly digital-based economy is the growing obsolescence of the aging electricity infrastructure throughout the world. x

Today's power delivery systems are as out of step with the needs of a competitive wholesale power market and a networked digital retail market as wagon trails were to automobiles.

x

Unless transformed, emerging technology will encourage by-pass, steadily reducing current assets to the provider of last resort, and ultimately to an industrial relic of the 20th Century.

x

There are many outside of today's energy industry who would like to encourage this outcome as a way of undermining the market position of today's suppliers while reducing fuel diversity and thereby increasing energy cost.

Plenary Session

x

There are few power users, large or small who want to be forced into electricity selfsufficiently. But unless reliability as well as cost is urgently addressed, that will be the inevitable result. In 2000, Silicon Valley firms exceeded the expected annual power demand growth every five weeks as the digital economy took hold. How can the energy industry cope as this trend expands across the nation?

As highlighted in the EPRI-sponsored nationwide Electricity Technology Roadmap effort, the role of electric power has grown steadily in both scope and importance over the past century. Developments in key technologies— including electric lighting, motors, computers, and telecommunications—have continuously reshaped American life and increased the productivity of its commercial and industrial foundations. In 2000, annual manufacturing labor productivity rose 7.1%, the largest increase since 1949. Historically, as technology has advanced, electricity has accounted for a progressively larger share of total energy consumption. It is now nearly 40% of the total in the United States and other countries with similar levels of economic development. This growth demonstrates that electricity-based innovation lies at the heart of economic progress. It is also the key to sustainable economic growth and opportunity in the developing world. With the dawn of the “always-connected” era, an “always-on” world of opportunities has emerged. This, in turn, has formed the foundation for the emergence of the digital economy. According to U.S. Department of Commerce data, GDP grew an average of 4.32% per year between 1996 and 2000, while one element of the digital 25

economy, Internet-based business, grew by 174.5% and created 1.2 million jobs alone. Today, the digital economy is no longer emerging—it is here. And there is a new digital society that has definitively crossed over into a new era of economics and social experience driven by digitally based technological changes that are producing new ways of working, new means and manners of communicating, new goods and services, and new forms of community—as well as a new set of challenges. Digitization of the global economy has proceeded in three phases. First came computers, which revolutionized information processing and fundamentally transformed the way most businesses operate. Next, as the cost of microprocessors plunged, individual silicon chips began appearing in all sorts of applications—from industrial process equipment and medical instrumentation to office machines and home appliances. This embedded-processor phase of digitization has currently progressed to the point that, for every chip in a computer, 30 more are in stand-alone applications.1 Now, phase three involves linking these computers and microprocessors together into networks. There are currently more than a million Web sites available on the Internet,2 potentially available to some 200 million computers around the world.3 As a result, Internet-based commerce already represents about 2% of the American GDP,4 and by the end of next year the revenues from e-commerce are expected to exceed those of

TkWh

the entire U.S. electric power industry.5 Eventually, many stand-alone microprocessors will also be linked to networks, supplying critical information on equipment operations and facilitating even more profound changes in daily life. The proliferation of networked digital technology raises two challenges for those who must supply the necessary electric power— quantity and quality. Together, microprocessors and the equipment they control have helped stimulate growth in the demand for electricity well beyond previous expectations. Just the information technology itself now accounts for an estimated 13% of the electrical energy consumed in the U.S.6, a proportion that is may grow to as much as 50% by 2020,7 as shown in the following figure. Even given the uncertainty in this estimate, it is clear that IT-related electricity usage is growing rapidly and could conceivably eclipse analog power in a few years. However, the amount of electricity used directly for computers and other digital devices represents just the tip of the iceberg: Virtually all of the commercial and industrial equipment controlled by tiny silicon chips also requires electricity, so digitization has simultaneously increased the electrification of the economy. More than 80% of the growth in total U.S. energy demand since 1990 has been met by electrical power,8 and within 25 years electricity is expected to account for more than half of the energy consumed in most industrial nations.9

4 13%

Digital Power 50%

Analog Power

2

1980

26

2000

2020


The demand for higher quality electricity has also become critical. An unprotected microprocessor will malfunction if power is interrupted for even a single AC cycle—1/60th of a second. On an annual basis, that means electricity must be available 99.9999999% of the time—“9nines reliability” as it’s sometimes called. The average reliability of power “at the plug,” however, is only about 3-nines (99.9%), so additional measures are required to prevent the malfunction of computers and other microprocessor-based equipment.10 This task of supplying digital-grade power is urgent. At a steel-rolling mill, for example, plate thickness is controlled by microprocessors and even a brief interruption causes rollers to get out of alignment so that the product must be remelted. Computer failure at a paper mill can create a mess that requires two work shifts to clean up and start over. And a 20-minute power outage at an integrated circuit fabrication facility can cost as much as $30 million.11 Need for a consortium The challenges just described come at a time of rapid change for the U.S. electric power industry and especially for its power delivery system. Federal deregulation has opened utility transmission networks for use by third parties,

resulting in a greatly increased volume of bulk power transactions and a host of new wholesale market players. Meanwhile, most states are now considering ways to increase competition in retail markets and provide customers with greater choice among electricity providers. Grid expansion and upgrades, however, have not kept up with the new demands brought by deregulation. Most transmission and distribution systems were designed more than a half-century ago, when long-distance power transfer was used mainly for economic exchange among a few utilities and when the reliability requirements of distribution systems were much less severe than in a digital economy. So far, the needed improvements in both capacity and reliability have not been made. During the last decade, for example, total electricity demand in the U.S. rose by nearly 30%12 but the nation’s transmission network grew by only 15%.13 Over the same period, expenditures by investor-owned utilities for distribution system construction fell by about 10% in real terms.14 The outlook for the next decade is even worse: Demand is expected to grow by 20%15 but planned transmission system growth is only 3.5%.16 The gap between demand growth and transmission capacity expansion is illustrated in the following diagram.

% 30 25 20 Electricity Demand

15 10

Transmission Capacity Expansion

5 0 1988-98

Plenary Session

1999-09

27

The effects of this lag between demand and infrastructure investment are already being felt. In four of the last five years, the U.S. has faced serious reliability problems. In August 1996, voltage disturbances cascaded through the West Coast transmission system, causing widespread blackouts that cost California alone more than $1 billion. In June 1998, transmission system constraints disrupted the wholesale power market in the Midwest, with pricing rising from an average of $30/MWh to peaks as high as $10,000/MWh. Similar price spikes also occurred in the summers of 1999 and 2000. Distribution

1965 1977 1994 1996 1997 1998

1999

2000

2001

– – – – – – – – – – – – – – – – – – – – – – – – –

November: Northeast blackout July: New York City blackout January: WSCC breakup (Northridge earthquake) December: WSCC breakup July 2: WSCC cascading outage August 10: WSCC cascading outage June: Minnesota-Wisconsin separation June: MAPP breakup July: Chicago (100,000 customers) July: Midwest price spikes to $10,000 MWh December: San Francisco tripoff July: New York City (200,000 customers) July: Chicago (100,000 customers) July: Midwest price spikes to $6,000 MWh August: Chicago (“Loop” business district) May: PJM power voltage reductions and curtailments May: New England price spikes to $6000 MWh June: Rotating blackouts in Silicon Valley Summer: San Diego prices soar November: California prices hit record highs December: California in stage 3 (1 day) January: California in stage 3 (18 days); rotating blackouts January: California PX folds February: California in stage 3 (16 days) March: California statewide rotating blackouts

A new mega-infrastructure is emerging from the convergence of energy, telecommunication, transportation, Internet, and electronic commerce. This is a cornerstone of the new “always-on” world and will require new, digital-grade quality infrastructure and services. The emergence of this mega-infrastructure creates a set of new and challenging needs which are captured in three principal destinations of EPRI’s Electricity Technology Roadmap: 1. Strengthening the power delivery infrastructure by increasing its ability to meet the demands of competition, and strengthening it against natural disasters and man-made threats. 28

system weaknesses have also become apparent, as major local blackouts have affected customers in New York City, Chicago, and San Francisco— sometimes with long-lasting consequences. For example, the August 1999 outage that affected businesses and government offices Chicago’s downtown “Loop” district has prompted the city’s utility, Commonwealth Edison, to launch a $1.5 billion distribution system upgrade program. A summary of major power disturbances, indicating their accelerating pace, is shown in the following figure.

2. Enabling customer-managed service networks that exploit the value of the electron to provide interactive, integrated energy- and communication-based services. 3. Boosting economic productivity and prosperity by fostering the development of innovative electrotechnologies that enable the digital economy. Achieving these goals will assure that digitalgrade electricity will be available to power the digital economy and contribute to the growth and prosperity of a digital society.


2. EPRI LEADERSHIP IN RESPONDING TO A DIGITAL SOCIETY CHALLENGE In response to the growing demand for high quality electricity supply fostered by the digital revolution, EPRI and its Electricity Innovation Institute (EII) have initiated an ambitious program designed to ensure high quality digital-grade electric power is delivered reliably to meet the needs of a digital society. The newly created Consortium for Electric Infrastructure to Support a Digital Society (CEIDS) will build public/private partnerships to explore state-of-the-art power delivery and end-use technologies, as well as yetto-be-developed technologies to meet the energy needs of high technology industries and businesses. Dregulation and rise of the digital economy have set electricity price, quality and reliability on a collision course. The main driving force behind efforts to increase competition in both wholesale and retail power markets was the need to make inexpensive electricity more widely available—in particular, to reduce regional price inequities. Already the effects of deregulation are being seen in the wholesale market, with both prices and price differentials declining rapidly. The effect on retail

markets will come more slowly, but over the next twenty years, the average real price of electricity is expected to fall by 10% for residential customers, 17% for commercial customers, and 14% for industrial customers.17 At the same time, however, industry restructuring has not yet provided adequate financial incentives for utilities to make the investments necessary to maintain—much less improve—power delivery quality and reliability. Meeting the energy requirements of an increasingly digital society will require applying a combination of advanced technologies—from generating devices (e.g., conventional power plants, fuel cells, microturbines) to interface devices to end-use equipment and circuit boards. Simply “gold plating” the present delivery system would not be a feasible way to provide the level of reliability, quality and availability required by microprocessors, nor will it address the vulnerability of the system to damage from natural disasters or terrorist activity. Neither will the ultimate customers themselves find traditional utility solutions satisfactory or optimal in supplying the ever-increasing reliability and quality of electric power they demand.

CEIDS Challenges CEIDS Power System Challenge

Customer Challenge

Economic Challenge

• Vulnerability due to changing system-use, system interdependence

• Increased access to electronics and telecom advances drives revolution in utility services

• Technology innovation counts for > 50% of economic growth

• Explosive growth in bulk power transactions increases demand for transmission access

• Mega-infrastructure emerging from convergence of electricity and communications

• Focusing on enabling technology platforms while building infratechnologies of the future

01/17/01. 1

Plenary Session

29

In addition, new technology is needed if society is to leverage the ever-expanding opportunities of the Internet and electric utilities’ natural connectivity to customers to revolutionize both the role of a rapidly changing industry and the way customers may be connected to electricity markets of the future. CEIDS can enable such a transformation and ushers the direction for building future infrastructure needed by a digital society. CEIDS can create and meet new levels of social expectations, business savvy and technical excellence by attracting players from the electric utility industry, manufacturers and end-users as well as federal and state agencies. In order to achieve this, CEIDS will be guided by the following key principles:

infrastructure in ways that both control cost and improve customer satisfaction. ¾ Government policy makers and their constituents by continuing the acceleration of productivity growth, facilitating consumer choice through the introduction of efficient technologies as the facilitation of truly open markets; and reducing overall energy needs and costs. ¾ Digital equipment producers and users by assuring the availability of highly reliable premium-quality power, reducing energy needs and costs involved in powering digital systems and related physical plant; and facilitating wide-scale choice of providers.

3. Create and foster opportunities that: Vision: To develop the science and technology that will ensure an adequate supply of high quality, reliable electricity to meet the energy needs of the digital society.

Mission: CEIDS provides the science and technology that will power a digital economy and integrate energy users and markets through a unique collaboration of public, private, and governmental stakeholders.

Goals: 1. Establish leadership in anticipating and meeting tomorrow's electric energy needs by: ¾ Applying a combination of the most advanced technologies ¾ Managing the vital link between economic productivity and the quality of electric power supply to customers ¾ Identify the “new” weak links, pressure points, and critical components being driven by the new uses of the nation’s electricity delivery system and making it vulnerable to failure or attack.

2. Enhance value for: ¾ Electricity industry participants by maximizing asset utilization, enabling digital loads to be served profitably, and leveraging the digital

30

¾ Enable digital-quality power supply by ¾ Leveraging the advantages of distributed resources ¾ Defining and facilitating electricity services

value-added

¾ Providing new direct current (DC) electricity supply technologies ¾ Developing and employing advanced power conditioning, power quality devices, and power electronics ¾ Establishing new service quality standards for electricity and related products By focusing on those objectives and applying them to the new concept of a digitally suited power system delivering “always-on” electricity to fulfill the ever changing needs of a digital society, CEIDS will created the following value proposition for the initiative’s participants, players, and society at large: 1. Establish a long-range strategy to ensure that the electric energy industry to continue to provide the quantity and quality of power required by the digital society by: ¾ Providing leadership in defining service quality standards for electricity and related products that conform to the reliability requirements of the digital society ¾ Leveraging resources of all stakeholders to evolve the existing power system to meet tomorrow’s needs ¾ Establishing the system and business flexibility to serve the customers of tomorrow Electrical Power Quality and Utilisation

¾ Ensuring that the system is sufficiently robust to withstand the threat of natural and manmade disaster

¾ Evolving new opportunities to provide DC power as a profitable business ¾ Creating business opportunities by developing new power quality mitigation technologies

2. Enhance stakeholder value by: ¾ Enhancing asset utilization in transmission systems by at least a factor of two

III. The Electric Infrastructure: System of the Digital Future or Relic of the Industrial Era?

¾ Improving system reliability and providing adequate power quality

Realizing the vision, mission and goals of the CEIDS program can help revolutionize the value of electricity service by ensuring that electricity can continue to be the engine driving economic progress. To do so requires the active participation of representatives from all of the key stakeholder communities including utilities, energy providers, end users, and government and quasi-governmental organizations. Therefore, CEIDS will reach out to all of these stakeholders to help define, refine, and fund the work necessary to achieve the vision of meeting the needs of the digital society.

¾ Allowing power providers and system operators to serve new digital loads profitably and reliably ¾ Leveraging the features of the digital infrastructure so as to control costs and improve customer satisfaction

3. Develop new markets by: ¾ Leveraging the advantages of distributed resources as a new business option ¾ Defining and facilitating the opportunity to provide a range of electricity-related products and services

Central Plant

Step-Up Transformer Distribution Substation

FACTS Device Gas Turbine

Distribution Substation

Receiving Station

DC MiniGrid

Microturbine

Set Top TV Box

RTP

Internet Service

CATV

End-Use Devices with Storage Personal Generators Networked Home Auto Demand Response Controllers

Combo Meters Elec/Gas & Water

Home Security

Telephone

Active Power Line Filters On-Line PQ Diagnostics Building Systems Tied to Market Signals Advanced Efficiency End-Use Devices AC Batteries Internet Metering

Cell Phone

Fuel Cell

FACTS Device

Internet Energy Buying

Wireless

Batteries Enhanced EMC Hardened End-Use Devices Multi-Locations Dispatched to Market Signals Gas Turbine

Internet Energy X-Change

Dispatched On-Site Gen

Integrated “e” Controllers On-Line PQ Forecasting Electronic Mfg. Integrated Fuel Cell Flywheel SMES

“e” Business Integrated With All Suppliers

PL Carrier

01/10/01 2929C4.24

Plenary Session

31

The existing electricity infrastructure was created to meet the demands of an analog, standalone industrial economy. As discussed earlier, the economy and society in general have rapidly evolved into a digital, networked paradigm. In this new paradigm, the systems designed to provide power to a static, analog load are inadequate to meet the needs of a dynamic, digital load. Power supply and delivery systems now need to conform to a model informed by drastically different operations, economic, and reliability requirements than those existing at the time the existing infrastructure was developed. In addition, potential terrorist threats now exist that were not remotely imagined when these systems were developed. Meeting the challenge of defining, designing and building the technologies that will provide the backbone for the infrastructure of the digital society requires a multi-disciplinary program of technology development. CEIDS will concentrate on developing technology and products to support the following four technology platforms: 1. The Self-Healing, Digital-Quality Electricity Superhighway – Develop science and technology that will increase the control, capacity, and reliability of power systems so as to supply consumers with the quantity and quality of energy they need at competitive rates. 2. Energy Solutions for End-Use Digital Applications – Provide options to energy consumers for meeting their energy needs by developing innovative technologies that provide greater tolerance to power disturbances. Options will include changes to the power requirements for

digital devices as well as integration with the power system, distributed resources, and power conditioning equipment. 3. Value-Added Electricity-Based Business Opportunities– Develop and implement technologies that will enable consumers to access a variety of electricity-related business opportunities. 4. Digitally Enabled Energy Efficiency – Develop innovative technologies that provide options for consumers to manage and use energy more efficiently. Options will include application of solid-state electronics for the control and utilization of electric power and load management programs. These platforms facilitate the transformation of electricity suppliers, markets and users, enabling a revolution in the way both suppliers and users approach the electricity market. The benefits that can be accelerated and realized through the CEIDS platforms include: ¾ Supporting wide-scale wholesale transactions through enhancement of the power delivery infrastructure ¾ Enabling the digital society through enhancement of power quality and reliability ¾ Enabling productivity through enhancement of reliable networked digital services ¾ Reducing energy needs through development of efficient technologies ¾ Enabling a wide range of electricity-related services to enhance customer choice and the value provided by electricity.

Revolutionizing the Value of Electricity Services Support Wide-scale Wholesale Transactions

Enable the Digital Society

Enable Effective Energy Markets

Increase Productivity

Reduce Energy Needs

Enable a wide range of electricityRelated services

TRANSFORMATION

The SelfHealing Digital Quality Electric Superhighway

Digital Power Solutions for Individual End-Users

Value-Added Electricity Services

Efficient Energy Markets

CEIDS PLATFORM 1/30/01 2976C1.9

32


Organizational Structure of CEIDS CEIDS has been launched through the formation of a 7-member core team housed at EPRI. In addition to support for this leadership, EPRI has allocated $2.5 million in Strategic Science & Technology funds to provide an immediate technical foundation. Formation of CEIDS is taking place against the background of mounting uncertainty about industry restructuring. At this time, no omnibus federal legislation is expected and deregulation of retail markets continues to be handled on a stateby-state basis. There are even proposals to reregulate parts of the wholesale power market—for example, by allowing utilities to own both generation and power delivery assets. This trend is most apparent in California, where the first customers to have price regulations fully lifted saw their electric bills double in just two months.18 As a result, considerable political pressure is being exerted on the California Independent System Operator, which has also been sharply criticized by the state’s Public Utilities Commission.19 In addition, the Federal Energy Regulatory Commission has launched a formal investigation to determine whether California’s free market experiment is “workably competitive” and whether possible re-regulation is required.20 In this case, EPRI proposes to form a publicprivate partnership. However, even EPRI in its perceived position as an instrument of the electric energy industry may have deficiencies in forming such a collaborative. First, EPRI needed to form a supporting not-for-profit organization, governed more broadly by public and private participants. Accordingly, it has formed the Electricity Innovation Institute. CEIDS will be established as an initiative of EPRI and the Electricity Innovation Institute. Once a credible level of private support has been assured, additional federal funds will be sought. Once underway, the Electricity Innovation Institute will provide the management of CEIDS. The Electricity Innovation Institute (EII) The deregulation of the electric power industry has resulted in a substantial decrease in strategic level research and development (R&D) funding on electricity and environmental issues. The EPRI Board, the EPRI Advisory Council and EPRI executive management developed the concept of a new public/private partnership to focus on the technical breakthroughs to achieve the goals of the Electricity Technology Roadmap.

Plenary Session

The Electric Innovation Institute has been established as a tax-exempt status 501c(3) supporting organization and as a scientific organization in the state of California. An application has been made to the Internal Revenue Service which seeks a determination that the Institute is a “publicly supported” organization. As a supporting organization, the Institute is linked to but not a part of EPRI 501 ©(3). As such it will have its own board of directors selected from a diverse group of stakeholders primarily outside the electricity industry, a corporate charter distinct from that of EPRI and a unique set of bylaws that govern its operation. As a non-profit corporation, the Institute will have no “owners,” but it will have a single member (EPRI). Since it is created and must operate exclusively for public benefit, the assets of the Institute will be held in trust for the benefit of the public. The mission of the Institute is to “lead public/private research collaboration to meet the needs of a growing world population for clean, efficient energy and quality of life opportunities that electricity provides.” CEIDS startup funds will be augmented as soon as possible with subscription fees from a broad spectrum of private stakeholders. These stakeholders would comprise companies and trade associations from industries whose productivity is clearly dependent on high-reliability electricity – including, for example, high-tech companies, telecoms, banks, automobile manufacturers, credit card transaction processors, etc. Appeals to such companies and their respective trade associations would be based on the benefits that CEIDS could provide to their long-term self-interests. Such benefits would potentially include a better understanding of the link between electric reliability and economic productivity, and a demonstration of technological solutions to current problems that threaten this linkage. By joining CEIDS, participants can help shape the Consortium’s research program to ensure that it meets the needs of their industry and can gain first access to the technologies developed by that program. Once sufficient private funds have been raised to mount a research effort that could at least make a credible study of current problems and propose viable solutions, EPRI would be in a position to seek public funds to demonstrate the technologies involved. Both CEIDS’s private membership fees and subsequent funds raised from public sources would represent an additional source of consolidated revenue to EPRI and thus help leverage the Institute’s base budget. 33

CONCLUSION Clearly, improving the power quality and reliability needs of the emerging digital society will integrate energy with information services and systems will require widespread cooperation among public and private interests. EPRI is in a unique position to form a consortium that could bring together these diverse interests and fulfill the goals discussed earlier. Among other strengths and experiences relevant to this effort, EPRI has been a traditional leader in developing the standards needed to integrate various types of equipment from numerous vendors into a smoothly functioning power system. EPRI has also pioneered many of the advanced technologies that are now being considered for widespread deployment on transmission and distribution networks and in end-use devices as a way to increase overall system reliability. Finally, EPRI’s credibility can be decisive in attracting sufficient support from diverse private and public sources to form CEIDS and enable it to make a significant contribution in ways that could potentially contribute billions of dollars in increased productivity to the American economy.

REFERENCES

7. The 50% figure comes from Mark Mills, who made the remark in a speech at the Center for Strategic and International Studies, quoted in The Energy Daily, 14 Jan 00, p. 4. According to the article, he was referring specifically to computers and the Internet – including desktops, servers, and routers. 8. Mark Mills, quoted by Peter Huber in National Review, 17 April 00 9. Background Paper for EPRI Summer Seminar 2000, p.3 10. The earliest discussion found so far on the “nines of reliability” concept is in the Inaugural Issue (1999) of The Huber Mills Power Report [published by GilderGroup] 11. Keynote speech made by Lew Platt, CEO of Hewlett-Packard, at an EPRI DA/DSM meeting, 28 Jan 97. 12. EEI Statistical Yearbook (1999), Table 8 13. EEI Statistical Yearbook (1989 and 1999), Table 86 14. Calculated from EEI Statistical Yearbook (1999), Table 70, using a deflator based on the Consumer Price Index, Table 82 15. Calculated from NERC Reliability Assessment (1999-2008), Figure 1

1. Kelly, Kevin. New Rules for the New Economy, p. 11

16. Ibid. Table 5

2. Ibid, p. 46

17. Energy Information Administration, Annual Energy Outlook 2000, p. 66

3. Ibid, p. 11 4. Warrick, Judith. Business-to-Business commerce – The Next Generation, p. 2

E-

5. Ibid. p. 3 6. The 13% figure comes from Robert Nardelli, CEO of GE Power Systems, who made the remark at a PO WER-GEN conference, quoted in Electric Light and Power, Jan 2000, p. 1. This number is hotly contested. Jonathan Koomey of LBL, for example, puts the figure at about 3% (private communication). In addition, Jay Hakes, head of the Energy Information Administration (EIA) is quoted as saying 3%, in Inside F.E.R.C., 7 Feb 00 [a McGraw-Hill publication].

34

18. Widely reported. See, for example, The Wall Street Journal, 3 August 2000, p. 2 19. The Wall Street Journal, 9 August 2000, p. CA 1, and 11 August 2000, p. A4 20. The Wall Street Journal, 24 August 2000, p. A3 21. The emerging 6-nines standard is mentioned by Skip Horvath, president of the Natural Gas supply Association, in Inside F.E.R.C., loc. cit. A later reference is found in remarks made by Pat Wood III, Chairman of the Public Utility Commission of Texas, to an ERCOT meeting on 16 August 2000.


Section 1 Power Quality Parameters: Evaluation and Standardisation

1.1. LO SCHIAVO L., MALAMAN R.: The Regulation of Quality of Supply in Italy (Italy).....................51 1.2. McEACHERN A., MONCRIEF W.A.: Revenue and Harmonics: An Evaluation of Some Proposed Rate Structures (USA) ...........................................................................................................63 1.3. WINKLER G., MEYER J.: Advanced Power Quality Rating Using Statistical Tolerance Intervals (Germany)...............................................................................................................................71 1.4. ZHEZHELENKO I.V., SAYENKO Y.L.: Some Aspects of Calculation of Power Quality in Accordance with European Standards (Ukraine) ..................................................................................79 1.5. VYSKOIL V., ŠPAEK Z., HRKOVÁ J.: Power Quality on the Boundaries Between Transmission and Distribution Network, Observations to the Standardization and Evaluation of PQ, Distortion of Currents in Characteristic Points of the Network (Czech Republic)........................85

Section 1. Power Quality Parameters: Evaluation and Standardisation

49

50



ELECTRICAL POWER QUALITY AND UTILISATION

September 19-21, 2001, Cracow, Poland

THE REGULATION OF QUALITY OF ELECTRICITY SUPPLY IN ITALY Luca LO SCHIAVO

Roberto MALAMAN *

Autorità per l’energia elettrica e il gas Milano (Italy)

Abstract – The economic regulation of utilities usually focuses on price regulation, but price regulation without quality regulation may give unintended and misleading incentives to quality levels. This paper describes reasons to introduce quality regulation for electricity supply, and analyses the Italian experience before and after the creation of the independent regulatory authority. Commercial quality and continuity of supply have been regulated by using overall and individual standards, compensation schemes and revenues-related incentives. 1. THE IMPORTANCE OF REGULATING THE QUALITY OF ELECTRICITY SUPPLY The major changes affecting the electricity sector throughout Europe require renewed attention by regulators to the quality of service provision. The following issues require special attention: i combining quality improvement and cost efficiency;

i devising adequate tools and economic incentives for each aspect of quality; i reducing quality gaps (if any) in the country; i considering the specific responsibilities of each phase of activity (generation, transmission, distribution and supply) as regards quality aspects; i involving customers and their associations in quality improvement. The quality of service in electricity supply has various components which can be grouped under three general headings: commercial relationships between supplier and user (commercial quality); the continuity of supply; and voltage quality. Commercial quality concerns the quality of the relationships between supplier and user. It is also important for potential customers before they select a supplier, because it starts from the moment when they ask for information or apply to be connected to the network. Commercial quality covers many aspects of the relationship, but only some of them can be measured and regulated by standards or other means. Standards may relate to the overall provision of services (often called

*

This paper is not an official document of the Italian Regulatory Authority for Electricity and Gas (Autorità per l’energia elettrica e il gas), and benefits from work by the Working Group on Quality of electricity supply, chaired by Roberto Malaman, constituted within the Council of European Energy Regulators (CEER). The authors wish to thank other members of the CEER Working Group. All the official documents of the Italian Regulatory Authority for Electricity and Gas and the final report of the CEER Working Group can be downloaded from the web site: www.autorita.energia.it


51

Overall Standards) or to the delivery of services to individual customers (often called Guaranteed Standards). Guaranteed Standards are usually associated with some kind of reimbursement to the user in the event of non-compliance. Standards can be defined, for example, in terms of the maximum time taken to provide supply, metering, reading and billing, to supply information, to respond to telephone enquiries, to make appointments, to deal with customers’ complaints, or to provide emergency and other services. Continuity of supply is characterised by the number and duration of interruptions 1 . Various indicators are used to evaluate the continuity of supply in transmission and distribution networks. The purpose of regulation can be to compensate customers for very long supply interruptions, to keep restoration times under control, and to create incentives to reduce the total number and duration of interruptions (and disincentives against any increase in them). The existence of different methods for measuring interruptions and estimating the number of users affected create problems in regulating the continuity of supply. Voltage quality is becoming an important issue for distributors and customers in some countries, both because of the sensitivity of end-user equipment and the increasing concern of some end-users. It is claimed that industrial equipment has become more vulnerable to voltage distortion, while at the same time the use of electronic devices in homes and small businesses has increased the sensitivity of a greater number of users. The main parameters of voltage quality are frequency, voltage magnitude and its variation, voltage dips, temporary or transient overvoltages and harmonic distortion. European Standard EN 50160 lists the main voltage characteristics in low and medium voltage networks, under normal operating conditions. The economic regulation of utilities usually focuses on price regulation, with relatively less attention paid to performance standards and social obligations. On the other hand, technical rules are not generally concerned with economic aspects and cost-efficiency. The linkage of economic and

1

Electric system reliability also depends on “adequacy”, i.e. the ability of the electric system to supply the aggregate electrical demand and energy requirements of the customers at all times, taking into account scheduled and unscheduled outages of system facilities (definition from NARUC, the U.S. National Association of Regulatory Utility Commissioners). Adequacy problems are not discussed in this paper.

52

technical regulation after liberalisation presents a challenge for regulators. Price regulation involves various incentives for maintaining or increasing the quality of supply. In rate-of-return regulation, companies usually define their own investment and quality levels. According to economic theory, this should create an implicit incentive to over-invest 2 in quality, and no incentive for cost-efficiency. In practice, excess quality does not seem to be the main effect of rateof-return regulation; an imbalance between different aspects of quality may sometimes arise, not necessarily reflecting customer preference, but rather the preferences of system operators. Simple price-cap regimes may incentivise a regulated company to reduce its quality of supply by cutting investments, maintenance, or personnel with the aim of increasing its profits. Both rate-ofreturn and price-cap regulation must therefore be accompanied by some kind of regulation of supply quality, the aim being to avoid distorted or excessive investment in the former case, and to prevent a deterioration of quality in the latter. Regulation may also encourage appropriate changes in quality in response to customer demands. Utility regulation must include a clear definition of the “product” supplied to the customer; price regulation without quality regulation may give unintended and misleading incentives to quality levels. Some authors claim to have found evidence of a decline in quality following the introduction of price-cap controls where no specific provision was made for quality regulation 3 . Quality incentives can ensure that cost cuts are not achieved at the expense of lower quality. This is particularly important because some aspects of quality have a long recovery time after deterioration. For this reason, quality regulation should be introduced when restructuring or during price control reviews in order to avoid unexpected quality reductions. For the reasons given above, Performance-Based Regulation 4 frequently includes quality incentives, 2

Over investment under rate-of-return regulation is usually referred to as the Averch-Johnson “over capitalisation” effect (see H. Averch and L. Johnson, The behaviour of the firm under regulatory constraints, in “American Economic Review”, 52, December 1962). 3 See section 4 in L. Rovizzi and D. Thompson The Regulation of Product Quality in the Public Utilities, in M. Bishop, J. Kay, C. Mayer Eds. “The Regulatory Challenge”, Oxford University Press, Oxford and New York, 1995. 4 Performance Based Regulation is any rate-setting mechanism which attempts to link rewards (generally profits) to desired results or targets. PBR sets rates, or components of rates, for a period of time based on external indices rather than a utility's cost-of-service.


even when price regulation was originally introduced without quality-saving or qualitypromotion mechanisms. Quality of service regulation is a governmental responsibility in some countries like Spain and Portugal; in other countries, among which Italy, an independent regulator has been instituted to regulate both tariffs and quality. 2. THE PREVIOUS SELF-REGULATORY FRAMEWORK IN ITALY (1996-1999) In Italy, under Law no. 481/95, the Regulatory Authority for Electricity and Gas (AEEG) is responsible for the regulation and control of quality in both sectors. The law which instituted the authority pays close attention to quality regulation. User protection and quality improvement are among the main goals pursued by the AEEG, which: i must set compulsory minimum quality standards, both overall and individual, for every aspect of the service in every phase (supply, distribution, transmission and generation); i may introduce automatic refunds if quality standards are not met by fault of the companies; i is required to “take account of” the recovery of quality gaps in the price-cap scheme; i may promote initiatives for quality improvement; i deals with claims by users concerning quality and tariff issues. The AEEG does not share its responsibilities for quality standards with any other institution, although central government may hypothetically set further standards besides those established by

the AEEG. As regards electricity, the central government is in fact responsible for concession schemes and is expected to determine universal public service obligations. On its constitution, the AEEG inherited the task of controlling the Citizen’s Charter (Carta dei servizi) scheme introduced before the Authority was created. According to the Citizen’s Charter, each electric utility may set its own quality standards and should indicate at least four individual guaranteed standards. Each utility may also define its own compensation scheme should its self-determined guaranteed standards not be met. The Citizen’s Charter scheme applies only to low voltage (LV) users, not to medium voltage (MV) and high voltage (HV) ones. A first list of quality indicators on which utilities may base their own quality standards was issued in 1995, before the AEEG started to operate. This list has proved to be poorly specified with regard to measurement methods and special clauses. The Citizen’s Charter should be applied by every electricity supplier. The situation in Italy is rather peculiar, in that one national electricity utility (ENEL) supplying 93% of LV users and almost all MV and HV users is matched by a further two hundred local suppliers, most of which are of small or micro size (see table 1). Since its constitution, AEEG has been collecting data on implementation of the Citizens’ Charter scheme by electric utilities and controlling the respect of quality standards. The Citizen’s Charter has been adopted by ENEL and most of the local electric companies. Some 99% of LV users are supplied by an electricity company which has adopted its own Citizen’s Charter (see table 2). The quality standards stated by electricity companies in their Charters differ greatly among the various parts of the country.

Table 1 - Electricity distribution in Italy Low Voltage (LV): about 32 million Medium Voltage (MV): about 110,000 Supply market shares ENEL 93.7 % (without selfOthers 6.3 % consumption) HV: 14 regional divisions ENEL structure: MV/LV: 74 distribution units > 100,000 LV users: 7 major local utilities (big municipalities) Local utilities 5,000 – 100,000 LV users: 30 local utilities < 5,000 LV users: about 160 small utilities (including small islands and remote Micro suppliers mountain valleys) Number of users


53

The situation is also differentiated within the ENEL distribution areas, because ENEL has chosen to set different local standards for different distribution areas, even if this differentiation is not always justified by the objective characteristics of the local area concerned. In general, the individual standards defined by companies in their Citizen’s Charters were not particularly challenging, and as a consequence the percentage of cases in which individual standards were not met is very low. AEEG has published figures comparing the actual quality levels achieved by the utilities against the local standards self-determined by the utilities themselves. As far as compensation to users is concerned, the Citizen’s Charter system has proved very ineffective, because the utilities have in general adopted schemes which pay compensation on request by users. The proportion between the number of compensation payments to the users affected and the number of cases in which the guaranteed standards were not met amounts to only

1 per 1,000 (see table 3). That users are unaware of the guaranteed standards has also been shown by a survey conducted on behalf of the AEEG on a sample of 3,500 domestic users. With regard to the continuity of supply, the data provided by the utilities show a highly differentiated situation, especially between the North and the South, even if account is taken of objective differences between urban and rural areas. The national yearly average conceals two distinct situations dividing the northern regions from those of the Centre and South (see table 4). Some differences have also been highlighted among the cities supplied by ENEL and those supplied by local utilities (generally municipalityowned), although the measurement methods used are not always homogeneous. The problem of homogeneity in continuity measurement rules and practices was addressed by the AEEG in 1998-99 prior to introduction of the new regulations on the continuity of supply (see next section).

Table 2 – Application of the Citizen’s Charter Adoption of “Carta dei servizi”

1996

1997

1998

1999

Number of Citizen’s Charters adopted ENEL

147

147

147

74*

Local utilities (> 5,000 LV users)

31

34

35

36

Micro suppliers (< 5,000 LV users)

33

46

57

64

28.6

28.8

29.2

29.6

2.4

2.5

2.5

2.5

< 0.1

< 0.1

< 0.1

< 0.1

Number of LV users supplied by utilities which have adopted a Citizen’s Charter (millions) ENEL Local utilities Micro suppliers

* Due to the reorganisation of distribution into 74 new operational zones.

Table 3 – Compensation (ENEL and local utilities, LV users) 1996 Standards not met by fault of the utilities

1997

1998

1999

23,846

6,099

4,176

8,418

Penalty payments requested by users

9

57

39

19

Penalty payments paid by companies

5

35

54*

22*

* Since 1998 two local utilities have spontaneously adopted automatic penalty payments 54


Table 4 – Continuity of service (ENEL, LV users) Continuity of supply indicators

1996

1997

1998

1999

Number of interruptions per LV user (> 3’, without notice) Italy (ENEL)

4.8

4.6

4.1

3.8

Northern regions

3.5

2.8

2.6

2.7

Central regions

5.1

5.7

4.9

5.2

Southern regions

6.1

6.1

5.4

5.4

Italy (ENEL)

272

209

196

191

Northern regions

159

125

121

145

Central regions

285

229

230

227

Southern regions

403

302

270

297*

Yearly duration of interruptions per LV user (> 3’, without notice), minutes lost

* The figure for the Southern regions in 1999 does not include 3 regions (out of 8) due to data collection problems

3. STRATEGY AND CRITERIA FOR REGULATION OF QUALITY OF ELECTRICITY SUPPLY To ensure the maximum level of transparency and accountability in its decisions, the AEEG issues consultation papers containing guidelines and proposals before it takes any decision on quality standards. These consultation papers are public (published on the AEEG web site) and are sent to all the stakeholders involved: electricity utilities, consumer associations (both domestic and industrial users), trade unions, environmental associations, technical bodies and associations, and others. During the consultation process, all the parties concerned may formulate questions, offer comments, and submit written proposals. Formal hearings are organised with the main stakeholders on the occasion of important decision-making, and no decision is taken without a consultation round. In March 1998, the AEEG issued a consultation paper setting out the guidelines for regulation of quality in the electricity supply. These guidelines focus on four main objectives. i First, guaranteed standards and service obligations must be introduced to protect users. Guaranteed standards must be homogeneous throughout the country, and they must be set

by the AEEG and no longer by the utilities themselves. The guaranteed standards do not concern the continuity of supply, except for medium voltage users. Automatic compensation payments will also be introduced, superseding the current situation of compensation paid only on demand by users. i Second, comparative mechanisms must be put in place to promote quality improvement. As far as overall standards are concerned, these comparative mechanisms should be limited to publication of the actual levels achieved as compared to the standards set. However, an economic incentive for the continuity of supply should be introduced which links the standard cost recognised to the utilities to actual levels of continuity and/or the recovery of existing gaps. Overall compensation in the form of a tariff discount may also be introduced if the continuity standards have not been met. Such compensation should involve all the low voltage users in the area affected. i Third, special effort must be devoted to making the continuity measurements more comparable and auditable. As a first step towards fulfilment of this commitment, a detailed method for the measurement of continuity indicators has been proposed by a second consultation paper. To ensure the trackability of the data provided by the utilities, this method requires that high and


55

medium voltage lines should be subject to a tele-control system able to detect and record every interruption in the high and medium voltage network. A similar requirement has not been applied to the low voltage network, because of the high costs involved, and because interruptions originating in the low voltage network have an average effect on the continuity indicators which is less than 10% of the total. i Fourth, information on quality standards should be improved through the involvement of consumer associations and new technologies like Internet. The main criteria followed by the AEEG in defining the new regulations on the quality of the electricity supply are the following. i Universality: it is essential to set the same quality standards for users in the same circumstances. However, it may be very difficult to assure quality standards for isolated networks (e.g. small islands and remote mountain areas), not least because small and micro distributors have problems in measuring quality. Moreover, the continuity of supply and voltage quality are objectively dependent on geographical features. Quality standards for continuity of supply and voltage quality must therefore be differentiated among urban, semi-urban and rural areas. i Graduality: implementing control systems and improving quality takes time (more for technical aspects than for relationships with users). The implementation lag should be estimated at 1 to 3 years, depending on the complexity of the automated control system to be introduced. The lag before the continuity of supply and voltage quality improve is not easily calculated. Graduality will be necessary to cope with substantial gaps among different zones, allowing longer recovery times for zones recording the worse levels of continuity. i Responsibility: responsibility for commercial quality, continuity of supply and voltage quality is a crucial issue because the final quality level for consumers usually reflects the behaviour of several players; regulators should clearly distinguish among the responsibilities of all players and use appropriate instruments for each of them. i Value of quality: modern quality regulation strategies tend to focus on outputs (effects on customers) rather than on input or expenditure. Regulatory bodies should not intervene in decisions on technical solutions or investment 56

plans; if outputs are measurable the regulator should focus on these. Setting the value of the energy not supplied is a necessary condition to establish compensation payments on continuity of supply. Given that the value of energy not supplied may differ greatly according to the kind of user, an average value of quality must be estimated. Furthermore, there are several criteria for estimating the average value of quality, which implies that discretionary decisions must be made by the competent authority. i Implementation and control: quality can only be measured by companies. The regulatory body determines the measurement rules and checks measurement procedures by means of sample inspections. Quality certification according to the Iso 9000 scheme is a useful device with which to introduce quality management procedures and systems, but it may not be enough per se to ensure the regular assessment of quality indicators. 4. NEW REGULATION OF QUALITY OF ELECTRICITY SUPPLY Since 2000, the Citizen’s Charter scheme has been replaced by the AEEG’s new quality regulations for commercial quality and the continuity of supply. Voltage quality has not yet been regulated by the AEEG, so that EN 50160 technical standards still apply. 4.1. New regulation of commercial quality The new regulations on commercial quality have been developed through the co-ordinated use of guaranteed standards (subject to automatic penalty payments in case of mismatch) and overall standards. After wide-ranging consultation, the AEEG has set six guaranteed standards for LV users and four for MV users. Guaranteed standards are homogeneous throughout the country (except for very small suppliers), and they supersede the previous standards defined by individual utilities (see table 5). However, companies can define their own standards if these are better than those defined by AEEG, or if they concern other quality aspects. If guaranteed standards are not respected by fault of the company, the users affected are entitled to receive penalty payments automatically through their bills; the amount of these penalty payments differs among LV-domestic, LV-non domestic and MV users (see table 6). Electricity utilities may avoid payment of compensation only under a strict Electrical Power Quality and Utilisation

list of conditions regarding, for instance, exceptional weather conditions or responsibility of the user. The new system of automatic penalty payment supersedes the previous ineffective system of penalties on demand. Users can appeal to the courts if the damage due to non respect of standards is greater than the penalty payment received.

annually, while actual performance is subject to comparative publishing. The overall standards for ENEL refer to provincial areas (there are about one hundred provinces in Italy), and they are distinguished by class of user (LV-domestic, LVnon domestic and MV user). Overall standards are not subject to penalty payments, nor do they produce any other economic effect.

The AEEG has established a set of overall standards (see table 7) which are monitored

Table 5 – Guaranteed standards (in force from July 1st, 2000) Services

LV USERS

MV USERS

Estimating charges (simple works)

15 working days

N/A

Providing supply (with simple works)

15 working days

N/A

Providing supply (without works)

5 working days

10 working days

Disconnection on request

5 working days

7 working days

1 day (1)

1 day (1)

3 hours

3 hours

Reconnecting users disconnected for non payment Maximum hour-band for customised appointments (2) (1) Except holidays

(2) Appointment fixed without respect of the relevant maximum time, only if the customer agrees

Table 6 – Automatic penalty payments paid to users in 2000 (July 1st – December 31st) LV DOMESTIC LV NON DOM. USERS USERS Amount of single penalty payment

MV USERS

LIT 50,000 (~25 Euro)*

LIT 100,000 (~ 50 Euro)*

LIT 200,000 (~ 100 Euro)*

2,733

621

8

1,675

403

5

169.1

40.3

1

1,488

1,059

1,459

1,007

99.5

146.3

ENEL Cases of standards non met Number of penalty payments paid Total amount (Millions of LIT) Other suppliers (> 5,000 LV users) Cases of standards non met Number of penalty payments paid Total amount (Millions of LIT)

4 1 0.2

* Some utilities have spontaneously fixed higher penalty payments than those defined by AEEG


57

Table 7 – Overall standards (in force from January 1st, 2001) OVERALL STANDARDS

MAX. TIME (LV&MV USERS)

MIN. % (LV USERS)

MIN % (MV USERS)

Estimating charges (complex works)

40 working days

85%

80%

Providing supply (with complex works)

60 working days

85%

80%

Billing problems resolution

15 working days

90%

95%

Verifying meter or voltage problems

10 working days

90%

95%

Responding to complaints and written queries

20 working days

90%

95%

Number of users with at least one meter reading (or self-reading) per year (1)

95%

N/A

Number of reading-based bills with a reasonable difference from estimates (2)

85%

N/A

90%

N/A

Appointments kept in the hour-band for estimating charges visits

3 hours

(1) Applicable only to LV users with estimated bills (2) The reading-based bill must not exceed more than one and half times the average expense paid by the user in the estimated bill; applicable only to LV users with estimated bills

AEEG has defined graduality to introduce new guaranteed and overall standards: i guaranteed standards have been in force since 1 July 2000 (except for a 3-hour band for customised appointments, which came into force on 1 January 2001); i overall standards came into force on 1 January 2001; suppliers with fewer than 5,000 users are not compelled to adopt the new standards defined by AEEG but a special mention is foreseen in the yearly report should they do so.

4.2. New regulation of continuity of supply Continuity of supply is the most important factor for both domestic and business users of the electricity supply. Because of its importance, the AEEG first paid close attention to systems for the measurement of continuity, in order to ensure that interruptions are measured in the same way throughout the country, and to make the continuity of supply measurements more comparable and auditable. The AEEG then drew up new regulations in order to reduce unplanned interruptions, introducing a link between the continuity of supply and the tariff. Law no. 481/95 requires the AEEG to recognise the costs already 58

incurred for quality in the tariff base, and to include? the costs incurred in closing quality gaps in the price-cap mechanism. The link between quality and tariffs has to be defined under a single national tariff constraint. As a consequence, an equalising fund is needed.

4.2.1. Measurement of interruptions AEEG has developed a method for standardising the measurement of interruptions among ENEL and other suppliers and, within ENEL, among its different operational zones. Each interruption must be recorded both automatically (by the SCADA system) and manually (for restoring operations). The number of customers affected must be actual for HV and MV users, but it may be estimated for LV users using estimate rules defined by the AEEG. AEEG has also defined three different types of cause of interruption, as follows: i acts of God (“force majeure”): emergencies or disasters attested to by local or national authorities, or weather conditions beyond design requirements for networks; i user or third party responsibility; i supplier’s responsibility. Electrical Power Quality and Utilisation

Interruptions have also been classified by origin in order to take account of different distribution network layouts: interruptions originating in the transmission grid, in the HV, MV, and LV distribution grid. In order to take differences between urban areas and rural areas into account, Italy has been classified into three density levels: i high density (urban) areas: municipalities with more than 50,000 inhabitants; i medium density (suburban) areas: municipalities with more than 5,000 and fewer than 50,000 inhabitants; i low density (rural) areas: municipalities with fewer than 5,000 inhabitants. Special attention has been paid to distinguish low and medium density areas (suburbs) from high density ones (downtown areas) in the largest cities (e.g. Rome).

4.2.2. Regulation of unplanned interruptions (long, i.e. >3’) Italy’s average actual level of unplanned interruptions is generally worse than in other EU countries. Moreover, there are major differences between northern regions and southern ones, even at the same density level. The MV network is the critical priority: interruptions originating in the MV network account for 85% of the minutes lost (in the HV network: 3%; in the LV network: 12%). In this situation, the AEEG’s objectives in defining the new regulations on the continuity of supply are the following: i enhance the overall level of continuity in Italy, and bring the country’s average level closer to European benchmarks; i bridge the gaps between north and south, reducing the differences among regional and district continuity levels; i protect consumers through automatic compensation, applying where possible individual compensations to guaranteed standards, and otherwise using overall standards with economic effects in the national tariff;

designed to solve the problems of industrial users with special quality needs. In order to highlight the responsibility of distribution suppliers and to simplify regulation, AEEG has decided to adopt a single indicator: minutes lost per LV user (customer minutes lost or cml), net of interruptions caused by acts of God and by users or third parties and net of interruptions originating in the EHV/HV networks. This indicator is measured the most close to LV users in about 300 districts (about 100 for urban areas, 100 for suburban, 100 for rural areas) covering almost all the country. The AEEG has devised two mechanisms to make the indicator more responsive to weather effects: economic effects are linked to the 2-year rolling average of the indicator and are neutralised if actual levels lie in the “deadband” (+/- 5%) around the standard level. The new regulations define nation-wide reference standards which do not depend on the actual level of each district and indicate the optimal quality levels expected: i urban areas: 30 minutes lost; i suburban areas: 45 minutes lost; i rural areas: 60 minutes lost. Because the nation-wide reference levels were greatly at variance from the actual situation when the new regulations were introduced (1 January 2000), district-wide compelling standards have been defined to identify the minimum improvement required in each district. These district-wide compelling standards are defined for each district and for each year according to a fixed set of yearly rates of improvement , starting from the actual 2-year 1998-1999 average. The minutes lost in each district must decrease year by year according to the improvement rates defined by the AEEG and which range from 0 to 16% according to the starting level. The worse the starting level, the greater the improvement rate required in order to force the district with most interruptions to converge on the performance of the best ones. Applying improvement rates to the actual situation of the 1998-1999 2-year period, AEEG has defined district-wide compelling rates for each of about 300 districts. Table 9 provides a synopsis at the national and macro-regional level.

i introduce flexibility for special quality needs via the possibility for companies to define – under the supervision of the Authority - new price options with special quality levels


59

Table 8 – Yearly rates of improvement required (2000-2003) Starting level (2-year average)

Yearly rate of improvement required

High-density districts

Medium-density districts

Low-density districts

Up to 30 minutes lost

Up to 45 minutes lost

Up to 60minutes lost

0%

30 to 60 minutes lost



5%




8%




10%




13%

Over 150 minutes

Over 271 minutes lost

Over 360 minutes lost

16%

Table 9 – District-wide compelling standards (2000-2003)

ITALY (ENEL)

Starting level 1998-1999

Standard

Standard

Standard

Standard

1999-2000

2000-2001

2001-2002

2002-2003

150 cml

133 cml

120 cml

109 cml

99 cml

11%

20%

28%

Average improvement Northern regions

96 cml

89 cml

Average improvement Central regions

180 cml

Average improvement Southern regions Average improvement

213 cml

83 cml

78 cml

34% 73 cml

8%

15%

21%

26%

157 cml

140 cml

125 cml

113 cml

13%

22%

31%

37%

186 cml

163 cml

145 cml

130 cml

13%

24%

32%

39%

Cml: customer minutes lost (2-year rolling average, net of interruptions due to external causes) By the end of March of each year, the utilities must provide the AEEG with their figures on the continuity indicators. After its sample controls, the AEEG calculates the 2-year rolling average and compare actual levels with district-wide standards. If the utilities have improved continuity more than required, they gain an extra incentive related to the extra-improvement. If they have improved continuity less than required, they must pay a penalty related to the less-than-required improvement. In both cases, the extraimprovement or the less-than-required improvement are evaluated using the parameter of 600 Lit/min/kw (about 18 Euro/kwh), which constitutes a strong incentive to reduce interruptions more than required, or at least to respect the district-wide compelling standards. The incentive system is funded by the penalties paid by utilities for districts in which the basic 60

improvement rates are not met, and for the net difference between incentives and penalties, through a Q-parameter in the price-cap formula:

'P = RPI - X + Q Because there is a single national tariff, an equalisation fund is needed to distribute incentives to utilities which are linked to different levels of quality without changing the final tariff district by district.

4.2.3. Controls Because the data on continuity levels are provided by utilities, the AEEG checks that interruptions have been recorded in a complete and satisfactory manner, according to the measurement rules defined by AEEG itself. Checks are carried out on a randomly selected sample of districts, in each of Electrical Power Quality and Utilisation

which AEEG officers examine a sample of interruptions to determine whether they have been correctly recorded, both automatically and manually, and that continuity indicators have been adequately calculated. The AEEG has defined two indexes to evaluate these controls: i an accuracy index, which measures whether all events have been recorded; a maximum 10% of inaccuracy is allowed; i a precision index, which refers only to the sampled interruptions and checks the calculation of the continuity indicator; a maximum +/- 3% of lack of precision is allowed.

4.2.4. Next steps The continuity of supply regulation must be completed with the following new rules: i requirements and costs for special connections with quality recorder to allow the individual measurement of interruptions and voltage dips; i individual guaranteed standards at least for HV and MV users; i some overall regulation is also expected for short (duration 1’’-3’) and transient ( 1000 100 < Isc/IL < 1000 50 < Isc/IL < 100

300 250 200 150

20 < Isc/IL < 50

100 50

Isc/IL < 20 49

45

41

37

33

29

25

21

17

13

9

5

1

0 Harmonic order - odd harmonic currents


N ^ 1.333

67

For common power conductors and low-order harmonics, the assumption of well-developed skin effect may not be appropriate. The approach taken by C57.110, Ref.[4], in derating transformers subjected to harmonic currents is to divide the winding resistance into two parts - a DC resistance that is independent of frequency, and an AC resistance that increases as frequency squared (a worst-case assumption). The AC resistance part is responsible for "winding eddy current losses," which include resistive skin effect and proximity effect. This derating approach is consistent with "k-factor" analysis of transformers [5] [6]. The current weighting suggested by C57.110 suggests

KN

1 x ( N 2 1)

(13)

where x is the ratio of resistance at the fundamental divided by the resistance at DC. Ranges for x are given in table V of Ref. [7], where it is shown that x=0.01 for most residential and commercial transformers, but x increases to 0.09 to 0.15 for substation transformers. This weighting may be most appropriate when transformer losses are the main concern. It has been suggested that the weights for triplen harmonic currents might be increased in environments downstream from delta-wye transformers to account for increased losses due to circulating currents. That matches the EN 50160 philosophy that has a different table for "multiples of 3" harmonics. Regarding voltage weighting, it is possible that the weights for voltages could be set to 1; alternatively, the weights for harmonic voltages could be less than one, or even zero, which would imply that voltage delivered at harmonic frequencies is worth less than voltage delivered at the fundamental frequency. 7. EQUATIONS FOR HARMONICADJUSTED POWER FACTOR The following equations may be useful for calculating harmonic-adjusted power factor.

EH EH: N: CN: EN: 68

ª 50 2 º «¦ C N E N » ¬ N =1 ¼

1/ 2

(14)

Harmonic-adjusted RMS voltage Harmonic order Adjustment factor from voltage adjustment table 3 Measured voltage at harmonic order N

IH IH: N: KN: IN:

ª 50 2 º «¦ K N I N » ¬ N =1 ¼

1/ 2

(15)

Harmonic-adjusted RMS current Harmonic order Adjustment factor from current adjustment Table 2 Measured current at harmonic order N

VA H

EH IH

(16)

VAH: Harmonic-adjusted instantaneous volt-amps EH: Harmonic-adjusted RMS voltage IH: Harmonic-adjusted RMS current

VAH hrs

³ VA dt H

(17)

VAH-hrs: Harmonic-adjusted volt-amp-hours VAH: Harmonic-adjusted volt-amps

hPF

w hrs VAH hrs

(18)

hPF: Harmonic-adjusted power factor w-hrs: watt-hours VAH-hrs: Harmonic-adjusted volt-amp-hours 8. CONCLUSIONS Increased costs resulting from harmonic currents and voltages are shared at present by all rate payers. Rate structures can be modified to more fairly allocate these increased costs. There are several approaches that may be taken to modifying rate structures to encourage load behavior that has minimal harmonic impact. One approach, harmonic-adjusted power factor, appears to be especially attractive. More work is required to achieve consensus on weighting factors for harmonic-adjusted power factor. More work is also required in investigating how direction of harmonic flow may influence the proper application of financial incentives. ACKNOWLEDGEMENTS This paper is based on an IEEE paper originally prepared by the authors and W.M. Grady, G.T. Heydt, and M. McGranaghan, whose original contributions the authors gratefully acknowledge.


9. REFERENCES 1. IEEE Standard 519-1992, IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems 2. IEEE Task Force (V.E. Wagner, Chairman), "Effects of Harmonics on Equipment", IEEE Trans. on Power Delivery, Vol. 8, No. 2, April 1993, pp. 672 - 680 3. G.T. Heydt, Electric Power Quality, Stars in a Circle Publications, Lafayette, Indiana, 1991 4. ANSI/IEEE Standard C57.110-1986, IEEE Recommended Practice for Establishing Transformer Capability when Supplying Nonsinusoidal Load Currents 5. D.A. Dini, "Testing and Rating of Transformers for Use with Nonlinear Loads", Underwriters Laboratories, Chicago, Illinois, 1992 6. I. Kerszenbaum, A. Mazur, M. Mistry, J. Frank, "Specifying dry-type distribution transformers for solid-state applications", IEEE Trans. on Industry Applications, vol. 27, no. 1, Jan/Feb 1991, pp.173-178

Alexander McEachern was born in Boston, Massachusetts in 1955. He studied Electrical Engineering and Computer Science at the University of California at Berkeley. He is a founder and director of Dranetz-BMI a manufacturer of precision powerrelated instruments. He is also the founder of Power Standards Laboratory, and represents the United States and the IEEE on several IEC working groups. W.A. (Bill) Moncrief was born in Atlanta, Georgia in 1947. He received the BEE degree in 1969 and MSEE degree in 1975 from Georgia Tech. He manages field services and other power quality research areas for EPRI, the Electric Power Research Institute.

7. D. E. Rice, "Adjustable Speed Drive and Power Rectifier Harmonics - Their Effect on Power Systems Components," IEEE Trans. on Industry Applications, Vol. IA-22, No. 1, January/February 1986, pp. 161-177. 8. M.S. Hwang, W.M. Grady, and H.W. Sanders, Jr., "Distribution Transformer Winding Losses Due to Nonsinusoidal Currents," IEEE Trans. Power Delivery, Vol. PWRD-2, Jan. 1987, pp. 140-146 9. EN 50160:1999 Voltage characteristics of electricity supplied by public distribution systems, CENELEC


69

70





SOME ASPECTS OF CALCULATION OF POWER QUALITY IN ACCORDANCE WITH EUROPEAN STANDARDS Igor V. ZHEZHELENKO Yuri L. SAYENKO Priazovskii State Technical University Mariupol (Ukraine)

Abstract - The authors consider problem-solving techniques for calculation of electric power quality parameters in accordance with EN 50160 European standards. Calculation of voltage fluctuation in rolling mills networks is treated in the first part of the paper. The second part deals with the problem of selecting the power of voltage fluctuation compensation devices. The third part is devoted to estimation of negative sequence impedance and valve converters in calculations of asymmetrical modes.

1. CALCULATION OF VOLTAGE FLUCTUATION IN ROLLING MILLS NETWORKS When designing electrical power systems (EPS) with quick-variable loads which may cause voltage fluctuation (VF) in the distributed circuit, it is necessary to define the values of flicker intensity (FI). Let us consider application of computing methods that can be used not only at the design stage, but in EPS operational conditions as well. In case of periodic VF of sinusoidal and triangular form in definite frequency ranges the waveform of the allowed values of VF amplitudes, depending on frequency and reduction coefficients to equivalent rectangular fluctuations for which the curve of the allowed values has been plotted. With rectangular 9 HZ (and higher) VF fundamental fluctuation, which is inside the rectangular, exceeds by 27 % the sinusoid of the same amplitude. That is why for frequencies over 9 HZ coefficient of 1.27 (e.g. at 25 HZ 0,84/0,66 = 1,27) can be applied for approximate equivalent replacing of sinusoid and rectangular VF.

At F = 3 HZ interharmonics (IH) of the meander voltage have a more critical for perception nature than the fundamental fluctuation. For example, rectangular VF at frequency F = 1 HZ is 3 times more dangerous than the similar sinusoid one. At fluctuations of triangular form the first harmonic is 19% less its amplitude. Thus, triangular VF are 19% less the critical value as related to the sinusoid signal of the same amplitude. Method of estimation of FI with the help of the curve of the allowed values and Fourier analysis may also be applied for analyzing other periodic VF. Flicker intensity may be defined with a reasonable facility for a certain type of single or repeated VF. A more general approach involves defining shorttime FI - Ps tk , caused by VF of different waveform. Then the resultant FI for T period is determined and calculated as the result of stepped form fluctuations Ps tk with Feq, coefficient called coefficient of equivalence (EN 50160):

Ps t k

Feq Ps t s .

(1)

Application of this method of estimation of FI is, in most cases too cumbersome. Calculation of VF in EPS of rolling mills can be carried out following a simplified procedure. With sloping VF having a negligible effect on FI value, it is permissible to allow in calculations variation of the voltage of rectangular form only. In case of non-periodic fluctuations, d 10 (min.) interval is considered for determination of the degree of a short-time flicker intensity. Calculation algorithm includes as follows: 1. Rectangular VF amplitudes are computed


79

G Ui .

di

(2)

scheme in Fig.1 is defined by the known equation:

2. Depending on the duration of T (min.) interval the equivalent fluctuation frequency is defined.

1 , min. T

r

(3)

k F

x sc , x sc x C

(7)

where sc is short-circuit impedance of EPS.

3. The allowable amplitude of VF is defined in line with Fig.2.2 [1] by r value. do

f (r )

(4)

4. Intensity of short-time flicker: n

3

¦

di

3

n

3

i 1

i 1

Pst

¦G U

do

3 i

.

do

(5) Fig.1. Simplified scheme of electrical supply of rolling mill

Error due to calculation performed by the above given algorithm does not exceed 1.5%. In design practice it is required to estimate FI within the network points, interconnected by transformers, air lines or cables with valve converters. Elements of EPS are basically of inductive nature (we are dealing with such here), that is why VF waveform in 220kV and 0.4 kV points correspond with great accuracy to the waveform of VF source; their amplitudes in complex network points decrease proportionally to k u( ) value, where k u( ) is the voltage-transfer coefficient or coefficient of voltage distribution between the points of VF source and n point. The structure of expression (5) allows to conclude that estimation of FI in the points connected to VF source can be, in the considered case, carried out using the expression [1]

Pst

( )

Pst

(u )

ku

()

,

(6)

where Pst(u ) - is FI in the point with VF source. Hereinafter coefficient k u( ) will be referred to as coefficient of FI distribution and designated with ( ) k uF . A simplified scheme of electric supply of a rolling mill is presented in Fig.1 as an example: VF source, the valve converter of a rolling mill, is quick-variable load. The scheme shows as a coupling transformer with the power system; 1…m as transformers, supplying other substation consumers. Coefficient of FI distribution on 220kV side of the 80

Presence of individual or commuted within filter compensation devices (FCD) capacitor banks reduces to a certain extent the accuracy of ( ) estimation k (F) ; in this case kF value must be decreased by 5-10%. It should be stressed again that the described solution, though it refers to a very important for practical work case, cannot be extended to the case when VF have the waveform different from that which is characteristic for quick-variable load of rolling mills. 2. DECREASE OF VF BY MEANS OF HIGHSPEED STATIC COMPENSATORS (SC) OF REACTIVE POWER (RP) Compensation of VF in the given case is carried out at the expense of the compensation of RP surge. To ensure compensating effect the delay time at generating RP by compensator should be minimal, not to cause increase of the VF level. This is, for example, the case of compensation of RP surge of rectangular form (Fig.2) with a certain delay time 't when instead of one there appear two RP surges (Fig.3) and VF level increases. No less important is the problem of selecting the SC power. Maximum compensating capability of SC is connected with the maximum VF amplitude, which can be compensated, is determined by the following expression [1]:

Qc max

GU c max 100 %

Ssc

It is quite obvious that

d c max Ssc . 100 % with

difference

(8) of


d i d c max VF will be totally compensated, and at

The last two equations yield

d i ! d c max they will be compensated to the value of d i d c max .

d c max

§ 1 · ¸ d max ¨¨1 Pst ¸¹ ©

(12)

Qc max

§ 1 Qmax ¨¨1 P st ©

(13)

or

· ¸¸ . ¹

It should be noted that computation by equation (13) yields overestimated results (5 y 10% allowance depending on the degree of differences of VF amplitudes).

Fig.2. Change of RP load (solid line) and SC (dashed line)

3. CONSIDERATION OF VALVE CONVERTERS PARAMETERS IN CALCULATING ASYMMETRIC MODES

Fig.3. Summary diagram of RP load and SC Flicker intensity after compensation

§ Fi 'd i ¨¨ d0 1 ©

n

Pst

3

¦ i

where 'd i

3

· ¸¸ , ¹

(9)

°d i d c max , d i ! d c max ; ® d i d d c max . °¯0,

Value of d c max and, consequently, maximum capacity of SC can be estimated when Pst

1.

However it does not seem possible to solve the equation analytically. Equivalent number VF req under the condition that all VF have maximum amplitude of d max is derived from the following equation

Feq d max d 0 (req )

u a (t ) U m sin Zt H 2U m sin(Zt M 2 ) ;

2S ) 3 2S H 2U m sin(Zt M 2 ); 3

(14)

u b (t ) U m sin(Zt

2S ) 3 2S H 2U m sin(Zt M 2 ), 3

(15)

u c (t ) U m sin(Zt

Pst .

(10)

On the other hand, after compensating these, equivalent by FI, maximal VF to the permissible level ( Pst 1 ) we arrive at:

Feq (d max d c max ) d 0 (req )

In calculating asymmetric modes of electrical supply systems, valve converters, as any other elements of electrical networks, are introduced into the equivalent circuit by negative sequence impedance. Estimation of the impedance for the 6 pulse non-controlled converter with Y/Y circuit diagram of transformer winding in particular is given in [2]. Consideration of a common case for various circuits of 6 and 12 pulse converters certainly presents scientific and practical interest. Let us first consider a 6 pulse converter with Y/Y circuit diagram of transformer winding. Let us assume that asymmetric system of voltages is applied to such converter

1.

(11)

(16)

where H 2 - coefficient of asymmetry; M 2 - angle of phase shift between voltages of positive and negative sequences. In case of infinitely high circuit inductivity of rectified current and neglecting the commutation phenomenon the curve of the network current will acquire rectangular-stepped form [1]


81

Fig.4. Voltage and network currents waveform of a converter with Y/Y scheme of transformer winding On calculating the amplitudes of fundamental current harmonics and their initial phases, it is possible to obtain the actual values of current phases in a complex form:

Ia

Ib

Ic

I ma 2

e jM a

2 2 Id

S

I m jM b e 2

2 2 Id

I mc

2 2 Id

2

e jM c

S

S

sin

D 2 D1

e

2

sin

D3 D 2

sin

D 3 D1

2

e

2

j(

j(

e

D1 D 2 S

2

) 2 ;(18)

) 2

.(19)

As angle D1 , at which the current begins to flow in phase A (Fig.4), corresponds to the moment in time t1, when u a (t1 ) u c (t1 ) , it can be derived from the following equation:

U m sin Zt1 H 2U m sin(Zt1 M 2 ) U m sin(Zt1 H 2U m sin(Zt1 M 2

2S ). 3

2S ) 3 (20)

On solving this equation we obtain:

D1 Zt1

arctg

1 H 2 ( 3 sin M 2 cosM 2 ) 3 H 2 ( 3 cosM 2 sin M 2 )

.

3 H 2 ( 3 cos M 2 sin M 2 )

S arctg

1 H 2 cos M 2 . H 2 sin M 2

; (22)

(23)

=2

| I |

| I aI a 2 I |

b c H 2 1 =1 H 2 a =1 . 2 | I2 | | I a a I b aI c |

In a particular case, at M 2 =2

(24)

0 , considered in [1],

(1 H 2 1 H 2 H 22 ) =1 | (2 1,5H 2 ) =1 .(25)

At M z 0 the expression for negative sequence impedance of a converter is too cumbersome. That is why to arrive at reasonable solution a series of calculations was performed on computer for various values of angle M 2 . Analysis of the results allowed us to conclude that at negative sequence voltage coefficient H 2 d 0,1 , or any angle values M2

(21)

Angles D 2 and D 3 may be obtained in a similar way from:

82

1 H 2 ( 3 sin M 2 cos M 2 )

Negative sequence impedance of a converter can be defined by the values of the currents of positive and negative sequence of the valve converter [2]

D1 D 3 S 2

S arctg

D3

) 2 ;(17)

2

D 2 D 3 S

j(

D2

= 2 | 2=1

(26)

For example at M 2 30 0 and H 2 0,04 the following values (in relative units) have been obtained Electrical Power Quality and Utilisation

D1

27 ,0 7 ; D 2

Ia

0,793e

Ic

0,766e

151,01 ; D 3

j 0,0 6

j120,0 6

; Ib ; I1

0,780e

271,0 2 ; j121,01

0,779e

u b (t1 ) uca (t1 ) ; ub (t 2 ) uca (t 2 ) ; u b (t3 ) ub (t3 ) .

;

j 0,0 001

;

(28)

Taking into consideration equations (14) – (16) and (27) we obtain:

1 j 31, 7 I2 0,016e ; = 2 1,998 = 1 . Dependence of negative sequence impedance on H 2 at M 2 30 0 is given in Fig.5 0

Let us now consider a 6 pulse converter with 8 / ' circuit diagram of transformer windings. In this case linear voltages are applied to the converter u b

u a (t ) ub (t ) ; ub (t ) ub (t ) uc (t ) ; u ca (t ) u c (t ) u a (t ) ,

(27)

and the network current waveform, in phase A for

H 2 sin M 2 ; 1 H 2 cos M 2

E1

arctg

E2

arctg

E3

S arctg

3 H 2 ( 3 cos M 2 sin M 2 ) 1 H 2 ( 3 sin M 2 cos M 2 )

(29)

(30)

;

3 H 2 ( 3 cos M 2 sin M 2 ) 1 H 2 ( 3 sin M 2 cos M 2 )

.

(31)

Amplitude of the fundamental harmonic current I m and initial phase M are determined from the following expressions:

example, will have the following view (Fig.6). Angles E 1 Zt1 , E 2 Zt 2 , E 3 Zt 3 can be derived from the equations:

Fig.5. Dependence = 2 (H 2 ) for 6 pulse converter with Y/Y circuit diagram of transformer windings


83

Fig.6 Network current of phase A of 6 pulse converter with 8 / ' circuit diagram of transformer windings. Im

2 Id 3S

E3

E2

³ E

( sin Zt dZt 1

³ E

E1 S

2 sin Zt dZt

E3

³ E

E1 S

³ E

( cos Zt dZt 2 cos Zt dZt 1

=

2 Id 3S

2

Ib Ic

2

1 2 1 2

arctg

2 sin E1 sin E 2 sin E 3 2 cos E1 cos E 2 cos E 3

I m ( E 1 , E 2 , E 3 )e jM ( E1 , E 2, E 3 ) ;

(34)

(32)

(33)

2. ..

. - !# X # \^ X# , #_`.{ 9, 2000 ., . 195-198.

I m ( E 3 , E 1 S , E 2 S )e jM ( E 3 , E1 S , E 2 S ) ;(35)

I m ( E 2 S , E 3 S , E 1 )e jM ( E 2 S , E 3 S , E1 ) .(36)

Calculation of negative sequence impedance (24) on computer in accordance with (29) – (36) confirmed appropriateness of equation (28) for a 6 pulse converter with 8 / ' circuit diagram of transformer windings. As network current for a 12 pulse valve converter is defined as the sum of expressions (17) – (19) and (34) – (36), it is quite obvious that expression (28) is right in this case too. Thus, analysis of expressions for negative sequence impedance of 6 and 12 pulse valve converters with different circuit diagrams of transformer windings, together with numerous calculations performed on computer, allow to conclude that in all cases at coefficient of voltage asymmetry H 2 1@) presumes that the satisfying of individual PQ parameters is checked up in all partial intervals of time (e.g. 10 minute intervals) within a certain period (e.g. one week) and when a certain parameter satisfies in the required percentage of intervals of this period (e.g. 95%), then this parameter is considered to satisfy for the given period as a whole.

Jana H RKOVÁ

If the electricity is supplied by the three-phase voltage, it is right to require that – in the interval denoted as such in which a certain PQ parameter satisfies the respective requirement – this parameter has to be compulsorily satisfying for all three voltages. For example, the value of the voltage must satisfy in all phases because a threephase appliance envisages it. It is an imperfection that this circumstance is being assumed only in silence and is not pointed out sufficiently in standards and technical reports. As opposed to this principle, the concept of treating the results of measurements by most standard programs of the suppliers of PQ analysers represents a simplification because the fulfilment of PQ parameters is evaluated for individual voltages. According to this concept we obtain three mutually uncoordinated values for the rate of fulfilment of each parameter so that it is not possible to arrive at an explicit conclusion that the given PQ parameter satisfies the requirements for the three-phase supply. It may be simply proved that the results obtained in this way are misleading and not applicable for assessing the PQ of the three-phase supply. TABLE 1.1 Evaluation of PQ parameter by phases and in a three-phase regime. U1 + + + + 4+; 1-

U2 + + + + 4+; 1-

U3 + + + + 4+; 1-

voltage U1 individually satisfies




U1, U2, U3 + + 2+;3voltages U1,U2,U3 simultaneously do not satisfiy

85

Table 1.1 illustrates an example of the evaluation of the voltage in individual phases and of the threephase voltage. It is required that the voltage should satisfy at least in four intervals of time from the total number of five. Though each voltage U1, U2, U3 satisfies individually when being evaluated by phases, the number of intervals in which some of three voltages does not satisfy surpasses the given requirement and the value of the three-phase voltage does not satisfy. When evaluating the PQ parameters at a threephase supply it is necessary to start from the premise that the interval in which the given PQ parameter satisfies is only that one in which the PQ parameter satisfies in all three phases. 1.2 Comments to IEC 1000-3-7 criteria for evaluating the flicker Pst, Plt We excerpt from this document >2@: “Tab.2. Indicative values of planning levels for Pst and Plt in MV, HV and EHV power systems planning levels MV HV EHV Pst Plt

0,9 0,7

0,8 0,6

Assessment procedure Measurements should be carried out according to IEC 868 with a minimum duration of one week,. From the Pst values measured during the observation week, the cumulative probability functions (CPF) of Pst and Plt should be obtained and the percentiles Pst 95%, Pst 99, Plt95 and Plt99 should be derived. -Pst 99% should not exceed the planning levels -Plt 99% should not exceed the planning levels Notes: 3. Planning levels in Tab.2 are not intended to control flicker arising from uncontrollable events such as faults in the power system, etc. 4. Comparing 99% and 95% percentiles may be useful. If the ratio between them is greater than 1,3; one should investigate the reason for the discrepancy. Possible abnormal results (e.g. due to thunderstorms) should be eliminated.” 1.2.1 Formulation: “minimum duration of one week” Example: One value Pst = 2.00 for the period of measurement; all other values Pst are small, e.g. 0.2.

86

It follows from the calculation that there will be Plt = 0.88 for the interval of 2 hours and Plt = 0.2 for the whole rest of the period of measurement. If the period of measurement will be one week. i.e. 168 hours, then 2 hours make more then 1% of this period. High values of Plt may not be subtracted when calculating Plt99 and Plt99 will be equal to 0.88. Consequently, Plt99 will not satisfy the given condition. If the period of measurement will be a little longer, e.g. 202 hours (i.e. 8.4 days), then 2 hours make less than 1% of this period. High values of Plt may be subtracted when calculating Plt99 and Plt99 will be equal 0.2 and it will satisfy the given condition. Therefore, in order that the evaluation of results may be univocal, the duration of the period of measurement must be fixed explicitly. 1.2.2 Formulation: “Comparing 99% and 95% percentiles may be useful ..”, “..uncontrollable events such as faults”; “.. abnormal results (e.g. due to thunderstorms) should be eliminated”. The voltage fluctuation and flicker are a regional phenomenon and most points in the network are not affected by it. On the contrary, faults in the network and e.g. the influences of thunderstorms are a general phenomenon and, therefore, if high values of flicker are measured especially in HV and EHV networks, this is mainly caused by these “uncontrollable events, such as faults” or by otherwise “abnormal results” (e.g. due to thunderstorms) which should be eliminated. The formulation “Comparing 99% and 95% percentiles may be useful” is thus incorrect and the comparison of 99% and 95% percentiles should become obligatory. If the comparison is not carried out it is even not possible to carry out the elimination of values caused by “uncontrollable events” or by “abnormal results” which is required in order to fulfil the intention to obtain and assess the values of flicker due to fluctuation of load. For the sake of evidence we shall give an example of the results of weekly measurements of flicker Plt at three 110 kV points between TS and DS during which high values of Plt99 were measured exceeding the required values (see Table 1.2). The comparison of 99% and 95% percentiles revealed Plt99/Plt95 ! 1,3 in all cases. That is why intervals in which “abnormal phenomena”, e.g. voltage dips occurred were eliminated. The flicker Plt99 decreased very significantly down to values well satisfying the requirements of the IEC Document.


TABLE 1.2 Values of flicker Plt99 and Plt95 at 110 kV points between TS and DS prior to and after the elimination of “abnormal results” Files Elimination

110-01 no

yes

110-02 no

110-03

yes

no

yes

Plt95

0,363 0,162 0,136 0,122 0,202 0,170

Plt99

1,069 0,333 1,628 0,153 1,465 0,202

Plt99/Plt95

2,94

1,27

11,97

1,25

For this purpose we may formulate the requirement of the standard as the requirement to sustain the voltage in a voltage band with width 2dn and band centre Un and, consequently, with the lower limit value of the band Un- dn and the upper limit value of the band Un+ dn(Table 1.3 - line 2). Analogically identical notions will be used for describing the really measured set being evaluated, e.g. the set of 95% mean 10minute rms values of the voltage: The voltage band with width 2 ds, with band centre which we shall denote as the real voltage Us and, consequently, with the lower limit value of the band Us – ds and the upper limit value of the band Us + ds. (Table 1.2 - line 3). From the results of measurements we assess the set of 95% mean rms values of the network voltage. Basing on its real lower value Umin95 and its real upper value Umax95 we assess the real width of the voltage band 2ds and the real voltage Us. Then we can easily compare the require limit values, i.e. Un ± 0.1Un for example and the real values of the voltage band is Us ± ds and, similarly, the band centre offered by the standard and the real voltage Us. Unequally large tolerances (unsymmetrical width of the band) as given on line 4 of Table 1.3 and / or another percentage of values which must satisfy may also be set. For the case of unsymmetrical tolerances we may determine the lower part of the band width dsd, the upper part of the band width dsh and the value of the real voltage Us which corresponds to Un given by the standard:

7,25

1,19

The formulation “comparing 99% and 95% percentiles may be useful” is incorrect and the comparison of 99% and 95% percentiles should be obligatory. Mutual comparison of individual points of the network according to the magnitude of flicker is not possible without the elimination of “abnormal results”. The carrying out of this elimination requires a careful observation of “uncontrollable events”, e.g. the measurement of dips >3@, as well as experience in analysing the flicker in the networks. 1.3 Supply voltage deviations The requirement concerning the voltage level or supply voltage deviation is normally formulated in such a way, so that the mean 10minute rms value of the supply voltage should be within a certain range (e.g. Un r 10% Un) during a certain part (e.g. 95%) of the period of measurement. The satisfying of this condition may be assessed by determining the number of mean 10 minute rms values of the voltage having satisfied this condition and their share in the whole period of measurement. However, we must know a little more if we want to quantify the rate of satisfying the given requirement and the reserve vis-à-vis to the required limit values and if we want to perform further analyses.

d sd

U s max 95 U s min 95

d nd d nd d nh

(1.3.1)

d sh

U s max 95 U s min 95

d nh d nd d nh

(1.3.2)

Us

U min 95 d sd

(1.3.3)

TABLE 1.3 Width of the band of supply voltage deviations, nominal voltage Un and real voltage Us lower limit value of the voltage requirement of the standard, equal tolerances

nominal voltage Un or real voltage Us

upper limit value of the voltage

width of the band of voltage deviations

Un-dn

Un

Un+dn

2dn

Usmin95=(Us-ds)

Us=Usmin95+ds

Usmax95=Us+ds

Usmax95+Usmin95= 2ds

requirement of the standard, unequal tolerances

Un-dnd

Un

Un+dnh

dnh+dnd

real set p=95%, unequal tolerances

Usmin95=Us+dsd

Us=Usmin95+dsd

Usmax95=Us+dsh

Usmax95-Usmin95= dsh+dsd

real set p=95%, equal tolerances


87

This way of expressing the voltage deviations supports a rarely used notion “width of the voltage band” which indicates in which tolerances the band of real voltages moves. Similarly illustrative but not numerically quantified results may be obtained by means of histograms. Fig. 1.1 shows an example of expressing the real set by means of the width of band of deviations 2 ds round the real voltage Us and by means of the histogram, as well as the comparison with the width of band ± 10% Un round the nominal voltage Un as required by the standard.

Fig. 1.1 Voltage deviations according to the reality and to standards It may be derived from this figure: x The width of band of the real voltage 2 ds is substantially narrower than the band 2 dn permitted by standard; x The real voltage Us is substantially higher than the nominal voltage Un offered by the standard, in spite of this the voltage tolerances are preserved; x The histogram offers the same view on the width and centre of the voltage band as calculation but without numerical quantification. The quantification of voltage deviations for a really assessed set may be advantageously carried out by introducing the notions “real voltage Us” and “width of the band of voltage deviations” (Usmax95 – Usmin95 = dsh + dsd). The width of the band of real voltage deviations corresponds to the required band width following from the required voltage tolerances. The band width makes easier the comparison of the really measured values with the required ones. The real voltage Us corresponds to the nominal voltage Un given in the definitions of requirements on voltage deviations.

88

2. PQ PARAMETERS AT 110 KV POINTS OF DELIVERY BETWEEN TS AND DS Then PQ parameters were chosen in compliance with the valid version of the Code of the Czech transmission system operation [4 ]. This Code is mainly based on the EN 50160 standard and on other EN, IEC and EURELECTRIC (UNIPEDE) documents and it fixes the binding parameters of PQ. Table 2.1 presents summarising results of the weekly checking measurements of known and less known parameters of PQ at six 110 kV points of delivery to DS from 400 kV and 220 kV systems. 2.1 Rms voltage level It is required that the mean 10 minute values should be within 110 kV ± 10% tolerances during 95% of the period of one week. The voltage is given in % of the nominal voltage Un in the table. Us95 ± ds95 represents the real band of tolerances including 95% of 10 minute values round the real voltage Us95. It has been generally revealed that, in reality, the band of 110 kV voltage deviations is much narrower than it is admitted by the rules and that with regard to reducing the losses, the band of deviations is shifted into higher values of the voltage. 2.2 Voltage unbalance It is required, that 95% of 10 minute values of unbalance should not exceed 2%. The real unbalance satisfies this requirement with a great margin. 2.3 Flicker Pst, Plt The permitted limit values for Pst, Plt are in compliance with IEC 1000-3-7 and they relate to 99% and 95% percentiles. A more detailed information concerning the methodology of this IEC Document is given in the first part of the paper. It became evident that the evaluation unconditionally requires a simultaneous observation of voltage dips and of other “uncontrollable” phenomena and the elimination of “abnormal results”. In general basing on the checking analyzis we may say that the level of flicker resulting from the fluctuation of consumption is very low at 110 kV points with high short-circuit power. 2.4 Harmonic distortion THD of the voltage and harmonic components of the voltage It is required that 95% of the mean 10 minute values should not exceed 2.5% for THD and 2% for individual harmonics. In reality, THD reached Electrical Power Quality and Utilisation

up to 1.693%. As for harmonics, the highest was mostly the 5th harmonic, namely up to 1.549%. Respecting the requirements, the reserve margin is thus relatively small.

The power factor was checked and the table indicates whether and in which percentage of the period of one week the value of the inductive power factor was cos } 0.95. This requirement was fulfilled at all points of checking.

2.5 Harmonic distortion THD of the current and harmonic components of the current Statistical data have been treated basing on 95% of the really measured 10 minute values. THD reached up to 8.246%. As for harmonic currents, even here the highest was mostly the 5th harmonic, namely up to 7.267%.

2.7 Variation of power supplied from the transmission system It is required that the variation of the real consumption of electricity supplied from the whole system per one minute should not exceed 2% approximately of the maximum daily load. The satisfying of this requirement dP/Pmax d 2% may be checked at individual points of delivery by the evaluation of changes of the magnitude of the active power supplied per minute. Table 2.1 indicates in which percentage of the day this requirement was not fulfilled and on which day of the week this non-fulfilment lasted for the longest time. In general we may state, that the requirement is not fulfilled at any checked points, the duration of nonfulfilment is longer on working days.

2.6 Reactive energy supplied from the transmission system It is required that the consumer may consume electricity permanently with the value of the inductive power factor cos } t 0.95. The value of the power factor is checked in hourly intervals by means of the consumed active and reactive energy.

TABLE 2.1 Survey of PQ parameters at 110 kV points of delivery in six 400/110 kV and 220/110 kV substations limits

Point 1

Point 4

Point 5

Point 6

Point 7

0,058

0,283

0,205

0,108

0,167 0,249 0,170 0,413

0,098 0,224 0,136 0,433

0,137 0,202 0,148 0,538

0,063 0,134 0,151 0,437

0,178 0,225 0,175 0,418

0,334 0,374 0,256 0,575

% % % % % % %

1,463 0,888 1,191 0,434 0,053 0,112 0,090

1,573 0,849 1,392 0,429 0,036 0,093 0,093

1,693 0,664 1,549 0,339 0,059 0,084 0,062

1,572 0,802 1,381 0,625 0,033 0,138 0,129

1,395 0,817 1,071 0,403 0,124 0,129 0,070

0,718 0,411 0,489 0,326 0,144 0,119 0,077

% % % % % % %

3,084 1,965 2,300 1,928 0,106 0,231 0,134

4,412 2,699 3,376 1,790 0,061 0,239 0,161

5,832 0,755 5,142 2,602 0,119 0,195 0,151

5,358 2,836 4,208 1,520 0,108 0,345 0,321

1,207 0,307 0,776 0,997 0,034 0,239 0,202

8,246 1,851 7,267 3,587 0,112 0,806 0,630

d 110 t 90

%Un %Un %Un %Un

107,52 1,07 108,58 106,45

105,65 0,61 106,26 105,04

103,95 1,15 105,10 102,81

104,18 0,79 104,96 103,39

107,62 1,29 108,91 106,32

106,11 1,04 107,15 105,07

cos fi

0

%t

0,00

0,00

0,00

0,00

0,00

38,03

dP/dt

0

%t den

3,75 Út

3,06 St DIP

4,58 Pá DIP

7,78 t

13,19 St DIP

66,88 So

voltage current

Pst95 Pst99 Plt95 Plt99

THDI 3ha 5ha 7ha 9ha 11ha 13ha s95 d95 max95 min95

%

Point 2 0,160

THDU 3ha 5ha 7ha 9ha 11ha 13ha

power

d2

0,404

voltage

flicker

ünbalance

d 0,8 d 0,6 d 2,5 d2 d2 d2 d2 d2 d2


89

3. DISTORTION OF CURRENTS IN THE DISTRIBUTION NETWORK The published papers and standards concerning EMC in the electric networks pay greater attention to the distortion of voltages than to the distortion of currents. In the following chapter, the authors preliminarily analyse the distortion of currents in distribution networks and its influence on the magnitude of the rms value of the current, as well as on the increase of voltage drop, on power and energy losses, and they try to identify the fields which deserve to be treated more profoundly from the economical point of view. 3.1 Some relations When the shape of the quantity M is distorted and its total harmonic distortion (THD) is THDM, then its effective (rms) value Mrms increases k-times compared to the value of the 1st harmonic M1. M rms k 1 THDM2 (3.1) (3.1) M1 Many physical processes in which some quantities liable to distortion are exercised are the function of the rms value of the respective quantity. For example, due to distortion of the current onto the value Irms compared to the undistorted current I1, the voltage drop on resistance R increases as follows: R.I rms 2 1 THD M k (3.2) R.I 1 The ohmic power losses Z increase quadraticly due to distortion of the current: 2 R.I rms R.I 12

Z rms Z1

1 THD 2

k2

(3.3)

The energy losses produced by distortion of the current acquire the following value for the period consisting of n intervals i: n

n

n

i 1

i 1

i 1

2 ¦ R.I rmsi ¦ R.I12i ¦ R.I i2 .THDi2

week, mainly the values being not exceeded for 95% of the time. Fig. 3.1 shows in a simplified way the most frequent scheme of the distribution network between the transmission system (point 4) and the voltage levels by which the majority of consumers are supplied (points 9, 10). 400 kV

4 110 kV 6 77

10 10

NN

Fig.3.1 Points of measurements of the distortion of currents; boundary with the transmission network point 4; supply voltages for majority of consumers - points 9, 10 3.3 Results of measurements and calculations The range of the values of THD95 and of harmonic components of the current following from the results of measurements in individual points of the network are summarized in Fig. 3.2. 100 THD HA (%) 30

10

A B (4)

A denotes losses corresponding to the undistorted current, B are additional losses caused by distortion of the current.

3

1 4

3.2 Test measurements and the choice of point of measurements in the networks The test measurements were made by using instruments measuring the parameters of voltages and currents, mostly of the class A (EN 61000-43), with a 16 bit AD converter, at sampling frequency 6.4 kHz and the measuring interval 10 ms for rms values and 320 ms for harmonics. The parameters were evaluated for 10 minute intervals. Further values were evaluated for the period of one

90

22 kV

99

6

7

9

10

Fig. 3.2 Range of THD95 values and of harmonic components (3th, 5th, 7th, 11th, 13th) of currents found in individual points of the network Table 3.1 presents the values calculated from the measured results according to relations 3.2, 3.3 and 3.4. It becomes evident how the voltage drops, power losses and energy losses increase due to the distortion of currents in individual points of the network.


TABLE 3.1 Increase of voltage drops, of power and energy losses due to distortion in individual points of the network Point 4 6 7 9 10

k voltage drop 1,00048 – 1,00146 1,00016 – 1,00378 1,00130 – 1,00694 1,00245 1,05622 1,00231 – 1,10923

k2 power losses 1,00096 – 1,00292 1,00032 – 1,00757 1,00260 – 1,01392 1,00490 – 1,11560 1,00462 – 1,23040

A+B/A energy losses 1,00039 – 1,00066 1,00023 – 1,00296 1,00098 – 1,00921 1,00318 – 1,04205 1,00323 – 1,09446

It follows from the performed investigations that the distortion of current in the points of coupling of the distribution systems to the transmission systems is mostly small. Higher values of the distortion of currents occur more likely in LV and MV networks (Fig.3.1). The magnitude of the distortion of current varies considerably during the day and during the week. High distortions take place more likely in the period of a lower load in the network, i.e. at lower values of the currents (Fig. 3.3).

6 THD (%) 5

800 I (A) 700 600

4

500

3

400 300

2

200 100

THD

1

I 0

0

Fig. 3.3 Weekly diagram of the THD and rms values of the current at the output side of a 110/22 kV transformer (point 7). The highest values are attained by the 5th harmonic (exceptionally by the 7th or the 11th harmonic). The impact on the increase of voltage drops, power losses and energy losses corresponds to the values of the distortion of the current. This seems to be significant for 110/MV but mainly for MV/LV transformers as well as for MV and LV lines. We may mention some cases of distortion of the current caused by municipal traction, by household and commercial

consumption and by consumption of administrative centres. The influence of the distortion of currents on the increase of the losses manifests itself in smaller extent than on increase of power losses, because the high distortion of currents more likely occurs during a lower load in the network (Fig. 3.3). From the point of view of economical impacts on the distribution of electricity, attention should be paid to the distortion of currents mainly in LV and MV networks. 4.

CONCLUSIONS

Standards dealing with PQ presume that when a certain parameter satisfies in the required percentage of intervals of the period of checking, then this parameter is considered to satisfy for the given period as a whole. If the electricity is supplied by the three-phase voltage, it is right to require that – in the interval denoted as such in which a certain PQ parameter satisfies the respective requirement – this parameter has to be compulsorily satisfying for all three voltages. The Technical Rapport IEC 1000-3-7 gives criteria and assessment procedure for the flicker Pst and Plt for HV systems. On the base of experience the authors present observations to some formulations: - In order that the calculation of results of flicker Pst, Plt may be univocal, the duration of the period of measurement must be fixed explicitly. - The formulation “comparing 99% and 95% percentiles may be useful” is not correct and the comparison of 99% and 95% percentiles should be obligatory. - Mutual comparison of individual points of the network according to the magnitude of flicker would not be possible without the elimination of “abnormal results”. - The carrying out of this elimination requires a careful observation of “uncontrollable events”, e.g. the measurement of dips, as well as experience in analysing the flicker in the networks. The quantification of rms voltage deviations may be advantageously carried out by introducing the notions “real voltage Us” and “width of the band of voltage deviations” (dsh + dsd). This makes easier the comparison of the really measured values of rms voltage with the required ones. The “Code of the Czech Transmission System Operation” fixes the binding parameters of PQ in the Czech transmission system and also at 110 kV


91

points of delivery between TS and DS. On the base of investigations in six 110 kV points of delivery from TS it may be shown, that: - the band of 110 kV voltage deviations is much narrower than it is admitted by the rules; with regard to reducing the losses the band of deviations is shifted into higher values of the voltage; - the values of unbalance satisfy requirements with a great margin; - the level of flicker resulting from the fluctuation of consumption is very low at 110 kV points of delivery (high short-circuit power); the evaluation of flicker unconditionally requires a simultaneous observation of voltage dips and of other “uncontrollable” phenomena and the elimination of “abnormal results”; - harmonic distortion THD of the voltage reached up to 1.693%, for harmonic components of voltage, the highest was mostly the 5th harmonic, namely up to 1.549%; respecting the requirements (2.5% for THD and 2% for individual harmonics), the reserve margin is relatively small; - harmonic distortion THD of the current reached up to 8.246%, for harmonic component of currents, even here the highest was mostly the 5th harmonic, namely up to 5.142%; - reactive energy supplied from the transmission system - it is required that the consumer may consume electricity permanently with the value of the inductive power factor cos } t 0.95. This requirement was fulfilled at all points of checking; - variation of power supplied from the transmission system - it is required that the variation of the real consumption of electricity supplied from the whole system per one minute should not exceed 2% approximately of the maximum daily load; this requirement is not fulfilled at any checked points. It follows from the performed investigations that the distortion of current in the points of coupling of the distribution systems to the transmission systems is mostly small. Higher values of the distortion of currents occur more likely in LV and MV networks. The magnitude of the distortion of current varies considerably during the day and during the week. High distortions take place more likely in the period of a lower load in the network. The highest values are attained by the 5th harmonic (exceptionally by the 7th or the 11th harmonic). The impact on the increase of voltage drops, power 92

losses and energy losses corresponds to the values of the distortion of current. This seems to be significant for 110/MV but mainly for MV/LV transformers as well as for MV and LV lines. We may mention some cases of distortion of the current caused by municipal traction, by household and commercial consumption and by consumption of administrative centres. The influence of the distortion of currents on the increase of losses manifests itself in smaller extent than on the increase of power losses, because the high distortion of currents more likely occurs during a lower load in the network . From the point of view of economical impacts on the distribution of electricity, attention should be paid to the distortion of currents mainly in LV and MV networks. 5. REFERENCES

[1] EN 50160 "Voltage characteristics of Electricity supplied by Public Distribution Systems" CENELEC, November 1994. [2] IEC 1000-3-7 "Electromagnetic Compatibility (EMC) Part 3: Limits - Section 7: Assessment of emission limits for fluctuating loads in MV and HV power systems – Basic EMC publication IEC, 1996-10. [3] prIEC 61000-4-30 "Electromagnetic Compatibility (EMC) Part 4- 30: Testing and measurement techniques – Power Quality Measurement Methods – Basic EMC publication. [4] Code of the Czech Transmission System Operation B Basic conditions for using the transmission system (in Czech) EPS Praha, Var. 27, 16.10.2000 [5] J. Arrillaga a.o.: Power System Quality Assessment John Willey Sons Ltd, 2000 Mailing addresses:

Ing. Václav Vyskoil, CSc. Ing. Zdenk Špaek, CSc. Ing. Jana Hrková EGÚ Brno, a.s., Hudcova 76a, PO Box 53 CZ 612 48 Brno, Czech Republic Phone: (+420) 5 413 212 01; Fax: (+420) 5 492 491 33 e-mail: [email protected] e-mail: [email protected] e- mail: [email protected]


Section 2 Methods of Power Quality Analysis: Modelling and Simulation

2.1. WILKOSZ K., SOBIERAJSKI M., KWASNICKI W.T.: Fourier Analysis of Voltage at the Terminal Node of STATCOM for Different Parameters of the Frequency-Dependent Line Model (Poland, Canada)....................................................................................................................................95 2.2. MIENSKI R., PAWELEK R., WASIAK I.: Compensation of Time-Varying Loads by Means of SVC – Modelling and Simulation (Poland) ..........................................................................................103 2.3. OZDEMIR S., OZDEMIR E., ERTUNC H.M.: The Simulation of Fuzzy Logic Based Advanced Static Var Compensator in Matlab (Turkey) .......................................................................................113 2.4. ZHEZHELENKO I.V., SAYENKO Y.L., NESTEROVICH V.V.: Using Wavelet Transform for Identification of Electric Circuits Parameters (Ukraine) ....................................................................119 2.5. KOVERNIKOVA L.I., SMIRNOV S.S.: Equivalenting of Electric Network when Calculating Modes of Harmonics (Russia)..............................................................................................................123 2.6. LEZHNYUK P., LUKIANENKO Y., YARNYH L., VYDMYSH V.: Simulation of the Higher Harmonics Spreading Process in Electrical Circuits With the Use of the Nodal Equation (Ukraine) ..............................................................................................................................................129 2.7. OLARU D., TRUSCA V.: Numerical Simulation of the Non-Linear Resonance Phenomena Using Spline Approximated Characteristic (Romania) .......................................................................133 2.8. RENNER H., FICKERT L., SAKULIN M.: Calculation of Voltage Dips in a Meshed 110-kVNetwork (Austria).................................................................................................................................139 2.9. LEVA S., MORANDO A.P., ZANINELLI D.: The Influence of the Zero-Sequence Component on the Line Voltage Drop (Italy)..........................................................................................................145 2.10. BANKO S., SABARNO L., SEVASTUK I., TRACH I.: Optimum of Rapidly Resorting of Voltage in Autonomy Electrical Power Systems (Ukraine).................................................................153 2.11. KUZNETSOV V., TUGAY Y., DMITRIEV E.: Suppression of Overvoltages Caused by OpenPhase Idle Operating Conditions in Subtransmission Network (Ukraine) ..........................................159 2.12. LEVA S., MORANDO A.P.: Power Properties of Single-Phase Non-Sinusoidal Systems: The Use of Generalised Rotating Vector Concept (Italy) ...........................................................................165 2.13. SAVINA N.V.: Modelling of Parameters of the Electric Power Quality Characterizing NonSinusoidal and Asymmetry of Voltage (Russia) ...................................................................................173 2.14. KOZLOV A., LOGINOV V.: Conditions of Steady-State Stability of Controllable AC Transmission Lines (Russia) ................................................................................................................179 2.15. PLOUTENKO A.D.: The Conception of a System of Analysis of Models of Access to Distributed Databases (Russia) ..............................................................................................................................187

Section 2. Methods of Power Quality Analysis: Modelling and Simulation

93

94





FOURIER ANALYSIS OF VOLTAGE AT THE TERMINAL NODE OF STATCOM FOR DIFFERENT PARAMETERS OF THE FREQUENCY-DEPENDENT LINE MODEL Kazimierz WILKOSZ, Marian SOBIERAJSKI Institute of Electrical Power Engineering Wroclaw University of Technology Wroclaw (Poland)

Wiesaw T. KWASNICKI Network Operation Services British Columbia Hydro Burnaby, B.C. (Canada)

Abstract - The paper deals with harmonic generation in a power system. In the paper impact of parameters of the frequencydependent modal power line model (FDML Model) on results of simulation investigations of harmonics generated by STATCOM is considered. At the beginning, assumptions are given, i.e. assumed model of a power system, the considered STATCOM, general simplifications, the changed parameters of FDML Model, principles of quantitative evaluation of the experimental results. Following, results of investigations are presented. Simulation investigations have been made using PSCAD/EMTDC electromagnetic transients simulation program. The Fourier analysis of the voltage at the terminal node of STATCOM has been performed for different values of parameters of the considered power line model. At the end of the paper, conclusions from the investigations are presented.

COM is considered. In the investigations the UNIX version of the PSCAD/EMTDC electromagnetic transients simulation program has been utilised [1]. In this program the following three basic transmission line modelling techniques are proposed: PI sections, the Bergeron Model and the FrequencyDependent Modal Power Line Model (the FDML Model) [2]. During investigations of the harmonic generation the FDML Model should be used. This model has many parameters that are related to it itself. The problem is suitable determination of those parameters. For different values of the parameters of the FDML Model the results of simulation investigations are discussed in this paper.

1. INTRODUCTION Results of investigations depend very much on the used models. Often theoretical considerations allow determining the field of utilisation of different models and then selecting a model for the considered case. Usually for such selected model there exists necessity to determine values of a number of parameters. These are not only parameters that are related to real object but also parameters related to the model itself. In the paper the problem of impact of parameters of the power line model on results of simulation investigations of harmonics generated by STAT-

2. ASSUMPTIONS 2.1. Model of the power system For the presented investigations a power system consisting of two subsystems: Sub 1 and Sub 2 connected by overhead lines: Line 1 and Line 2 (Fig. 1), is considered. The considered STATCOM is connected to node Nm at the ends of lines Line 1 and Line 2. The STATCOM regulates the bus voltage rated at 110 kV. Each subsystem: Sub 1, Sub 2 is represented by a 3-phase voltage source (frequency f = 60 Hz, source reactance Xs = 2Sf*0.012 :, source resistance Rs = 0.44 :). Each power line is characterised by the following data: – conductor data: each phase consists of only a single conductor AFL-6 of radius 1.481 cm; DC resistance/length of the conductor: 0.05906 :/km; the horizontal distance of the adjacent conductors: 5.4864 m; the height of the middle


95

Es1

Zs1

N2

N1

Zs2

Es2

Line 2

Line 1 Nm

Sub 1

Sub 2

STATCOM

Fig. 1. The model of a power system conductor at the tower: 15.24 m; the height of the other conductors at the tower: 13.716 m; the midspan sag of the conductor: 6.096 m; – ground wire data: the number of ground wires: 2; the conductor name: AFL 1,7, the conductor radius: 0.55245 cm; DC resistance/length of the conductor: 0.6136 :/km; the horizontal distance of the adjacent conductors: 5.4864 m; the height of each conductor at the tower: 18.897 m; the midspan sag of the conductor: 3.048 m. 2.2. The considered STATCOM A 12-pulse STATCOM rated at 110 kV ±100 MVA [3] has been considered for the experiments. The model of STATCOM was built using basic components from the standard library of the PSCAD/EMTDC program. In the investigations of harmonic generation by STATCOM, the fundamental frequency modulation firing technique (FFM) was applied. Utilisation of harmonic filters has not been considered, for the purpose of preserving an exact harmonic content resulted from the STATCOM alone operating in the meshed power network. 2.3. General simplifications In order to make the set of variable parameters of a reasonable size, the following simplifications have been made: – all lines are ideally transposed, – the length of Line 1 and Line 2 is the same and is equal to L, – equivalent impedances of both the subsystems are the same and are equal to Z, where Z = Rs + Xs, (Rs and Xs given in Subsection 2.1), – active power flow into Line 1 (at the beginning) is equal to 100MW. 2.4. The FDML Model When the FDML Model is used the following pa96

rameters should be determined: – a maximum error of approximation of the surge impedance Zc(), – a maximum error of approximation of the attenuation constant A() (A() = e-() L ; () is the propagation constant, = 2 f ), – a maximum error of approximation of the current transformation, – a maximum number of poles/zeroes for approximation function of Zc(Z), – a maximum number of poles/zeroes for approximation function of A(Z), – a maximum number of poles/zeroes for approximation function of the current transformation, – a starting frequency fl, – an end frequency fu. Also the following options should be selected: – frequency dependent current transformation Ti or constant transformation Ti, – non-transposed line or ideally transposed line, – interpolating travelling time or do not interpolating travelling time. During investigation ideally transposed line has been considered and then current transformation Ti has been constant. Also interpolating travelling time has been taken into account. To reduce number of cases it was assumed that all abovementioned maximum errors are the same and also all above-mentioned maximum number of poles/zeroes are the same. The maximum error affects the number of poles/zeroes used in approximation, and therefore changes of maximum errors have been considered during investigations. 2.5. Quantitative evaluation of the experimental results Changes of the magnitudes of characteristic harmonics were observed when varying parameters of the FDML Model representing lines in the power system model of Fig.1. The following indices were calculated as means of quantitative evaluation of Electrical Power Quality and Utilisation

the experimental results: Gh

100

Vmha Vmhr , max Vmhr

(1)

L

stant A() changes when the length of the line changes. As an example in Fig. 4 the indices GAM, GAP as functions of L are given for the frequency 23*60 Hz. The indices GAM, GAP are defined in the a)

In (1) subscript r denotes reference case (Case Ref) that is characterised by the following values of the parameters mentioned in subsection 2.4: – maximum error of approximation of Zc() and A(): 1%, – a starting frequency fl: 0.5 Hz, – an end frequency fu: : 1 MHz. Subscript a denote one of the cases: Case E change of a maximum error of approximation of Zc() and A(), Case FL - change of a starting frequency for curve fitting for Zc() and A(), Case FH - change of an end frequency for curve fitting for Zc() and A(). To determine impact of the above-given parameters on the magnitudes of characteristic harmonics the conditions of performing simulation experiments were kept unchanged, in particular the time step during simulation was constant. 3. RESULTS OF THE INVESTIGATIONS 3.1. Change of maximum error of approximation of Zc() and A() The investigations have been performed for the maximum error of approximation of Zc() and A() equal to 1% and 2%. Results of the investigations are presented in Fig. 2 – 5 and Tables 1 - 4. In Fig. 2 magnitudes of the characteristic harmonics of the voltage at the node Nm in Case Ref as a function of L are given. The indices GZM, GZP (Fig. 3) are defined as follows: G ZM

100

Z ca Z cr , G ZP max Z cr Z

100

M Za M Zr , (3) max M Zr Z

20 h = 11 h = 13

15 10 5 0 25

20

jM Z

100

h = 23 h = 25

15 10 5 0 25

50 75 Length L, km

100

c)

10 h = 35 h = 37

5

0 25

50 75 Length L, km

100

d)

20 h = 47 h = 49

10

0 25

where:

Z c (Z) Z c e

50 75 Length L, km

b) Magnitude of Vmh, %

Vmxh (x {A, B, C}; x indicates phase) is the RMS value of the harmonic voltage of order h at node Nmx (in phase x).

Magnitude of Vmh, %

(2)

Magnitude of Vmh, %

Vmh

VmAh VmBh VmCh , 3

Magnitude of Vmh, %

where:

50 75 Length L, km

100

.

Zc() is the same for different lengths of the line in contrast to the constant A(). In general, the con-

Fig. 2. Magnitudes of the characteristic harmonics of the voltage at the node Nm in Case Ref versus the length L.


97

a)

a)

10 Index Gh, %

Index GZM, %

5

0

-5

5 0 h = 11 h = 13

-5

-10 0

2.5

5

7.5

25

Log10Z b)

Index Gh, %

Index GZP, %

50

5 0 -5

25 0 -25

2.5

5

7.5

25

Log10Z c)

a)

0.25

50 75 Length L, km

100

h = 35 h = 37

20

Index Gh, %

Fig. 3.Indices GZM and GZP versus Log10Z for Case E; Mode 1, Mode 2 (Mode 3).

Index GAM, %

h = 23 h = 25

-50 0

10 0

-10

0

-20 -0.25

25

-0.5 25

50 75 Length L, km

100

d) Index Gh, %

0.25

0

50 75 Length L, km

100

h = 47 h = 49

50

b) Index GAP, %

100

b)

10

-10

25 0 -25 -50 25

-0.25 25

50 75 Length L, km

100

Fig. 4.Indices GAM and GAP versus Log10Z for Case E; Mode 1, Mode 2 (Mode 3). same way as the indices GZM, GZP. In Fig. 5a – 5d the index Gh versus the length L is presented, when the maximum error of approxima98

50 75 Length L, km

50 75 Length L, km

100

Fig. 5. Impact of the maximum error of approximation of Zc and A on magnitudes of the characteristic harmonics.

tion of Zc() and A() changes from 1% to 2%. In all the figures dashed lines limit the range of values r5%. Electrical Power Quality and Utilisation

Max 3.0E+04 2.3E+03 2.2E+06 2.8E+03

5 2 2 2

5 2 2 2

Zeroes

Min 6.5E+00 8.7E+00 3.5E+02 9.6E+00

Max 6.3E+01 4.6E+01 5.2E+03 4.7E+01

Max 5.6E+01 2.3E+01 5.1E+03 2.5E+01

No. of zeroes

No. of poles

L, km

TABLE 3. Characteristics of the functions approximating A() for mode 1 the Case Ref.

2 3 4 4 4

5 6 7 7 7

Zeroes

Min 2.9E+03 2.9E+02 1.2E+02 1.0E+02 8.4E+01

Max 8.7E+05 5.6E+05 3.4E+05 2.1E+05 1.9E+05

4 2 2 1 1

5 3 4 2 4

25 40 60 80 100

Zeroes

Min 2.2E+04 5.0E+04 2.8E+04 1.4E+04 3.5E+04

Max 3.8E+06 1.0E+06 5.3E+05 1.4E+04 3.5E+04

Poles

Min 2.1E+04 4.8E+04 2.7E+04 1.3E+04 3.1E+04

Max 6.3E+06 2.3E+06 5.3E+06 7.1E+05 4.5E+06

3.2. Change of starting frequency for curve fitting for Zc() and A()

Min 2.1E+04 4.8E+04 2.7E+04 1.3E+04 3.1E+04

Max 6.3E+06 2.3E+06 5.3E+06 7.1E+05 4.5E+06

25 40 60 80 100

No. of poles

5 3 4 2 4

Max 3.8E+06 1.0E+06 5.3E+05 1.4E+04 3.5E+04

No. of zeroes

4 2 2 1 1

Min 2.2E+04 5.0E+04 2.8E+04 1.4E+04 3.5E+04

Poles

L, km

L, km

No. of poles

TABLE 7. Characteristics of the functions approximating A() for mode 1 in the Case FL.

No. of zeroes

TABLE 4. Characteristics of the functions approximating A() for mode 2 (3) in Case Ref.

Zeroes

Min 2.9E+03 2.9E+02 1.2E+02 1.0E+02 8.4E+01

Max 3.3E+04 1.5E+04 9.2E+03 6.4E+03 3.8E+03

Results of the investigations, when a starting frequency for curve fitting for Zc() and A() changes from 0.5 Hz to 60 Hz are presented in the Fig. 6 – 7 and Tables 7 - 8.

25 40 60 80 100

Max 3.3E+04 1.5E+04 9.2E+03 6.4E+03 3.8E+03

Min 3.1E+03 3.0E+02 1.3E+02 1.1E+02 8.6E+01

Poles

Max 8.7E+05 5.6E+05 3.4E+05 2.1E+05 1.9E+05

25 40 60 80 100

Min 3.1E+03 3.0E+02 1.3E+02 1.0E+02 8.6E+01

Poles

5 6 7 7 7

Zeroes

TABLE 6. Characteristics of the function approximating A() for mode 2 (3) in Case E.

Poles

Min 2.7E+00 2.8E+00 3.5E+02 3.1E+00

2 3 4 4 4

25 40 60 80 100

L, km

No. of poles

Ref E FL FH

No. of zeroes

Case

TABLE 2. Characteristics of the functions approximating Zc() for mode 2 (3).

No. of poles

Min 2.7E+00 2.7E+00 5.0E+02 3.0E+00

No. of poles

Max 3.4E+04 2.9E+03 2.3E+06 3.4E+03

No. of zeroes

Min Ref 6 6 6.6E+00 E 4 4 6.5E+00 FL 18 18 5.4E+02 FH 4 4 7.1E+00

Poles

No. of zeroes

Zeroes

TABLE 5. Characteristics of the functions approximating A() for mode 1 in Case E. L, km

No. of poles

No. of zeroes

Case

TABLE 1. Characteristics of the functions approximating Zc() for mode 1.

3 2 5 4 4

6 5 8 7 7

Zeroes Min 1.6E+03 6.1E+02 3.0E+02 4.3E+02 3.9E+02


Max 1.5E+05 5.5E+03 9.3E+03 5.0E+04 3.6E+04

Poles Min 1.5E+03 5.9E+02 2.9E+02 4.2E+02 3.8E+02

Max 9.9E+05 5.7E+05 3.1E+05 1.9E+05 1.5E+05

99

No. of poles

4 2 3 2 0

5 3 5 4 3

Min 3.0E+05 1.1E+05 3.5E+04 3.2E+04 -

Max 3.3E+06 1.1E+06 1.8E+06 2.0E+05 -

Poles Min 2.8E+05 1.1E+05 3.4E+04 3.1E+04 5.3E+05

Max 5.8E+06 2.3E+06 3.7E+06 3.3E+06 4.8E+06

a) Index GZM, %

2

h = 11 h = 13

10

Index Gh, %

No. of zeroes

25 40 60 80 100

Zeroes

a)

0

-10 -20 25

b)

100

h = 23 h = 25

25 0

-25

1

-50

0

25

50

75

100

Length L, km

-1 -2

c)

0

2.5

5

50

7.5

b)

10

h = 35 h = 37

25

Index Gh, %

Log10Z

0

-25

5

-50

0

25

-5

-10

d)

0

2.5

5 Log10Z

Fig. 6.Indices GZM (a) and GZP (b) versus Log10Z for Case FL; Mode 1, Mode 2 (Mode 3). 3.3. Change of an end frequency for curve fitting for Zc() and A() One has investigated change of an end frequency from 1 MHz to 100 kHz. The obtained results are presented in Fig. 8 – 9 and Tables 9 - 10.

50 75 Length L, km

100

h = 47 h = 49

20

7.5 Index Gh, %

Index GZP, %

50 75 Length L, km

50

Index Gh, %

L, km

TABLE 8. Characteristics of the functions approximating A() for mode 2 (3) in the Case FL.

10 0

-10 -20 25

50 75 Length L, km

100

Fig. 7. Impact of the starting frequency for curves fitting for Zc and A on magnitudes of the characteristic harmonics.

4. CONCLUSIONS Selection of a modelling technique and also values of parameters for the selected model is an essential problem of each process of modelling. Once again this ascertainment is confirmed by the investigations that are here described. 100

The FDML Model is recommended for the harmonic generation investigations. One has observed that changes of the parameters of this model can have significant effect on results of investigations especially when investigations are conducted near


a)

2 1 0 -1 -2 -3

-20 2.5 Log10Z

5

25

7.5

b)

b)

10 5 0 -5

100

25 0

-25

-10

-50 0

2.5

5

7.5

25

Log10Z

No. of poles

2 1 3 5 5

4 4 6 8 8

L, km

No. of zeroes

No. of poles

5 3 7 3 1

5 3 7 3 2

100

h = 35 h = 37

25 0

-25 -50

Zeroes Min 2.4E+03 1.5E+03 3.7E+02 1.9E+02 1.3E+02

Max 1.1E+05 1.5E+03 7.7E+04 6.7E+03 6.0E+03

Poles Min 2.2E+03 1.4E+03 3.5E+02 1.8E+02 1.2E+02

Max 3.4E+05 5.2E+05 2.7E+05 2.4E+05 1.9E+05

TABLE 10.Characteristics of the functions approximating A() for mode 2 (3) in the Case FH.

25 40 60 80 100

50 75 Length L, km

50

Zeroes Min 1.2E+04 2.4E+04 7.6E+02 3.6E+03 2.5E+04

Max 6.2E+05 5.6E+05 6.3E+05 6.0E+05 2.5E+04

25

d)

50 75 Length L, km

100

h = 47 h = 49

50

Index Gh, %

No. of zeroes

TABLE 9. Characteristics of the functions approximating A() for mode 1 in the Case FH.

c) Index Gh, %

Fig. 8.Indices GZM (a) and GZP (b) versus Log10Z for Case FH; Mode 1, Mode 2 (Mode 3).

L, km

50 75 Length L, km h = 23 h = 25

50

Index Gh, %

Index GZP, %

0

-10

0

25 40 60 80 100

h = 11 h = 13

10

Index Gh, %

Index GZM, %

a)

25 0

-25 -50 25

50 75 Length L, km

100

Poles Min 1.2E+04 2.3E+04 7.5E+02 3.5E+03 2.3E+04

Max 5.9E+05 5.1E+05 5.4E+05 3.9E+05 6.0E+05

Fig. 9. Impact of the end frequency for curves fitting for Zc and A on magnitudes of the characteristic harmonics. plained by ascertainment that assuming other parameters, in fact, we model other power line, for which resonance conditions are fulfilled for other length. It should be noted that a change of the maximum

resonance conditions. This conclusion can be exSection 2. Methods of Power Quality Analysis: Modelling and Simulation

101

error of approximation of Zc() and A() (e.g. of 1%) can results in much more change of values of harmonic magnitudes. However it should be emphasised that it is possible to determine such ranges of the length L, for which the relative change of harmonic magnitudes is relatively small (e.g. within r5%). It can be observed that in most cases larger changes of harmonic magnitudes (e.g. more than 5%) occur for the smallest and also for the largest considered values L. Selection of the line model is essential from viewpoint of the precision and efficiency of calculations. Less number of poles/zeroes of the functions approximating Zc() and A() or in general a larger error of approximation of Zc() and A() will result in more efficient model, but the precision of the simulation will be worse. The mentioned error of approximation is calculated as a percentage of the maximum of the curve. For the A() it is consistent as 1.0. For Zc(), the maximum value will depend on the starting frequency. Zc() will get larger as the starting frequency is lowered. This fact can result in decreasing of the number of poles/zeroes what has been observed during the conducted investigations when fl = 0.5 Hz. This fact can also result in smaller values of poles (see section 3) and then in larger time constants in the line model which means a longer simulation time to reach steady state. The basic principle should be such a selection of parameters fl, fh that enable to fulfil the condition f [fl, fh], where f is a frequency which ought to be represented during investigations. However, the mentioned selection should guarantee relatively short time of reaching steady state, the possible high precision of required approximation and also possibility of conducting a simulation without numerical problems. 5. REFERENCES 1 The manual of PSCAD/EMTDC. Winnipeg: Manitoba HVDC Research Centre, 1994. 2 Pritindra Chowdhuri: Electromagnetic transients in Power Systems, John Wiley & Sons, New York 1996. 3 Wilkosz K., Sobierajski M., Kwanicki W.: The Analysis of Harmonic Generation of SVC and STATCOM by EMTDC/PSCAD Simulations. Proc. of the 8th International Conference on Harmonics and Quality of Power (ICHQP'98), Athens, Greece, October 14 - 16, 1998, pp. 853858. Dr. Wiesaw Kwanicki was born in Czestochowa, Poland in 1954. He received his M.Sc. degree in Applied Mathematics from the Wroclaw University of Technology, Poland in 1979. He also received the M.Sc. and Ph.D. degrees in Electrical

102

and Computer Engineering from the University of Manitoba, Canada in 1986 and 1998 respectively. He has worked for the Manitoba HVDC Research Centre (Winnipeg, Canada) between 1986 and 1999 where he was engaged in studies of various aspects of power systems, power electronics, applications of artificial intelligence, and development of electromagnetic transients simulation and high speed transient stability programs. At present he is a Senior Engineer with the British Columbia Hydro (Vancouver, Canada). He is a member of IEEE and a registered Professional Engineer in the Province of British Columbia. Mailing address: Wiesaw KWANICKI British Columbia Hydro SCC-Burnaby Mountain C/o 6911 Southpoint Drive Burnaby, B.C., Canada V3N 4X8 e-mail: [email protected] Prof. Marian Sobierajski was born in 1947 in Poland. He received his MSc, PhD and DSc in Electrical Engineering from Wrocaw University of Technology, Poland respectively in 1971, 1976 and 1988. He is working in Wrocaw University of Technology since 1971, where he is presently Professor. He is a member of Power System Group of Polish Academy of Sciences. He has been responsible for several research projects in the field of planning and control of power systems. His recent works include the probabilistic aspects of voltage stability and the application of FACTS in modern power systems. Mailing address: Marian SOBIERAJSKI Institute of Electrical Power Engineering Wrocaw University of Technology Wyb. Wyspiaskiego 27, 50-370 Wrocaw, Poland phone: +48 71 348 10 94, fax: +48 71 348 53 51 e-mail: [email protected] Prof. Kazimierz Wilkosz was born in Jaworzno, Poland in. 1952. He received an M.Sc. degree in Engineering Cybernetics from Wrocaw University of Technology, Poland in 1976. In 1979 he received the Ph.D. degree, and in 1991 the D.Sc. degree in Electrical Engineering from Wrocaw University of Technology. At present, he is Associate Professor and Assistant Manager of Institute of Electrical Power Engineering. He is a member of Power System Group of Polish Academy of Sciences. His special areas of interest being at the same time of teaching and research are power system analysis, computer systems in power engineering. His recent works include problems of harmonics and application of FACTS in power systems. Mailing address: Kazimierz WILKOSZ Institute of Electrical Power Engineering Wrocaw University of Technology Wyb. Wyspiaskiego 27, 50-370 Wrocaw, Poland phone: +48 71 348 10 94, fax: +48 71 348 53 51 e-mail: [email protected]





COMPENSATION OF TIME-VARYING LOADS BY MEANS OF SVC – MODELLING AND SIMULATION Rozmysaw MIENSKI

Ryszard PAWELEK

Irena WASIAK

Technical University of Lodz Lodz, Poland

Abstract – The paper deals with compensation of frequently time-varying loads by means of Static Var Compensators (SVC). As a varying load an arc furnace is considered. The SVC system used for compensation consists of thyristor- switched capacitors and thyristor - controlled reactors (TSC/TCR). Models of a load and a SVC useful for digital simulation of transients are discussed. A model of a measurement blocks for power quality assessment is also proposed. All models were elaborated by means of PSCAD/EMTPDC programme. Simulation of the system performance was done and some results of it are presented in the paper. 1. INTRODUCTION The systematic growth in number and power of non-linear and frequently time-variable loads has resulted in degradation of power quality in electrical networks. Disturbances such as voltage fluctuation, flicker, harmonics or unbalance may cause problems for both domestic and industrial consumers. It has an effect on decreasing reliability of supply and increasing network losses. In recent years consumers demands for power quality have increased considerably due to an increasing sensitivity of applied receivers and process controls. Many customers experience severe consequences of bad power quality, which can prevent appliances from proper operating and make some industrial processes shut down, which can result in big economical losses. To meet consumers requirements some kind of distortion compensation is necessary in those points of power system where disturbing loads are connected. For this aim Static Var Compensators (SVC) have been widely used in distribution power networks of many countries. Providing fast reactive power compensation, they perform the task of securing

adequate power quality in the points of common coupling (PCC’s) from which various disturbing load devices are supplied. SVCs prevent fluctuations in the supply network which are detrimental to neighbouring consumers, maintain constant voltage on loads’ buses and reduce voltage flicker, keep good and stable power factor and balance the reactive power consumption. Different conventional SVC configurations are applied: fixed capacitor with thyristor controlled reactor (FC/TCR), thyristor switched capacitor (TSC) and combined thyristor switched capacitor with thyristor controlled reactor (TSC/TCR). The last solution is the most universal one, giving reduced power losses at zero var output [12]. Simulation of transients plays an important role in analysis of power system behaviour. It is very useful in solving power quality problems, it is needed in evaluating equipment performance and it seems to be necessary when designing the compensation devices. There are various computer programs available nowadays for this aim. One of them is PSCAD/EMTPDC programme, which is particularly useful in simulation of networks with power electronics elements and systems [1]. EMTPDC (electromagnetic transients and DC) is an implementation of the EMTP-type method of solving the transient response of circuits. Offering accurate representation of power system equipment like sources, transformers, lines, switches, etc. as well as Flexible AC Transmission System (FACTS) devices, it enables the efficient simulation of a wide variety of power systems and networks. The authors have used this program in examination of using SVC systems in electrical networks for compensation of power quality disturbances. It has been assumed that the SVC cooperates in loads whose demand for active and reactive power rapidly varies. It has a task of providing the power


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quality indices in the PCC according to the binding Standards. The paper presents some results of modelling and simulations for the system including: a supply network, an arc furnace as a heavily disturbing load, a SVC of the TCR/TSC type and a measurement system for power quality assessment. 2. SYSTEM MODELLING 2.1. Electrical network The network under study is presented in Fig. 1, in the graphical form obtained from PSCAD/EMTPDC. A substation of 30 kV is supplied from the transmission network of 220 kV through a transformer. The network is represented by an equivalent voltage source which reactance results from a shortcircuit power on the high voltage side of the transformer. The substation load is an arc furnace, which is connected to the substation busbars through a transformer 30/1,1 kV. Electric arc furnace is a major source of disturbance such as voltage fluctuation, harmonics and phase unbalance which may cause problems for other customers on the grid. Moreover, the reactive power drawn by the furnace leads to power losses and limits the flow of active power in the network. Also, the physical process inside the furnace is erratic in nature, with one or several electrodes sticking electrics arcs between furnace and scrap. As a

consequence, the consumption especially of reactive power becomes strongly fluctuating in a stochastic manner. The voltage drop caused by reactive power flowing through circuit reactances in the electrodes, electrode arms and furnace transformer therefore fluctuates erratically as well. This is called voltage flicker and is visualized most clearly in the flickering light of incandescent lamps fed from the polluted grid. Flicker is an annoying sensation which easily becomes a source of complaint. To maintain the proper power quality at the substation busbars, which is a point of common coupling (PCC), the substation is also equipped with a SVC system. The way of modelling for the arc furnace and the compensator is described below. Arc furnace The basis for constructing the furnace model was information published in [13]. A furnace characteristic was modelled as a function of an arc voltage instantaneous value and an instantaneous value of the arc current. Parameters of the function depends on the furnace power. The function variable of the stochastic nature is an arc length. Three arc length stochastic generators were constructed experimentally with the criterion to obtain typical active and reactive power flow, known from measurements. The furnace model was developed with using typical PSCAD modules. Its diagram is shown in Fig. 2.

Fig. 1. Network under study

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Three arc models

Fig.2. Diagram of the arc furnace model constructed with the PSCAD/EMTPDC SVC PSCAD contains an in-built SVC model, which employs the state variable method of simulation [4, 12]. The circuit, illustrated in Fig. 3, encompasses the electrical components of a transformer with two phase-displaced secondary windings connected in star and delta, a 12-pulse TCR and up to 10 TSC banks. This 12-pulse design gives better harmonic compensation. The principal harmonics are 11th and 13th of relatively small value and harmonics filter may be avoided. The SVC transformer is modelled as nine mutually coupled windings on a common core, and saturation is represented by an additional current injection obtained from a flux/magnetising relationship. In the TCR branches the thyristor switches are modelled as changing resistances. RC snubber circuits are included in parallel with each thyristor. The TSC bank is represented by a single capacitor per phase regardless of the number of TSC branches in operation at a given time. When a bank is switched the capacitance value and initial voltage are adjusted accordingly. The advantage of this approach is reduction of state variables. Signals to add or remove a TSC bank, and the a firing angle order for the TCR are provided from the external control system.

The TCR firing model includes a phase-locked loop (PLL) and a firing-controller. The instants of valve firing are determined when a reference angle derived from the PLL equals the ordered angle passed from an external controller. Firing angle D can change from 90 to 180 degrees. Full conduction is obtained with D = 90. The current is equal 0 when D = 180q. Accurate determination of switching instants is obtained by employing an integration step length, which is a submultiple of that employed in the EMTPDC main loop. The SVC model is a separate subroutine which interfaces to a parent EMTPDC programme as a current source. SVC control circuit Control blocks are modelled individually by an user with the control system modelling facilities of the programme. The SVC control is designed to compensate the reactive power drawn by the arc furnace. It is assumed at this stage of work that the considered system is symmetrical. The input signals to the control circuit are 3-phase RMS voltage and 3phase reactive power. The circuit consists of: filtering blocks, PID blocks, non-linear susceptance characteristic, TSC allocator and TCR firing pulses block. The control signal which is a


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SVC module

PI controler

Measured voltage

Rated reactive power

filtering

Measured reactive power

Alfa order

Control Non-linear susceptance characteristic

Capacitor switching logic

Fig. 3. Diagram of the SVC and its control reactive power order for the SVC is split by an allocator into a capacitor on/off signal for the TSC, and a reactive power demand from the TCR.A nonlinear reference is used to convert the TCR reactive power demand into a firing order for the TCR firing controller to generate the appropriate inductive current. 2.2. Power quality assessment The SVC system is designed to ensure the adequate power quality at the point of common coupling (PCC). Both the level and waveform shape of the supply voltage characterise the quality of supply. For its assessment international and native standards and regulations have been made. They define some characteristics of the voltage at the supply terminals, give their admissible values and the method of their measurement and evaluation. The basic document is the European and Polish Standard [2]. A special blocks has been elaborated in the PSCAD/EMTPDC for power quality assessment according to the binding standards. From the measured instantaneous voltage signals the following power quality indices are determined: x voltage RMS, x asymmetry factor, x THD factor and harmonic spectrum, x flicker. 106

The power quality assessment blocks are shown in Fig. 4 and 5. Signals are sampled with the rate of 400 samples per one period. The data window is rectangular, and subsequent windows are in contact. For the voltage parameter meter the PSCAD module of on-line frequency scanner was used. The flickermeter block is the digital realisation of the system described in detail in the Standard [3]. Its first part is a voltage adaptor, which allows to reduce the influence of the very slow voltage fluctuation on measurement results. The second part represents a combined reaction of the bulb-eyebrain channel to supply voltage variation. It is called IEC flickermeter. 3. RESULTS OF SIMULATION The block elements described above compose a simulator of the examined network built by means of the PSCAD/EMTPDC programme. It allows to investigate different cases of the network operation with the assessment of power quality at the PCC. To examine the presented model and illustrate operation of the system some simulation were performed including the SVC switched off and on, and for different settings of controls. Some results of simulation are presented in Fig. 6 and 7. It should be noticed that the simulation period as well as averaging time for the power quality indices were chosen optionally. They can be changed according to the requirements of the EN 50160 Standard, which are one week measurement period and 10 min. averaging time.


Fig. 4. Diagram of the voltage parameter meter

Fig. 5. Diagram of the IEC flickermeter


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Fig. 6. Power quality indices at the PCC with the SVC switched off

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Fig. 7. Power quality indices at the PCC with the SVC switched on


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4. CONCLUDING REMARKS Reactive power compensation is one of the tasks for the SVC systems applied in distribution networks and co-operated with non-linear and unbalanced loads. The system is designed to meet the quality demands. The results presented in Fig. 6 and 7 prove that the SVC is an effective way for reducing voltage fluctuation, however it increases the voltage distortion at the same time. Therefore, when assessed the compensation effectiveness one should examine the all power quality parameters simultaneously. For examination of electrical power networks the method of simulation can be applied effectively and PSCAD/EMTPDC has been recognised as a good tool. The network under consideration model was constructed using some modules which are available in the program in the form of built-in procedures and completed by the modules worked out by the authors. Such a simulator allows evaluating the power quality at the PCC according to the EN 50160 Standard. It may be also useful for studies and transient analysis of power networks with non-linear and unbalanced loads.

7.

8.

9.

10.

11.

5. ACKNOWLEDGEMENT 12. This work has been supported by the State Committee for Scientific Research under Contracts No. 1459/T10/2000/18 and No. 1659/T10/2001/20

13.

6. REFERENCES 1. 2.

3.

4.

5.

6.

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Arrilaga J., Smith B.: AC-DC Power System Analysis. IEE, 1998. EN-PN 50160: : Voltage characteristics of electicity supplied by public distribution systems, 1998. (in Polish) European Standard EN 60868: Flickermeter - Functional and design specifications, 1993. Gole A. M., Sood V.K.: A Static Compensator Model for Use with Electromagnetic Transients Simulation Programs. IEEE Transaction on Power Delivery, Vol. 5, No. 3, July 1990. Gole A.M. and others: Guidelines for Modeling Power Electronics in Electric Power Engineering Applications. IEEE Transactions on Power Delivery, Vol. 12, No. 1, 1997 Lasseter R. H., Lee S. Y.: Digital simulation of static var system transients. IEEE Transactions on Power Apparatus and Systems, vol. PAS-101, no. 10, 1982.

14.

15.

16.

Lee S. Y., Bhattacharya S., Lejonberg T., Hammad A., Lefebvre S.: Detailed modelling of static var compensators using the electromagnetic transients program (EMTP). IEEE Transactions on Power Delivery, vol. 7, no. 2, 1992. Lefebvre S., Gerin-Lajoie L.: A static compensator model for the EMTP. IEEE Transactions on Power Systems, vol. 7, no. 2, 1992. Lima m.C., Ramos A.J.P., Baltar F.J.: Performance of Static Var Compensators in Degradated Transmission System Conditions: Dynamic Studies Versus Electromagnetic Transient Studies. International Conference on Power Systems Transients IPST’99, 07, 1999, Budapest, Hungary. Maguire T.L., Gole A.M.: Digital Simulation of Flexible Topology Power Electronic Apparatus in Power Systems. IEEE Transactions on Power Delivery, Vol. 6, No. 4, October 1991. Ramos A.J.P., Tyll H.: Dynamic Performance of a Radial Weak Power System with Multiple Static Var Compensator. IEEE Transactions on Power Systems, Vol. 4, No. 4, October 1989. Song Y. H., Johns A.T.: Flexible AC Transmission Systems (FACTS). IEE, 1999. Sousa J., Correia de Barros M.T., Covas M., Simoes A.: Harmonic and Flicker Analysis in Arc Furnace Power System. International Conference on Power Systems Transients IPST’99, 07, 1999, Budapest, Hungary. Szostak A.J.M., Ramirez A., Martinez M.L.B.: transient Studies of the Static VAR Compensator of San LorenzoParaguay. International Conference on Power Systems Transients IPST’99, 07, 1999, Budapest, Hungary. Tenorio A. R. M., Monteiro J.S., Vasconcelos A.N.: Exhanced Modeling of the Fortaleza SVC Incorporating a PLL-based Firing System Validated Against Laboratory Tests. International Conference on Power Systems Transients IPST’99, 07, 1999, Budapest, Hungary. Vasconcelos A.N., Ramos A.J.P., Monteiro J.S., Lima M.V.B.C., Silva H.D., Lins L.R.: Detailed Modeling of an Actual Static VAR Compensator for Electromagnetic Transient Studies. Transactions on Power Systems, Vol. 7, No. 1, February 1992.


Rozmysaw Mienski received M.Sc. and Ph.D. degrees from Technical University of Lodz. At present he is a senior lecturer at the Institute of Electrical Power Engineering of Technical University of Lodz. His area of interest is power quality and AC/DC power network simulator. e-mail: [email protected] Ryszard Pawelek was born in 1952 in Chocz, Poland. He received M.Sc. and Ph.D. degrees from Technical University of Lodz. At present he is a senior lecturer at the Institute of Electrical Power Engineering of Technical University of Lodz. He is a secretary of the Editorial Board of Polish periodical “Electrical Power Quality and Utilisation”. His field of interest is power quality. e-mail: [email protected]

Irena Wasiak graduated from the Technical University of Lodz, Poland. There she received the Ph.D. degree in electrical power engineering. Presently she is a senior lecturer at the Institute of Electrical Power Engineering, Technical University of Lodz. She is a secretary of the Program Board and a member of the Editorial Board of Polish periodical “Electrical Power Quality and Utilisation”. Her area of interest includes modelling and simulation of transients in power systems, and power supply quality. e-mail: [email protected]


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THE SIMULATION OF FUZZY LOGIC BASED ADVANCED STATIC VAR COMPENSATOR IN MATLAB Sule OZDEMIR

Engin OZDEMIR

H. Metin ERTUNC

Kocaeli University, Izmit (Turkey)

Abstract - It is proposed a fuzzy logic controller for an ASVC system with mathematical analysis, and simulation model developed in MATLAB environment. Many simulations are performed to analyze the effect for increasing transmission capability, damping low frequency oscillation, and improving transient stability. The performance of the proposed controller is investigated by the digital computer simulations of a sample system. The implementation of simulation has been carried out using the graphical capabilities of MATLAB.

1. INTRODUCTION Power electronic technologies are finding increasing applications in high-voltage transmission for improving both transient and steady-state performances, and in low-voltage distribution systems to improve on the reliability and quality of power flow. Nowadays, customers are increasingly concerned about the vulnerability of their sensitive electronic equipment to power disturbances. Since %90 of outages affecting customers originate on the power system, there is a need for utilities and industrial customers to find new ways to offer premium quality power to customers with sensitive loads. Reactive power compensation is a must in many industrial sites and, for stabilization purposes, in power system.

systems and large, fluctuating industrial loads. It can draw or supply the reactive current from or to the power system. This function is identical to the dynamic synchronous condenser, but its response time is extremely faster than that of the condenser. In this paper a fuzzy logic based controller design with mathematical analysis and simulation model development with the MATLAB environment are performed to verify the dynamic operation of ASVC. The simulations are performed to analyze the effect for improving transient capability, damping low frequency oscillation assuming that the ASVC is connected to the harmonic producing inductive load. Computer simulation results obtained with Matlab are provided and discussed to validate the performance of the proposed control algorithm.

2. ASVC ASVC utilizes modern power electronics technology to provide stepless reactive power compensation for power factor control and also voltage regulation. Figure 1 shows a basic block diagram of an ASVC. It consists of a three-phase voltage source inverter (VSI) with the input inductors (Lc,Rc) and a DC capacitor (Cdc). Three phase AC mains feeds power to the load. The capacitor Cdc is used as an energy storage element to obtain a self-supporting DC bus of the ASVC for an effective control of the system.

The ASVC was proposed by several researchers [1,2,3] to compensate the reactive power in power Section 2. Methods of Power Quality Analysis: Modelling and Simulation

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a aa 3-phase AC mains

isa

iLa

isb

iLb

isc isn

iLc iLn

icn

Balanced/unbalanced linear/nonlinear loads

Cdc

icc icb ica

a

b

c

0 Cdc

Figure 1. Block diagram of an ASVC. ASVC has multifunctional capability towards different types of loads; such as static var generator, load balancer and three-phase active filter. For a three-phase power system shown in Figure 1, instantaneous voltages va, vb and vc and instantaneous currents ia, ib and ic are expressed as instantaneous space vectors v and i, i.e.,

v

ªv a º « » «v b » i «¬ v c »¼

ªi a º « » «i b » «¬i c »¼

(1)

The "a", "b", and "c", three phases are mutually orthogonal three phase coordinates. The instantaneous active power of a three-phase circuit p can be given by p= v.i or

p= va.ia + vb.ib + vc.ic

(2)

ªi D º ªv a º « » >C@ «v b » and ««i E »» «i » «¬ v c »¼ ¬ 0¼

ªv D º « » «v E » «v » ¬ 0¼

ªi a º >C@ ««i b »» «¬i c »¼

(4)

where

ª « « 2« 3« « « ¬

>C@

1 2 3 2 1

1 0 1 2

2

1 º 2 » » 3» 2 » 1 » » 2 ¼

(5)

The input of the system is the control variable D and the output is the generated reactive power Qc. In this respect,

Ax BD

(6)

where "." denotes the dot (internal product or scalar product of vectors. A new instantaneous space vector q is defined by

dx dt Qc

Cx

(7)

q=vxi

where

(3)

where "x" denotes the cross (exterior) product of vectors or vector product. Vector q is designated as the instantaneous reactive power vector of the three-phase circuit. These three equation can be written for multiphase system. For three phase voltages and currents, the D and E and zero components are expressed as

114

x

>i

A

ª Rs « L « « Z « « « 0 ¬

C

> Vs

q

id

v dc

@

T

Z Rs L D 2C

0 0@

(8) º 0 » ª Vs º « L » D »» B « 0 » » « L» » « 0 » 0 » »¼ «¬ ¼

(9)

(10)


From equation (6) and (7), the transfer function of the system is given by

and adaptive features of fuzzy logic based control system.

Q c (s) D(s)

From the transfer function of (11), a fuzzy logic based controller can be designed in order that that var compensator system has fast dynamic characteristics.

C(sI A) 1 B

N(s) M(s)

(11)

where 2 Vs ª 2 R s D2 º s s « » L ¬ L 2LC ¼

N(s)

s3

M(s)

(12)

2 ½° D 2 R 2R s 2 °ª R s º D2 s Z 2 ¾s s ®« » 2 L L 2 LC 2L C °¯¬ ¼ °¿

(13) Figure 2 shows the unit step response of the openloop system with the circuit parameters given in Table 1. It can be seen that ASVC system takes about 15 cycles, 250ms, to reach another steady state. x 10

To achieve fast dynamic response of the closedloop system with the circuit parameters given in Table I, the PI controller parameters are determined as follows[4]: Kp=1x10-5, Ki=1.7x10-4

(14)

Figure 3 and Figure 4 show the unit step response of the closed-loop system for designed control parameters in (14) and for fuzzy logic based controller respectively. The ASVC system takes two cycles to reach steady state in PI controller.

5

1

8

0.8 6

0.6 0.4

4

0.2

2

0 0

0

0.05

0.1

0.15

0.2

0.25

0.3

0

time (sec)

Figure 2. Unit step response of the open-loop ASVC system. TABLE 1. Circuit parameters. Symbol Value f 60 Hz 2Sf(red/sec) Z Vs 550V Rs 0.4: L 10mH C 1000PF 1.2 2 / 3D

0.02

0.04

0.06

0.08

0.1

Figure 3. Transient response for a step change in the var command for a PI controller. 1.2 1 0.8 0.6 0.4 0.2

3. FUZZY LOGIC BASED CONTROL 0

Fuzzy logic based control is basically nonlinear and adaptive in nature. This gives a robust performance in cases where the effects of variations in controller parameters are present. In this study, it is proposed a fuzzy PI (FPI) controller, since simplicity of a PI controller is combined with intelligence

0

0.02

0.04

0.06

0.08

0.1

Figure 4. Transient response for a step change in the var command for a fuzzy logic based controller.


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Step Load Fuzzy Logic Controller Step 1 n1.s2+n2.s+n3

Step Input PI Switch Sum

PI Controller

d1.s3+d2.s2 +d3.s+d4 Transfer Fcn

Sum2 Auto-Scale Graph

Figure 5. Block diagram of the proposed control system. The fuzzy logic based control consists of a first section that generates the inputs, a fuzzy core that performs fuzzyfication, inference and defuzzyfication and a third section that transforms the output in the inverter driving signals. The fuzzy core output variable is the duty cycle that defines the sampling period portion. Inputs of the fuzzy logic based controller are categorized as various linguistic variables with their corresponding membership values as shown in Figure 6. Depending on range (large, medium, small and zero) and sign (positive or negative) of the error signals, FPI controller determines the corresponding output from the linguistic rule base table.

controller. Figure 5 shows the computer simulation block diagram of the proposed control system. 5. CONCLUSION

In addition, based on the open-loop transfer function of the system obtained from the analysis of the DQ-transformed equivalent circuit, the fuzzy logic based controller is designed in order that the ASVC system has fast dynamic response. It is compared to PI controller. The performance of the proposed fuzzy logic based controller is investigated by the digital computer simulations of a sample system, and it is proved to be very effective and robust in damping power system oscillations. 6. REFERENCES

NL

-e

NM

-e2

NS

1

-e1

Z

PS

e1

PM

e2

PL

e

Error signals Figure 6. Membership values of fuzzy variables. Z = Zero NL = negative large NM = negative medium NS = negative small

PL = positive small PM= positive medium PS = positive small

4. SIMULATION RESULTS

Computer simulations are performed for the steady state performance and dynamic response of the proposed controller with respect to conventional PI

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1. Hingorani, N. G. ; Gyugyi, L.: Understanding FACTS. IEEE Press. New York, 2000. 2. Singh, B., Chandra, A., Al-Haddad K.: DSPBased Indirect Current Controlled STATCOM Part 1: Evaluation of Current Control Techniques. IEE Proc-Electr. Power Appl. Vol. 147 No.2 pp. 107-112, 2000. 3. Miller J.E.: Reactive power control in electric systems. John Wiley & Sons, New York 1982. 4. Guk C.C., and et.al.: Analysis and Controller Design of Static Var Compensator Using Three-level GTO Inverter. IEEE Transactions on Power Electronics, Vol.11, No.1, January 1996. 5. Ozaslan S. : Reactive power compensation with Static Var Compensator. Kocaeli University, Ph.D. thesis, 1997.


Dr. Engin Ozdemir was born in 1970 in Izmit, Turkey. He received the B.Sc. and M. Sc. degree in electrical engineering from Technical University of Yildiz. He received his Ph.D. and degree from Kocaeli University. Presently, he is Assistant Professor Dr. Vise Chairman of Electrical Education Department of the Technical Education Faculty of the Kocaeli University. His areas of interest include active power filters and electric power quality.

Dr. H. Metin Ertunç was born in 1969 in Denizli, Turkey. He received the B.Sc. degree in electrical engineering from University of Hacettepe. He received his M.Sc. and Ph.D. degree from Case Reserve Western University. Presently, he is Assistant Professor Dr. of Electronics and Communication Engineering Department of the Engineering Faculty of the Kocaeli University. His areas of interest include simulation of electric power systems.

Mailing address: Dr. Engin Ozdemir Kocaeli University Technical Education Faculty, Electrical Education Department, 41100 Izmit. TURKEY phone:(+90)(262) 324 99 10-202, fax :(+90)(262) 331 39 09 e-mail: [email protected]

Mailing address: Dr. H. Metin Ertunç Kocaeli University,Engineering Faculty, Electronics and Communication Department, 41100 Izmit. TURKEY phone:(+90)(262)335 1148 e-mail: [email protected]

Sule Ozdemir was born in 1973 in Izmit, Turkey. He received the B.Sc. degree in electrical education department from Marmara University of Istanbul. He received his M.Sc. and degree from Kocaeli University. Presently, he is a Ph.D. student of Electrical Education Department of the Kocaeli University. His areas of interest include active power filters and electric power quality.

Mailing address: Sule Ozdemir Kocaeli University, Technical Education Faculty, Electrical Education Department, 41100 Izmit. TURKEY phone:(+90)(262) 324 99 10 - 270, fax :(+90)(262) 331 39 09 e-mail: [email protected]


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USING WAVELET TRANSFORM FOR IDENTIFICATION OF ELECTRIC CIRCUITS PARAMETERS Igor V. ZHEZHELENKO

Yuri L. SAYENKO

Viktor V. NESTEROVICH

Priazovskii State Technical University Mariupol (Ukraine) Abstract - The paper describes the possibility for identifying electrical circuits parameters with wavelet transform. It is shown that if the circuit parameters change during the experiment the wavelet transform possesses a number of advantages as compared to Fourier transform. 1. INTRODUCTION At present the problem of electromagnetic compatibility of electric receivers and power supply networks is becoming more and more acute. One of the constituents of this problem is distortion of the voltage waveform in the electric circuit points and distortion of currents that flow through circuit branches. Connection to the circuit points of electrical receivers having non-linear volt-ampere characteristics or those with variable in time parameters is the main reason which causes such distortions. Now it is a common to present non-sinusoidal voltages and currents in the form of Fourier-series expansion. In this case non-linear elements are regarded as voltage or current sources at higher harmonics frequencies. To estimate the values of voltages and currents waveform distortion and to be able to develop procedures for their decreasing it is required to have possibility to simulate electrical circuit behavior in different modes with higher harmonics sources connected to it. This entails possessing information on the circuit parameters, its frequency characteristics in particular. In a general case the information on electrical circuit parameters can be obtained by way of computing or experimentally. The degree of adequacy of the information obtained by computing depends, to a great extent, on the parameters of the electrical circuit components

and the adequacy of the mathematical models used for presentation of these components at higher harmonics frequencies. This results in a considerable error in defining by way of computing the frequency characteristics of electrical circuit impedance and admittance. Besides, these characteristics, in the majority of cases, have stochastic, not deterministic, nature. The latter stipulates for the urgent development of the methods for experimental identifying of these characteristics. More often in case of experimental identification of the electrical circuit frequency characteristics the circuit is considered as a linear deterministic or stochastic system which is invariant in time. This allows to use widely the methods of spectral analysis, based on application of Fourier transform [1]. However real electrical circuits cannot always be represented as systems invariant in time. Their parameters may evolve during the experiment and may sometimes vary spasmodically (e.g. at commutation of certain elements). In this case Fourier transform fails to sufficiently meet the task requirements because of non-locality of trigonometrical functions. One of the possible ways of elimination of this defect is application of Fourier transform alongside with the window function, localized in time (Gabor transform), but this method has one drawback: resolution in time is a constant and does not depend on frequency (scale). So it seems to be of interest to apply wavelet transform for solving the problem of identification of electrical circuit parameters. Unlike Fourier transform, wavelet transform retains locality in signal representation [2].


119

2. ESTIMATION TECHNIQUE Continuous wavelet transform can be determined as follows [2-4]: f

t b x(t ) g ( )dt , ³ a a f

1

. W ( a, b)

(1)

g - a conjugate complex of the basic wavelet function; a - scale parameter (the degree of compression of wavelet function); b - parameter defining wavelet function shift along time axis. Function g (t ) must meet the admissibility condition [2]

2S

³

G (Z )

f

Z

2

dZ f ,

(2)

where G (Z ) - Fourier image of function g (t ) ; Z - frequency. Hereinafter it is assumed that Morlet function defined by the following equation will be used as a wavelet function:

e jZ 0 t e t

g (t )

2

/2

(3)

,

where Z 0 - frequency of the wavelet-function (usually it is agreed Z 0 =6 or Z 0 2S ). Fourier transform of the wavelet function (3) is:

e ( aZ Z0 ) . This function has a peak at aZ Z 0 . G (aZ )

2

(4)

It is possible to show that wavelet transform of the harmonic signal with variable amplitude k (t )

x(t )

k (t ) cos(Z t ) ,

(5)

where Z - signal frequency, is equal to

(6) a k (t )e (Z Z0 ) e jZ b , For the given value of parameter a (corresponding

W ( a, b)

i

2

2

In the general case N

iu

¦i

(10)

ki

k 1

and Ak e ck t cos(Z k t M k ) ,

ik

(11)

where Ak , Z k , M k - initial amplitude, proper frequency and phase of the damped oscillation. Having applied wavelet transform to equation (9) and selecting the parameter value a , which makes the influence of the neighboring resonance frequencies negligible, we obtain

I (t ) Z k b M k >W (a, b)@ . (12) Frequency Z k can be found with the help of derivation of expression (12) by parameter b .

Thus, the proposed method allows to determine resonance frequencies by analyzing transient processes, which occur at execution both active and passive experiment. More comprehensive data on frequency characteristics of the parameters of the circuit being investigated can be obtained from analyzing the nature of variation of L2 module of wavelet coefficients, which defines energy density of the investigated signal. 3. SIMULATED EXAMPLE To test the proposed method simulation of processes which occur in simple electrical circuit at commutation of a number of branches has been performed. Simulation was carried out using Matlab software. Fig.1 shows the diagram of the circuit being investigated.

Ls

(7) (8)

Let us consider the electrical circuit which includes elements with linear volt-ampere characteristics and emf (electromotive force) sources of ac current with Z frequency. Let us assume that there occurred connection or disconnection of an element. Due to the linear nature of the equations associating currents and voltages, the forced values of currents in all circuit branches and of voltages in all of its sections, will 120

(9)

where i f - forced current component;

to a definite frequency Z ) its module and phase appear as:

| W (a, b) | a k (t )e (Z Z 0 ) , >W (a, b)@ Z b .

i f iu ,

iu - its free component.

where x (t ) - the signal under investigation;

f

also be harmonic functions with the same frequency Z . Current in any of the branches

Es R1

R2

R3

L2

L3

C2

C3

Fig. 1. Diagram of the investigated circuit Electrical Power Quality and Utilisation

The branch with resistance R1 was the first to be connected followed by branches with complex load R2 L2 C 2 . Fig.2 shows the corresponding circuit model. Branch currents and voltages obtained in the result of simulation were analyzed with the

help of wavelet transform basing on complex wavelet Morlet. The results of the wavelet analysis of transient process current which consists of a forced component with frequency 50HZ and two high-frequency free components, damping by exponential law with various time constants, are given in Fig.3

Fig. 2. Circuit model

Fig.3 Results of wavelet analysis of transient process (variations in wavelet transform amplitude intensity are indicated by isolines): 1 – area corresponding to forced component, 2 and 3 – areas corresponding to free components


121

Wavelet transform of the processed signal is a complex function of two variables W ( a, b) and its module can be presented by a surface in threedimensional space. In the diagram the surface is depicted by its plane projections with isolines which permit to follow variation of wavelet amplitude intensity. The plots are defined by coordinates ln a and t as there is a unique correspondence between parameter b and time t . Connection of branches occurred at points in time 0.045 sec. and 0.115 sec. It is obvious from Fig.3 that wavelet transform allows to evolve individual transient process current components, define their frequencies (which are connected with parameter a in a unique manner) and on the basis of this information identify electrical circuit parameters. 4. CONCLUSION When identifying electrical circuits parameters it is possible to use wavelet transform along with Fourier transform. Wavelet transform possesses a number of advantages in the cases when circuit parameters change during conducting experiment. 5. REFERENCES 1. Macedo F.X.: Power system harmonic impedance measurement using natural disturbances. Proc. of Third international conference on sources and effects of power system disturbances.- London, New York 1982, pp.183-188. 2. Chui C.K.: Wavelets analysis and applications: an introduction to wavelets. Vol. 1. Academic Press, New York 1992. 3. Staszewski W.J.: Identification of damping in MDOF system using time-scale decomposition. Journal of Sound and Vibration, vol.203, no. 2, 1997, pp. 283-305. 4. Ruzzene M., Fasana A., Garibaldi L., Piombo B.: Natural frequencies and dampings identification using wavelet transform: application to real data. Mechanical Systems and Signal Processing, vol.11, no.2, 1997, pp.207-218.

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Prof. Igor V. Zhezhelenko was born in1930 in Mariupol, Ukraine. He received Ph.D. and D.Sc. degrees from Novocherkask Polytechnical Institute. Presently, he is rector of State Technical University, Member of Academy of Science of high Education of Ukraine. His areas of interest include electric power quality and electromagnetic compatibility. Mailing address: Igor V. Zhezhelenko

Priazovskii State Technical University Universitetska Str. 7 87500 Mariupol UKRAINE phone: (38) (0629) 33-21-08, fax: (38) (0629) 34-31-27 e-mail: [email protected] Prof. Yuri L. Sayenko was born in 1962 in Mariupol, Ukraine. He received Ph.D. degree from Institute of electrodynamics of Ukraine National Academy of Science. D.Sc. degree he received from Silesia Polytechnical Institute of Gliwice. Presently, he is Professor of Priazovskii State Technical University, Member IEEE. His areas of interest include electric power quality and electromagnetic compatibility. Mailing address: Yuri L. Sayenko Priazovskii State Technical University Universitetska Str. 7 87500 Mariupol UKRAINE phone: (38) (0629) 52-85-99, fax: (38) (0629) 34-31-27 e-mail: [email protected] Viktor V. Nesterovich was born in 1957 in Mariupol, Ukraine. Lecturer at department of Industrial Power Supply at Priazovskii State Technical University. Graduated from PSTU in 1979. Main research area – Quality of electric power. Universitetska Str. 7 Mailing address: Viktor V. Nesterovich Priazovskii State Technical University Universitetska Str. 7 87500 Mariupol UKRAINE phone: (38) (0629) 31-65-51, fax: (38) (0629) 34-31-27 e-mail: [email protected]





EQUIVALENTING OF ELECTRIC NETWORK WHEN CALCULATING MODES OF HARMONICS Lidia I. KOVERNIKOVA

Sergei S. SMIRNOV

Energy Systems Institute Siberian Branch of the Russian Academy of Sciences Irkutsk (Russia)

Abstract – Measurements of harmonic voltage levels in electric networks of a complex configuration with long transmission lines, as is the case in Russia, show that they may be high in the network nodes situated at a large distance from harmonic sources. This fact makes it necessary to calculate large-sized networks for the analysis of harmonic conditions in some limited area. Calculation of large-sized electric circuits for ten harmonics simultaneously requires much computer time, especially in the multivariant calculations, when a larger part of the circuit remains unchanged. The calculation of such circuits can be accelerated by formation of the rated circuits of a smaller size on the base of equivalenting that are stored as initial data. Experience of calculations performed shows that in the analysis of harmonic conditions in the large-sized circuits consideration is given as a rule to a separate network area with 100-200 nodes. Therefore, in the initial data for the network circuit the studied area can be separated and stored with its dimensions and the remaining part can be transformed and represented as equivalents. The transformed initial data as an equivalent rated circuit may be stored in individual files as initial data for further calculations. The paper presents approaches to formation of equivalent rated circuits to analyze harmonic conditions and algorithms for calculation of harmonic voltages in the studied section of the network on their base. 1. INTRODUCTION In Russia HV electric networks even in one power system are located on a vast territory, are rather

long and have several voltage levels starting with 110 kV. Power systems are interconnected to form the unified power system of the country. Several decades ago large non-linear consumers were connected to a power system without analysis of their impact on the harmonic voltage levels. Measures to normalize their levels in accordance with the standards on power quality were not taken. As a consequence the power quality indices, characterizing harmonic levels, as is seen from measurements, exceed the established standards. These situations are typical particularly of the Siberian region of Russia. In Siberia the electrified railway transport, large energy-intensive enterprises, large industrial cities are scattered on a vast territory and interconnected by extended electric networks. The results of measurements of harmonic condition parameters that were made in electric networks of East Siberia show that the harmonic distortions cover great distances [1]. In the extended networks they are spread in such a way that the large harmonic sources located at a distance of several hundreds of kilometers may make a larger contribution to the harmonic voltage level than the nearby sources. These peculiarities necessitate the use of network circuits consisting of hundreds and thousands of nodes and lines to calculate harmonic conditions. In the multi-variant calculations, for example, when the choice is made of devices for normalization of harmonic conditions in some network area, such a circuit is inconvenient for application. A larger part of the scheme does not change in the calculation, however the computer time increases, visualization during analysis of the results is lost, etc. Therefore, a more convenient circuit is required to perform calculations. From the above said it follows that before making calculations it is suggested to construct equivalent rated circuits of a


123

much smaller size and store them as initial information for the multi-variant calculations. The paper describes two approaches to constructing rated circuits for the analysis of harmonic conditions.

2. FORMATION OF EQUIVALENT RATED CIRCUITS If it is necessary to calculate harmonic conditions in a limited network area with non-linear distortion loads and to take into consideration impact of nonlinear sources located beyond the area of concern, the equivalent rated circuits can be obtained by two methods. First, the equivalent circuit may be formed for the whole network from the available information and then simplified by reduction to the bounds of the studied network area. Second, the equivalent circuit may be formed for the considered area from the available information and the external network may be taken into account as equivalents determined by the results of measuring the parameters of conditions and network. This approach allows for both large non-linear loads and numerous small ones scattered on the electric network and having a profound effect on harmonic conditions, if taken together. 2.1. Two algorithms for forming equivalent rated network circuits The suggested algorithms for forming equivalent network circuits to calculate harmonic conditions are based on the methods of network equivalenting that are properly developed for calculations of conditions for a basic frequency [4]. What are the equivalent circuits of an electric network that are applied to calculate harmonic conditions? First, this is a set of passive (in terms of harmonic distortion generation) network elements (transmission lines, condensers, reactors, activeinductive loads, etc.) connected in accordance with the electric circuit. Second, the active elements, i.e. the sources of harmonic distortions, are non-linear loads that are represented in the equivalent circuits as current sources with the known current values and phases. In the multi-variant calculations of harmonic conditions one may be interested in the network conditions in a bounded geographical area or of a definite voltage or in some definite nodes. Thus, the network circuit can be represented by two subcircuits: the studied subcircuit in which changes are performed and the conditions are analyzed; the equivalented subcircuit in which there are no changes and whose conditions in the calculations made are of no interest. 124

Dimensions of the equivalent circuit for the initial network can be reduced by transformations, leading to decrease of its dimensions, i.e. by equivalenting. For this purpose the second subcircuit will be used. In what form can the equivalent rated network circuits be represented after transformations? First, all nodes can be excluded in the equivalented network subcircuit and in the rated equivalent subcircuit the number of nodes will correspond to the number of nodes in the studied subcircuit. Second, the equivalented subcircuit can be represented as a multi-port circuit. If we are interested in the fraction of current and voltage in the studied network from each harmonic source located in the equivalented subcircuit, or some definite sources, then the nodes they are connected to can be retained. Two algorithms for forming equivalent rated network circuits are described below. All nodes of the equivalented subcircuit are excluded in the first algorithm, in the second the equivalented subcircuit is represented by the multiport circuit. The first algorithm. The harmonic voltage levels in electric networks are calculated by a system of nodal equations in the form

Y U n n

I , n

(1)

where: n – the number of harmonic; Yn – the matrix of nodal admittances of the n-th harmonic;

U n – the matrix of unknown nodal voltages of the n-th harmonic; I – the matrix of the n -th harmonic current n of non-linear loads.

For simplification of representation the index n will be omitted in formulas. We divide the current matrix In into two submatrices I1 and I2

In

I1 , I2

(2)

where: I1 – the submatrix of current harmonic sources in the studied subcircuit, I 2 – the submatrix of current harmonic sources in the equivalented subcircuit.


The nodal voltage submatrix U n will also be split into two submatrices U1 and U 2

U1 , U 2

U n

(3)

where: U1 – the nodal voltage submatrix for the studied subcircuit, U 2 – the nodal voltage submatrix for the equivalented subcicuit. Equations (1) will be rewritten in the form

Y11Y12 Y21Y22

I1 . I2

U1 U 2

(4)

I1 . I2

(5)

The following system of matrix equations is obtained

Y11U1 Y12U 2 Y21U1 Y22U 2

I1 I2 .

(6)

The parameters of the equivalent rated network circuit are obtained by elimination of network nodes belonging to the equivalented subcircuit. After elimination of the submatrix U 2 from this system we will have

(Y 11Y12Y221Y21 )U1

I1 Y12Y221I2 .

Y1eU1

(7)

Thus, expression (7) is a nodal equation for the transformed circuit with eliminated nodes belonging to the equivalented subcircuit. Elements of the matices Y1e and I1e are parameters of the equivalent rated circuit. Elements of the matrix Y1e are self- and mutual nodal admittances of the studied subcircuit after transformation. Elements of the matrrices Y1e and I1e are stored in files as initial data for the multi-variant calculations. If the parameters of elements of the studied subcircuit are changed or switching of elements are performed, the elements of the matrices Y1e and of the matrix elements are known [3]. The equivalented subcircuit may retain nodes, to which nonlinear loads are connected. The contribution of these loads to voltage distortion in the studied subcircuit is of interest. The second algorithm. The harmonic voltage levels in network nodes are also calculated by a system of nodal equations (1). After simplifying transformations the equivalented subcircuit will be represented by the equivalent multi-port circuit with the number of ports equal to the number of lines connecting the studied and equivalented subcircuits. We eliminate lines connecting two subcircuits and replace them by currents with the corresponding signs. Nodes of both subcircuits will be classified into internal and boundary nodes. The boundary nodes are the nodes to which the eliminated lines are connected, the internal nodes include the rest of them. Put down the system of nodal voltage equations for each subcircuit

Y Y ikk ikm U ik Y Yimm U im imk

I ik , I I im sj

(8)

where:

i Denote

Y1e Y 11Y12Y221Y21 I1e I1 Y12Y221I2 .

I1e .

I1e must be adjusted. Procedures for adjustment

The nodal admittance matrix Yn is split into blocks in accordance with division of nodes in the studied and equivalented subcircuits. Elements of the square matrices Y11 and Y22 are the self- and mutual admittances in the studied and equivalented network subcircuits. The mutual admittances between the studied and equivalented subcircuits are elements of the rectangular matrices Y21 and Y12 . After multiplying the matrices in (4) we have

Y11U1 Y12U 2 Y21U1 Y22U 2

The system of equations will have the form

Y , ikk

– the network number (1 – for the studied subcircuit, 2 – for the equivalented one); Yimm – the nodal admittance submatrices of the internal and boundary nodes of subcircuit i ;


125

Y ,Y – the submatrices of mutual nodal ikm imk U

ik

, U im

admittances of the internal and boundary nodes of subcircuit i ; – the nodal voltage submatrices of

In terms of the boundary conditions (10) the system of equations (9) for the network represented as multi-port circuits takes the form

Y1eU1m

Y2e (U1m Z sj Isj )

the internal and boundary nodes in subcircuit i ; – the submatrix of current harmonic

I ik Iim Isj

sources connected to the internal nodes of subcircuit i ; – the submatrix of current harmonic sources connected to the boundary nodes of subcircuit i and eliminated lines.

To represent the equivalented subcircuit as a multiport circuit with respect to the boundary nodes the internal nodes are eliminated. After elimination of the voltage matrix for internal nodes and transformations the system of equations (8) for each subcircuit takes the form

YieU im

Iie Isj ,

(9)

I1m Isj I2e Isj .

(11)

After some transformations this system of equations can be written as follows:

YeU1m Ye Ie

where

Ie ,

(12)

Y2e (1 Y2e Z sj )Y1e

I2e (1 Y2e Z sj) I1e .

Determine the harmonic voltages U1m in the boundary nodes, to which the studied subcircuit is connected, from the system of equations (11). The system of equations (8) for the studied subcircuit is written in the form

U Y U 1m I kk 1k 1km 1k Y U Y1mmU1m I1m Isj . 1mk 1k Y

1

where:

Yie Iie

Yimm Y Y 1 Y ; imk kki ikm I Y Y 1 I im imk ikk ik

(13)

From the first equation of system (13) we determine U – the voltage values in the internal

kk

1

parameters of the multi-port circuits for the studied and equivalented subcircuits. The equivalented subcircuit will be stored in the file of initial data as the matrices Y2e and I2e . The calculation algorithm of harmonic voltage levels in nodes of the studied subcircuit on the base of Y2e and I2e of the equivalented subcircuit is described below. Transformations will be equivalent, if the boundary conditions for the studied and equivalented subcircuits are met

I1m U1m U sj where:

U sj Z sj

I2 m U 2 m ,

Z sj I – the matrix of voltage drop in the sj eliminated lines, – the diagonal matrix of resistances in the eliminated lines.

126

(10)

nodes, i.e. nodes of the studied subcircuit which are of interest for the analysis. Thus, the equivalented part of the network can be represented for each harmonic as two matrices Y2e and I2e . The matrix dimensions are determined by the amount of lines connecting the studied and equivalented subcircuits. The algorithm can be described in short as follows: 1. The electric network circuit is divided into two parts: studied and equivalented. The boundary nodes and lines connecting both subcircuits are separated. 2. The system of equations (8) is set up for each subcircuit, the internal nodes are eliminated and parameters of the equivalent multi-port circuits Y1e , I1e and Y2e , I2e are obtained.

The matrices Y2e and I2e belonging to the equivalented subcircuit are stored as initial data in a separate file. 3. In terms of the boundary conditions (10) the system of equations (12) is set up, from which the harmonic voltages in the boundary nodes of the studied subcircuit U1m are determined. Electrical Power Quality and Utilisation

4. Harmonic voltages in the internal nodes of the studied subcircuit are determined from the first equation of system (13). 5. Calculation terminates, but if changes were performed in the studied subcircuit, we pass to point 2 to calculate the parameters Y1e and

I1e , of the multi-port circuit of the studied

subcircuit and then to points 3, 4, 5. Initial data for the studied subcircuit are specified in a usual way, therefore the setting of changes in the parameters of elements and their switchings does not involve difficulties. Elimination of nodes in the described algorithms can be done, for example, by the Gauss method. The procedure of node elimination is well developed for electric networks for the basic frequency. Specific feature of harmonics is the current and voltage matrices. Since the harmonic conditions are of stochastic character and described by probabilistic characteristics, currents of the harmonic sources should be represented by mathematical expectations and variances [2]. The network circuit also changes stochastically, but to a lesser extent than the currents. Hence, the currents of harmonic sources and the voltages in network nodes are considered in calculations as probabilistic parameters. The random vectors of currents and voltages are described for each harmonic by the mathematical expectations of their magnitudes and phases and the corresponding variances. 2.2. Formation of equivalent rated circuits on the base of measurements If the studied and equivalented subcircuits are connected by one line, the equivalented subcircuit can be replaced by the equivalent two-port circuit on the base of the results of daily measurements of harmonic condition parameters (currents in lines and voltages in the boundary node, for example). The equivalent two-port circuit is the current source with the admittance placed in parallel. When taking measurements the following condition should be satisfied: no switchings of elements and variations of the parameters should be performed in the equivalented subcircuit. In practice this condition is surely hard to satisfy, however, approximate estimates can be obtained. The studied network subcircuit is subjected to changes that lead to considerable variations of the power flow by the line connecting the subcircuits. These variations in turn cause changes in harmonic currents on the line and harmonic voltage levels in the boundary node.

Let us search for parameters of the equivalent twoport circuit for each harmonic in the form of I2e

and Y2e , where I2e is the current of the current source, Y2e is the admittance connected in parallel

to the current source. Current Is through the line connecting the studied and equivalented subcircuits should be equal to current of the two-port circuit. The current of the two-port circuit is determined via the parameters of its elements as the relation:

I d

I2e U mY2e ,

(14)

where U m – the harmonic voltage in the boundary node that is equal to the voltage on two-port circuit taps. The admittance Y2e is determined from the set of measured magnitudes Is and U m by choosing two subsets of harmonic currents and voltages which differ essentially in the values of corresponding magnitudes, for example subsets corresponding to the maximum and minimum volumes of power transmitted by the line. The admittance Y2e is determined from the expression

Y2e

M 1 ( Is ) M 2 ( I s) b, M 1 (U m ) M 2 (U m )

(15)

where M 1 ( Is ) , M 2 ( Is ) , M 1 (U m ) , M 2 (U s ) – the mathematical expectations of harmonic currents and voltages of the first and second subsets. Current of the current source from expression (14) is I2e I U mY2e .

d

In terms of the whole set of measured currents and voltages this expression takes the form

M ( I

2

e)

M ( Is ) Y2e M (U m ) ,

where:

M ( Is )

– the mathematical expectation of the n-th harmonic current vector for a daily measurement, M (U m ) - the mathematical expectation of the n-th harmonic voltage vector for a daily measurement.

Variance of the n-th harmonic current vector of the source is taken as Section 2. Methods of Power Quality Analysis: Modelling and Simulation

127

D ( I2e )

D ( Is ) .

Thus, parameters of the two-port circuit of the equivalented subcircuit that are to be stored in the file and applied with the initial data of the studied subcircuit in multi-variant calculations are determined. 3. CONCLUSIONS 1. Two algorithms of forming the rated equivalent circuits of a large-sized electric network by the equivalenting methods are suggested. 2. An algorithm of obtaining an equivalent of the part of the large-sized network by using the measured information on the parameters of harmonic conditions is suggested. 4. ACKNOWLEDGEMENTS The study is performed with the support of Russian Foundation for Basic Research, the grant for the support of the leading scientific schools { 00-1599035. 5. REFERENCES 1. Smirnov S.S., Kovernikova L.I. High harmonics in HV networks// Elektrichestvo. 1999, { 6 (in Russian). 2. Smirnov S.S., Kovernikova L.I. Studies on harmonics in high voltage networks. Proceedings of international conference “Power quality-Assessment of in fact”, CIGRE Regional meetings – Asia and Middle East, September 10-11, 1997, New Delhi, India 3. Sendy K. Current methods of power system analysis. – M.: Energiya, 1971 (in Russian). 4. Zhukov L.A., Stratan I.P. Steady-state conditions of bulk electric networks and power systems: Calculation methods. – Energiya, 1979 (in Russian).

128

6. BIOGRAPHIES Dr. Lidia I. KOVERNIKOVA was born in Novo-Aleksandrovka, Russia. She is a graduate from the Novosibirsk Power Engineering Institute. She defended thesis of Candidate of Technical Sciences and received the scientific degree. At present she is Researcher in the Siberian Energy Institute in the Russian Academy of Sciences. She is also a Doctor of the Power Supply Department of the Irkutsk Institute of Engineers of Railway Transport. Her field of interest is in higher harmonics. Mailing address: Lidia I. Kovernikova Energy Systems Institute Siberian Branch of the Russian Academy of Sciences 130, Lermontov Str., Irkutsk, 664033, RUSSIA phone:(395-2) 46-24-95, fax: (395-2) 46-27-96, e-mail: [email protected] Dr. Sergei S. SMIRNOV was born in 1936 in Berendeevo, Russia. He is a graduate from the Moscow Power Engineering Institute. He defended thesis of Candidate of Technical Sciences and received the scientific degree. At present he is Researcher in the Siberian Energy Institute in the Russian Academy of Sciences. His field of interest is harmonics. Mailing address: Sergei S. Smirnov Energy Systems Institute Siberian Branch of the Russian Academy of Sciences 130, Lermontov Str., Irkutsk, 664033, RUSSIA phone:(395-2) 46-24-95, fax: (395-2) 46-27-96




SIMULATION OF THE HIGHER HARMONICS SPREADING PROCESS IN ELECTRICAL CIRCUITS WITH THE USE OF THE NODAL EQUATION Piter LEZHNYUK

Yury LUKIANENKO Leonyd YARNYH Vinnytsia State Technical University Vinnytsia (Ukraine)

Abstract – The paper suggests an algorithm of higher harmonics calculation in an electrical network. It is shown that the process of high harmonics spreading in the network can be efficiently simulated by a system of nodal equations and regime parameters of all harmonics can be determined by means of Hauss method. In recent years, the technologies which use controlled rectifiers have been highly development. This resulted in the increase of harmonics in electrical networks. As usual, when constructing equipment, the starting point is that current in the junction is sinusoidal. Therefore, consumers supplied by such a network will suffer loses from negative effect of harmonics on the equipment [1]. The problem in question is divided into three parts: determination of the high harmonics source, examination of the harmonics spreading space and level of their conformity with the standards, estimation of the higher harmonic influence on electrical receivers and development of the arrangements for this influence elimination. The given paper considers the method and algorithm of higher harmonics calculation in certain elements of electrical nets. The calculation algorithm is developed with the assumption that electrical network is passive and symmetric, that is it could be applied superposition theorem which allows to separately consider each harmonic. As equivalent circuit in fig. 1 shows, the power flows of fundamental harmonic and power flows on the harmonic frequences are interconnected. Feeding system is a source of the major sinusoidal current. Through the system resistance R + jX it feeds the load R + jX through controlled static converter. System power S is transmitted to consumers through common junction (CJ). The greater part of this power S as usual feeds the loading, and the smaller one S` – the converter.

Volodymyr VYDMYSH

The fig. 1.b shows the harmonic flows in equivalent circuit. In this circuit the fundamental harmonic generator is presented by its harmonic resistance. The source of current harmonics is the converter, from which the part of power S`, transferred into harmonics power, returns to the system S and generator S. The largest part of the harmonics power is consumed by the load S. On running through CJ, the powers S and S are distributed between parallel elements of electrical net (lines, transformers), entail extra losses and bending the voltage, impair the working conditions of electric power consumers in the system. The measure of harmonics effect on the electrical net regime and electrical power consumers is evaluated by the non-sinusoidal voltage ratio, which is calculated by the formula [1] n

n

¦ U Q2

k

Q 2

U1c

¦ U Q2

Q 2

100 |

U c

100[%],

(1)

where UQ – active value of Q-th harmonic voltage; U'1 – voltage of the direct sequence of the major frequency; n – number of the latest of the harmonics accounted. The power flow values on harmonic frequences in the elements of electric net are determined by formula: n

S Qi

#\ U #\ I , 3¦ U Qi Qi Qi Q 1


(2)

129

Rc+jX

J`

Rc+jX

CJ

CJ S

a

S`

S

S

S

S

R+jX

R+jX

R +jX

a)

b)

Fig.1. Power flows in electrical system with harmonics

Input of initial data

Formation of CM of basic frequency

Reading frequency characteristics of loading of lines, transformers, generators and filters from DB Determination of specifying currents and nodal currents of Q-th harmonics

Shaping of the matrix of nodal conductances YQ on the basic of frequency relationship

Solving of the system of equations – YQ UQ = JQ

Determination of the currents in branches, loop currents and calculation of power flows Recording of Q-th harmonic regime to DB

Qdn

Determination of non-sinusoidal nodal ratios

Analysis of harmonics spreading and graphical processing of calculation results Fig. 2. Algorithm of examination of harmonics spreading

130


#\ where U #\ Qi , U Qi – input and output voltages of of the two-terminal by which the i-th element of the network [2] is simulated; I Qi – Q-th harmonics current in the i-th element of the network. Taking into account (1) and (2) and requirements for the process of harmonics spreading [2], there has been developed the methods and algorithm for calculation of electrical nets regime on each of the harmonics. The algorithm, presented in [3] was taken as basic. The fig.2 shows the block-logic diagram of the algorithm adopted to harmonic analysis. Since loadings are specified by powers, taking into account their static characteristics, the calculation is effected by few iterations. On inner iteration the system of nodal equations is solved by Gauss method:

YQ U Q

J Q ,

U Q2 R 2 R 2 XC J2

J

Q2 N2 N2 Q

.

Rc+jX S’

J` J

S

’

S

R+jX

R+jX

R+jX

(3)

where YQ – matrix of nodal conductances of electrical net on Q-th harmonic, U Q – voltage phasor of Q-th harmonic nodes relatively to basic node; J Q – phasor of specifying node currents which include harmonics sources currents. On the outer iteration the node powers are refined according to the calculated voltages and current harmonic number as well as components of phasor J Q are redetermined. Calculation results of each harmonic regime are transferred to data bank (DB) for further analysis. On completing the calculations, the final results are transferred to the graphical shell of the program and are dispensed to the elements of electrical net. The mnemonic diagrams directly presents the ratio of non-sinusoidal node voltage and values of full currents and powers in the network branches. Regime parameters of any harmonic, including the most influential one, are presented on call. The program, which realises the algorithm in question, stipulates for examination of the process of harmonic level limitation in the system by means of parallel filters (fig. 3). Since filters, as a rule, are specified by their resistances on the harmonic, which they are adjusted for, and the calculation is effected in “power”, the filter data are recalculated in the powers of each harmonic. Under condition that in the filter adjusted for N-th harmonic NXL = XC/N, filter powers of Q-th harmonic are determined: PQ

where R, XL, XC are active, inductive and capacitor filter resistances, PQ – losses of active power in the filter; QQ – power of filter generation;

,

QQ

U Q2 X C J 2 R 2 XC J2

Fig. 3. Harmonic power flows in electrical net with filter The calculation result in determination of power flows in network elements S' and S' which differ from power flows in network without filters. If there is a task to determine optimal placement and power of filters, then the correspondent mathematical model is formulated and the problem is solved by simplex-method. In this case when modelling the process of harmonic spreading in electrical net, the examined algorithm and program are used as separate program module. Algorithm allows to simultaneously analyse flowing of the current harmonics in the net from unlimited number of sources of harmonics with any given range. The number of filters is unlimited either. So, total generation of current harmonics by some converters and filters, connected to the electrical net buses, is used to analyse the current harmonics penetration to the given network. The developed program can be used in designing practice, for monitoring of the harmonic level in electrical power supply systems as well as when examining real conditions of electrical power supply in junctions of electrical power consumers and determination of harmonics conformity to the standards.

,


131

1.

Shidlovski A.K., Kuznetsov V.G.. Improvment of power quality in electrical nets. – K.: Naukova dumka, 1985. – 268 p.

2. Arrillaga J., Bradley D., Bodger P.. Harmonics in electric nets: Trans. from English. – M.: Energoatomizdat, 1990. – 320 p. 3. Determination of the optimal regimes of electrical nets /Astahov Y.N., Lezhnyuk P.D., Nahul V.I., Yarhyh L.V. // Izv. of AS of USSR. Power engineering and transport. – 1983. – N 1. p 48 – 59. Professor Piter Lezhnyuk was born in 1946 in Rivne, Ukraine. He graduated from Lviv Technical University. ”. He received the degree of Candidate of Science in the Moscow Electrical Engineering Institute on the subject “Improvement of regulation of voltage quality in electrical systems by criteria method”. He received the D.Sc. degree on the subject “Methods and remedies of criteria modelling in automation tasks of optimum control of electrical systems” . His research directions are mathematical modelling and automation of optimum control by regimes of electrical systems. Mailing address: Piter Lezhnyuk Vinnytsia State Technical University Institute of Electrical Systems and Networks 33/5 Yunosty Av., 21021 Vinnytsia, UKRAINE phone: (+380) (432) 43-52-33

Docent Leonyd Yarnyh was born in 1938 in Chernivtsi, Ukraine. He received the degree of Candidate of Science from the Moscow Electrical Engineering Institute. His research direction is analysis and improvement regimes of functioning of arched stoves and researching their influence on feeding electrical systems. Mailing address: Leonyd Yarnyh Vinnitsa State Technical University Institute of Electrical Systems and Networks 233/9 3-th Frunzenska Str., 119270 Moscow, RUSSIA phone: (095) 242-23-80 Assistant Volodymyr Vydmysh was born in 1978 in Vinnytsia, Ukraine. He received the M.Sc. degree in electrical engineering from the Vinnytsia State Technical University on the subject “Development mathematical models and algorithms of accounting for influences in optimum control of electric power system” in 2000. His research directions are mathematical modelling and automation of electric power systems. Mailing address: Volodymyr Vydmysh Vinnytsia State Technical University Institute of Electrical Systems and Networks 605/100 Keletska Str., 21021 Vinnytsia, UKRAINE phone: (+380) (432) 44-03-38

Docent Yury Lukianenko was born in 1965 in Vinnytsia, Ukraine. He graduated from the Vinnytsia State Technical University on the speciality “Electrical Systems and Networks”. He received the degree of Candidate of Science in the subject “Models and algorithms for decision making in dynamic systems with balance restrictions in graphical environment” in 1995. His research directions are mathematical modelling and automation of electric power systems. Mailing address: Yury Lukianenko Vinnytsia State Technical University Institute of Electrical Systems and Networks 25/85 Chervonoarmiyska Str., 21009 Vinnytsia, UKRAINE phone: (+380) (432) 44-76-78 e-mail: [email protected]

132




NUMERICAL SIMULATION OF THE NON-LINEAR RESONANCE PHENOMENA USING SPLINE APPROXIMATED CHARACTERISTIC Dan OLARU Vasile TRUSCA University "Politehnica" of Bucharest Bucharest (Romania)

Abstract - The presented paper studies the over voltage stress caused by the non-linear resonance, arisen in a power delivery network region, containing a non-linear inductance and a capacitive behaviour device. These kind of phenomena are difficult to be explained and modelled from the theoretical point of view. The well conditioned mathematical problem statement, giving a unique defined solution, isn't complete solved until now. The over voltage level is dependent on the electrical parameters of the devices, on the evolution law of the non-linear characteristic, on the initial condition concerning the supply voltage and its time variation. To analyse the problem, it may be used different tools concerning the mathematical model. Because the non-linearity of the problem, it is very important to have a good approximation of the saturable inductance behaviour and its characteristic curve. For this goal, it is used a differential equation mathematical model, including the magnetic flux as variable. As non-linear parameter it is used the magnetic reluctance of the core. The Spline approximation of the current-magnetic reluctance dependence is a very good choice, because the resulting smoothing proprieties. The paper presents the differential mathematical model, the MathCad implementation and a representative numerical simulation example. By these, the authors try to find the relations between the non-linear reluctance behaviour and the maximal obtained over-voltage level. 1. INTRODUCTION

The power delivery network may be affected by over voltages caused by different phenomena.

The commutation and lightning transient over voltages are the most dangerous, but in a certain sense, the most well known. For this, there are defined testing procedures, based on standardised pulse waves. By difference, the non-linear resonance may have long time effects, having causes and evolution that are not very well determined. The maximal amplitude and its occurring place are difficult to be previewed. The non-linear resonance is an oscillating phenomena which takes place in a region of the network, where there are reactive complementary devices with at least one having a non-linear characteristic. The most simple case that can be imagined, is a monophased circuit containing a linear capacitor and a non-linear inductance. The non-linear behaviour of the inductance is caused by the ferromagnetic core. Because of the non-linear behaviour, the real electric parameters of the phenomena may be very different, depending on the working condition type and on the supply voltage of the line. Because of this, it is very difficult to make a testing procedure, trying to cover all the possibilities. The mathematical model may be a good solution when the dominant parameters of the circuit can be well approximated. The non-linear resonance phenomena may be usually approximated using a single or a double LCR circuit, which is a physical model of the real circuit. The most dangerous case, from the over-voltage point of view, is the series type topology situation. A practical interesting situation, when over-voltage phenomena may arise , caused by the non-linear resonance, is the input circuit of a non-charged transformer. The mathematical model isn't a precise tool, because its simplicity and the lumped constant approximation. However it may be useful, because by this way a correlation can be stated between the non-linear characteristic, the voltage source parameters and the circuit resulting parameters. By


133

the simulation of the circuit evolution, it is interesting to evaluate the sensibility of the nonlinear phenomena on the voltage source values. This paper describes a mathematical method, easy to be programmed in the MathCad language, which can model the non-linear inductance behaviour, based on the core magnetisation characteristic, using the Spline approximation. The case taken under consideration permits to study the interaction between the transient switching phenomena and the non-linear resonance arisen in the circuit. 2. THE CIRCUIT EQUATIONS

We take under consideration a simple LCR series circuit (fig.1) containing a sinusoidal voltage source E(t) and a current dependent non-linear inductance L(i). For transient simulation purposes, it is modelled by a differential equation system. The non-linearity is taken under consideration by variable coefficients. It is very important to choose the state variables in order to have only first order equations and to have a good representation of the non-linearity. A good way to achieve this goal is to use a reluctance function with a magnetic flux dependence, Rm()). Thus, for a certain current flowing in the inductor we have the relation:

i

Rm ( )) )

(1)

Because of this we take as state variables, the capacitor voltage u and the magnetic flux ).

ª du º « dt » « d) » « » ¬ dt ¼

Rm () ) º 0 ª ª º «0 »« » C « 1 R R () ) » ¬ E ¼ m ¬ ¼

3. THE RELUCTANCE FUNCTION EVALUATION

To analyse the non-linear resonance it is necessary to find a correspondence between the reluctance and the magnetisation function Rm()) characteristic of the core material B=B(H). This is an experimental curve, creating a biunivoque correspondence between the B and H values. To estimate the reluctance function Rm()) we need a numerical evaluation (depending on the magnetic state). Thus the B-H dependence can be described by a non-linear permeability function P (H):

B P( H) H

E(t)

i

iL

C L(i)

)

P( H) A H

Thus, using the Kirchhoff II theorem and the capacitor equation, we have the non-linear differential system:

AH

(2)

d) dt

u R Rm ( )) ) E

(3)

(7)

1 i 2 S r

1 ) P ( H)

(8)

where r is the radius of the winding. After the simplifications it results:

2 ) r P( H )

(9)

This equation is similar with the first equation concerning the reluctance function:

In the matriceal form, it is represented as:

134

1 ) P( H )

Without any loss of generality we can consider an inductor with a single wind. If we explicitate the magnetic field H by the circuit magnetic law, we obtain:

i

Rm ( ) ) ) C

du dt

(6)

If we isolate the A H factor, we obtain:

S r2 Fig.1. The elementary circuit model

(5)

If we multiply by the core section equivalent surface area A, we can find the magnetic flux magnitude:

u R

(4)

i

Rm ( )) )

(10) Eliminating the current magnitude from the both equations , it results a new representation for the reluctance function: Electrical Power Quality and Utilisation

Rm ( ) )

2 rP ( H )

giving

Rm ( ) )

1 P ( H)

for

(11)

r =2

(12)

As before, to simplify the calculus, we considered r=2 (as an arbitrary value for the radius winding). It is easy to observe that this equation has sense only when Rm()) and P(H) are numerical calculated values. From the functional point of view the relation will have sense only if the two functions have the same dependence. This is a very important thing, when we use the global interpolation Spline function. The function P(H) may be evaluated, for every point by the B, H ratio:

P( H)

B( H) H

(13)

We can consider B as a function. Let us describe this function by the equation y=F(x), which in fact is the magnetising characteristic. Thus, it results:

P( H)

F( x) x

(14)

But we need to have a flux dependence. We notify that B(H) function is identical to the )(H) function if A=1. Thus the definition domain and the function values domain of F(x) must be ( I) function: reversed obtaining a P

P ( ) )

y 1

(15)

F ( y)

where x=F-1(y) is the describing equation for the inverse function of F(x). It must be specified that P ( I) have the same local values, but (H) and P represent different functions. Now, for the same ipotesis as before, we can state that:

Rm ( ) )

F1( y) y

To realise the numerical simulation of the circuit, by the differential system we need a continuous dependence of the magnetic reluctance function Rm()). For this, we need a continuous dependence of the P()) function. If we use the Spline interpolation of the magnetisation characteristic B(H) we obtain a P(H) dependence. If we need the P ( I) function we need the inverse of the Spline function, which is unknown. ( I) function we must use the Thus to have the P following procedure: * Determine the inverse of the material characteristic function, in the discrete form; This can be done very simple by the reciprocal substitution of the argument and the function values; * Interpolate by Spline function the discrete inverse function; ( I) and the Rm()) functions. * Evaluate the P 4. THE ALGORITHM

For the purposes of the algorithm implementation, it is necessary not only to interpolate the material characteristic, but also to condition the function variation, as to be well defined during the calculus. There are three problems that need to be fixed for the numerical simulation: * the material characteristic function must be the same for the positive and negative values of the argument, so we need to use the absolute value representation; * the singular values must be eliminated; * the definition domain of the reluctance function must be extrapolated. To illustrate this process we take under consideration a magnetisation characteristic defined by a particular exponential form, having the origin variation rate defined by a constant T. But this procedure may be applied in the same way, for no important point-by-point described experimental curve. For example let be F(z) and G(z) the inverse of F(z), where z is a generic argument used for the both functions. In the following we use directly the MathCad language to describe the calculus.

(16)

where F-1(y) is the inverse of the material characteristic function. From the numerical point of view we must solve the following problem. Section 2. Methods of Power Quality Analysis: Modelling and Simulation

135

z F( z): 1 e T

(17)

G( z): T ln(1 z )

(18)

Fig.2. The Spline interpolated G(z) function

Fig.3. The P(z) variation after the Spline interpolation To interpolate by Spline functions we need two vectors corresponding to the arguments and the values of G(z):

argi : zi

vali : G( zi G )

(19)

So the reluctance function must be restricted, using the low valued G quantity (G:=0.000001 for example):

ª 1 1 º , Rm ( z ) : if « z t G , P ( z ) P (G ) »¼ ¬

(25)

It is interesting to note that in the simulation process, the following numerical problem arises: * The magnetisation characteristic is saturated for high values of the magnetic field. In the example we consider a unitary asymptotical value. * So the definition domain of the corresponding inverse function becomes limited for the unitary value. At this point the permeability becomes zero and the reluctance becomes infinite. So in a realistic situation the field value must be limited. * By difference, in the numerical simulation the values are always finite, so the field may exceed the values corresponding to the end of the reluctance variation; In conclusion, it is justified to extrapolate the last values of the reluctance function. So the reluctance function must be extrapolated by a value near the end of the variation:

ª 1 º Rm ( z ) : if « z d 1, Rm ( z ), P ( E ) »¼ ¬

(26)

where E value is near the unity (E:=0.999999).

where zi values covers the definition domain of G(z) and G is a very low value, stopping the logarithm argument to nullify. The Spline interpolation is done by the MathCad commands: w:=cspline(arg,val)

(20)

S(z):=interp(w,arg,val,z)

(21)

where w is a intermediary variable and z is now the Spline generic variable. Now the reluctance function may be expressed by:

Rm ( z)

1 P ( z)

(22)

using:

P(z):=

z S( z )

(24)

as an intermediary variable. But it is easy to observe that the function Rm(z) becomes singular for z=0.

136

Fig.4. The evolution of the reluctance function By this it is obtained a reluctance function well defined for all the positive real axis. For the normal domain of variation [0,1], corresponding to the Spline interpolated G(z) function we obtain the variation of the reluctance showed in fig.2. 5. THE NUMERICAL SIMULATION

In the following there are presented some simulation numerical results concerning the series LCR type circuit model. For the numerical simulation we have the following goals: ¾ to show the possibilities of the procedure described above; Electrical Power Quality and Utilisation

¾ to show the non-linear behaviour for a simple typical case; ¾ to obtain some information about the criteria that must be taken under consideration when we deal with this type of problem; ¾ to obtain some conclusions concerning the non-linear resonance phenomena. For the simulation purposes it is used a MathCad implemented Runge-Kutta 4-order procedure. For the capacitor value it is chosen a sub resonant value (corresponding to the minimal value of the reluctance function). For the simulation it is applied a linear increasing amplitude sinusoidal source voltage. So we can eliminate the transient switch-on phenomena and we can observe the typical non-linear phenomena (fig.5). Concerning the non-linear behaviour we have the following conclusions: * the non-linear resonance phenomena becomes important when the variation of the reluctance function becomes high non-linear (fig.4); * We can observe a region, before that the nonlinear resonance is negligible; * For relative low values of the supply voltage it is observed an exponential non-linear behaviour; * The exponential evolution is followed by an amortised oscillating amplitude evolution; * The amplitude minimal values have an asymptotically type increasing evolution;

* The amplitude maximal values are relatively uniform after the non-linear resonance takes place; * The over-voltage values can exceed several times the voltage level in the absence of the non-linear resonance ; * Because of the non-linear resonance phenomena are produced high values, low frequencies components; * For a single series type LCR circuit the nonlinear contribution has an unstable behaviour. 6. CONCLUSIONS

In order to avoid the non-linear resonance overvoltage problems, we must stop the supply voltage variation before the value where the exponential increasing evolution may arise. If by occurrence this value is reached, further precautions aren't important. From the numerical experiment it can be observed, that for the relatively low and the high values of the supply voltage, the over voltages caused by the non-linear phenomena aren't important. This is, because for the low values, the reluctance has low variations and for the high values, it increases at values where the inductance and the corresponding magnetic energy become negligible.

Fig.5. The capacitor voltage evolution, under non-linear resonance, for linear increasing supply


137

7. REFERENCES

1. Van Craenenbroeck T., Michiels W., Van Dommelen D., Lust K. - Bifurcation Analysis of the Three-Phase Ferroresonant Oscillations in Ungrounded Power System; IEEE Transactions on Power Delivery, Vol.14, No.2, April 1999. 2. Kieny C. - Application of the Bifurcation Theory in studyng and understanding the Global Behavior of a Ferroresonant Electric Power Circuit; IEEE Transactions on Power Delivery, Vol.6, No.2, April 1991. 3. Chakravarthy S.K., Nayar C.V. - FrequencyLocked and Quasiperiodic Oscillations in Power Systems; IEEE Transactions on Power Delivery, Vol.13, No.2, April 1998

138

Prof. Vasile Trusca was born in 1937 in Zavideni, Romania. He received the Ph.D. degree in electrical engineering from University "Politehnica" of Bucharest. Presently, he is Professor at the Electrical Power Engineering Section of the University "Politehnica" of Bucharest. His areas of interest include electrical power system and electric switchgear analysis. Dr. Dan Olaru was born in 1956 in Bucharest, Romania. He received the Ph.D. degree in electrical engineering from University "Politehnica" of Bucharest. Presently, he is Senior Lecturer at the Electrical Power Engineering Section of the University "Politehnica" of Bucharest. His areas of interest include computer analysis of the electrical power system. Mailing address: Dan Olaru Electrical Engineering Faculty Dept. Electrical Switchgear & Measurement University "Politehnica" of Bucharest Splaiul Independentei 313 77206 BUCHAREST phone:(+40)(1)4.10.04.00, fax: (+40)(1)4.10.43.55 e-mail: vtrusca @ apel.apar.pub.ro





CALCULATION OF VOLTAGE DIPS IN A MESHED 110-kV-NETWORK Herwig RENNER

Lothar FICKERT

Manfred SAKULIN

University of Technology Graz (Austria) Abstract –Knowing the network parameters and the failure statistics of a network, the number of dips per year and the distributions of remaining voltages and dip durations can be estimated. Two approaches have been chosen by the authors: Using a Monte-Carlo-Method, the remaining voltages for random failure locations in the network are calculated. The simulation can be done for an arbitrary number of years, the number of dips per year is taken from the fault statistics. Using the second method, the calculation of the remaining voltage is carried through for failures in every node of the network. The results are weighted according to the failure probability of each node. The dip duration is derived from the network's protection system. These methods have been applied to a real network. The results are compared with a 2-years-measurement in the network. 1. INTRODUCTION Voltage dips can be very harmful for specific industrial consumers like semiconductor production or paper production, as they may cause production failures and outages. The distributions of x remaining voltage VR during dips x duration of dips in the connection point of a customer determine the number of failures to be expected. In practice, the duration of dips has less influence on the causation of production outages than the magnitude of the remaining voltage. With the knowledge of the sensitivity of the production process regarding voltage dips, the number of 'harmful' voltage dips per year can be estimated. With the average costs of an outage one can make an economical evaluation of possible

counter measures and estimate the pay back period of such an investment [1,2,3]. Furthermore, the dip parameters are necessary to design the dip-ride-through-capability of a dynamic voltage restorer (DVR). These parameters could be obtained by long term measurements over at least 2 years. Immediate results can be achieved by appropriate simulation methods. Knowing the network configuration and the fault statistics of the network, the number of events per year and the distributions of remaining voltage can be calculated. Most of the dips are caused by faults in the supplying network and are in overhead line dominated grids mainly initiated by lightning strokes. The fault clearing times depend on the setting of the protection relays and the protection scheme. In compensated networks with the star point grounded via a Petersen coil, only 3-phase faults, phase-to-phase-faults and double-phase-to-ground faults affect the consumer. The zero sequence component of the voltage is normally not transferred across the transformer of the customer. A further influence on the remaining voltage comes from the operation of the power plants in the network. In Austria the operation of the power plants strongly depend on the season. Thermal power plants are only operating during winter time. Run-of–river power plants operate in winter according to the amount of water only with a reduced number of their generators connected to the grid. During summer time, usually all generators of a run-of-river-power plant are synchronised, which means a greater contribution to the short circuit capacity. Storage power plants are operating in summer and winter for peak demand.


139

Since the calculation method is node oriented, a separate algorithm is used, to divide long lines and insert additional nodes on these lines.

2. DURATION OF VOLATGE DIPS Most of the dips are caused by faults in the supplying network and are mainly initiated by lightning strokes. The fault clearing times determine the duration of the voltage dip and depend on the setting of the protection relays, the protection scheme and the time delay of the breakers. Usually in meshed high voltage grids, distance relays with different stages are used. Each stage has its specific fault impedance range and tripping time. Typical total fault clearing times in high voltage networks are in the range of 200 ms (1st stage of distance relay) and 500 ms (2nd stage). Since usually only 85 % of the line are in the impedance range of the 1st stage, approximately 30 % of all faults (15 % at the beginning of the line and 15 % at the end of the line) are completely switched off from both sides with the time of the 2nd stage. This corresponds in general to the measurement example in fig. 2.

The calculation of the remaining voltage VR in an arbitrary node – expressed in symmetrical components – is shown in the following equations. Zff means the driving point impedance in the faulted node f whereas Zif stands for the transfer impedance between faulted node f and investigated node i. VL represents the driving prefault voltage of the network. The zero sequence component VR0 is not needed, since it has usually no effect on consumers in medium and low voltage networks due to the connection group of the transformers 3-phase-fault V 1R

§ Z1 ¨1 if ¨ Z1 ff ©

V 2R

0

x

impedance

x

type (overhead line, cable, transformer, breaker,..),

x

nodes

x

length

x

specific fault probability (faults/km,year, faults/year)

x

protection relay setting

140

(2)

V 1R

§ 1 Z1iF ¨1 ¨ 2 Z1FF ©

V 2R

1 1 Z iF 1 VL 2 Z FF

3.1. Fundamentals

The branch elements of the network characterised by the following parameters:

(1)

phase-to-phase-(-to-ground-)-fault

3. CALCULATION OF THE REMAINING VOLTAGE

A network with can be described by its impedance matrix Z as it is well known from short circuit analysis. Using the method of an equivalent prefault voltage source at the faulted node, all infeeds are represented by their short circuit impedance, connected to ground. Regarding unbalanced faults, the impedance matrix must be built for the positive (Z1), negative (Z2) and zero sequence system (Z0).

· ¸V ¸ L ¹

· ¸V ¸ L ¹

(3)

(4)

double-phase to-ground-fault

·¸ V

V1R

1 1 1 1 2 § ¨1 1 a Z ff a Z fg Z if / Z ff ¨ Z 0ff Z 0gg 2 Z 0fg 2Z1ff 2 Z1gg 2Z1fg ©

V 2R

1 a Z 2

1 ff

a 2 Z1fg Z1if / Z1ff

Z 0ff Z 0gg 2 Z 0fg 2 Z1ff 2 Z1gg

¸ ¹

V

2 Z1fg

L

L

(5)

(6)

are j

a

e

2S 3

(7)

As a result, the phase-to-phase voltages VR12, VR23 and VR31 are calculated out of the symmetrical components. The remaining voltage VR is defined as VR

Min^ VR12 , VR 23 , VR 31

`


(8)

Quasi-Deterministic-Method Using the Quasi-Deterministic-Method, a fault in every node in the network – including the additional nodes – is simulated and the remaining voltage in the node of interest is calculated. The calculation is done for 3-phase, phase-to-phase and double-phase-to-ground faults. Additionally, a probability, which depends on length and specific fault probability of the connected branches, is assigned to each calculated voltage. As result, the cumulative probability function of all voltages VR is calculated. The result is the same, as it could be expected from a long term measurement.

significantly higher fault frequency. These lines are highly exposed overhead lines through alpine terrain. The soil resistance in this region is very high, so that almost every lightning strike into the ground wire causes a back strike into the phase wires.

voltage dip measurement

3.3. Monte-Carlo-Method The Monte-Carlo-Method is a probabilistic simulation method with a simulation time range of several years. From the total line length in the network and the specific fault probability the average number Fa of failures per year in the network is calculated. In a next step, for every simulation year y, the number of failures Fy is fixed, based on Fa and an assumed yearly standard deviation V. In our case, V was chosen according to the yearly variation of lightning strokes. At last, the simulation program spreads the faults over all nodes of the network, where the probability of a node to be hit depends on length and specific fault probability of the connected branches. With increasing simulation times, the results approach the results of the Quasi-DeterministicMethod.

4. INVESTIGATED NETWORK

fault locations sub station feeding 220 kV-networks power plant

Fig. 1 Fault locations in analysed network The rest of the faults seems to be – as far as it can be stated after 2 years observation period – rather equally distributed in the network. Table 1 gives a summary of the specific fault probabilities in the network Table 1: Fault statistics for overhead lines of the investigated network fault

The voltage dip measurement was carried out in a substation of a 110-kV-network, with 1200 km overhead lines. The measurement period was 2 years. Fig. 1 gives an overview of the network. In these two years, 61 faults have been registered. At least 80 % of the faults were due to lightning strokes. Therefore most of the faults occurred during the thunderstorm season from May to September. The fault locations are also shown in fig. 1. Two overhead lines in the south of the area show a

a

b

c

3-phase

0,66

5,1

0,5

phase-to-phase

1,42

12,0

0,91

double-phaseto-ground

0,05

0

0

total

2.13

17.1

1.41

[1/100km,a]

[1/100km,a]

[1/100km,a]

a: Long term fault statistics (over all) b: 2 years statistics of 'exposed' lines c: 2 years statistics of 'normal' lines


141

A summary of the registered voltage dips is shown in fig. 2. The diagram shows the duration of the voltage dips and the remaining voltage.

For control purposes, the second simulation was done with the fault statistics derived from the measurement (Table 1, column b and c). Fig. 4 gives a comparison of the calculation results and the measurement, which shows an excellent correspondence.

Fig. 2 Magnitude of remaining voltage VR (lowest phase to phase voltage) and duration of measured voltage dips

5. SIMULATION RESULTS Fig. 3 shows the results calculated with the QuasiDeterministic-Method. The cumulated probability function of the remaining voltage is compared with the two-years on-site-measurement. Basis for the calculation was the long term fault statistics (Table 1, column a) which was assigned to all lines. The calculated voltages tend to be below the measured voltages. This is due to neglecting the effect of the exposed lines. The spread of the calculated data originates from the assumptions of minimal and maximal operation of storage power plants.

Fig.4 Simulation results taking into account the increased fault frequency of exposed lines

A different way to present the results is shown in fig. 5. Five classes with a class width of 20% of the nominal voltage have been defined. The results of a Monte-Carlo-Simulation over a time range of 10 years have been classified as well as the results from the measurement and the QuasiDeterministic-Method. Also shown in the diagram is the cumulative mean value, calculated for the results of the MonteCarlo-Method. As it can be seen, this mean value approaches the result of the Quasi-DeterministicMethod with increasing simulation time. The results of the measurement (1 year) are inside the variation band of the Monte-Carlo-Method.

Fig.3 Simulation results with equally distributed fault frequency

142


Fig. 5 Comparison of

Monte-Carlo-method

10. year Quasi-Deterministic method

measurement 1. year

cumulative mean

- measurement results (year 1999) - simulation results (Monte-Carlo-Method) - simulation results (Quasi-Deterministic Method)

6. CONCLUSION In this paper, two methods are described, to calculate the distribution of the remaining voltage in a specific node of an electric network. Those methods are based on long term fault statistics and a short circuit calculation algorithm. The calculation methods have been applied to a real network. The results are compared with a 2-yearsmeasurement in the network and show a good correspondence in general. Special care must be taken in the case, that the specific fault probability of the network elements is not uniformly distributed over the network. 7. REFERENCES 1 L. Fickert, H. Renner: Kosten der Versorgungsqualität, Proceedings of Internationale Energiewirtschaftstagung, Wien, Febr. 2001 2 A. Haber: Analysis of voltage dips and remedial measures in an industrial plant, Master thesis at the department of Electrical Power Systems and High Voltage Engineering, TU Graz, 2001

3 R. C. Degeneff, R. Barrs, D. Carnovale, S. Raedy: Reducing the Effect of Sags and Momentary Interruptions: A Total Owning Cost Prospective, Proceedings of IX. International Conference on Harmonics and Quality of Power, 1.-4.10.2000, Orlando, S. 397-403 4 R. Mienski, R. Pawelek, I. Wasiak: A Simulation Method For Estimating Supply Voltage Dips in Electrical Power Networks, Proceedings of IX. International Conference on Harmonics and Quality of Power, 1.-4.10.2000, Orlando, S. 739-744

Herwig Renner was born in Graz, Austria, in 1965. He completed his doctoral degree in 1995 at the University of Technology Graz, where he works as assistant professor at the Department for Electrical Power Systems His main research work is in the field of electrical power quality. Lothar Fickert Study of electrical engineering and Ph.D. at the University of Technology Vienna (1975), national and international industrial experience (1975 – 1998), head of department of Electrical Power Systems at the University of Technology Graz since 1998.


143

Manfred Sakulin was born in Ischl, Austria, in 1944. He is professor and senior researcher at the Department for Electric Power System at Graz, University of Technology. His main activities are in the fields of energy efficiency, and power quality.

Mailing address: Herwig Renner, Lothar Fickert. Manfred Sakulin : Department for Electrical Power Systems and High Voltage Engineering, University of Technology Graz Inffeldgasse 18, 8010 Graz, AUSTRIA phone: (+43)316 873-7557, fax: (+43)316 873-7553, e-mail: [email protected]

144





THE INFLUENCE OF THE ZERO-SEQUENCE COMPONENT ON THE LINE VOLTAGE DROP Sonia LEVA

Adriano P. MORANDO Dario ZANINELLI Dipartimento di Elettrotecnica Politecnico di Milano Milano (Italy)

Abstract – The paper deals with the analysis of the influence of the zero-sequence components on the line voltage drop calculation for unbalanced and polluted power systems. The Park approach permits to define a calculation expression with a structure very close to the classical one typically employed in case of balanced systems. Some examples are reported to verify the proposed procedure with the results given by a commercial software on Power Quality.

1. INTRODUCTION The voltage drop is one of the most important quantities in the characterization of electric power transmission and distribution systems. In fact it represents in a certain way the indicator of the effectiveness of the connection between the loads and the generation centers. The voltage drop control is also an essential task both for the stability and the economy of the power system and its calculation, even with the introduction of simplified procedures and approximations, is fundamental for the power system analysis [10]. In the presence of unbalances or/and distortions the accuracy of the voltage drop calculation becomes a burden and the introduction of some simplifying hypotheses can bring to wrong results neglecting some disturbance’s contributions. In a previous paper the Authors introduced the Park transformation for taking into account the simultaneous contributions of harmonic, interharmonic and sequence components in the voltage drop [3]. In particular, the analysis in terms of the Park vector confirms the single-phase nature of the Park variables and stresses the role of the Park imaginary power. This last quantity appears

to be a generalization of the reactive power concept to any operating condition as far as the voltage drop is considered [2]. The formulation previously proposed and tested did not consider the simultaneous presence of zerosequence instantaneous components. These latter are very important especially in case of low voltage three-phase four-wire distribution systems where unbalances or third order harmonics are present in the system. The present paper introduces the simultaneous presence of zero-sequence instantaneous components on the power line for the voltage drop calculation taking into account the disturbances that can occur in case of power distribution lines with earth conductor. The approach is particularly addressed to the following aspects: - to consider all the Power Quality aspects in the voltage drop calculation on the methodological point of view; - to carry out compatible models for representing the distribution systems in any possible perturbation due to disturbances or faults. The results obtained – also applied to some examples – show the role of the instantaneous power components (real and imaginary by Park and real by zero-sequence) on the line voltage drop and permit a formalism very close to the one used for single-phase equivalent systems in the case of balanced and non polluted circuits. 2. THE PARK APPROACH In the most general case, the time-domain equations for a three-phase distribution line (see Fig.1) are differential equations. For the purposes of this work, they can be more clearly written in


145

(a)

Fig.1. Three-phase line with physical symmetry.

term of Heaviside operator [6,8] p d / dt , so the line impedance vector z, written in terms of this operator, becomes z(p). Therefore, the timedomain equations for the line in Fig.1 are:

>v m t @ >v v t @ >z p @ >it @

(1)

By applying the Park transformation (see Appendix A), the following relationships can be obtained. 1. Park vector:

v t v t z i t "p i t v °° m ®z R j- " R jX ° ' °¯L M "

(2)

These equations are the Park equations describing the two-port network shown in Fig.2a. They differ from those describing a single-phase line under sinusoidal conditions because of the presence of the dynamic term "pit . 2. zero-sequence component:

°v m 0 t v v 0 t R 0 i 0 t " 0 pi 0 t ' ® °¯L 2M " 0

(3)

These equations describe the two-port network shown in Fig.2b. Considering the rms three-phase instantaneous voltage the relevant per unit three-phase voltage drop, expressed in term of Park vector (see Appendix A), can be written as:

'v T t

v Tm t v Tv t v Tv t

(4)

2 t v Tv2 t (5) 1 v Tm 2 2 t v Tv

1 ' v T t 2 2 v Tv t

2

' v T t

Deriving the contribution due to the Park vector and the one associated to the zero-sequence, the (5) becomes:

1 'vt 1 'v 0 t 2 2 2 v v t v v 0 t 2 v 2v t v 2v 0 t 2

2

'v T t

'vt 'v 0 t

(6) The voltage drop in a four-wire three-phase distribution line is composed by two terms. The first one depends on the instantaneous positive and negative sequences through the Park vector; the second one represents the contribution of the instantaneous zero-sequence. With reference to Fig.2: 'vt

v m t v v t 2

2

2Rp p t 2Xq p t

2

2"v v t u p i t z 2 it z i t "p i * t 2

z * i * t "p i t " 2 p i t p i * t

'v 0 t

2

v m 0 t v v 0 t 2

2 R 0 p 0 t

2

2" 0 v v 0 t pi 0 t R i t 2R 0 " 0 pi 02 t " 20 pi 0 t pi 0 t

Representing the difference between rms output and input instantaneous voltages [2] by using Taylor Series approximation, we obtain: 146

(b) Fig.2. Park representation of a two-port network representing a three-phase short line.

2 0 0

2

(7) By applying to the first term of eq.(6) the geometrical approach typical of the sinusoidal case in terms of Park vector [2, 3], and observing that


2 t t v 2v t v 2v 0 t , the following expression v Tv

is obtained: Rp P t Xq P t

'vt

v t v 2 v

2 v0

" i t u pv v t

t

" p p P t

v t v 2v 0 t 2 v

(8)

v 2v t v 2v 0 t

This result can be extended to the second term of (6), taking into account the usual value of the ratio between the impedance of the distribution system and that of the load. This is in accordance with the results obtained by the Authors in different examples and applications [2,3]. Then, we have:

'v 0 t

R 0 p 0 t

v t v 2 v

2 v0

t

" 0 v v 0 t pi 0 t v 2v t v 2v 0 t

(9)

On the point of view of energy flow, it is possible to affirm that the voltage drop expressed by (6) depends on the real pp(t) and imaginary qp(t) power in terms of Park vectors, and depends on the power p0(t) as concerns the instantaneous zero-sequence component. The result confirms once again the important role played by the imaginary power where, considering a four-wire system, the contribution due to the zero-sequence power p0(t) must be added. Equations (6,8,9) allow the voltage drop calculation in presence of zero-sequence components in the power line. In this way is possible to take into account harmonics, interharmonics and all sequence components that can occur in case of distribution lines with earth conductor. 3. THE EFFECT OF HARMONIC AND SEQUENCE COMPONENTS ON THE VOLTAGE DROP UNDER NONSINUSOIDAL AND UNBALANCED CONDITIONS The definition of the role performed by harmonics and sequence components present in the network becomes very important on the application point of view. Equations (6,8,9) can be reconsidered in order to clarify the dependence of pp(t), qp(t), i t , v v t , p0(t), i0(t) and vv0(t) on the harmonic and sequence components. The first term of (6), represented as in (8), is already deeply investigated in [3] with the only difference of the zero-sequence voltage contribution vv0(t) at the denominator. Instead, the second term of (6), represented as in (9), concerns

the contribution of the zero-sequence component and will be developed in the following. Under the hypothesis of periodic steady-state, it is convenient to apply the Fourier series analysis. In this way, for each harmonic it is possible to use the theory related to sinusoidal system and counterrotating vectors. This procedure brings to the expressions (10): º ª ° f » « f ° R 0 e « ¦ V0 h I0* h ¦ V0 h I0* k e j( h k )Zt » ° f » « h f hzk ° R 0 p 0 t »¼ «¬ h 0 ,3,6 h , k 0 , 3, 6 ° 2 f f 2 ° v v t v v 0 t Vh2 ¦ V02h ¦ ° h f h f h 0 , 3, 6 h z 0 , 3, 6 ° ° f °° " 0 v v 0 t pi 0 t ª« " hZ Q h ® 2 ¦ 2 « 0 ° v v t v v 0 t « hh 0f,3,6 ¬ ° ° 1 ½º º ° f °° f °°» ª f Z * ( ) j h k t 2 2 « » ° e® ¦ j " 0 kZ Ik Vh e ¾» « ¦ Vh ¦ V0 h » ° h f °h fz k °» «hh z 0f,3,6 h 0 , 3, 6 ¼» ° °¯h , k 0,3, 6 °¿»¼ ¬ ° ° ° °¯ (10)

The results obtained can be discussed as in the following: - the formal unification of the effects associated to harmonics and sequence components on the voltage drop, already shown in [3] for the instantaneous positive and negative sequences, extends to the zero sequence one due to the presence of the earth conductor; - also the zero sequence contribution to the voltage drop presents a pulsating component, giving evidence of zero-sequence harmonic component additional contribution with respect to the sinusoidal, positive sequence, balanced three-phase case. 3.1 Voltage drop average value Starting from the above general expression of the voltage drop in time domain, it is possible to evaluate the following average value with reference to the interval [t, t+T]: 'v T

1 t T 'v T t dt T ³t 'v ' v 0


1 t T >'vt 'v 0 t @dt T ³t

(11)

147

0.105

0.12

0.1

(vm-vv)/vv

(vm-vv)/vv

'vT 'v 'v0

'vT 'V 'VH

0.1

0.08 0.095

0.06

'v

'v 0.04

0.09

0.02 0.085

0

-0.02 0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.08 0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

t (s)

t (s)

(b)

(a)

Fig.3. Numerical example where the voltages at the line output port are sinusoidal of positive-, negative- and zerosequence components. (a) Voltage drop waveform. The solid line represents the voltage drop given by (4), the dashed line represent the voltage drop given by the proposed formula (6), the dotted line represents the voltage drop evaluated by using Park vector only, and the point-dashed line is the voltage drop given by instantaneous zero-sequence component only. (b) Voltage drop waveform. The solid line represents the voltage drop given by (4), the dashed line represent the voltage drop given by the proposed formula (6), the dotted line represents the classical voltage drop, and the point-dashed line is the voltage drop given by a commercial software.

This calculation is justified by the importance of the voltage rms value on the load and distribution system operation. Applying (11) to the formula (6), it is possible to obtain for the two different terms (8,9) respectively: R 'v

f

¦P

Z"

h

¦V

2 h

f

'v 0

h

(12)

¦V

2 0h

h f h 0 , 3, 6..

h f h z 0 , 3, 6

R0

¦ hQ

h f h z 0 , 3, 6

h f h z 0 , 3, 6 f

f

f

¦ P0h Z" 0

h f h 0 , 3, 6 f

¦V

h f h z 0 , 3, 6

2 h

f

¦ hQ

R 0 P0 Z" 0 Q 0

(13)

V12 V22 V02

Equation (13) represents a generalisation of the voltage drop classical expression typically used in case of positive sequence component only. In this case the reactive power associated to positive-, negative- and zero-sequence components (Q1, Q2 and Q0 respectively) appears. 4. EXAMPLES

¦V

2 0k

h f h 0 , 3, 6..

The unbalanced sinusoidal case is of particular interest. In these conditions the presence of contributions associated to positive-, negative- and zero-sequence components brings to the following:

148

V12 V22 V02

0h

h f h 0 , 3, 6

f

R P1 P2 Z" Q 1 Q 2

'v T

Some typical examples are considered that refer to a short 4-conductor line supplying a passive load. The different examples differ for the voltage and current waveforms imposed to output port of the distribution system feeding the load. In particular, the following situations are considered. a) The voltages at the line output port are sinusoidal of positive- (f=50 Hz and magnitude V), negative- (magnitude V/4) and zero(magnitude V/2) sequence components. The current drawn by the passive load is consequently unbalanced. The voltages at the line-input port are obtained by means of (4).


0.12

0.105

0.1

(v m-v v)/v v

(v m-v v)/v v

'v T 'v

'v T 'V 'VH

'v0

0.1

0.08

0.095

0.06

'v

'v 0.04

0.09

0.02

0.085 0

-0.02 0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.08 0

0.01

0.02

0.03

t (s)

0.04

0.05

0.06

0.07

0.08

0.09

0.1

t (s)

(a)

(b)

Fig.4. Numerical example where the voltages at the line output port are periodical with subharmonic, interharmonic and 3td harmonic components. (a) Voltage drop waveform. The solid line represents the voltage drop given by (4), the dashed line represent the voltage drop given by the proposed formula (6), the dotted line represents the voltage drop evaluated by using Park vector only, and the point-dashed line is the voltage drop given by instantaneous zero-sequence component only. (b) Voltage drop waveform. The solid line represents the voltage drop given by (4), the dashed line represent the voltage drop given by the proposed formula (6), the dotted line represents the classical voltage drop, and the point-dashed line is the voltage drop given by a commercial software.

b) The voltages at the line output port are periodical with subharmonic (fs=20 Hz Vs=0.1V), interharmonic (fi=320 Hz Vi=0.2V) and third harmonic (f3=150Hz V3=V) components, the basic frequency is 50 Hz with magnitude V. The current drawn by the passive load is consequently unbalanced and distorted. The voltages at the line-input port are obtained by means of (4). The values of the harmonic/interharmonic components in the example are greater than the usual values in the distribution network. This is for exalting the distortion phenomena in order to point out their effects on the voltage drop calculation. The following diagrams show the results of the numerical simulation on the above mentioned cases. a) Figures 3a and 4a show the different waveforms of the line voltage drop calculated as in (4) and in (6). In addition the figures report also the terms of (6) respectively associated to the Park vector and the zerosequence component only. b) Figures 3b and 4b show the voltage drop waveforms evaluated by means of the usual definition (4) and the proposed formula (6), compared with the classical formulation and the value given by a commercial software [6]. The above diagrams show that:

1) The comparison of the diagrams in Figures 3a, and 4a proves that the proposed algorithm, based on the Park approach, is correct, since the voltage drop diagram evaluated as in (6) is the same as that computed using (4). 2) The zero-sequence component plays an important role in the voltage drop calculation as evidenced in Figures 3a and 4a. 3) The average value of the voltage drop calculated as in (13) results at an intermediate value between the classical calculation and the evaluation obtained by the commercial software. In particular in the case 4b this average value is very close to the one calculated with the classical voltage drop formula. 5. CONCLUSIONS In the present paper the problem of voltage drop calculation in three-phase, four-wire distribution lines is pointed out and deeply investigated with reference to dynamic state, in which unbalances and distortions are taken into account. The use of Park transformation permits the expression - in a separate form and generalising the results of the classical theory – of the contribution due to the three different instantaneous sequences. In particular, the positive, negative and zero-sequence components are associated in terms of power


149

quality and energy flow to the classical Park power. The analysis developed in the paper, consisting in the investigation of the zero-sequence component contribution to the voltage drop under non sinusoidal and unbalanced conditions, completes and gives a defined frame to the studies performed by the Authors for evaluating the dynamic behaviour of power line delivery systems in any operation and condition.

6. APPENDIX A. PARK TRANSFORMATION APPROACH If the following Park transformation [T] [4,5] is employed: ª cos-1 2 « ° sin-1 °°>T @ 3 « «¬ 1 2 ® ° °- - k 1 2S °¯ k 3

cos-3 sin-3 1 2

cos- 2 º sin- 2 »» 1 2 »¼

(A1)

k 1,2,3

(1) can be expressed by means of the ^d,q,o` variables. Then the Park vectors are defined as (Fig.5):

w t w d t jw q t

2 w a t D w b t D 2 w c t e j- t 3 w DE e j-t (A2) where D e j2 S / 3 . The zero-sequence component is expressed as: w 0 t

1 w a t w b t w c t 3

(A3)

The formal time-domain generalization of the rms three-phase value under sinusoidal condition can be written in the following form: w a2 t w 2b t w c2 t w t w * t w 02 t

w d2 t w q2 t w 02 t w DE t w *DE t w 02 t

(A4) that is invariant with the axis choice. The axis can be fixed, ^D,E`, or rotating at speed - d- / dt , ^d,q` (Fig.5).

150

Fig.5. Geometric interpretation of Park transformation.

7. REFERENCES 1. Arrilaga J., Bradley D.A., Bodger P.S.: Power System Harmonics. John Wiley & Sons, 1985. p.116. 2. Ferrero A., Leva S., Morando A.P.: About the role of the Park Imaginary Power on the Three-Phase Line Voltage Drop. ETEP Eur. Trans. on Electr. Power, vol. 10, no.5, September/October 2000, pp.287-296. 3. Leva S., Morando A.P., Zaninelli D.: A New Formulation of Line Voltage Drop in Unbalanced and Distorted Systems. Proc. Of IEEE ICHQP2000, October 1-4, 2000, Orlando (Florida) USA 4. Ferrero A., Morando A.P., Ottoboni R., Superti Furga G.: On the meaning of the Park power components in three-phase system under non sinusoidal conditions. ETEP Eur. Trans. on Electr. Power, vol.3, no. 1, January/February 1993, pp.33-43. 5. Kupfmuller K.: Einführung in die theoretische Elektrotecnik. Heidelberg: Springer Verlag, 1968, p.623 6. Harmflow+: Harmonic Simulation and Analysis Tools. EPRI/Electrotek, version 2.0, Knoxville (TN), USA, February 1995 7. Morando A.P., Superti Furga G.: Park Power in non sinusoidal three-phase system. l’Energia Elettrica, LXVII, no.2, 1990, pp.6570, In Italian 8. Simonyi K.: Foundations of Electrical Engineering. Oxford: Pergamon Press, 1963, p.385 9. Stokvis L. G.: Diagramme de la chute de tension dans un conducteur de ligne triphasée de tension moyenne. Cas général d’un système


déséquilibré géométriquement et électriquement. Revue Gènèrale de l’Electricité, 1923, pp.3-5 10. Weedy B.: Electric Power Systems. London: John Wiley and Sons, third edition 1990

Sonia Leva received the M.S. degree (1997) and the PhD degree (2001) in Electrical Engineering from the “Politecnico di Milano”, Milano, Italy. Since 1999 she is Assistant Professor in Elettrotecnica at the same Department. Her current research interests are concerned with the electromagnetic compatibility, the power quality and the foundation of electromagnetic theory of electric network. Dr. Leva is a member of IEEE, Mailing address: Sonia Leva Dipartimento di Elettrotecnica – Politecnico di Milano Piazza Leonardo da Vinci, 32 20133 Milano – Italy Phone: +39-02-23993709, Fax: +39-02-23993703 e-mail: [email protected] Adriano Paolo Morando received the M.S. degree in Electrical Engineering from the “Politecnico di Milano”, Milano, Italy. From 1984 to 1989 he was with ASEA Brown Boveri, where he was concerned with AC drives for electrical traction. Since 1989 he is Assistant Professor of Elettrotecnica at

the Electrical Engineering Department of the “Politecnico di Milano”. His current research interests are concerned with the electromagnetic compatibility, the power quality and the foundation of electromagnetic theory of electric network. Mailing address: Adriano Paolo Morando Dipartimento di Elettrotecnica – Politecnico di Milano Piazza Leonardo da Vinci, 32 20133 Milano – Italy Phone: +39-02-23993729, Fax: +39-02-23993703 e-mail: [email protected] Dario Zaninelli received the Ph.D degree in Electrical Engineering from the Politecnico di Milano, in 1989, and he is now Associated Professor in the Electrical Engineering Department of the Politecnico di Milano. His areas of research include power system harmonics and power system analysis. Dr. Zaninelli is a senior member of IEEE, a member of AEI and a member of the Italian National Research Council (C.N.R.) group of Electrical Power Systems. Mailing address: Dario Zaninelli Dipartimento di Elettrotecnica – Politecnico di Milano Piazza Leonardo da Vinci, 32 20133 Milano – Italy Phone: +39-02-23993721, Fax: +39-02-23993703 e-mail: [email protected]


151

152




OPTIMUM OF RAPIDLY RESTORING OF VOLTAGE IN AUTONOMY ELECTRICAL POWER SYSTEMS Sergey BANKO

Ludmila SABARNO Igor SEVASTUK Institute of Electrodynamic Kiev (Ukraine)

Abstract - The quality of the electric power in isolated systems can be increased by lowering amplitude of dips and cruption outbursts of voltage called by shock load, and diminishing their duration. The extreme possible quality of power (QP) in such systems are accessible only at optimum on speed regulation of excitation of synchronous generator. At development such AVR rationally to use a maximum principle by L.Pontriagin. The matching with traditional controller displays, that optimum on speed AVR ensure the best results. 1. INTRODUCTION The transient processes in an autonomy electric power system caused by suddenly turn on or cutting off a considerable load, essentially reduce output power quality parameters. First of all it falls into magnitude and duration of voltage dips. Therefor there is a problem to reduce magnitude for voltage dips, and then to retrieve it for minimum time. Thus ability an adjusting system of excitation synchronous generator (SG) and automatic voltage regulator (AVR) should be used by a maximum fashion. Apparently, that the problem of the quickest restoring AC generator terminal voltage most effectively can be decided only in a class of regulating systems, optimum control. In such systems the minimumly possible transient period is ensured at the expense reverse stage control of the excitation SG. Transition numbers from forcing to deexcitation of field and their duration (control actions switching moments t1 , t 2 ...t n ) depends on many factors. Among them parameters of regulation plant (time constants and gains, associated with SG and exciter), perturbations magnitude, limiting value of

Igor TRACH

control action. And also from state variables of regulated coordinate in initial and final points of phase trajectory 2. DESCRIPTION FAST RESPONSE VOLTAGE CONTROL STRATEGY Algorithm and searching procedure the moments of switching t1 , t 2 ...t n for the solution a problem in a general view are available in an arsenal of the modern control theory. It is a method of a dynamic programming by R. Bellman and maximum principle by L.Pontriagin [1, 2]. The indicated methods give the program of a numerical solution the problem. At searching functions supplying an extremum to a functional if there is limitations the methods of a classic calculus variations are used, as a rule. However, as against usual problems a calculus of variations, where all required functions are equivalent, basically of maxima by L.Pontriagin the phase coordinates and control actions are disjointed. This separation is specially convenient when the limitations are superimposed only on control, and the required functionals both coupling equations are linear also radicals of a characteristic equation are real [2]. Maxima the principle by L.Pontriagin with backed butt joint of the solutions of the differential equations with an alternating right member in switchings moment can be utilised for a sectional problem. 2.1. Self-exciting SG From a vectogram a isolatied SG [3], reduced in a fig. 1, it is possible to note:


153

where ( x dc X n )( x q X n ) ( Rn ra ) 2 Td Tdo , ( x d X n )( x q X n ) ( Rn ra ) 2

q Eq V

k

T

M

Iq

I

V t

Fig. 1

Eq

U cosT xd I d ra I q ;

Eq

k nZ I f ; I

V

Id

I sin(T M );

Iq

I cos(T M );

cos M tgT

Rn

Zn

; sin M

(1)

xq Rn ra X n Z n2

ra Rn xq X n

r fd ( I f Td

Zn

;

Eq VR

.

Vf )e

t Td

,

(3)

kVR ,

Z n ( xq X n ) 2 ( Rn ra ) 2

k cEqc

( xdc X n )( xq X n ) ( Rn ra ) 2

Eqc .

When the generator is weighted from an idling, Eqc Eq Vo 1. and his voltage was supported by rated:

;

( x dc X pr )( x q X pr ) ( R pr ra ) 2

E qc

k dI ) Vf n d ; dt ZG dt

ZG

If the generator carried a fractional load R pr , X pr

dI f

r fd Td ( xd xd' )

kn

Voc

Xn

Vf (Vo'

where Vf

;

Zn

( xd X n )( xq X n ) ( Rn ra )

2

Solving the equation (2) in a general view, we shall receive:

d

Id

Z n ( xq X n ) 2 ( Rn ra ) 2

Z pr ( x q X pr ) 2 ( R pr ra ) 2

.

(4)

Z.

Where E q - EMF of no-load operation;

For reaching optimum on speed of regulation terminal voltage SG V at the moment turn on a

V , I - terminal voltage and current the generator; Td , r fd - time constant and resistance of field

load is indispensable force of excitation by duration t1 . After the voltage V will reach the rate

winding, respectively; V f VR , I f - out exiter voltage and current

value the installation of a rate value of excitation VRn on an exit AVR is indispensable. The time t1 is determined from (3) at V (t1 ) Vf 1 : t1 Td ln 1 1 V0c / V Rm / V Rn

(voltage and current of field winding), respectively; ra , xd , xdc , xq , Rn , X n - parameters of the generator and his load; Z - angular velocity of rotating (frequency), which one starts by a stationary value; ZG - base angular velocity; p d / dt - derivative operator; Stationary value of time Td , and time t , express in seconds, and remaining magnitudes in relative unities at ZG = 314 rad / sec. If to accept ZG =1, all magnitudes should be dimensional. Simplifying, it is possible to receive the equation of the generator terminal voltage at turn on a load:

Where 1 Vo' - magnitude of initial voltage fall; k f VRm VRn - field forcing ratio, equal relation maximum limitation of excitation voltage to rated. At moderate and technically acceptable forcing excitation: 1 Vo' k f VRm VRn d 5 , the relation 1 and kf also can be noted, using property of a log: t1

Td

dV V dt

154

kVR ,

Td ln 1 1 V0c / k f | Td 1 V0c / k f .

(5)

(2)


Voltage transient after turn on a nominal series connected active - inductive load Rn 0,8 ; X n 0,6 to the diesel power generator type MC 117-4(Russia) Pn 100 kW, V 400 V; I 180 A;

cos M ra

0,8; xd

1,25; xd'

0,10; xq 2; T

0,02; Tdo 1,63s; Eq

0,701;

21o

from an idling and field forcing ratio, equal 1, 2 and 4, are exhibited on fig 2. By introduced curve of 1 regulation is characteristic for self-energizing generators (with a power current compounding). Apparently, that the heightening field forcing ratio of excitation even up to 2 allows essentially to reduce duration of voltage dip.

V

1.0

kf

4

2

1

0.9 0

VR

Td Te

d 2V dt

2

(Td Te )

dV V dt

k 2VR

(7)

where k 2 k k e . For reaching optimum on speed of response voltage V the control procedure will contain two intervals. On the first period of time (from to till t1 ) voltage V R V R max - field forcing, and on second (from t1 till t 2 ) - V R VR min V R min d 0 - reversed of excitation with a consequent exit on steady-stated VR VRn . Using a procedure [2], it is possible to find moment of switching t1 and t 2 solving the equation (7) on each interval with allowance for of requirements of backed butt joint (lacing) these solutions for an instant t1 by the way equalities of voltages and their left-hand derivative and on the right. The final requirements of transient process for an instant t 2 are beforehand known: V Vf , and derivative dV / dt t t2 0 . The starting conditions for t o are determined by known property of persistence a EMF Eqc

4

synchronous machines

behind transient resistance xdc at the commutation

2

moment Voc

1

t Fig. 2

2.2. SG with dc-or brushless exciter

In case for excitation GS is used dc - or brashless exciter that to the equations (2), circumscribing transient process in the generator, the equation of an exciter is added. In a general view the machine exciter can be represented by an aperiodic link of the first order with a transfer function:

Vf

Therefore equation for voltage SG will have the second order:

ke VR , Te p 1

(6)

k e k c Eqc . In a moment t o

0

begun forsed field AVR on magnitude of a required derivative does not influence, as the equation (7) in a right member does not contain derivatives. The directed rotor curent on the part of a stator after turn on a load damps from a time constant Td . The voltage dip starts with the same stationary value of time. Without regulation of excitation: V t V

'

(Vo' f

where Vfc

V

' f

)e

t Td

ke k Eq , and

,

(8) Eq

Vo 1,

if the

generator was weighted from an idling. If he carried an initial load and its voltage was supported by rated, Eq Eqc see (4). Therefore magnitude of a derivative in an initial instant will be peer:

where V f ; Td ; k e exciter out put voltage, time constant and gain respectively. Section 2. Methods of Power Quality Analysis: Modelling and Simulation

155

dV dt (t 0)

k eVo ' (k k ) Td

1 (Vo' Vf' ) Td

Vo .

(9)

The solution of the equation (7) on the first interval will look like: V (t )

Ao' A1' e Dt A2' e Et ,

where A0' k 2VR max ; D 1 / Td ; E For

Vo

t 0 It is k eVo ' (k k ). Td

(10)

1 / Te .

known:

V

Vo' ,

and

1000(Russia) Pn cos M

Then: Vo' Ao' A1' A2' ; Vo DA1' EA2' . Whence: A1' Vo E Vo' Ao' E D , A' V D V ' A' E D . 2

>

>

o

o

o

@

decided this system by the numerical method. At cutting off a load the order and the course of reasonings will be similar. It is necessary only to take into account, that the field current of an exciter very promptly will decrease to zero point. Further in calculations the plant of control - SG with a dynamoelectric exciter should be represented by model of the first order from a stationary value of time Tdo . The calculation of transient processes under the formulas (10) and (11) at an inductive load X n 1, cos M 0 turn on to SG MC 310-

@

ra

0,8; x d

0,02; Tdo

250 kW, V

1,15;

x d'

400 V; I

0,18; x q

448 A;

0,55;

1,50 s;

is reduced in a fig. 3. Exciter BCM24,5/13 parameters are: Te 0,2 s, k 2 20.

On the second interval:

V (t )

Aocc A1cc eDt A2cc e Et ,

where A0" For Vf (t )

(11)

V 1

k2VR min . t

Vf ; V

t2 : V

kf

10

kf

10 kf

0,

4

0.85

0 DA1"e Dt 2 EA2" e Et 2 .

VR

0

Whence: A1" E Vf Aocc eDt 2 /( E D ),

@ Acc @e

Ao" A1"e Dt1 A2" e Et1 ,

DA1' e Dt1 EA2' e Et1 DA1"e Dt1 EA2" e Et1 . Simplifying we have a system of the nonlinear algebraic equations concerning unknowns t1 and t2 : eDt1 e Et1

>V / E V >V / D V o

o

' o ' o

@ @/( A

Ao' (Vf Ao" )eDt2 /( Ao' Ao" ); Ao' (Vf Ao" )eDt2

' o

Ao" ).

The time - t1 and t 2 relevant to intervals forcing and deforcing of excitation can be received having 156

2

kf

1 const

0 .4

0 .2

t

s

Fig. 3

Et 2 A2" /( E D ). f o Using a procedure from [2], we shall joint (lacing) the solutions for voltage SG at the moment of switching t1 by the way equalities of voltage V and his derivative:

Ao' A1' e Dt1 A2' e Et1

kf

and

Ao" A1"e Dt 2 A2" e Et 2 ;

> D >V

4

kf

The moment of switchings are marked by points on each transient-response curve. The curve of voltage SG exhibited by a dotted line corresponds to a condition of excitation without inverting (VR min 0) . As it is visible, the inverting AVR is essential at diminution for voltage recovery time. If the inverting of excitation misses or there is his frustrating efficiency AVR in transient regimes is reduced in 2-2,5 times. That is, the regulating system with AVR and field forcing ratio 10 but without inverting, ensures restoring voltage SG approximately for the same time, as with field forcing and deforcing ratio of excitation equal 4. It is necessary to underline, that in the second case the power of the executive board AVR and, first of all, power transformer in 2,5 times is less. After the voltage SG will enter a band of a given exactitude (for example r 0,5% ) for further regulation can be used aperiodic AVR. Such controller ensures an effective vibration damping and high scale of a stability at small transient voltage deviations from steady-stated. Electrical Power Quality and Utilisation

Under production conditions connected load is not a stationary value and varies, being by function of many parameters. Therefore phase trajectories of coordinates of regulation deflect from optimum. Said to the full falls into a load from asynchronous short-circuited motors, the pure resistance which one in a dual circuit of substitution depends on slip (rotational speed). Therefore switchings of moment the correction on an aberration of a phase trajectory from calculated, is obligatory. It is obliged will be fulfilled continuously in an adjustment process. With allowance for development othe modern controlling means and regulations on microprocessor element basis can not arise of problems at technical embodying such optimum on speed of a controller. 3. CONCLUSIONS

A fast restoring method of voltage SG for design of optimal AVR is presented with two case studies. Results have been obtained using a approximately model of SG and the strategy with ultra highspeed control by L.Pontriagin. 4. REFERENCES

1. Bellman R.E., Glicksberg I., Gross O.A. Some aspects of the mathematical theory of control processes. The Rand Corporation, Santa Monica, California, 1958. 2. ! .., .., .., ..

. #, , 1961. 3. Concordia Ch. Synchronous Machines. Theory and Performance, John Wiley and Sons, New York, 1951.

Sergey Banko was born in 1955 in Tula, Russia. He received the M.Sc. degree in electrical engineering from Kiev Polytechnical institute in 1978. He received his Ph.D. degrees from Institute of electrodynamics of Kiev. Presently, he is Senior research worker of Institute of electrodynamics in Kiev of the National Ukrainian Academy of science. His areas of interest include computer analysis of power system and non-linear optimization. Ludmila Sabarno was born in 1965 in Kiev, Ukraine. He received the M.Sc. degree in electrical engineering from Kiev Polytechnical institute in 1988. At present, she is Reseacher in Institute of electrodynamics of Kiev of the National Ukrainian Academy of science. His areas of interest include computer science of power system and electric power quality. Igor Sevastuk was born in 1955 in Kiev, Ukraine. He received the M.Sc. degree in electrical engineering from Kiev Polytechnical institute in 1978. At present, she is Reseacher in Institute of electrodynamics of Kiev of the National Ukrainian Academy of science. His research includes computer science of power system analysis and automatic control. Igor Trach was born in 1954 in Kiev, Ukraine. He received the M.Sc. degree in electrical engineering from Kiev Polytechnical institute in 1978. He received his Ph.D. degrees from Institute of electrodynamics of Kiev. Presently, he is Senior research worker of Institute of electrodynamics in Kiev of the National Ukrainian Academy of science. His field of interest is in computer analysis of power system and telemechanics. Mailing address: For all: Sergey Banko National Ukrainian Academy of Science, Institute of electrodynamics 56, Peremohy Av., 03680, Kiev-57, Ukraine phone:(+38)(044)441-26-29,441-26-59 fax:(+38)(044) 446-94-94 e-mail: [email protected]


157

158





SUPPRESSION OF OVERVOLTAGES CAUSED BY OPEN-PHASE IDLE OPERATING CONDITIONS IN SUBTRANSMISSION NETWORK Vladimir KUZNETSOV, Yuriy TUGAY

Eugenie DMITRIEV

Institute of Electrodynamics NAS Kiev (Ukraine)

Academy of water-borne transportation Novosibirsk (Russia)

Abstract – Ferroresonance in electrical power systems currently attracts a lot of attention in both the electric utility industry and the academic sector. One major reason is ferroresonance has been found to occur more frequently in electrical systems in the recent years. Another reason is when it occurs; ferroresonance could lead to substantial damages on power or voltage transformers, and subsequently power outage, losses of production etc. Consequently, ferroresonance should deserve more attention when planning an electrical distribution system, particularly on how to prevent its occurrence, or reduce its damaging effects if it cannot be entirely avoided. The features of ferroresonance overvoltages in practically idle transformers have been presented. Special measures for prevention and suppression this dangerous phenomenon has been proposed in this paper. The development of special devices for detecting of ferroresonance is very important.

Ferroresonance is a physical phenomenon, which can occur in a sinusoidally excited non-linear system, as result of an interaction between a nonlinear inductance and a capacitance. Whereby an electrical power system, consisting of capacitive and non-linear inductive circuit elements, is driven into a series or parallel resonance condition by a disturbance that results in temporary or sustained overvoltages across these circuit elements [1]. The ferroresonance phenomenon in power network can cause dangerous overvoltages and overcurrents, which can lead to serious damage to the equipment and longer-term failure of power supply. Therefore, it must be avoided by all means. Capacitive elements in the system may include an overhead line of considerable length, an underground cable of much shorter length, power factor

correction capacitors, etc. The non-linear inductive element is very often an iron-cored element such as a reactor, a power or voltage transformer, hence the term "ferroresonance". Ferroresonance is also realised as a “jump” phenomenon. The system suddenly jumps from one stable state - the “normal state”, to another stable state - the “resonant state” upon a system disturbance, such as a small change in system voltage, circuit capacitance, or transients. This sudden jump is sometimes referred to as “bifurcation”. Researchers have tried to tackle it analytically by using time domain methods, such as numerical integration techniques. They are now trying to further understand this phenomenon by applying the theory of non-linear dynamics and chaos [2,3]. Depending upon the values of the capacitance and the inductance, electrical system may go into series resonance when ZL=1/ZC (Z=2Sf, where f system frequency), or at one of the sub-harmonic or higher order harmonic frequencies. The nonlinear transformer introduces these current and voltage harmonics. Ferroresonance emerges when the capacitance and the inductance tune in with one of these harmonics. Ferroresonance is found to occur mainly at the fundamental frequency while sometimes sub-harmonic ferroresonance may also happen. In the latter case the ferroresonant overvoltages are less severe and more difficult to mitigate. This ferroresonance has sometimes occurred in an extra high voltage transmission network where a voltage transformer is connected to a deenergised transmission line that runs alongside another energised line. Three essential conditions have to be fulfilled for the emergence of ferroresonant overvoltages: x there must exist a relatively large circuit capacitance, say, due to a very long distribution overhead line, such as that which exists in a rural distribution system, or a shorter but relatively more capacitive underground cable


159

which is now very often found in an urban distribution system. In some cases, the internal capacitance of the transformer could also be large enough to trigger ferroresonance; x the power transformer involved is in an unloaded or a very light load condition. Load causes damping which reduces the ferroresonant overvoltages, or even prevents ferroresonance from occurring at all; x a system disturbance that precedes and induces the ferroresonance. Usually ferroresonance occurs in power distribution system where one or more of the supply conductors to unloaded (or very lightly loaded) power transformer are interrupted, leaving a transformer coil energised through the capacitive coupling with the other phases and (or) with parallel lines. The linear phase-to-earth and phase-to-phase capacitance of the transmission line conductor and the non-linear inductance of the transformer could then establish a series resonant circuit. Some typical system disturbances that caused ferroresonance in power distribution system: x sudden de-energization of one phase while the other two phases remain energised. For instance, when a phase conductor is accidentally broken, or upon loss of a phase(s) due to a blown fuse(s) as a result of a phase-to-earth fault Sudden de-energization of two phases can have the same effect; x out-of-phase closing, for instance, two phases are closed simultaneously when the third phase remains open for a few cycles. This could happen in distribution systems where single phase switching is employed. Single phase fusing is still commonly found in rural systems due to its relatively low capital cost and also because it enables limited power transfer to be maintained after the frequent single phase-to-earth fault is cleared. Out-of-phase closing also happens with the older three-phase circuit breakers or switches, which can have a pole spread of several cycles. x out-of-phase opening could lead to similar ferroresonant overvoltages as out-of-phase closing. As mentioned above, such asymmetrical switching operation can lead to essential overvoltage when the transformer with low loading or without the loading is connected to the end of a line and it works with not grounded neutral point. But the situation gets more difficult when ferroresonance circuit is supplied not only by line’s capacitance, but also by electromagnetic coupling, principally. This situation occurs in the subtransmission substations with semi-unit circuit design where the step-

160

down transformer is connected to line by means of shorting device and disconnector, not high-voltage circuit breaker. Usually, the step-down transformer has got a high-voltage star-connected windings with grounded neutral point and a low-voltage winding with delta circuit (Fig.1). There are possible such dangerous switching in these circuitry [4]: x switching-off or switching-on by the line’s

Fig. 3

Fig. 2 switch 1 with non-loaded or low loading transformer and with open switch 2 (Fig. 1a); x switching-off or switching-on by the switch 2 with non-loaded or low loading transformer and with open line’s switches 1,3 (Fig. 1b). For instance, phase C of circuit breaker has remained open after fail switching–on (Fig. 2). The capacity currents appear in this circuit (solid pointers). But the currents in the phases A and B of high-voltage star-connected winding (dotted pointers) induce EMF in appropriate phase of lowvoltage winding with delta circuit (bold pointers).

The angle between these EMF equals 120q. Therefore equivalent EMF equals E too. It induces the current in delta circuit which generate flux in Fig. 1 magnetic core of phase C. As result, the EMF –E is induced in primary circuit. We could convert this three-phase circuit to equivalent one-phase circuit (Fig.3), with passive Electrical Power Quality and Utilisation

elements: Cp - phase-to-phase capacitance; Cg – phase-to-ground capacitance; Ll - leakage inductance of low-voltage winding per phase; LP - nonlinear inductance of transformer. The equivalent circuit is simplified one with some assumption (the voltage supply has unlimited power, the circuit is linear basically, wave behaviour do not keep in mind, etc.). So it can’t be used for a punctual investigation, but it is illustrative example of physical nature this type of resonance overvoltages. If EMF phase C equals (-E), then EMF other phase equal (-aE) and (-a2E), respectively. Therefore equivalent EMF phase A and B equals (0,5E). We can see, that the potential at the break point is determined on the one hand capacitance coupling between open phase and close ones (EMF 0,5E), and on the other hand electromagnetic coupling between windings (EMF E with internal impedance X=3ZLl,). So, the criterion of linear resonance (LP don’t take into account) is:

got failure (out-of-phase opening). In-substation HC-2 personnel heard specific loud noise in the voltage transformer TV for about fifteen minutes. Then the noise had stopped. Jankoy’s high-voltage circuit breaker was opened properly after thirty minutes since starting of commutation. And the voltage transformer TV exploded, while line was switching-on after repair. A subsequent analysis test indicated decomposition of oil and solid insulation. Close examination of transformer’s details also reported a smell of burnt insulation The simulation of process caused by open-phase operation was performed for this network and it

The application of three-winding transformers produces favourable conditions for realisation of a resonance, because the tertiary has considerably Fig. 4 lower power, than other windings. Therefore leakage inductance of a winding of the lowest voltage, which reduced to power of the transformer, is very high. Usually, the corona discharge restricts overvoltages significantly, because the coupling EMFresonance circuit is weak (large capacity reactance) and resistance losses have greater effect. On the contrary, the electromagnetic coupling is stronger and sensitivity behaviour of circuit for resistance loss is lower, so the overvoltages are more significant. A substantial increase of a voltage under the conditions closes to resonant raises a role a non-linear

1 Z (C g 2C p )

3ZLl

current of magnetisation of the transformer. Therefore the problem become non-linear and special method should be used to find its solution. Obviously, the most suitable is computer simulation lately. It has been used for investigation of accident in Crimean network (Fig.4). The substation HC-2 is connected to line 220 kV Jankoy-Evpatoriya by means of shorting device and disconnector. The line was made dead for some repair. Evpatoriya’s high-voltage circuit breaker had worked properly, but Jankoy’s one had

showed that ferroresonance had occurred in both transformers connected in parallel (power and voltage). But the voltage transformer was damaged first, because it is weaker. The current in a winding of high voltage of the transformer sharply increased up to several amperes after starting of a ferroresonance. The short-circuits had arisen in a winding of high voltage as result of overheating. These short-circuits changed an inductance TV, the ferroresonance loop was detuned and process was terminated. But the damage of winding caused explosion subsequently. The results of simulation have shown impossibility of suppression with the aid of valve-type arresters. When the voltage of the neutral point reaches magnitude sufficient for operation of a valve-type arrester, the spark gaps are punched and a current pulse is appeared. It's promptly reduced to the minimum value, further it's augmented and then rather slowly diminishes. The voltage is less at the second and third operation of a valve-type arrester, than at first. Therefore the amplitude highfrequency component of a current of a valve-type arrester is arisen at consequent operations of a valve-type arrester and there are requirements for prompt abruption of a current in a valve-type arrester. In turn, it accelerates the recurring origin of a ferroresonance of voltages. The calculation has shown that after operation of a valve-type arrester and starvation of an arc of an accompanying current, the overvoltages were recovered, and it reduced in new operation of a valve-type arrester


161

with a consequent starvation of an arc etc. If the ferroresonant overvoltages were not reduced as time goes by, sooner or later the surge arresters would be destroyed due to thermal runaway. The classification of possible means of struggle with ferroresonance overvoltages is developed as result both theoretical and experimental researches [6]. The analysis of emergency processes in living electrical networks and assessment of works of resistors in a ground wire and non-linear overvoltage suppressor is carried out too. As results of this analysis is fact, that the above-mentioned ferroresonance can be prevented by ground connection of a neutral of the idle transformer through the resistor during open-phase operation of power line [5]. The amount of the resistor in a neutral should be determined from some requirements: ensure preventing a ferroresonance, don't influence to operation of relay guards and ensure a lightning protection of a neutral and to correspond to condition of manufacture. These requirements are corresponding to the resistor by amount 1000... 2000 Ohms. To supplement the resistor protection of the transformer from the ferroresonance it is suggested that current through the resistor should be continuously monitored, otherwise it may blow-out. By the way, if we can open delta winding of power transformer, the conditions of ferroresonance would be eliminated. It is offered for the interruption of ferroresonance to use the standard shorting device. Two variants of its operation are in this case: x short-circuiter is on live phase, so its close-in caused open this phase by means of highvoltage circuit breaker; x short-circuiter is on open phase and current, which appears in a circuit, equals approximately idle current of transformer and voltage on other open phase equals 0.5 phase voltage. Therefore utilisation of standard shorting device gives capability to open line (if eliminator is blocked) or to interrupt dangerous state.

4. \# ..: !" # " . , #, 1972. 5. X# .., # .., # ..: & " '

#"

' * # . ` \ , _`.4, 1994 , . 94-97. 6. Kuznetsov V., Tugay Y., Dmitriev E.: Elimination of the ferroresonance overvoltages in electrical networks of the high voltage. Proc. of the 5th international conference electrical power quality and utilisation, Cracow, Poland September 15-17, 1999 pp. 357-364.

1. Marti J.R., Soudack A.C.: Ferroresonance in power systems: Fundamental solutions. IEE Proc.-C, vol. 138, no. 4, 1991, pp. 321-328. 2. Bodger P.S., Irwin G.D., Woodford D.A., Gole A.M.: Bifurcation route to chaos for a ferroresonant circuit using an electromagnetic transients program. IEE Proc. - Gener., Transm., Distrib., vol. 143, no. 3, May 1996, pp. 238242. 3. Emin Z., Al Zahavi B.A.T., Auckland D.W., Tong Y.K.: Ferroresonance in electromagnetic voltage transformers: A study based on nonlinear dynamics. IEE Proc. - Gener., Transm., Distrib., vol. 144, no. 4, July 1997, pp. 383-387. 162


Prof. Vladimir Kuznetsov was born in 1935 in Krymsk, Krasnodarsky kraj, Russia. He graduated Kiev Polytechnic Institute in 1958 (Faculty of Electrical Energy). He received his Ph. D. and D. Sc. degrees from Institute of Electrodynamics of the National Academy of Sciences of Ukraine. Presently, he is Vice Director of Institute of Electrodynamics of the National Academy of Sciences of Ukraine and Professor in Kiev Polytechnic Institute. His areas of interest include electric power quality, normal and abnormal steady states; electrical networks and supply systems.

Prof. Evgeniy Dmitriev was born in 1937 in Baku. He graduated Azerbaijan Institute of Petroleum and Gas in 1962. He received his Ph. D. degree from Institute of Physics Academy of Sciences of Azerbaijan and D. Sc. degree from Institute of Electrodynamics of the National Academy of Sciences of Ukraine. He was Chief of Laboratory of Institute of physics Academy of Sciences of Azerbaijan until recently. Now he is Professor of Academy of waterborne transportation (Novosibirsk, Russia). His areas of interest include lectrophysics, transmission of the electric power, engineering of high voltages.

Mailing address: Vladimir Kuznetcov Institute of Electrodynamics of the National Academy of Sciences of Ukraine 56 Peremogy av., 03680 Kiev-58 UKRAINE phone (+380) (44) 446-3341, fax (+380) (44) 446-9494 e-mail: [email protected]

Mailing address: Evgeniy Dmitriev Academy of water-borne transportation 33 Shchetininskaya str., 630099 Novosibirsk RUSSIA phone (+7)(383) 222-07-65

Dr. Yuriy Tugay was born in 1947 in Kiev, Ukraine. He graduated Kiev Polytechnic Institute in 1971 (Faculty of Electrical Energy). He received his Ph. D. degree from Kiev Polytechnic Institute. Presently, he is Leading Scientist of Institute of Electrodynamics of the National Academy of Sciences of Ukraine and Senior Lecturer in Kiev Polytechnic Institute. His areas of interest include computer simulation of power system and electrical networks, energy losses and electric power quality. Mailing address: Yuriy Tugay Institute of Electrodynamics of the National Academy of Sciences of Ukraine 56 Peremogy av., 252680 Kiev-58 UKRAINE phone (+380)(44) 441-2623, fax (+380)(44) 446-9494 e-mail: [email protected]


163

164





POWER PROPERTIES OF SINGLE-PHASE NON-SINUSOIDAL SYSTEMS: THE USE OF GENERALISED ROTATING VECTOR CONCEPT Sonia LEVA Adriano P. MORANDO Dipartimento di Elettrotecnica Politecnico di Milano Milano (Italy)

Abstract – The Fourier transform extends to the non-sinusoidal system, in terms of rotating vectors, the phasor approach typical of the sinusoidal case. The obtained method is valid under any condition. In fact, it is able to analyse any waveform transformable by Fourier by using a complex unified form. The sinusoidal case is only a particular case of this one. This generalisation is particularly important under the energy analysis point of view. In fact, in terms of rotating vector, an instantaneous complex power and, as its component, an instantaneous imaginary power, are definable. These quantities have, with reference to any waveform, a role similar to that performed by complex S and reactive Q power in sinusoidal case. Therefore their use can be a conceptual point of reference for power quality problems analysis. 1. INTRODUCTION The single-phase circuit energy balance, under sinusoidal conditions, is developed by using phasor algebra. In this case the phasor notion identifies, over the complex plane, three different quantities (the active P, reactive Q and complex S powers) able to totally define the energy balance of the system itself. Under non-sinusoidal condition the researches, in agreement with Budeanu and Fryze theories, are respectively developed in the frequency and time domains. This research, still under way, has not reached consolidated results yet [1]. In three-phase systems, under any conditions, the Park transformation is applied. This leads, tanks to the use of the space vector theory, to a single-phase complex formulation of the energy balance. This approach is expressed in time-domain and it is

valid under any possible condition. In it, close to the real term pp(t), that represents the instantaneous real power, an instantaneous imaginary power qp(t) appears. This component is very important under power quality aspects [2,3,4]. The effectiveness deriving from the representation of the quantities based on the rotating vector notion suggest the extension, in time domain, of the use of rotating vector approach to the single-phase case also under any condition. Under the strictly mathematics point of view this extension is possible, univocally, by using the Fourier transform. The present paper uses the rotating vector notion for the representation of single-phase nonsinusoidal quantities. From these quantities the correspondent energy balance is deduced. It results, in time domain, a complex quantity. Its real and imaginary terms represent the conceptual extension to non-sinusoidal condition of the power already defined in phasor domain. Furthermore to the imaginary part of the obtained complex power can be attached a similar meaning, under the power quality point of view, to that assumed by the reactive power (in sinusoidal condition) and by Park imaginary power (in the three-phase system). The paper presents also an applicative example. 2. THE PHASORIAL DOMAIN A general time function f(t) can be represented as a projection of a suitable rotating vector f(t) on a axis (see Fig.1). On the analytical point of view we can write:

^

f t e^f t ` e Ft e j- t

` Ft cos-t

(1)

Under the sinusoidal case, the following conditions can be applied (Fig.2):


165

°Ft 2 F ® °¯-t Zt -o

(2)

Therefore this approach leads to the following relations, characteristic of the phasor algebra: f t e^f t ` 2FcosZt - o ° 2 e F e jZt ° ° 2 e p Fe jZt ®p f t p jZ ° ° d °p dt ¯

^

^

`

`

(a)

(3)

Tanks to this algorithm, the integral-differentials equations, proper of a linear, timeindependent network, becomes as concern the Fig.1. The rotating vector steady-state solution f(t) as representation of a – algebraic, linear general function f(t). and with complex variables. Particularly important is their use in the energy analysis. In fact, on this point of view, a single term, the following complex power: v t i * t V I S VI j-V -I VIe jM

P jQ S

(4)

is able to summarise by means of the following relation: pt P >1 cos 2Zt 2- I @ Q sin 2Zt 2- I (5)

all the parameters necessary to identify the time evolution of the energy process. Under nonsinusoidal conditions, the Fig.2. The use of rotating phasor notion loses vectors in the sinusoidal its validity. On the case. contrary, the vector notion expressed by (1), can be still applied to the network analysis. Given the advantages characteristic of the phasor algebra method, it is a worthwhile effort to study in depth the formal and

166

(b)

Fig.3. The rotating vector limits in respect to the phasor notion. The rotating vector (a) is nonunivocal and (b) its time derivative operation is nonalgebraic.

conceptual implication peculiar of eq. (1). With the following objectives: 1) A methodological point of view: the development of a rotating vectors algebra that allows to study any non-sinusoidal system. 2) An energy point of view: the definition, in terms of rotating vector notion, of an instantaneous complex power s(t), and its real and imaginary power components. This investigation must be done taking in to account that the rotating vector tool: It is not univocally defined (see Fig.3a): if the function f(t) is known, one of the two quantities {F(t),-(t)} is arbitrary and the other one is consequently deduced; Entails a time derivative operation that, t and on the depending on the factors jtime derivative p, is non-algebraic (Fig.3b):

^

pf t pe^f t ` e j:t f t pFt e j- t (6)

`

Therefore, the rotating vector algebra formulation implies: The deduction of an expression for the rotating vector (1) that is univocal; The analysis, on the circuit point of view, of the implications related to the rotating vector notion application. This paper covers the following items: a) Elaboration of a univocal definition of the vector transformation (1). b) Vector formulation of the Ohm equation for the perfect one-port elements. c) Deduction, in a vector way, of the energy balance for the perfect one-port elements. d) Definition of the complex power and verification of its conservation properties; interpretation of the role played by its real and imaginary components in the energy balance formulation. e) Exemplification of the obtained results with reference to elementary cases. Electrical Power Quality and Utilisation

f t

m

m

n 1

n 1

¦ Fn cosZ n t - n e¦ Fn e jZn t -n e^f t `

(7) we have: At e jMt

f t

resistance. induttance

The univocal condition requested by transformation (1) is subject to the following properties: 1) formal: the transformation (1) must turn into an identy when f(t) is a sinusoidal function; 2) distributive: the rotating vector corresponding to a functions sum must be coincident to the addition of each term. The first condition leads to the phasor notation use for a sinusoidal function. As concern to the distributive property, if a periodic f(t) consists of a finite sum of sinusoidal functions, being:

v t R i t x ° j- t ® v t Lp i t L pIt e I j -I t L i t x ° j- V t j -V t C v t ¯it Cp v t C pV t e (11)

capacitor

3. THE GENERASID ROTATING VECTOR CONCEPT

(8)

The non-periodic general time function f(t) analysis is possible by using the Fourier Transform (FT) instead of Fourier series. Tanks to FT, it is possible to generalise eq.(7). In fact, every time function f(t), satisfying all the analytical conditions required by this kind of transformation [5], can be univocally decomposed in the sum of an infinite number of sinusoidal functions. In these terms, representing the spectrum density in the following rotating vector form:

Fig.4. Vector Ohm equations for the o elements.

In the reactive case, because of the non algebraic nature of the derivation operation (6), the following terms appear: A “radial term” linked to the derivative of the istantaneous amplitude of the vector; A “tangential term” linked to the istantaneous angular speed of the vector. In sinusoidal regime, characterised by a constant angular frequency, only the second term, with x

F jZ

f

1 f [ e jZ[ d[ S ³f

FZ e jJ Z

(9)

adding all the infinite vectors obtained in this way, the function f(t) can be univocally represented as follows: ½ f f t e®³ F jZ e jZt dZ¾ ¿ ¯0 j- t e At e e^f t `

^

(10)

`

4. THE OHM EQUATIONS EXPRESSED IN TERMS OF ROTATING VECTOR With reference to the three perfect one-port elements R,L,C, taking into account eq. (6), we have respectively:

-

Z , appears. From these relations the diagrams and the equivalent circuits shown in fig.4 can be deduced. Because of the non algebraic nature of the equation (6), the voltage and current rotating vectors for the reactive elements are not displaced with the phase of S/2. Therefore, in the equivalent electric circuit a generator, representing the real component, linked to the change of the rotating vector instantaneous magnitude, appears. As concern the network dynamic model integration, it leads, for linear and timeindependent system, to a solution expressible in closed form. With reference, for example, to an R,L series circuit, the state equation: p i t

1 R it v t L L


(12)

167

leads to the following solution: i t , A

t

t

where wPI(t) is the “radial” component of the instantaneous magnetic stored energy. t

1 W W e ³ e v t dt A e W L t t t j- V t 1 W e ³ V t e W dt A e W L

(13)

The following considerations apply. 1) this type of solution can be extended – by matrix analysis - to the higher order networks; 2) the obtained result represents a sinusoidal case generalisation; 3) it holds the closed form too: if time function v(t) satisfies the conditions required by FT [5], then the equation (13) can be performed by classical analysis approach.

capacitor: 1 2 1 2 2 °w H t 2 Cv t 2 CV t cos -V t ° ° w HV cos 2-V t ® 2 °p H t pw H t >pw HV @ cos -V t ° x °¯ -V t w HV sin 2-V t

(18) where wHV(t) is the “radial” component of the instantaneous dielectric stored energy. Equations (17) and (18) are perfectly singleminded between them.

5. THE ENERGY BALANCE The instantaneous power general formulation, observing that (Fig.4):

Taking into account the time function: yt Yt cos- y t

(14)

its r.m.s. value, with respect to the observation interval T, expressed in terms of rotating vector, can be defined as:

v t v* t ° v t 2 ® it i* t °it 2 ¯

(19)

results: Ye T

T

1 Y 2 t cos 2 -t dt T ³0

(15)

This quantity depends on the time variable T. The instantaneous power expression for the three perfect one-port components {R,L,C}, results respectively: resistance: p R Ri 2 t RI 2 t cos 2 - I t ° ® PVI t cos 2 - I t °P pR RI e2 ¯ R

(16)

where PVI(t) represents the “radial” component of instantaneous power pR(t). inductance: 1 2 1 2 2 °w P t 2 Li t 2 LI t cos - I t ° ° w PI t cos 2 - I t (17) ® 2 °p P t pw P t >pw PI t @ cos - I t ° x ° I t w PI t sin 2- I t ¯

168

p t

1 ^v i* v* i v i v* i*` 1 e^v i* v i` 4 2 (20)

Meanwhile the following instantaneous vector complex power (Fig.5): st vt i* t Vt It e jMt p o t jq o t ° °p o t V( t ) I f t Vd ( t ) I d ( t ) Vq ( t ) I q ( t ) ®q t Vt I t V ( t ) I ( t ) V ( t ) I ( t ) q q d d q ° o ° p o2 t q o2 t ¯st Vt It (21)

can be defined. This complex quantity, in the case of the three perfect one-port, can be expressed as

(a)

(b)

Fig.5. Rotating vector use in the energy balance analysis. (a) Voltage and current vectors, (b) complex power and its real and imaginary components.


follows:

we have:

° °s R vt i* t RI 2 t p VI t p o °s vt i* t Lp i t i* t pw PI t ° L (22) x ® ° j - I t 2 w PI t p o jq o °s vt i* t vt Cp v* t pw HV t ° C x ° ¯ j - V t 2 w HV t

st v t i t * p 0 jq 0

Therefore the instantaneous power results: pt

^

Taking in to account that: i i * It

p 02 q 02

2

Vt

`

1 e st 1 e j2 - I t 2 1 p o t 1 cos 2- I t q o t sin 2- I t 2 (23)

In particular, for R-L series circuit case, this power is expressed in the following way: 1 p VI t pw PI 1 cos 2- I t 2

(24)

x

- I t w PI t sin 2- I t

The following considerations apply. 1) Eq.(23) generalised the instantaneous power formula (5) typical of sinusoidal steady-state case. In fact, it considers the contributions due to both the magnitude and the phase variation. 2) The instantaneous real power po(t) is equal to the addition of the resistive contribution pVI and the time derivative of radial component of the stored energy. 3) The instantaneous imaginary power qo(t) is equal to the product of the instantaneous radial component of the energy multiplied by the instantaneous angular speed of the current or voltage vectors. This quantity can be regarded as an “instantaneous reactive power” associated to a general non-sinusoidal system. 4) The instantaneous powers po(t) and qo(t) are in agreement with the instantaneous real pp(t) and imaginary qp(t) Park power. As for the imaginary single phase power, it reproposes the same meaning of the sinusoidal case. With reference, for example, to Fig.6 network, in terms of general rotating vectors, observing that: v2

R i Lp i

(27)

2

we have: 'pt Ri 2 t RIt cos 2 - I t 2

Rcos 2 - I t Rcos 2 - I t V t 2

pt

(26)

x 1 2 2 2 RIt p LIt j - I LIt 2

(25)

p 02 t q 02 t

(28)

V 2 t

p 02 t

Rcos 2 - I t V t 2

q 02 t

The presence of the imaginary term q0 increases the amplitude of the current rotating vector. Than, as a result of this aspect, it Fig.6. The power transmission can be considered in terms of rotating vector: the as an index of the imaginary power increases the amplitude of the line current instantaneous rotating vector. network reactivity. In other words, the presence of the imaginary power, in agreement with Park approach, is directly related to the occurrence of events that cause an increment in instantaneous line losses under the same voltages and the same instantaneous consumption from the load. 5.1. The complex power invariance If [A] is the incidence matrix of a network, observing that: >A@ >i*@ 0 ® ¯>v@ >A@t >u@

(29)

we obtain: s

>v@t >i*@ >>A@t >u@@t >i*@ >u@ >A@ >i*@

0 (30)

The rotating vector theory defines a complex power in agreement with the Tellegen theorem.


169

of the instantaneous power absorbed by the {C,G} 6. AN EXAMPLE The example proposed in the present paper is based on the dynamical analysis of the linear timeinvariant network represented in Fig.7. Assuming at this regard: Fig.7. The linear, time-invariant, second order network analysed in the proposed example.

ª R / L 1 / L º ' >A@ °« G / C»¼ °¬ 1 / C °ª1 / L º ' ®« » >B@ °¬ 0 ¼ °ª i L t º ' °« » >x t @ ¯¬ v C t ¼

(31)

pt v C t i L t

the following second order state equation is obtained: p>x t @ ® ¯x0

p G t G v t

(35)

Fig.8. adopted input.

The voltage

(32)

0

and in a conservative one:

The voltage input waveform e(t) is represented in Fig.8. By Fourier series method, its analytical representation assumes the following approximate expression: et #

(34)

This power can be decomposed in a dissipative component: 2 C

>A@ >xt @ >B@ et

> @ >x0 @

one port:

4E 2S 1 6S 1 10 S ½ t cos t¾ t cos ®cos S ¯ T 3 T 5 T ¿

4E 1 1 ½ e®e jZt e j3Zt e j5Zt ¾ S 3 5 ¯ ¿

p C t pt p G t

(36)

The evolution of the obtained functions {iL(t),vC(t)} and {p(t),pG(t),pC(t)} is represented, in a cartesian form, by the diagrams of Fig.9. If the rotating vectors general method is adopted, the network of Fig.7 assumes the configuration represented in Fig.10. In this case the state variables, expressed in an exponential form, became:

(33) The solution of the state equation, expressed in a close form in time-domain, allows the calculation 1.5

0.14

iL vC

p pG pC

0.12

1 0.1

0.5

iL vC

0.08 0.06

0

0.04

-0.5 0.02 0

-1

-0.02

-1.5 0

0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02

t

(a)

-0.04 0

0.002 0.004 0.006 0.008

0.01

0.012 0.014 0.016 0.018

0.02

t

(b) Fig.9. Cartesian representation of the obtained waveforms. (a) iL and vC waveforms; (b) waveform of the related p, pG, pC power.

170


8. REFERENCES

Fig.10. The analysed network expressed in rotating vectors domain.

^ ^

` `

i L t e I L t e j-L t ® j- C t ¯v C t e VC t e

e^i L t ` e^v C t `

(37)

The corresponding state variables and complex power components are represented in Fig.11. 7. CONCLUSIONS In the present paper an extension, in general terms of rotating vectors, of the usual phasor algebra has been proposed. The approach, based on the Fourier transform, can be applied to a vast area of waveforms and regimes. With reference to its statement, represented by vectors in time-domain, this approach can be regarded as an intermediate stage between the frequency domain analysis (Budeanu) and the time domain analysis (Fryze). The energy analysis derivable by this approach confirms this possibility: the power balance generalises the classic sinusoidal one. The use of a vector algebra makes a natural step the definition of the instantaneous complex power. By this approach an imaginary power, formally in agreement with the Park theory, can be also obtained. The proposed example confirms this conceptual path: at the present the close examination of a great many of applications becomes a fixed course.

2.5

1. Emanuel A.E.: Power in non sinusoidal situations a review of definitions and physical meaning. IEEE Trans. on Power Delivery, Vol.5, no. 3, July, 1990, pp.1377-1385 2. Ferrero A., Morando A.P., Ottoboni R., Superti Furga G.: On the meaning of the Park power components in three-phase system under non sinusoidal conditions. ETEP Eur. Trans. on Electr. Power, vol.3, no. 1, January/February 1993, pp.33-43. 3. Ferrero A., S. Leva, Morando A.P.: An approach to the non-active power concept in terms of the Poynting-Park vector. 5th International Workshop on Power Definitions and Measurements under Nonsinusoidal Conditions, Milano, Italy, Oct. 16 – 18, 2000 4. Morando A.P., Superti Furga G.: Park Power in non sinusoidal three-phase system. l’Energia Elettrica, LXVII, no.2, 1990, pp.6570, In Italian 5. Skilling H.: Electric Networks. McGraw Hill, New York, 1970

0.02

iL vC

2

0

1.5 -0.02 1 -0.04

0.5

I m0

-0.06

I ms

-0.5

-0.08

-1 -0.1 -1.5 -0.12

-2 -2.5 -1.5

-1

-0.5

0

0.5

1

1.5

-0.14 0

0.05

0.1

0.15

0.2

Re

(a)

0.25

0.3

0.35

0.4

R es

(b)

Fig.11. Rotating vector rapresentation. (a) state variable iL and vc; (b) complex power s.


171

Sonia Leva received the M.S. degree (1997) and the PhD degree (2001) in Electrical Engineering from the “Politecnico di Milano”, Milano, Italy. Since 1999 she is Assistant Professor in Elettrotecnica at the same department. Her current research interests are concerned with the electromagnetic compatibility, the power quality and the foundation of electromagnetic theory of electric network. Dr. Leva is a member of IEEE, Mailing address: Sonia Leva Dipartimento di Elettrotecnica – Politecnico di Milano Piazza Leonardo da Vinci, 32 20133 Milano – Italy Phone: +39-02-23993709, Fax: +39-02-23993703 e-mail: [email protected]

172

Adriano Paolo Morando received the M.S. degree in Electrical Engineering from the “Politecnico di Milano”, Milano, Italy. From 1984 to 1989 he was with ASEA Brown Boveri, where he was concerned with AC drives for electrical traction. Since 1989 he is Assistant Professor of Elettrotecnica at the Electrical Engineering Department of the “Politecnico di Milano”. His current research interests are concerned with the electromagnetic compatibility, the power quality and the foundation of electromagnetic theory of electric network. Mailing address: Adriano Paolo Morando Dipartimento di Elettrotecnica – Politecnico di Milano Piazza Leonardo da Vinci, 32 20133 Milano – Italy Phone: +39-02-23993729, Fax: +39-02-23993703 e-mail: [email protected]





MODELLING OF PARAMETERS OF THE ELECTRIC POWER QUALITY CHARACTERIZING NON-SINUSOIDAL AND ASYMMETRY OF VOLTAGE Nataly V. SAVINA Amur State University Blagovestchensk (Russia)

Abstract - Low quality of the electric power reduces functional reliability of functioning any energy company. For control of the electric power quality the method of modelling of parameters of the electric power quality (PEPQ), which are characterizing nonsinusoidal and asymmetry of voltage, was designed. The main analytical expressions, which are describing models of parameters of quality of the electrical power, were brought.

At present many power companies work in conditions of the low quality of the electric power. Main sources of its distortion are non-linear electrical receivers, single-phase electrical loads and three- phase users of the electric power, working in the asymmetrical mode, and users with sharply variable working modes. Low quality of the electric power, and in particular non-sinusoidal and asymmetry of voltage, is negative phenomena for any power companies, because reduces its functional reliability and efficiency of functioning. At functioning of the electric power systems in low quality of the electric power, as well as in electrical networks of users of all levels of the voltage and all technological structures, is observed observable growing of losses of the electrical power, sharp reduction of the lifetime of electrical apparatuses and cable lines, reduction of function reliability of electrical supply, failures and false operations of the relay-type protection and automation. At dispatched and technological control of power system and at functioning automation system of control the information is distorted and gets lost, becomes an incorrect account of the electrical power.

In ditto time an electric power, as goods, subjects to obligatory certification. Consequently, power companies must ensure a quality delivered user of the electrical power. Considering that cost of the electric power in the crude structure of the prime cost of the product great, and observance of respective rates and requirements on the quality of the electrical power for enterprises sometimes is not economic advantageously, necessary monitoring and control of quality of electric power. The main of present paper is a development of the strategy of the determination of regularities of changing the parameters of the quality of the electric power in power companies, which are characterizing non-sinusoidal and asymmetry of voltage. Main difference in the plan of the electrical power quality between power companies and systems of electrical supply of enterprises is an ensemble of sources of garbling of its quality, portioned both during, and territorial. This explains instability of parameters of the quality of the electric power. So PEPQ represent itself casual processes. Consequently, theory of casual functions and method of the planning of the experiment for the determination of regularities of their changing are used. Experimental studies PEPQ for revealing the regularities of their change must be realized on the following plan: determination of the main purpose of the study; choice of probabilistic features required for achievements supplied to purposes; evaluation degrees of the influence of casual factors, output on need of their account; evaluation degrees of the influence of changing the conditions when writing a different realization (revealing ergodicity of process); evaluation degrees of the


173

influence of changing the conditions when writing of each realization (determination stationary process); choice necessary and sufficient accuracy of studies, accepted probability not excess of possible inaccuracy; determination of the type of the law of sharing probability; the analyse correlation functions and spectral of density of the process; determination necessary and sufficient amount of the realization for ensuring given accuracy of the study, step of the slicing on the level and sampling on a time. For revealing the regularities of change PEPQ it is necessary to use a daily realization. Herewith required amount of the realization for the reception of reliable information is defined by duration of the observation on allowing abilities of the power spectrum, i.e. difference of two nearby peaks of this spectrum. For power companies, in which main source of garbling a quality of the electric power is electrified rail-freight traffic (electrical traction of alternating current), system of primary electrical supply of tractive substations is component part of district electrical networks; so it is necessary to research a quality of the electric power on buses 110 - 220 kV of tractive substations; working at the total tractive network 27,5 kV. For power companies, in which source of garbling a quality of the electric power are nonlinear loads of the industrial enterprises, quality of the electric power should define on the border of balance accessories with the power system. As a realization in the same way should take day. How multiple studies have shown, duration of one realization in power companies with the prevalence of the tractive load varies within from 3 hours before 18 hours. In power companies with the prevalence industrial non-linear load (rectifiers, welding, arc steel-melting stoves) - within from 2 hours before 12 hours. For the determination of the time of questioning the automatic facilities of the measurement PEPQ it is necessary to know a step of the slicing:

't d

2 2H , S n0

(1)

where n0 - an average of zeroes of per unit time, which defined by number of crossing by the casual process of factor of the quality of the electric power of its population mean; H - maximum relative inaccuracy of the mathematical description of the casual process. Step of the slicing for the study PEPQ in power companies with the prevalence removable nonlinear and asymmetrical load varies within from 5 minutes before 20 minutes. Definitively optimum 174

time of questioning the facilities of measurements PEPQ must be defined with provision for technical possibilities of the system of supervision of the electric power quality. The following stage of modelling of factors of the quality of the electric power is a determination of point and interval evaluations of numeric features: population mean, average square-law deflection, asymmetries, kurtosis on well-known expressions, on /1/ for instance. For the determination of analytical expressions, which are describing casual process of changing a factor of the quality of the electric power, for the reason their further use for tasks of control and supervision of the electric power quality it is necessary to define stationary and ergodicity of the process. For this is conducted analyse normalized correlation functions of each daily realization of modelling parameter of the quality of the electric power. Correlation function of the process characterizes in the quantitative form to probabilistic communications between ordinates of the process (its sections). If correlation function of the valance, the process has brightly denominated dependency between sections on a time and correlation communications disappear only under significant differences between them. Limiting case is that, under which correlation function does not fade for all period of observations, i.e. process is prototyped "white noise". In this case more suitable to use spectral density. Quick fading process is indicative of its sharply oscillatory nature with unceasing and disorderly fluctuations. To stationary processes can be referred that PEPQ, in which functions of sharing of consequent their values f PEPQ ( t1 W ), f PEPQ ( t2 W ),

f PEPQ ( tn W ) do not hang from a time. Here W - an interval of sampling. Moment and correlation functions of the first-order have one and same type for any time, i.e. population mean and dispersion are constant in time. Correlation function depends only from the gap of the time between first and second section of the casual process, i.e. W t2 t1 . If population mean and density of sharing probability are changed depending on a time, PEPQ must be described non stationary by the casual process that highly obstructs its modelling. Important characteristic of some casual processes is ergodicity, since presence of ergodicity allows to go over to averaging on a time of one realization. If considered PEPQ inherent ergodicity, for it sufficiently episodic supervision in the quarter once. Electrical Power Quality and Utilisation

Sufficient and necessary term of ergodicity for stationary processes at the determination of firstorder features is condition, under which function of the correlation goes to zero: T

lim

1 R(W )dW T ³0

0,

(2)

where T - a time of studies PEPQ; R(W ) - correlation function normalized. "Generalized " property of ergodicity, i.e. possibility to judge on non-stationary process on one realization, is observed beside non stationary processes of the type X ( t ) A( t ) X S ( t ), where X ( t ) - non stationary casual process of change PEPQ; A( t ) - deterministic or casual function of the time; X S ( t ) - the stationary casual process selected. Necessary and sufficient term of existence beside non stationary process "generalized" characteristics of ergodicity is

lim D ª¬C M (T ) º¼ where C M (T )

1 T

0,

(3)

T 2

³ M > X ( t )@ dt -

an evaluation

T 2

of the function M > X ( t )@ defined by the term, which defines a dependency of the sought statistical feature.

D ª¬ C M (T ) º¼ - dispersion of this evaluation. Study of casual processes of changing the factors of the quality of the electric power, characterizing non-sinusoidal and asymmetry of voltage, has allowed to separate them on 4 groups: stationary ergodicity, stationary non ergodicity, non stationary and non stationary with a generalize ergodicity. 1) Stationary ergodicity process of changing a factor of distortion sinusoidal crooked voltage U, factors asymmetry on inverse 2U and zero sequences 0U characteristic of elements of electrical network power companies, to which connected non-linear or asymmetrical load, main level and nature of change which during the day little depends on changing the conditions of functioning (working) specific electrical receivers and from the location of the day on axis of the time within a year. It differs an unlimited decrease a

modulo correlation functions with the growing of the argument. For the analyse to sufficiently one realization. In correlation functions often is present a periodic component, which possible select. The given process is observed in points of the connection to network or in points of the total connecting with the uniform specific load. For instance, for tractive of alternating current -, as a rule, buses of the high voltage of tractive substations, finding inwardly power company, i.e. not border with elements of network adjacent power companies. 2) Stationary non-ergodicity process specified above PEPQ inherent elements of the network, in which main level of the specific load and nature of its change constant only under determined conditions of functioning. Such process differs non identical of density of sharing of each realization, computable averaging on a time (non-ergodicity), but regardless of a time of density of distribution, computable averaged on the ensemble of the realization for different moments of the time t1, t2, … tn (stationary). Such process of change PEPQ characteristic of elements, containing specific uniform load and showing adjacent with other power companies, in which quality of the electric power also low. 3) Non stationary processes of change PEPQ are observed in elements, which bordered with other power companies, having lumpy load, and in elements, in which main level and nature of changing a specific load depends and from a time and from conditions of functioning as electrical receivers themselves, so and networks as a whole. 4) Non stationary processes with "generalised" ergodicity characteristic of elements of network, in which main level of the specific load and nature of its change greatly depends on a time, but stays to be constant when changing the conditions of functioning. Density of sharing probability computable at different moments of the time, not alike (nonstationary), but density of sharing a realization, computable averaging on a time, alike (generalised ergodicity). Non-stationary is caused by presence in the casual process under investigation PEPQ values forming, put into garbling a quality of the electrical power uniform on the mode specific electrical receivers, receiving power supply from networks of adjacent power systems. The given forming possible select, is described it random quantity all considered above processes are represented by normalized correlation function, NCF, on fig.1-4.


175

1 0,8 0,6

R(W)

0,4 0,2 0 -0,2 -0,4 0

20

40

60

80

100

120

min

W

R (W )

e

0,38 W

cos 0, 425W

Fig.1 NCF of the stationary ergodicity process KU

1 0,8 0,6

R(W)

0,4 0,2 0 -0,2 -0,4 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 u10 min

W

R (W )

e

0,99 W

0,56 cos(0,03W 0,5S )

Fig.2 NCF of the stationary non-ergodicity process KU

The following step in modelling PEPQ is spectral analyse - i.e. study an spectral of density of the process. Such approach allows to select an amount and frequency hidden periodic forming in the casual process that vastly simplifies an approximation normalized corelian functions. If in spectral of density is present one obviously denominated peak, process has not periodic or harmonic component.

176

The question is decomposition of the function of changing under investigation PEPQ during on frequencies of the spectrum. But if in spectral of density there is several obviously denominated peaks, in the process there are hidden to periodicity, moreover number of peaks quantifies harmonic functions, falling into approximating expressions for NCF.


1 0,8 0,6

R(W)

0,4 0,2 0

-0,2 -0,4 -0,6 0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

W

R (W )

0,35e

0,62 W

u10 min

cos(0,07W 0,17)

Fig.3 NCF of the non-stationary process with "generalised" ergodicity K2U 1 0,8 0,6

R(W)

0,4 0,2 0

-0,2 -0,4

u10 min

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

W

R (W )

e

0,3W

S· S· § § 0, 2 cos ¨ 0,06W ¸ 0,05cos ¨ 0,14W ¸ 2¹ 2¹ © ©

Fig.4 NCF of the non-stationary process K2U Under variety of change PEPQ, characterizing non-sinusoidal and asymmetry of voltage, many from them inherent of total characters, but all of these feel an influence of casual factors, have a connection with the technological process specific electrical receivers, with non-deterministic by conditions of its designing, as well as with the probabilistic character of modes of considered process and adjacency with power company. Main law of sharing probability of sections of stationary casual processes - Gauss distribution (fig.5). So their possible is refer to Winer casual processes. Non-stationary processes distinctive are described of density of sharing probability statistical beside. For tasks of governing quality of electric power are reasonable to use differentiate functions, so at

approximations NCF for the reception of models under investigation PEPQ must be used exponential-cosine of the dependency. How a multiple analyse has shown, correlation function of ergodicity processes U, 2U, 0U are described by analytical functions of the following type: in the absence of hidden periodicity (one peak in functions an spectral of density S())

R(W )

e

D W

; R (W )

e

aD 2 W

2

;

(4)

at presence of hiding periodicity:

R (W )

e

R (W )

e

R (W )

e

D W

cos Z 0 W ;

aD 2 W D W

2

sin Z 0 W ;

(5)

(b cos Z 0 W c sin Z 0 W ).


177

0,5 0,4 0,3

f(KU) 0,2 0,1 0 0

0,07

1,32 1,35

1,5

1,6

2

2,5

3

3,94

KU Fig.5 Histogram of KU For non-ergodicity of processes are typical following expressions for NCF:

R(W )

A(W ) e

D W

; R(W )

R(W )

A(W ) e

D W

cos Z0 W ;

R(W )

A(W ) e

aD 2 W

R(W )

A(W ) e

D W

R(W )

2S cG W .

2

A(W ) e

aD 2 W

2

;

sin Z0 W ;

(6)

(b cos Z0 W c sin Z0 W );

where ,, - constant values, got as result statistical processing. - factor of fading; 0 - own frequency of the spectrum, defined on the graph spectral of density by the generally accepted way, on /2/ for instance. () - casual function, or collection of casual functions, are brought in the considered process of specific electrical receivers of belong different productions or (and) connected to networks of adjacent power systems. At the analyse of non-stationary non-ergodicity casual processes PEPQ, for the reason allocations of casual function, put into the considered process other specific electrical receivers, which not connected to the point of the total joining in the considered group of loads, reasonable use a theory of fuzzy sets. Power spectrum of processes of change PEPQ is conclude in the range from 0 before 12 radian/hour. FINDINGS. 1. The strategy of modelling PEPQ, characterizing non-sinusoidal and asymmetry of voltage, for governing quality of electric power in power companies, based on probabilistic-spectral presentation of process of their change is offered. 2. The analytical expressions, describing correlation functions of the process of change PEPQ on the base of their parameters are determined. 178

3. The categorization of casual processes of change PEPQ in power companies on stationary and ergodicity, based on experimental studies with provision for physical conditions of shaping PEPQ in the element of the load at the imposition and differences of "mixed" high harmonicas, inverse and zero sequences of the tension, generated different specific electrical receivers are brought. REFERENCES 1. Ventcel E. S. Theory of probabilities. .: High school, 1998 - 576 p. 2. Sveshnikov A.A.: Applied methods of the theory of casual functions - .: Science, 1968 – 463 p.

Docent Nataly V. Savina was born in 1956 in t.Kazimagomed, Azerbaijan. She received the candidate of science degree at 1983 years in Dnepropetrovsk mining institute, Ukraine. In 1990 her was conferred rank of docent of the chair of electrical supply of industrial enterprises. At present – chief of the chair of Power engineering of the Amur State University. Her areas of interests include the quality of the electric power in power systems.

Mailing address: Nataly V. Savina Amur State University Chair of Power engineering 21 Ignatevskoe shosse, 675027 Blagovestchensk Amur region RUSSIA phone: (4162) 35-05-56 e-mail: [email protected] [email protected]





CONDITIONS OF STEADY-STATE STABILITY OF CONTROLLABLE AC TRANSMISSION LINES

Alexander KOZLOV Vadim LOGINOV Amur State University Blagoveschensk (Russia)

Abstract – The leading indexes of work of a transmission line are defined for the longlasting normal regimes and the regimes after damage. However, except these regimes on undamaged line so-called “special” regimes can exist. As a rule, these regimes are short. For examples: the transmission line, working with small load; self-excitation of generators, which works at transmission line without load, etc. The new principles of management regimes of electrical power possible to realize, using FACTS-technology. But realization the desired controlling influence can require change the parameters FACTS-regulators in very greater limits. As a result the equivalent parameters flexible AC transmission lines can fall into zone of border values under the provisions of selfexcitations, or other “special” regimes. The zones of steady-state stability and the corresponding range of the parameters of a controllable AC transmission lines are determined. Keywords: controllable AC transmission line, limits of regulation, generator, self-excitation, investigation. Maximum using the AC transmission lines to the account of increasing reception capacity and mode control of their functioning always be of interest for the electrical engineers. Insertions of the direct current (IDC) ensure an electromagnetic sectioning of the system and conservation given power flow on communications. Electromechanic insertions of alternating current (EIAC), except this, ensure a get fat localization of electromagnetic and electromechanic connecting processes within the framework of each of united lectric power systems (EPS). But as far as IDC and EIAC are

high-priced, settle problems of control but leave for frames increasing of reception capacity of the high-voltage transmission line (HVTL) - justified searching and other ways of the decision of the problem - an use for making operated HVTL controllable compensatory devices. In one or another form these variants are discussed sufficiently often, more exactly - as soon as appear perspective offers and possibilities to their technical realization. So was with devices of longitudal capacitive compensations, controllable shunting reactors, sources of reactive powers, multifunction and self-compensating HVTL, phase-shifter devices, drives of the electric power. Some ideas realized, others or did not find a broad using, or were remitted before best temporary. Progress in the development of the technology of semiconductors has allowed at the beginning initially 90-h years to proceed with the fabrication of latched thyristors and high-speed diodes, which scale covers tensions from 2500 before 6000 V and hanged up current from 1500 before 4000 A. As a result the electrotechnic companies of the world sharply actuate a development of the new class of powerful converters of the tension and different steady-state devices on their base. The lines equipped by these devices, have got FACTS name (Flexible AC Transmission Systems) [4]. Under flexible control of the regimes functioning of HVTL understand a possibility of change transferred on her active powers, as well as accompanying reactive powers to the account of the influence on values of voltages, impedance and corner of the transmission. As far as cost of equipment FACTS forms 10-30% cost of equipping the transfers and insertions of the direct current, idea of flexible control HVTL becomes attractive and from economic positions.


179

At first thought, using the flexible AC transmission lines make enables: to increase an active power flow on transmission lines; to ensure forced sharing a power in closed network (including power flow reverse); to raise stability of the systems. However all these advantages require a careful test. Realization of desired control influences can require a changing the parameters of the regulator, stated in lines, in very greater limits. As a result equivalent parameters of the controllable highvoltage transmission line can fall into the area of frontier values under the provisions of selfexcitations, self-swings or other "particular" mode. So main will take aim functioning is a searching for the algorithm of discovery "areas of frontier modes" - region of changing the parameters of regulators, enclosed inline, at the hit in which possible breach (or observable change) of the state of working EPS. In given item this searching for is executed for controllable transmission lines with two circuits, with the escalated mutual influence of circuits, got naming "self-compensating transmission line" (SCTL). Studies such transmission lines are conducted in Russia with the beginning 1970-h years [1]. Constructive the SCTL represents itself the transmission line with two circuits with phases, which are drawing near together (for the reinforcement of the mutual influence); at least in one of the circuits the phaseshifter device was installed, ensuring regulation of the corner T of the shift between vectors of voltages phases. In general event the circuits of the line can be executed on different nominal voltages. Study of changing the equivalent parameters of circuits at the regulation of the corner T has allowed to reveal, that for some combinations of voltages of the circuits, when the line functioning (working) with small T , equivalent reactive resistance of the circuit of the undermost voltage (circuit UV) becomes negative. The generator, connected to this circuit, in this case works to the a)

equivalent capacitive load and, consequently, in the system, containing circuit UV, a self-excitation can develop. For the analytical decision of the task to necessary dependencies < eqv I ck ,T and reqv f ck ,T ,

which can be received when using equivalent constants of the eight-pole, which the selfcompensating transmission line are substitution. Use a known formula, allowing define a value of the resistance of the line through factors of the quadrupole: Z SCTL A12eqv where

A12eqv

- a factor of the equivalent

quadrupole, substituting corresponding to circuit SCTL or line as a whole. In that event, when circuits work separately, each circuit is substituted by the equivalent quadrupole. Border parameters are voltages and currents transmitting and receiving systems: U Cc 1 , U Ccc1 , U , U , I c , Icc , I , I , - a fig. 1, b. C2

C3

1

1

C2

C3

Communication between parameters of the mode beginning and end of each circuit is expressed by known equations:

U Cc 1 I1c

c eqv A11 Ac 21eqv

c eqv U C 2 A12 ; c eqv IC 2 A 22

(1)

U Cc 2 I1cc

cc eqv A11 cc eqv A 21

cc eqv U C 3 A12 , cc eqv I> 3 A 22

(2)

where

c , Aeqv

cc Aeqv

-

factors

of

equivalent

quadrupoles, substituting, accordingly, upper and lower circuits SCTL. On the other hand, presenting the self-compensating transmission line by the eight-pole: b)

Fig. 1. Rated scheme (a) and vector diagram of voltages (b) of circuits SCTL under their functioning on own loads.

180


K T 1

U Cc 1 I1c U Ccc 2 I1cc

0

0 1 Kˆ T 1

0 0

0 0

U C 2 I C2 U

0 0 KT 3

2

T2

T1

4

T2

T3

A1eqv A 2eqv A3eqv A 4 eqv

where

4

B1 B 2 B3 B 4

B1eqv B 2eqv B3eqv B 4 eqv

C1eqv C 2 eqv C3eqv C 4 eqv

Ai , Bi , C i , D i - constant of the six-pole, which substituting SCTL; K Ti - ideal transformations: K T 1 K T 3

U Cc 1 ; U1@ U Ccc1 ; U

K T 2

K T 4

1 B 1eqv D 3eqv D 1eqv B 3eqv

C1 C 2 C 3 C 4

0 D1 K T 2 0 1 Kˆ T 2 D2 0 0 D 3 0 0 D 4

U1[ U > 2 U 2 [ U

0

0

0 KT 4

0 0 1 Kˆ T 4

0

D1 K T 1 / Kˆ T 4 U C 2 D 2 /( Kˆ T 1 Kˆ T 4 ) IC 2 = D 3 K T 3 / Kˆ T 4 U C 3 D /( Kˆ Kˆ ) I T3

4

D1eqv U C 2 D 2eqv IC 2 , D 3 U C 3 D 4eqv IC 3

C3

T4

(3)

Communication between factors of equivalent quadrupoles - (1, 2) and eight-pole - (3) - is fixed as follows. Dependency (1) in Y form:

Y11c eqv Y21c eqv

I1c I> 2

; .

Y12c eqv U Cc 1 . Y22c eqv U C 2

(4)

For the six-pole, deciding ( 3 ) for currents, will get:

>3

2@

I 1c I 1cc I C 2 I

A1 A 2 A3 A

B1 K T 1 / Kˆ T 2 C1 K T 1 K T 4 B 2 /( Kˆ T 1 Kˆ T 2 ) C 2 K T 4 / Kˆ T 1 B3 K T 3 / Kˆ T 2 C 3 K T 3 K T 4 B 4 /( Kˆ T 2 Kˆ T 3 ) C 4 K T 4 / Kˆ T 3

A3 K T 2 K T 3 A K / Kˆ

C3

0 1 Kˆ T 3

0

A1 K T 1 K T 2 A K / Kˆ

IC 3

0 0

u

C3

A1eqv D 3eqv D1eqv A3eqv B A A B u 1eqv 3eqv 1eqv 3eqv D3eqv B 3eqv

D 3eqvC1eqv C 3eqv D1eqv B1eqvC 3eqv C1eqv B3eqv D1eqv B 1eqv

D 3eqv B3eqv D 1eqv A3eqv A1eqv D 3eqv A B B A 1eqv

3eqv

1eqv

3eqv

U Cc 1 U cc u C1 U C 2 U C3

D1eqv B1eqv u D1eqvC 3eqv C1eqv D 3eqv B C C B 3eqv 1eqv

Y11 Y21 Y31 Y 41


3eqv 1eqv

Y12 Y13 Y22 Y23 Y32 Y33 Y42 Y43

Y14 U Cc 1 Y24 U Ccc1 u , Y34 U C 2 Y44 U C 3

(5)

181

Comparison ( 4 ) and ( 5 ) allows to write: U cc Y11 Y12 C1 ; U Cc 1 U Y13 Y14 C 3 ; U

Y11c eqv Y12c eqv

as the characteristics transmission line influences cut-in of inline additional compensatory devices. Simplify ( 6 ) and ( 7 ) with following admissions: - Modulas of voltages on terminals of circuits are U1H U1K U1 between itself, i.e. and U 2 H

C2

Y21c eqv Y22c eqv

Factor

§ U cc · ¨¨ Y31 Y32 C1 ¸¸ ; U Cc 1 ¹ © § U · ¨¨ Y33 Y34 C 3 ¸¸ . U C 2 ¹ ©

c eqv A12

of

the

-

equivalent

quadrupole, substituting upper circuit SCTL: c eqv A12

Z HVeqv

1 Y21c eqv

1

U cc Y31 Y32 C1 U Cc 1

Replaceable Y31 and Y32 expressions, containing equivalent factors of the eightpole - ( 5 ), after transformations will get: Z HVeqv

c eqv A12

-

cc eqv A12

B1 D 3 D 1 B 3 U c Kˆ Kˆ B1 T 4 B 3 T 4 C1 K T 3 K T 1 U Ccc 1

(7) Formulas ( 6 ), ( 7 ) allow to define a value Z eqv and, consequently, Pmax for corresponding to circuits of the line, under given values of voltages. As far as formulas contains a general constants of the eight-pole, possible track,

Z HVeqv

j

182

( r1 jx1 )( r2 jx 2 ) ( jx12 ) 2 r2 jx 2 jx12 k e

jT

k e jT .

Under these admissions K T 1 K T 2 1 , 1 jT K T 3 e , KT 4 k e jT . k Formulas ( 6 ) and ( 7 ) are converted to the following type:

(6)

ZUVeqv

U 2 ; attitude U1H / U 2 H

In transferring system - C1 - voltages U Cc 1 and U Ccc1 are in size and coincide on the phase. This admission allows to fix a value T at the beginning initially SCTL and hereunder reduce a scheme a SCTL, which circuits enclosed in different branches of network - a fig. 1, to the rated scheme with electrical will at the beginning of initially line. Voltages at the beginning initially and end of upper circuit comply with voltages transferring and receiving systems, i.e. U Cc 1 U1H , U1K U C 2 .

B1 D 3 D 1 B 3 U cc Kˆ Kˆ D 3 T 2 D 1 T 2 C1 K T 1 K T 3 U Cc 1

For circuit UV, similarly:

U 2 K

Z HVeqv

B1D 3 D1B3 ; D 3 D1 k e jT

(8)

ZUVeqv

1 B1D 3 D1B3 . k B1 k B3 e jT

(9)

In that event, when length of the SCTL does not exceed 500 km, expressions (8, 9) possible vastly simplify, presenting circuits of the line only active and reactive resistances with mutual inductance will and neglecting conductivities of circuits. Falling into (8, 9) constant of the eight-pole, substituting line, are written as follows: B1 r1 jx1 r01 l jZL01 l ; D r jx r l jZL l ; 3

2

2

02

02

B3 D1 jx12 jZM 012 l , where l - a length of the line. Then

2 ( r1 r2 x1 x 2 x12 )( r2 x12 k sin T ) ( r1 x 2 r2 x1 )( x 2 x12 k cos T ) 2 r22 x 22 x12 k 2 2r2 x12 k sin T 2 x 2 x12 k cos T

2 ( x1 x 2 r1r2 x12 ) ( x 2 x12 k cos T ) ( r1 x 2 r2 x1 ) ( r2 x12 k sin T ) 2 r22 x 22 x12 k 2 2r2 x12 k sin T 2 x 2 x12 k cos T

rHVeqv jxHVeqv ;

(10)


ZUVeqv

j

( r1 jx1 )( r2 jx2 ) ( jx12 )2 1 k k ( r1 jx1 ) jx12 e jT

2 ( r1r2 x1x2 x12 )( kr1 x12 sin T ) ( r1x2 r2 x1 )( kx1 x12 cos T ) 1 2 2 2 k k ( r1 x12 ) x12 2r1x12 k sin T 2 x1x12 k cos T

2 ( x1 x 2 r1 r2 x12 ) ( kx1 x12 cos T ) ( r1 x 2 r2 x1 ) ( kr1 x12 sin T ) 1 2 2 2 k 2r1 x12 k sin T 2 x1 x12 k cos T k ( r1 x12 ) x12

Self-excitation of the generator appears when running on the winding of stator capacitive current, overtaking on the phase voltage of the generator. In [3] determined term, approximate characterizing possibility beginning of the process: a). Synchronous generator with salient-poles: - region of the synchronous self-excitation ("area I"): xd6 xq6 ; (12) xd6 ! xeqv ! xq6 ; reqv d 2 - region of the asynchronous self-excitation ("area II"): xq6 xdc 6 ; (13) xq6 ! xeqv ! xdc 6 ; reqv d 2 - an area III asynchronous self-excitation dependent from parameters of damping windings of the machine. For arising a self-excitation in this area it is necessary presence of slide between the field of the stator and the field of the rotor. Borders of the area III are defined at the presentation of the synchronous machine by frequency features. Preliminary test will conduct at the admission that Z Z 0 const . Term of arising a self-excitation in this case: x xcd6 ;

In formulas (12) - (16) accepted following indications: reqv - equivalent active resistance of the SCTL;

(11)

of the corner T , close to 0 o in that event only, if attitude of voltages of circuits k U 2 /U1 turns out to be less critical k Lcc x12 / x1 . Test will conduct for the scheme, shown on the fig. 1 - circuits electrically unconnected, each circuit works on the block scheme "generator - a transformer - a circuit SCTL - a system of the big power", but corner T is fixed at the beginning of initially line - between vectors of voltages U1H and U 2 H . Terms (12) – (16) in general type: x ! xeqv ! x ; (17)

reqv d

x x . 2

(18)

Value T , under which can appear a self-excitation, will define, substituting in (17, 18) of the value reqv and xeqv from (11) and deciding inequalities comparatively T . Term x ! xeqv is executed, if

T c ! 2arctg where A

A A2 B 2 C 2 , BC

(19)

(r1 x2 r2 x1 ) x12 x 2r1 x12 k 2 ; 2 ( x1 x2 x12 ) kx1 kx2 r12

B

>

@

2 x k k 2 (r12 x12 ) x12 ;

(14)

b). Synchronous generator without salient-poles: - area II: x xdc 6 xd6 ! xeqv ! xdc 6 ; reqv d d6 ; (15) 2 - III area: xeqv xcd6 . (16)

rUVeqv jxUVeqv

C

2 ( x1 x2 x12 ) x12 r1r2 x12 x 2 x1 x12 k 2 .

Term x eqv ! x is kept, if

T c 2arctg where Ac

Ac ( Ac) 2 ( Bc) 2 (C c) 2 , Bc C c

(20)

(r1 x2 r2 x1 ) x12 x 2r1 x12 k 2 ;

2 Bc ( x1 x2 x12 ) kx1 kx2 r12

>

@

2 x k k 2 (r12 x12 ) x12 ;

x eqv - an equivalent reactive resistance of the SCTL, having capacitive nature; x d6 , x q6 , x cd6 - reactive resistances of the synchronous machine (with provision for resistances of the transformer). Changing a sign of the equivalent reactive resistance of the circuit UV SCTL occurs at values

2 C c ( x1 x2 x12 ) x12 r1r2 x12 x 2 x1 x12 k 2 . Inequality ( 18 ) is kept, if

T cc d 2arctg

Acc ( Acc) 2 ( Bcc) 2 (C cc) 2 , Bcc C cc


(21)

183

where 2 Acc ( x1 x2 x12 ) x12 x12 r1r2 r1 x12 k 2 ( x x ) x x 2 Bcc k k 2 (r12 x12 ) x12 ; 2 2 (r1r2 x12 ) kr1 kr2 x12 ;

>

C cc

a)

@

( x x ) k 2 x1 x12 (r1 x2 r2 x1 ) x12 .

Substituting in formulas (19) - (21) corresponding to values x and x , possible define values T , characterizing borders of areas of the selfexcitation. Changing a sign of UVeqv is accompanied by increasing an equivalent active resistance rUVeqv . This circumstance reduces probability of arising a self-excitation, which can exist only then, when region T c and T cc overlay each other. Payments of these regions have allowed to reveal, that changing a sign of equivalent parameters of the circuit UV SCTL can become a reason of the self-excitation of the synchronous generator, working at this circuit, if length SCTL does not exceed 80-120 km. Limiting length of the line depends on powers and type of the generator. Results of payments are illustrated on the fig. 2a and 2b, where shown region T c and T cc for different lengths of circuits of the undermost voltage of the self-compensating transmission line. Rated parameters of the generator and transformers, corresponding to brought graphic dependencies, following: - Synchronous generator with salient-poles: xd = 0.93; xq =0.63; xdc = 0.35.

b)

- Synchronous generator without salient-poles: xd = 1.606; xdc = 0.28. - Resistance of transformer e k = 0.105. Brought schedules confirm that because of the growing rUVeqv an area of the self-excitation noticeably decreases; fall values of the corner T , under which it can appear. If length of the circuit UV SCTL - less than 2530 km, exists probability of origin at the regulation of the asynchronous self-excitation in the area III a fig. 2, b - that must be checked in addition, with provision for slides (i.e. under Z z Z 0 ). As far as self-excitation possible under determined range of lengths of the line, previously than define borders of areas, should realize, possible this mode for the machine with known parameters, connected to circuit UV SCTL, working in the inphase mode. Limiting length possible crudely to value, comparing reactive resistance of the generator with the transformer

184

Fig. 2. Changing the limits of the regulation of the corner T , defining borders of areas of synchronous and asynchronous self-excitation of the synchronous generator with salient-poles a) and of the synchronous generator without salient-poles b) depending on lengths of circuit UV SCTL.


x d6 and the equivalent reactive resistance xUVeqv , determined under T x d6

whence

llim

00 :

x@@ llim

2 x01 x02 x012 llim x012 k x01

xd6 x012 k x01 . 2 x01 x02 x012

( 22 )

If length of the considered line l ! llim , selfexcitations not will. In the system, containing upper circuit of the line, or SCTL as a whole (circuits work parallel, or be an electrical communication between them at the beginning initially transmission line) selfexcitation also can not develop. For lines of other class - FACTS - it is necessary to expect similar results, as far as in [2 and 4] is noted possibility the active power flow reverse on such lines that mathematically possible represent as a changing a sign of the equivalent reactive resistance of the transmission line. Since received information insufficient, in this article it does not communicate. Thereby, brought above information on possibility of origin "particular" modes self-compensating transmission line, which circuits work at own loads and electrically unconnected, allow to note the following: 1. Changing a sign of equivalent parameters of the circuit UV, appearing at the regulation of the corner of the shift between systems of vectors of voltage of circuits T , can bring about the breach of firm functioning a system, containing this circuit, because of the self-excitation of synchronous generators. Possibility of arising of this phenomena of vagary imposes a restriction on limits of the regulation of the corner T - a functioning SCTL in the inphase mode not possible in any cases. 2. Development of the self-excitation possible when functioning (working) a generator on the circuit UV SCTL by the length before 80-120 km. Range of lengths of the line depends on parameters of the generator and external network. Received formulas, allowing value a limiting length an SCTL and define borders of the regulation under the provisions of arising a self-excitation. 3. Growing of the equivalent active resistance of the circuit UV, having place at the change T to limits 00 y 90 0 , vastly reduces an area, in which possible self-

excitation, and in ditto time enlarges probability of the self-swing appearance. References

1. Venikov V.A., Astachov J.N., Postolati V.M. Controllable AC transmission lines with the increased reception capacity. – Electricity, 1969, N 12. 2. Kochkin V.I., Shacarjan Y.G. States of working of the controllable transmission lines. - Electricity, 1997, N 9. 3. Self-excitation and self-swing in the electrical power systems / V.A. Venikov, N.D. Anisimova, A.I. Dolginov, D.A. Fedorov. - Moscow, “Higher school”, 1964. 4. The Unified Power Controller: A new approach to Power Transmission Control / L. Gyugyi, C.D. Schauder, S.L. Williams, et al. – IEEE Transactions on Power Delivery. April 1995. Vol. 10, N 2. docent Alexander Kozlov was Born at 1951 year in Frunze town, Kirghizia. Candidate's thesis has protected in the Moscow energy institute. At present – docent of the power engineering chair of the Amur state university. Scientific interests - a control of electrical power system modes. engineer Vadim Loginov was born at 1975 year in Svobodny town, Russia. Graduate student of the Amur state university. Scientific interests - a control of electrical power system modes. Address: Alexander Kozlov, Vadim Loginov Amur state university The power engineering chair Blagoveschensk, Ignatjevskoje freeway, 21 RUSSIA Telephone (4162) 35-05-56 E-mail: [email protected]


185

186





THE CONCEPTION OF A SYSTEM OF ANALYSIS OF MODELS OF ACCESS TO DISTRIBUTED DATABASES Andrey D. PLOUTENKO Amur State University Blagovestchensk (Russia)

Abstract – In the article we present a conception of a complex of tools for analysis of the access models to databases of distributed data manipulation systems (DDMS). The complex is intended for computing (model) experiments for the purpose of analyzing the time indicators of a DDMS, based on distributed databases and applications.

1. INTRODUCTION When generating distributed data manipulation systems analysis of time indicators is of great significance. What makes these characteristics particular are difficulties in theirs forecasting, because on the whole, time indicators depend on decisions taken on the first stages of a distributed system design. That is why it is important to give a designer, who makes the elaboration in a DDMS environment, a tool to forecast time indicators of distributed systems and to reveal bottlenecks of a DDMS on basis of project decisions descriptions. At the Amur State University in association with the Moscow State Technical University of N.E.Bauman has been worked out a Complex of tools for analysis of Models of access to distributed databases (TAM). CTAM is intended to conduct calculating experiments, which aim to analyze time indicators of a DDMS, these experiments base on distributed databases and applications. This complex belongs to expert systems and is realized in a Personal Oracle end Developer/2000 environment. 2. CTAM structure CTAM includes correlative subsystems providing the description of a conceptual DDMS project:

a conceptual database schema of a DDMS and database fillings (a forecasted number of records in tables and database cardinalities); queries and transactions of a DDMS, which can apply to other transactions of a distributed system; architectures of a DDMS: a topology and characteristics of nodes and networks from registers of TPC tests results and network parameters; tables distribution (with the account of the record dubbing) and transactions distribution on DDMS nodes; intensities of workstation addresses to transactions. In Figure 1 we present a realized in a CTAM schema of connections organization between the components of a projecting distributed system. With the help of unbroken pointers we show the connections “an address to”, and dotted lines illustrate the connections “enter and dispose in”. Thus the queries enter the transactions structure (2), during their execution SQL operators apply to database tables (1). The description of database tables, queries and transactions form a conceptual project of a DDMS, while projecting we aspire to make the conceptual project independent of realization, in other words, of the architecture of a future DDMS (a complex of tools, general system packets and so on.). After the choice of TC, OS, DBMS we perform a distribution of database tables and transactions on the nodes of a distributed system (pointers 3, 4, 5, 6). The tables are stored on database servers and transactions may be placed on workstations, applications servers, database servers (stored procedures). Pointers 7, 8, 9 illustrate addresses of transactions to database tables. It should be mentioned, applications servers can not exist or be placed on the same nodes as database servers.


187

Queries

1

Database tables

select update insert delete

Conceptual project

2

Transactions

Technical project

5 3

Application servers

4

Database servers

6 7 8 9

W orkstations

LANs, M ANs, W ANs

Fig. 1. The developed scheme of distributed system component interconnections

On the basis of description of DDMS parameters the complex analyses the characteristics of productivity and reveals bottlenecks of a distributed system. CTAM calculates the following characteristics for each architecture variant: the load of nodes (servers and clients) and networks and their components on queries and transactions; the average execution time of transactions and its components on nodes, networks, queries and called transactions; the average time of access from nodes to transactions which accounts the time of transmission of input and output data from workstation; the average time of blocks, manipulated during the execution of SQL queries. CTAM belongs to a class of expert systems (ES). It is known [2], ES include the following main components: a database, a knowledge base, a knowledge derivation mechanism, derivation machine, explanation mechanism. A CTAM database is a base of metadata, which stores data about a projected distributed system of data manipulation. It is known, that projection of most of complex systems is performed on the condition of a great uncertainty of primary data. That is why the description of DDMS parameters 188

in a STAM and the calculation of characteristics can be executed with the help of fuzzy figures. A CTAM knowledge base is organized in the form of frames. A frame is a combination of slots, describing the conceptual project properties (a concept) [2]. The connection between CTAM frames is determined by the sequence of execution of SQL queries of transactions and transactions connections between each other, and also by sequence of used resources of a network. A CTAM derivation machine provides the filling of frames with data out of the database, organizes their dynamic connection and executes the calculation of time indicators of a projected DDMS. The calculation of components of time characteristics supports the explanation mechanism and points out apparatus and program resources that can be critical. 3. CTAM database schema During the elaboration of a CTAM database schema we decided the following purposes: ¾ a description of thesaurus of a subject area, as well as parameters of database schema and filling; ¾ SQL queries description to a projected DDMS database with the help of special structures, a Electrical Power Quality and Utilisation

definition of a structure of transactions, a SQL queries connection with transactions; ¾ a provision of a description of nodes, channels, a DDMS topology, virtual channels of connection and their characteristics; ¾ a determination of nodes and channels configurations on the basis of CTAM registers; ¾ a distribution of transactions with SQL queries descriptions and DDMS database tables on nodes; ¾ a provision of storage and leading of components of loads mechanism and transactions time; these components are used to support an ES explanation mechanism; ¾ a support of a CTAM database consistency ( non – contradiction). We managed to decide these purposes by means of a distinction of demanded ER–diagram entities and connections between them. In fig. 2 we present a CTAM database subschema, used to describe a DDMS conceptual project. The entities “Subject area – A “ and “Thesaurus – B” are used to identify a thesaurus on every subject area (subsystem) of a projected DDMS. These thesauruses are used to generate table schemas (entity “Table schema – D ”). Tables characteristics are descried in the entity “C Schema – DB ”. The names of queries to a DDMS database are defined with the help of the entity “Queries – F”, and the entities “Query tables – E” and “Query attributes – V” let to describe tables and attributes used in a query, and also to point out a condition which will be satisfied by the database records, which take part in the operation. Entities of “Transactions – H” and “ Transaction content – G” are used to define transactions characteristics and connections of queries to these transactions. While elaborating a CTAM database schema we used identification connections. We determined the rules of Referential Integrity (RI) during the connections assignment. We used the following rules, which put restrictions on the value of an attribute of a connection: During the Parent Delete of a tuple all the tuples of a daughter entity with the same attribute value of the connection (CASCADE) are deleted automatically. During the Parent Update of any attribute of a tuple key, the appropriate attributes of the tuples of a daughter entity are updated automatically with same (old) value of a connection attribute (CASCADE) The use of the rules mentioned above helped to decide a problem of a consistency provision (non – contradiction) of a CTAM database.

A knowledge base is organized in the form of frames. Each frame consists of slots. A slot includes the following elements [2]: 1) A slot name. The first two slots of a frame are service, and they have definite names: IS–A – shows, that a slot contains a pointer of a parent frame, DDESEN-DANTS – a slot contains a pointer of a daughter frame. 2) A pointer of inheritance. It indicates the way of a value determination, in case that a parent frame has a slot with the same name. It takes one of the following values: SAME – a slot takes the same value with the appropriate slot in a parent frame, UNIQUE – a slot can take the value different from the value of the appropriate slot in a parent frame.

4. Organization of a knowledge base of a CTAM Section 2. Methods of Power Quality Analysis: Modelling and Simulation

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3) A type of a slot. It indicates what data type it belongs to: INTEGER (an integral number), REAL (a real number), BOOL (a boolean number), TEXT (a text), BINARY (a binary massive), LIST (a list), TABLE (a table), EXPRESSION (a language expression), FRAME (a frame pointer), LISP (a connected procedure, which gets and processes the information given to a corresponding slot from another frame with the help of MSG function). 4) A slot value. A type of value should account with the indicated slot type. 5) Daemons. Daemon – is a procedure, automatically launched during the execution of a condition in the time of the address to a slot. IF – NEEDED – this daemon is launched if its value is not established at the moments of address, IF – ADDED – is launched during the substitution of a value in a slot, IF – REMOVED – is launched during the slot value delete. LISP – procedures and procedures–daemons define procedural knowledge of an expert system on the basis of frame. During the elaboration of this procedural knowledge the original theoretical results, obtained by the authors of CTAM , was used. 5. Derivation machine The elaboration algorithm of a CTAM derivation machine is defined by a structure of frames and connections between them. A frame “Process” is a root one. After its activization a corresponding LISP–procedure reads the records of a table “To a transaction from a U–node” and analyses transactions which DDMS nodes address to. 190

Transactions can contain SQL-queries to a distributed database and addresses to other transactions. For the next SQL-query a frame “SQL” is activated. When we substituted query name in a slot a correspondent IF – ADDED procedure– daemon, is launched, this procedure executes a decomposition of a query on subqueries and fills slots of a given frame which are used in the next daughter frame. Every subquery is connected with a table, pointed in a query. The connection between homogeneous “SQL” is defined by a sequence of SQL queries execution of transactions and connections of transactions between each other. Later a frame “Virtual channel” is activated. During the substitution of names of query tables in a slot of LIST type, we lunch a procedure – daemon, which defines virtual channel (VC) for each subquery on the basis of information from a CTAM database. VC presents a way from the primary node, when the primary SQL query is executed, to a node, where a table is stored, this table is connected with a subquery. As CTAM lets to simulate a computing network with account of a record dubbing (replication) of data, than in a general case VC used for operations of research and renewal can be different for one DDMS table. Frames “Node” and “Channel” are activated for every VC. For a primary and end VC node a node load is accumulated and the time of cursor and subquery execution is evaluated on the basis of data, obtained from “Virtual channel” (a list of VC nodes and their parameters) and “SQL” (a volume of output and input data of subqueries) frames. The daughter “Components of channel load on Electrical Power Quality and Utilisation

transactions” and “Components of channel load on queries” frames store the components of channels load in a database CTAM. A “Transaction execution time” frame, which is a daughter one with a reference to “Node” and “Channel” frames, evaluates the execution time of a current transaction, and with the help of its daughter frames A11, A12, A13, A13 stores the components of this time in database CTAM. A lot of CTAM parameters can be set in the form of fuzzy figures. The arithmetical operations with fuzzy figures were realized to work with such parameters. In a CTAM we used a trapezoidal form of a fuzzy figure presentation, which is characterized by four figures (R1, R2, R3, R4). In this case the operations over fuzzy figures A and B can be brought to operations over the elements (A1, A2, A3, A4) and (B1, B2, B3, B4) of these fuzzy figures [1, 3]. Let C = AqB, where q is a symbol of an arithmetical operation. Then, approximating C by a trapezoidal fuzzy figure, we get x (C1, C2, C3, C4) = (A1qB1, A2qB2, A3qB3, A4qB4) – for operations +, *, x (C1, C2, C3, C4) = (A1qB4, A2qB3, A3qB2, A4qB1) – for operations , /. And besides for operations + and we get a sharp function of belonging. Procedure of approximation mentioned above lets to evaluate fuzzy values of output characteristics within one running of the model. The offered CTAM means of analysis are flexible enough and let to simulate various access models to databases: models of a database server and applications, an access model over the technology of Internet/Intranet (CGI and API interfaces, an access out of Java-applets and ActiveX objects), models of systems with architectures CORBA and DCOM.

Prof. Andrey Ploutenko was born in 1961 in Krasnoyarskiy region, Russia. He graduated from the Mechanical department of the Amur State University. He received the candidate of science degree in the field of the technological processes automation from the University of Technology and Design (St.Petersburg), he is a professor of the Chair of Informational and Control systems of the Amur State University. His areas of interests include the distributed informational systems design, the analysis of mechanisms of an access to distributed databases.

Mailing address: Andrey Ploutenko Amur State University 21 Ignatevskoe road, 675027 Blagovestchensk RUSSIA phone: (4162) 35-05-56 e-mail: [email protected]

6. REFERENCES 1. Dubois D., Prade H. Fuzzy Real Algebra: Some Results // Fuzzy Sets a. Systems. - 1979. - Vol. 2, {4. - P. 327-348. 2. Presentation and usage knowledge / Ed. H. Ueno, I. Isudzuka. - Moscow: Mir, 1989. - 220 . 3. Handling information with uncertainty in decision support systems / .N. Borisov, et. Al. - Moscow: Radio i Svyaz, 1989. - 304 p.


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3.1. SANTARIUS P., GAVLAS J., BIOVSKÁ B.: Evaluation of Power Quality in Regional Distribution Networks (Czech Republic) .............................................................................................195 3.2. PROCHÁZKA K., KYSNAR F., VYSKOIL V., HRKOVÁ J.: Properties of Instruments for the Measurement of Power Quality on the Boundary Between the Transmission System and the Distribution Systems and the Results of Their Testing (Czech Republic)............................................203 3.3. WACLAWIAK M., McGRANAGHAN M.: Substation Power Quality Performance Monitoring and the Internet (USA) ........................................................................................................................211 3.4. KOPONEN P., VEHVILÄINEN S.: Improved Power Quality Monitoring kWh-Meter (Finland) .....217 3.5. CAROLSFELD R., SEMCZUK M.: Pass or Fail: The Future of Power Quality Monitoring (Canada, Poland)..................................................................................................................................225 3.6. HASHAD M., HARTMAN M., HANZELKA Z., BIEN A.: The Hypothesis for the Wrong Measurements Results Obtained During the Flickermeter Comparative Test (Poland)......................231 3.7. BIEN A., ROZKRUT A.: A Measurement Scale for the Light Flickering Phenomenon (Poland)......237 3.8. BARKAN J., LESCHENKO S., VASILJEV A.: Factors Having Influence on the Accuracy of Flicker Determination (Latvia) ............................................................................................................241 3.9. KUMIEREK Z., KORCZYSKI M.J.: Subharmonics in Electrical Power System Identification Problems (Poland) .........................................................................................................245 3.10. ZHEZHELENKO I.V., SAYENKO Y.L., BARANENKO T.K.: Calculation of Interharmonic Voltage Spectrum in the Points of Industrial Power Supply Systems (Ukraine)..................................253 3.11. PACHOLSKI K.: The High-Speed Overloading of Measuring Transducers of Distorted Electrical Signals (Poland) ..................................................................................................................259

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EVALUATION OF POWER QUALITY IN REGIONAL DISTRIBUTION NETWORKS Pavel SANTARIUS

Josef GAVLAS Technical University Ostrava Ostrava(Czech Republic)

Abstract – The wider use of appliances with nonlinear characteristic in the LV distribution networks (such as TV sets, computers, compact fluorescent lamps, etc.) causes degradation of a supplied power quality. For the evaluation of current status and for the design of needed measures a series measuring was performed in the distribution networks which were focused mostly on a complex evaluation of some important quality parameters (harmonics, voltage fluctuation and voltage unbalance) in the distribution network of a regional distribution company in the Czech Republic. The importance of the power quality monitoring is increasing because the new energetic law will start opening the electricity market since the year 2002. The measures and the evaluation of quality were realised on more than 50 supply nodes of the distribution network according to the standard SN EN 50160. In the paper the results of a complex research are summarised with relevant evaluation in relation to the standard SN EN 50160. 1. INTRODUCTION The new power engineering law, which came into effect in the Czech Republic from the year 2001, gradually liberalises the electrical energy market in such a way that from 2006 it should be liberalised wholly. This fact will also significantly influence the view of the quality of supplied electrical energy. The energy will gradually become one of the dominant factors based on which a consumer will make a decision when choosing a supplier of electrical energy. The expansion of facilities with non-linear characteristics in the LV distribution networks (e.g. TV sets, computers, copy machines, compact

Blanka BIOVSKÁ

lamps, and the like) makes the quality of the supplied electrical energy much worse. For the evaluation of current state, and for the possible suggestion of the corrective action plan, a number of measurements in the distribution networks was done. The measurements concentrated mainly on the complex evaluation of some significant parameters of the quality of supplied electrical energy (harmonic, voltage fluctuation and unbalance) in the distribution network of the regional power engineering company in the Czech Republic. It feeds the area of ca 10 000 km2 with about 2 million inhabitants. The regional power engineering company is aware of these factors which worsen the quality of the supplied electrical energy, and is preparing itself for the liberalised environment of electrical energy market well in advance. That is why the program of complex monitoring of the quality of supplied electrical energy was prepared already in 1996, and in 1997 the complex monitoring of chosen parameters of the quality of voltage in this company was started. Gradually, the single feeding points of 110 kV in six parts of the company were measured. According to the agreed program two parts of the company were gradually measured in those years, thus the whole cycle was finished at the end of the year 1999. In 2000 a new cycle of complex monitoring in the same feeding nods was started, thus the information about changes within three years are gradually obtained. 2. THE METHOD EVALUATION

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Monitoring of the quality of electrical energy supply was gradually done in single parts of the company. The measuring was done in a complex way within the HV, MV and LV distribution. The


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program of complex quality of electrical energy evaluation was done in 54 feeding points 110 kV. The measuring was practically ensured in a way that the quality of feeding switching stations 110 kV and 22 kV was monitored by means of quality monitors QN (producer EGÚ Brno). The quality of the LV outlet of chosen distribution transformer station, which is fed by the measured 22 kV switching station, was monitored by means of a BK 500 analyser (producer ELCOM Praha). At the same time the parameters of quality were also monitored by means of quality monitor QN. The composition of consumption in the LV network was similar in all measured localisation's – i. e. a mix of family houses and blocks of flats. In harmony with the Standard SN EN 50 160, the measuring and evaluation of the quality of single points was done in one week intervals, while the parameters of quality were evaluated for 10 minute intervals in the course of measuring. As single parts of the company have 8-10 feeding nods 110 kV, the monitoring was organised in half-year cycles, thus the whole program lasted for 3 years. Before the beginning of the program of complex monitoring a number of measuring were done in the distribution networks, and it was proved that in the voltage curve there are only some dominant harmonics, and that is why only evaluation of the chosen ( 3., 5., 7., 9. and 11. ) harmonic was done. In single feeder points they evaluate measured data in all phases and on all voltage levels a) harmonic voltage x maximum and 95% values of the selected harmonics ( 3., 5., 7., and 11. harmonic) in all phases and on all voltage levels in one week intervals x one week time sequences of selected harmonics b) voltage fluctuation and voltage unbalance

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maximum and 95% values of Pst and Plt parameters for evaluation of voltage fluctuation in all phases and on all voltage levels in one week intervals one week time sequences of Pst and Plt parameters maximum and 95% values of voltage unbalance on all voltage levels in one week intervals.

3. STANDARD SN EN 50 160 The Standard SN EN 50 160 defines properties of the feeding voltage supplied to consumers at the transferring place. The Standard SN EN 50 160 is valid for LV as well as HV network. This Standard was accepted by the EU countries in 1994, and in 2000 by the Czech Republic. The Standard is not valid for these conditions: x Operation after breakdown x Extraordinary situations due to the weather and natural catastrophes x Extraordinary situations due to the measures of public authorities and the like For power interference (harmonic, flicker, unbalance) this standard prescribes: Flicker intensity – size of the coefficient of a long-term degree of perception of Plt flicker must not overcome the value of 1,0 for 95% of the time of the week under normal operating conditions. Voltage unbalance - under normal operating conditions the ratio of a 10 minute effective value of feedback voltage component and respective component should not overcome the size of 2% in 95% cases of measuring intervals of the week. Voltage harmonics – under normal operating conditions it should not overcome 95% of 10 minute average of effective values of single harmonics in arbitrary one week interval of the size shown in the Table.

Odd harmonic Odd harmonic Even harmonic except multiples 3 multiples 3 Order Voltage Order Voltage Order Voltage harmonic harmonic harmonic harmonic harmonic harmonic h % h % h % 5 6 3 5* 2 2 7 5 9 1,5 4 1 11 3,5 15 0,5 6...24 0,5 13 3 21 0,5 17 2 19,23,25 1,5 In relation to the network design the values of 3rd harmonic in HV networks may be significantly lower


4. CONCLUSIONS The complex monitoring of selected parameters of the quality of supplied electrical energy was done in one week intervals on all three voltage levels at the same time in six local distribution parts of the company (altogether 54 feeding points). The measuring was not successful in all the cases (failures, drop – outs, etc.). The measured values are shown in bar charts by means of spreadsheet Microsoft Excel. 4.1. Harmonics When measuring, before the beginning of the program of complex monitoring all voltage harmonics components were evaluated. It was proved that in distribution networks there are only some distinctive harmonics, and that is why only the evaluation of the 3., 5., 7., and 11. harmonic was done in the program of the complex evaluation of harmonics. As it was stated in Chapter 1, maximum and 95% values of the above mentioned harmonic components in one week intervals were evaluated. We can summarise: x the most considerable harmonics were in the LV network x the 5th harmonic was the most considerable, on all voltage levels (evaluation of 5th harmonic is shown in the appendix) x the 3rd harmonic (according to SN EN 50 160, the limit is 5%), the values were around 1% in the LV networks. In one case, the limits according to the Standard were exceeded (over 10%). In this case, the limits for the 3rd harmonic on all voltage levels were exceeded for unknown reasons. Repeated measuring in this feeding point did not prove these results. From the time sequences we may admit that the resonance took place due to compensating condensers of the street lighting. Otherwise, the values below 1% were in the HV and MV networks. x the 5th harmonic (according to SN EN 50 160, the limit is 6%), the values in the LV networks were mostly in the range of 2-4% (in the above mentioned feeding point around 5%). In the MV networks the values were in the range of 2-3%, exceptionally slightly above 3%, and in the HV networks around 1%, exceptionally slightly above 2%. x the 7th harmonic (according to SN EN 50 160, the limit is 5%), the values in the LV networks were around 1%, exceptionally up to 2%. In the MV networks the values were

similar – around 1%, exceptionally up to 2%, and in the HV networks mostly up to 1%. x the 11th harmonic (according to SN EN 50 160, the limit is 3,5%), in the LV networks the values were up to 0,5%, exceptionally up to 1%. In the MV networks the values were also up to 0,5%, exceptionally up to 1%, and in the HV networks significantly below 0,5%. As for the harmonics, we can conclude that nearly in all cases the measured values, in some cases even with the big reserve, comply with the requirements according to SN EN 50 160. The exceptional case in the above mentioned feeding point has not been proved when measuring repeatedly, but this fact highlights the point that one week measurement does not necessarily characterise parameters in the given area reliably. 4.2. Flicker Measuring and consequential evaluation of a short–term and long-term flicker is necessary first of all from the point of view of unfavourable impact of light sources on human beings. From the measured values on the above mentioned feeding points, the maximum and 95% values of Plt and Pst were evaluated. In the appendix, the results of maximum and 95% of the Plt value are shown. We can summarise: x the requirements according to SN EN 50 160 were exceeded for the 95% values of Plt (the Standard requires Plt less than 1) in two places – in one case the limit was exceeded on the LV level, but only insignificantly (1,13). In the second case, also on the LV level, the limit was exceeded more significantly (2,4 – 2,9), however this has not been proved either when measuring repeatedly. x on the HV level the values of Plt were mostly up to 0,5 and exceptionally up to 0,8; thus below the limit according to the Standard x for the values of Plt (95%) on the HV level, the SN EN 50 160 Standard does not state any limits. The measured values were mostly up to 0,5 exceptionally up to 0,8. x for Pst (95%) on all levels, the SN EN 50 160 Standard does not state any limits. x when comparing maximum and 95% values for Plt and Pst , where we can see significant decrease of 95% values against the maximum (see graphs in appendix) 4.3. Unbalance The SN EN 50 160 Standard states the limit of unbalance up to 2%, in exceptional cases up to 3%. From the graphs values of unbalance (which


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are in appendix) we can conclude that the values stated by the Standard have been exceeded in one case on the HV level (2,9%) and in one case on the MV level (3,2%). Unfortunately, the explanation for such high values has not been found. 4.4. Results of repeated measuring As it was stated, in 2000 we gradually started the repeated monitoring in the same feeding points. The first results show some changes of the quality parameters, and these are the increase as well as the decrease. It is probable that most changes are conditioned to the changes of the structure of industry in the monitored feeding areas. In the last appendix, an example of the evaluation of the differences of 5th harmonic in one distribution plant is shown. 5. REFERENCES [1] Santarius P., Gavlas J., Kužela M.: Power quality parameters in distribution networks, 5th International Conference EPQU 99, Krakow, Poland 1995 [2] Vyskoil V., Santarius P.: Some problems of power quality in Czech distribution networks, International Conference CIRED, Nice France, 1999 [3] Biovská B., Gavlas J., Santarius P., Vašenka P.: Komplexní hodnocení kvality dodávané elektrické energie v distribuních sítích energetické spolenosti, IV. konference ERU 2000, Brno, listopad 2000

This publication was elaborated with the support of GAR grant No 102/99/1000.

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Prof. Pavel Santarius was born in 1941 in Bludovice, Czech Republic. He graduated from the Technical University Brno. In 1976 he completed his PhD.studies at TU Brno. Since 1970 he has been employed by the Department of Electrical Power Engineering of VSB-TU Ostrava, since 1993 as a professor. His research work deals with solving the problems of EMC. He is a member of the National Committees CIRED and CIGRE. Mailing address: Pavel Santarius VSB-Technical University of Ostrava, Department of Electrical Power Engineering 17. listopadu 15, 708 33 Ostrava-Poruba phone: (++42)(069) 699-4279 e-mail: [email protected] Ing. Josef Gavlas was born in 1941 in Dobra, Czech Republic. In 1977 he graduated from the VSB-Technical University of Ostrava. His research interests are currently in the sphere of EMC. Mailing address: Josef Gavlas VSB-Technical University of Ostrava, Department of Electrical Power Engineering 17. listopadu 15, 708 33 Ostrava-Poruba phone: (++42)(069) 699-4362 e-mail: [email protected] Ing. Blanka Biovská Was born in 1948, Czech republic. In 1972 he graduated from the Technical University Brno. In 2001 he completed his PhD.studies at VŠB TU Ostrava. His research interests are currently in the sphere of EMC. Mailing address: Ing.Blanka Biovská VSB-Technical University of Ostrava, Department of Electrical Measurements 17. listopadu 15, 708 33 Ostrava-Poruba phone: (++42)(069) 699-3229 e-mail: [email protected]





PROPERTIES OF INSTRUMENTS FOR THE MEASUREMENT OF POWER QUALITY ON THE BOUNDARY BETWEEN THE TRANSMISSION SYSTEM AND THE DISTRIBUTION SYSTEMS AND THE RESULTS OF THEIR TESTING Karel PROCHÁZKA František KYSNAR EGC ENERGOCONSULT B eské Budjovice (Czech Republic)

Václav VYSKOIL Jana H RKOVÁ EGU Power Institute Brno (Czech Republic)

Abstract - The paper describes the reason of formulating the requirements, of acquiring and then submitting to test operation of four types of PQ analysers for the points of delivery between the transmission (TS) and the distribution system (DS). During the tests the data from these analysers were transmitted to the point of evaluation by modems and a telephone link. The data from the measurement carried out during the chosen week were evaluated. The accuracy of measurement of some parameters was verified by means of a precise source of voltages and currents.

The agreed PQ parameters and the admissible limit values for the TS/DS boundary are given in the draft of the Code of Transmission System Operation [6] and in that of the Code of Distribution Systems Operation [7].

1.

INTRODUCTION

The new Czech Energy Act [1] imposes the task upon electricity generators, upon transmission system operator (TSO) and distribution system operators (DSOs) to supply electricity with a defined quality (PQ). The delivery of electricity from TS into DS is mainly realized on the 110 kV level (at output point of 400/110 and/or 220/110 kV transformation) for which no general standards similar to EN 50160 [2] have been adopted till now. Therefore, it was necessary to define the PQ parameters and to come to an agreement between TSO and DSOs concerning the PQ parameters and their limit magnitudes for the delivery of electricity from TS into DS. When choosing the PQ parameters within the study [3] we started from those which must be guaranteed by DSOs for their customers and which are defined in [2], as well as from IEC 61000-3-6 [4] and IEC 61000-3-7 [5] standards.

2.

REQUIREMENTS ON PQ ANALYSERS

The comparability of results provided by PQ analysers of different suppliers to be used at the points of delivery between TS and DS belongs to basic requirements on PQ analysers. The requirements on instruments suitable for a continuous observation of PQ are presented in the UNIPEDE publication [8] in which the methods of measurement, environment, required accuracy and the methods of evaluating the measured results are specified. The standard IEC 61000-4-30 [9] is now under preparation in which, among others, some definitions of PQ parameters, algorithms of measurement and the method of evaluation are made more precise and the requirements on the measurement of currents are introduced. Though it is yet in the stage of a draft, we have decided – for the given purpose – to apply its requirements when acquiring the analysers, mainly the requirements concerning the intervals of measurement and the evaluation of individual parameters. 2.1 Power Quality parameters being measured The PQ analyser must be able to measure simultaneously the PQ parameters defined in [2] (beside transient overvoltages between live conductors and ground) and to measure the


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magnitudes of currents and of other quantities derived from them (according to the respective voltages). The measurement of currents and of quantities derived from them is important for assessing the origin of impairing the voltage quality (TS or DS). The analyser should also indicate the intervals of measurement with the occurrence of voltage dips and swells and it should signalize the exceeding of the set limits of chosen parameters. The following requirements were formulated for programs for communication and evaluation: x x x

2.3 Requirements on accuracy The requirements on accuracy which should be satisfied by the analysers, comply with the requirements set in [9] for instruments of class A. The required accuracy refers only to voltages and to quantities derived from them. In case of current outputs and of quantities derived from them we respected the values declared by the manufacturers. 3. TEST OPERATION 3.1 Analysers being tested

compatibility with Windows 9x/NT basic aggregation of the measured PQ data as a component part of the analyser local and remote setting of limits the exceeding of which is being evaluated and recorded (and/or signalized) as an event.

The following four companies from the total number of seven contacted lent us the PQ analysers for test operation: x ELCOM, a.s.s - analyser BK 550 x TMVSS, s.r.o. - analyser UP-2210, the product of the Swedish company Unipower x UPS Servis, s.r.o. - multipurpose measuring instrument 7600 ION, the product of the Canadian company Power Measurement x UTES Instruments, s.r.o. - measuring instruments QWave Power and QWave Premium manufactured by the Belgian company LEM. The instrument QWave Power was lent us for test operation. Beside these analysers, for the purpose of comparison we have also used the instrument TOPAS 1000 – the product of the company LEM designed for the measurement of PQ in various applications in the power system.

2.2 Intervals of measurement The defined intervals of measurement are given in [9]. The following intervals of time are required for various PQ parameters: - for mains frequency 10 seconds - for flicker 10 minutes and 2 hours - for supply voltage magnitude, for harmonic/ interharmonic voltages and unbalance - 3 seconds, 10 minutes and 2 hours - for signals on the network: 3 seconds, 10 minutes. The evaluation of rms values of voltages in each period with revolving after each half-period is required for voltage dips and swells [9].

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switchboard for measurement

Fig. 1 Connection of analysers in Dasný substation


3.2 Installation of analysers and securing the test operation The analysers being offered enable various methods of communication – see Table 2. As it is not our aim to discuss the particular properties of individual analysers being evaluated but only the approach to their verification, we denote them by letters A to E in our subsequent analyses. TABLE 1 Methods of communication A RS 485 Ex. modem Ethernet

B RS 232 RS 485 Ex. modem In. modem Ethernet

C RS 232 RS 485 Ex. modem Infrared Ethernet

D RS 232 RS 485 Ex. modem Ethernet

For simplicity and for equality of conditions, a uniform communication between the point of evaluation (EGC-EnerGoConsult B, s.r.o.) and the analysers using a telephone link via modem was chosen for test operation at the 400/110 kV substation Dasný. One telephone number was reserved at the exchange of Dasný substation and each of analysers being tested had its own shutter. The connection of the analysers and the scheme of the test operation is illustrated in. Fig. 1. A data server with the following parameters was used at the point of evaluation for scanning and administrating the data from the analysers: Motherboard – Pentium II 350 MHz Memory – 128 MB Disc – 4 x Samsung 8 GB Graphic card – ATI 3D RAGE IIC AGB Modem – GVC 56 K Dual band. In case of analysers with signalization outputs, the information about the appearance of voltage dips and interruptions was transmitted into the dispatch centre of the regional utility JE. As each analyser being tested imposed different requirements on the operation system, each analyser had its own (exchangeable) hard disc 8 GB in order to eliminate the risk of potential interferences 3.3 PQ parameters being evaluated It has been agreed to start from the data for the following rms values in the interval of 10 minutes and for Pst and Plt when comparing the properties of the installed analysers: x phase voltage (phase-to phase voltage for final observation)

x x x

x x x x x x x x x x x

x

4.

negative-sequence component of the voltage total harmonic distortion of phase voltages (phase-to-phase voltages for final observation) phase voltage harmonics of the order up to 25 (3rd, 5th, 7th, 11th and 13th harmonic at minimum) (phase-to-phase voltage for final observation) phase currents cumulative active, apparent and reactive power negative-sequence component of the current angle of the negative-sequence component of the current total harmonic distortion of the current current harmonics of the order up to 25 (3rd, 5th, 7th, 11th and 13th harmonic at minimum) angle of harmonics of the order up to 25 (or power) flicker Pst in the interval of 10 minutes flicker Plt in the interval of 2 hours dips, swells and interruptions indication of “non-valid” intervals of measurement at the appearance of dips and swells for Pst, Plt, the negative-sequence component of the voltage and harmonics indication of ”non-valid” intervals of measurement of 10 periods for the voltage when the rms value of the voltage exceeds 150% Un or is smaller than 50% Un.

RESULTS OF TEST OPERATION

The intention to submit the instruments being offered to test operation has been successively extended by the aim of evaluating the measured values of various parameters. The instrument TOPAS 1000 which is not expected to be used for checking the PQ parameters at the points of delivery from TS to DS but which, nevertheless, can provide additional data for assessing the properties of instruments being offered was also included into test operation. The period of time from 26.02. 00:00 h to 04.03. 24:00 h has been chosen as decisive for comparing the results of measurement. This period comprises 1008 ten minute intervals corresponding to the duration of one week as it is required by standards for the majority of quantities.


205

TABLE 2 Selected statistical values of quantities for the period of one week – the highest and the lowest assessed statistical values and the difference between them – quantity

unit

current

voltage

flic ker

powe r

Us95 rms value of current - mean active - mean apparent - mean reactive - mean Pst 99 Plt 99 unbalance 95 negative-sequence component 95 THDU 95 3ha 95 5ha 95 7ha 95 11 95 13 95 THDI 95 3ha 95 5ha 95 7ha 95 11ha 95 13ha 95

values of quantities from individual instruments

permitted value

dUs V d A MW MVA MVAr d d % d V % d % d % d % d % d % d % % % % % % -

max

min

difference

B

66469

A

66131

338

B B B A D A C A D D D A D D B B E D D D

427,48 82,316 83,370 7,640 0,439 0,526 0,300 176,4 1,575 0,649 1,478 0,446 0,116 0,111 5,830 2,624 4,584 2,857 0,237 0,202

D D D E B B B B C A A D C C C C C A A C

406,77 79,596 80,128 1,565 0,210 0,408 0,160 93,6 1,407 0,571 1,394 0,423 0,081 0,060 4,996 0,499 2,496 2,639 0,193 0,128

20,71 2,720 3,242 6,075 0,229 0,118 0,140 82,8 0,168 0,078 0,084 0,024 0,035 0,051 0,834 2,125 2,088 0,217 0,044 0,074

current

voltage

flicker

powe r

TABLE 3 Values of quantities and of the interval within the period of one week when the difference between these values was the greatest

206

quantity RMS value of voltage RMS value of current active apparent reactive Pst Plt unbalance negativesequence component THDU 3ha 5ha 7ha 11ha 13ha THDI 3ha 5ha 7ha 11ha 13ha

unit

date

E

D

A

B

C

max

min

differ.

V

26.2.01

20:20

66353

65362

64671

65574

65438

66353

64671

1682

A

28.2.01

15:40

528,72

521,15

417,83

543,04

532,50

543,04

417,83

125,21

MW MVA MVAr

15:40 15:40 16:30 6:50 8:50 3:50

104,08 104,46 -0,01 1,084 0,122

%

28.2.01 28.2.01 26.2.01 1.3.01 1.3.01 26.2.01

102,78 103,46 7,76 0,439 0,488 0,064

81,48 82,27 14,33 1,197 0,156 0,209

105,08 106,17 8,16 0,090 0,408 0,000

104,47 105,49 9,82 0,199 0,525 0,300

105,08 106,17 14,33 1,197 0,525 0,300

81,48 82,27 -0,01 0,090 0,122 0,000

23,60 23,89 14,34 1,107 0,402 0,300

V

2.3.01

23:20

69,68

58,44

163,90

13,37

163,90

13,37

150,53

% % % % % % % % % % % %

1.3.01 28.2.01 28.2.01 2.3.01 26.2.01 2.3.01 3.3.01 4.3.01 3.3.01 3.3.01 26.2.01 26.2.01

6:00 2:20 16:00 20:40 15:40 18:20 14:30 7:10 14:30 14:30 19:00 19:40

0,626 0,646 0,973 0,344 0,066 0,048 3,992 0,770 3,377 2,099 0,100 0,059

0,635 0,654 1,072 0,170 0,153 0,128 5,970 2,088 4,882 3,125 0,728 0,474

0,761 0,513 0,801 0,287 0,090 0,041 5,562 0,757 4,499 2,627 0,100 0,068

0,640 0,642 1,004 0,241 0,080 0,020 6,210 3,155 4,648 2,817 0,094 0,000

0,761 0,654 1,072 0,344 0,153 0,128 6,210 3,155 4,882 3,125 0,728 0,474

0,340 0,513 0,801 0,170 0,066 0,020 3,992 0,728 3,377 2,099 0,094 0,000

0,421 0,140 0,271 0,174 0,087 0,108 2,218 2,426 1,505 1,025 0,634 0,474

0,340

5,375 0,728 4,686


4.1 Unification of results provided by individual analysers

4.3 Comparison of results of measurement performed by individual instruments

The instruments indicate the measured values in a different way and, therefore, a unification is necessary for the purpose of comparison. For example:

a) Statistical values of quantities for the period of one week

x x

x

quantities are being indicated as - rms values or peak values - absolute or relative values relative values may be related to - a variable quantity, e.g. to the rms value or the height of the 1st harmonic (THD), or to a constant value (TDD) values for a 10 minute (2 hour for Plt) interval may be affiliated to the beginning or to the end of the interval.

4.2 Method of measurement and of treating the measured data The data for individual quantities were treated in a usual manner with some complements complying with standards and respected documents. a) Three phase values of the quantities were calculated from the set of the most unfavourable values in individual 10 minute intervals. b) Plt values were calculated from Pst for sliding intervals by a unique method. During the calculation we eliminated the values for those intervals in which a voltage dip was revealed >5@. c) For a better general characterization of the sets of measured values of individual quantities, we have also assessed the mean value (p=50%) and the standard deviation £ of the set , together with statistical values max (p=100%), p=99%, p = 95% and min (0%) which characterize only the marginal areas of the set of measured values. The values for p=99%; p=95% were not used for quantities for which it is not required by the standards or for which it has no physical sense (currents, power). d) For mains voltage, the mean level and a symmetrical tolerance for voltage deviations Us ± d (%) for 95% of values of the set are given in order to enable a better comparison with the permitted value Un ± 10% Un.

For each quantity, 1008 values for ten minute intervals are at disposal from each instrument being tested for the period of one week. From 1008 values measured by individual instruments, we have determined for each quantity the statistical values that follow from the expected purpose and from the approach applied in relevant Czech and international documents >2,4,5,6,8,9@. The maximum and the minimum value obtained from the measurement performed by various instruments ( A – E) and, for the purpose of comparison, the permitted limit value are given in Table 2 for the characteristic statistical value of each quantity. The advantage of evaluating the statistical values consists in that they characterize the whole period of the week and reduce the impacts of possible deviations of time which may occur at individual instruments. b) Values of quantities in ten minute intervals Values measured by individual instruments being tested (A – E) are at disposal for the given 10 minute interval. From these values we try to find for each quantity the maximum and the minimum measured value and the difference between them. All this shall be carried out for all quantities and for each from 1008 ten minute intervals. Table 3 summarizes the data for each quantity provided for it by individual instruments in that interval from the total number of 1008 in which the difference between the data obtained by individual instruments was the greatest. The matter then concerns the worst case that has occurred. For the purpose of comparison the table also indicates the permitted limit value. c) Records of events – dips and swells for the defined limits All instruments recorded one dip during the period of measurement, namely on 27.02.2001.


207

TABLE 4 – Values of the dip recorded by individual instruments Instrument E D A C B

Time of beginning 07:13:18 07:14:14 07:15:27 07:13:21 07:13:23

Duration 49 ms 52 ms 60 ms 50 ms 50 ms

Phase L1 L1 L1 L1 L1

Residual voltage 83,4% Un 84,3% Un 83,0% Un 83,4% Un 83,7% Un

4.4 Comments on the results of measurement performed during the test operation Rms value of the voltage The differences between the values measured by individual instruments are small. Instrument A has been reset during the measurement. In the remaining period the greatest difference of the statistical (p=95%) value from individual instruments decreased to 0.36% Un. Rms value of the current The differences of values measured by individual instruments are significant and the greatest difference of the statistical (mean, i.e. p=50%) value of the current amounted to about 5%. Active power (MW), apparent power (MVA), reactive power (MVAr) The differences of the statistical (mean) value for the measurement during one week performed by individual instruments attain for the active power 2.72 MW (up to about 3.4%) for the apparent power 3.24 MVA (up to about 3.4%) for the reactive power 6.08 MVAr (up to about 7.6% of the apparent power) The differences are significant and the greatest ones occur in the reactive power.. Flicker Pst and flicker Plt (calculated from Pst) In order to arrive at a real answer to the question whether the height of the flicker satisfies the requirements, it is unconditionally necessary to eliminate those intervals in which a voltage dip was revealed >5@. In spite of this, the differences of statistical (99%, 95%) as well as individual values of the flicker obtained from the measurement by individual instruments are considerable. The method of

208

measurement and of evaluating the flicker Pst seems to be insufficiently unified and it should be paid further attention. Value (relative) of voltage unbalance The differences of values measured by individual instruments are considerable and the difference of the statistical value of the unbalance (95%) reached up to 0.140%. The discrimination of values being measured is insufficient at some instruments. Rms value of the negative-sequence component of the voltage The differences of values measured by individual instruments are considerable. The difference of the statistical (95%) value moved in the range from 93.6 to 176.4 V. Value (relative) of the total harmonic distortion of the voltage THDU The differences of values measured by individual instruments are small and the greatest difference of the statistical (95%) value reached up to 0.168%. Value (relative) of voltage harmonics (of the 3th, 5th,7th,11th,13th order at minimum) The differences of values measured by individual instruments are small and the greatest difference of the statistical (95%) value was revealed for the 5th harmonic and it attained 0.209%. Value (relative) of the total harmonic distortion of the current THDI The differences of values measured by individual instruments are higher than of THDU values. The greatest difference of the statistical value (p=95%) attained 0.834%. Value (relative) of current harmonics (of the 3th, 5th,7th,11th,13th order at minimum) The differences of values measured by individual instruments are small. The greatest difference of the statistical value (p=95%) occurred for the 5th harmonic (2.088%). A defect on one of the instruments was revealed later on.


5. ANOTHER EXPERIENCE RESULTING FROM TEST OPERATION 5.1 Demands on time necessary for data transmission One of the investigated parameters of the analysers were demands on time necessary for the transmission of the measured data. For this comparison, measurements during one day were carried out. A uniform data transmission rate 9.6 kBd was used. The results given in Table 5 are partly influenced by the fact that the range of transmitted data was different at some analysers and included some parameters which were not required. TABLE 5 Instruments

Time of transmission

A C D

25 min 17 min 13 min

B

2 min

Table 7 Requirement Online connection U- accuracy of 16A, CEI, 1994 Prof. Jacob Barkan was born in 1929 in Kraslava, Latvia. He received the Diploma Engineer degree in electrical engineering, Candidate of Technical Sciences (Ph.D.), from Latvian State University (Riga Polytechnical Institute) in Riga and Dr. hab. eng. from Electric Power Institute of Academy of Sciences, Moscow. For more than 20 years J. Barkan was operator, chief operator in Latvian power system Latvenergo. Presently, he is professor in Riga Technical University and principal research investigator. His areas of interest are power system regimes and automation, power quality. Mailing address: J. Barkan Faculty of Power and Electrical Engineering, Riga Technical University, 1, Kronvalda blv., Riga, LV 1010, LATVIA phone: (371) 7089937 e-mail: [email protected]

err,%

30 20 10 0 1

1.5

2

2.5

3

FLr

Fig.8 The resultant error for the case IFL1v(t ) v1(t ) @ dt 2

0

100

U1 V 2 V12

100

V1

The factor DF is equal to ratio of root mean square of the difference between the actual voltage and its fundamental, to the fundamental (somewhat similar approach is mentioned in the motor drives standards) [7,14]. If the interharmonics do not occur in the investigated circu,it, then the factors THD and HD are equal. The following nomenclature is proposed here: Let the factor describing harmonic distortion be HDh and HDi the factor describing interharmonics distortion and THD total factor describing both distortions. Factor describing distortion caused by interharmonics is defined by Eq. 9, which is as follows: f

¦Vihk2

HDi %

n 2

V1

100 (9)

f

2 ¦Vhhn

3. FACTORS DESCRIBING HIGHER HARMONICS AND INTERHARMONICS

HDh %

The Total harmonic distortion (THD) given by formula (7) describes the higher harmonics content of voltage or current in the non-linear circuit. For voltage: THDV %

V22 V32 ... V1 f

¦

k 2

V1

100

100

k 2

V1

100

Thus the coefficient described by formula 7, according to the new nomenclature should be named as Higher Harmonic Distortion, HDh. The relation between HDh, HDi and THD coefficients is given by Formula 10. THD

(7)

2 Vhhk

(8)

HDh2 HDi2

(10)

If THD and HDh can be measured then the DHi can be calculated from Eq.10. It is reasonable to assume that if HDi HDh then HDi can be neglected as small and THD^ HDh.

The magnitude of THD can be measured by available instruments like Fluke 41B, F27 Chauvin Arnoux etc. The THD as given by Eq.7 describes fully the distortion of voltage, eventually current, when Section 3. Power Quality Measurements: Techniques, Instruments, Results

249

4. DETERMINATION OF HARMONICS AND INTERHARMONICS.

Nowadays the favoured method of harmonic analysis seems to be a numerical one. Fast Fourier Transform (FFT), a faster form of DFT is an efficient and accurate enough method. The FFT measuring equipment can be used to: x quasistationary harmonics & interharmonics x time variable harmonics & interharmonics x rapidly variable harmonics & interharmonics In numerical analysis of distorted voltage and current the FFT method needs a measurement window, which is a time interval within which the samples are taken. Two types of windows Hanning and rectangular are commonly used and chosen according to the time dependence of harmonics. 5. CONCLUSIONS.

a)

b)

c)

There is a current shortage of precision in defining various types of distortions caused by non-linear load There is an inherent difficulty in discrete treatment of these harmonics and interharmonics and subharmonics The authors propose a method of determination of the interharmonic distortion.

6. REFERENCES

[1] Andria and others.: Analysis of Distorted Unbalanced Waveforms in Inverter Devices. Conf. EPE2. Grenoble 1987. [2] Beites L. F.., Mayordomo J.G., Hermantez A. Asensi R. Harmonics, Interharmonics and Unbalances of Arc Furnaces; a New Frequency Domain Approach. 8th International Conference on Harmonics and Quality of Power, ICHQP’98 Athens, Greece, 1998, pp.1071-1078.

250

[3] Carbone R. Meniti D., Sorrentino N., Tesla a.: Interactive Harmonic and Interharmonic Analysis in Multiconvertor Industrial System. 8th International Conference on Harmonics and Quality of Power, ICHQP’98 Athens, Greece, 1998, pp.432-438. [4] Czarnecki L.S. wietlicki T.: Power in nonsinusoidal networks, their analysis, interpretation and measurement. IEEE Trans. Instrum. Measur. 1990, vol.IM-39.2 pp 340-345. [5] Quality of Power, ICHQP’98 Athens, Greece, 1998, pp.749-754.. [6] Hammon J., Van Der Merwe F.S.: Voltage Generated by Voltage-Fed Inverters Using PWM Natural Sampling, IEEE Trans. On Power Electronics. 1988. Vol. 3 no.3 [7] IEEE-519. Recommended Practise and Requirements for Harmonic Control in electrical Power system. Standard-1992 [8] Matllavelli P., Fellin L., Bordignon P., Perna M.: Analisis of Interharmonics in DC Arc Furnances Instalations. 8th International Conference on Harmonics and Quality of power, ICHQP’98 Athens, Greece, 1998, pp.1092-1099. [9] Loskarn M., Tost K. D., Unger C., Witzmann R., Mitigation of Interharmonics due to large Cycloconverter-Fed Milli Drives 8th International Conference on Harmonics and Quality of power, ICHQP’98 Athens, Greece, 1998, pp.122-126. [10] Nowacki Z. Pulse Width Modulation in AC Electrical Frequency Converters. (in Polish) PWN, Warsaw, 1991 ISBN - 83-01-10402-3 [11] Tunia H., Winiarski B., Power Electronics (Energoelektronika ) (in polish, WNT, warsaw 1994. ISBN 83-204-1648-5 [12] Peretko L.: Interharmonics Frequencies Measurement and the Relation with Light Flicker. 7th International conference on Harmonics and Quality of Power. Las Vegas, USA, 1996 [13] Varadan S. Grigis A.S., Makram E.B.: A Fast Fourier Transform (FFT) Perspective of Interharmonics in Power System. 7th International Conference on Harmonics and Quality of Power. Las Vegas. USA. 1996. [14] IEC 61036 Alternating current static wattmeters for active energy. (classes 1&2)


Prof. Zygmunt Kusmierek Was born in 1936 in Poland. He received the M.Sc. degree in electrical engineering from the Technical University of ód¦, Ph.D. (1996) and D.Sc. (1982). Presently, he is a Professor and Head of the Division of Electrical Metrology and Motor-car Electrotechnics, of the Institute of Theoretical Electrotechnics, Metrology and Material Science in the Technical University of ód¦. His areas of interest include power quality, analysis harmonics and energy components in power. systems and their measurements under nonsinusoidal conditions. Mailing address:

Zygmunt Kusmierek Technical University of ód¦, I-12 Stefanowskiego 18/22, 90-924 ód¦, Poland Phone: +48 42 6312518), fax: +48 42 6362281 E-mail: [email protected]

Dr M. Jerzy Korczyski Was born in 1949 in ód¦, Poland. His M.Sc degree was in Electrical Engineering (Electronic Circuits and Power Devices), Ph.D. in Measurement and Instrumentation from the Technical University of ód¦. At present he is a senior lecturer in the Institute of Theoretical Electrotrechnics, Metrology and Material Science, (Division of Metrology and Motor-car Electrotechnics). His areas of interest include, theoretical metrology, evaluation of measurements, uncertainties, signal processing, components of power in non-linear circuits, computer based measurement systems. He is a co-author of the books: "Scientific Metrology" "Measurement Data Handling", and he lectures at the International Faculty of Engineering. He has a wide industrial experience in Automation and Visualisation of Production, Quality Engineering and Electrical Drives. Mailing address: M. Jerzy Korczyski Technical University of ód¦, I-12s Stefanowskiego 18/22, 90-924 ód¦, Poland Phone: +48 42 6312518), fax: +48 42 6362281, E-mail: [email protected]


251

252





CALCULATION OF INTERHARMONICS VOLTAGE SPECTRUM IN THE POINTS OF INDUSTRIAL POWER SUPPLY SYSTEMS Igor V. ZHEZHELENKO Yuri L. SAYENKO Tatiana K. BARANENKO Priazovskii State Technical University Mariupol (Ukraine) Abstract - The paper describes the method for calculating interharmonic voltages spectrum in the points of industrial power supply systems. The method is based on spectral-correlation theory of random functions. Correlation function parameters of a number of quickvariable loads at industrial enterprises are given. The authors have developed a program for computer-generated calculations; the sample of calculation is presented in the paper.

Calculation of interharmonic voltages spectrum in the points of industrial electrical power systems (EPS) is needed to solve the problem of normalization of interharmonics with further decreasing their levels. Interharmonics appear during various non-linear loads which are exorbitant at all industrial enterprises of today. Interharmonics sources are, first of all, such quickvariable loads as arc furnaces (AF), aggregates of electric arc and impulse welding, various rolling mills. During AF operation, especially within the heating period, the form of the supply network voltage curve undergoes distortion, and current curves, different in distortion phases, occur. These distortions are random by their nature, which is connected with the technology of heating metal by electric arc. Analysis of operation of different types of welding aggregates shows that for all types of electric welding current consumption is of a pulse character with random or determined variations in alternating pulses and their form [1]. During operation of various rolling mills equipped with thyristor drives amplitude modulation of curves of network currents by random law can be observed. Thus during operation of the above mentioned

quick-variable loads the curve of the network current may be presented in the form of amplitudemodulated fluctuation (oscillation) with a random law of changing the amplitude and the initial phase. f

I ¦ Q

i (t ) [[ (t ) 1]

mQ

sin(QZ 0 t IQ ) ,

(1)

1

where [ (t ) - centered stationary random process with a null mean value and a given correlation function K [ (W ) ; I mQ - constant amplitudes; Z 0 - angular frequency; IQ - mutually independent random initial phases uniformly distributed at an interval of (S , S ) . The correlation function of a random process of the non-linear load current i (t ) will consequently be [2] f I m2 Q K i (W ) [ K [ (W ) 1] cosQZ 0W , (2) Q 1 2

¦

where K [ (W ) - a given correlation function of modulating random process [ (t ) . By a known correlation function K i (W ) it is possible to obtain the spectral density of a nonlinear load current, using the Fourier transform (FT). S i (Z )

f

³ K (W )e i

jZW

dW .

(3)

f

In this case the spectral density is the sum of the two components - discrete and continuous [2]. The


253

discrete component is the sum of delta-functions and characterizes distribution of higher harmonics. Distribution of interharmonics is characterized by a continuous component, that is why further on we will consider only the continuous component of the spectral density. As a given correlation function K [ (W ) for the majority of non-linear loads correlation functions of the following type can be used: K [ (W ) K [ (W )

K [ (W )

D[ e

D[ e

D[ e

D W

D W

D W

;

(4)

cos Z1W ;

(5)

(cos Z1W

D sin Z1 W ) , Z1

º »; D (QZ 0 Z ) ¼» 1

D[ D

2

¦

(7)

I m2 Q

º 1 ; 2» D (QZ 0 Z1 Z ) ¼» 2

D[ D

f

I ¦ Q

2 mQ

º QZ 0 Z ; 2 » D (QZ 0 Z 1 Z ) »¼ 2

2

k o i (1 k o i )

i 1

,

n

¦P

2

i

(11)

t ci

i 1

where Pi - nominal active power of a single ER ; t ci - cycle time; t o - operation time to tc .

ko

(12)

For one-type electric receivers

TC

k o. av (1 k o. av )t c.av .

(13)

The frequency of periodic oscillations Z 1 is determined by the number of large welding machines n and the time of the cycles t c.av

2S n. t c.av

Z1

(14)

Load variance of ER group with pulse operation mode is determined by the equation

¦ I n

D

2 rms .i

2 I av .i

(15)

i 1

(9)

Correlation functions of AF load current are very well approximated with equation (5). Static parameters of variance, attenuation coefficient and proper frequency of the correlation function for different stages of a heating period of AF by each 254

TC

(8)

ª QZ 0 2Z 1 Z « 2 2 4Z 1 1 ¬«D (QZ 0 Z 1 Z ) QZ 0 2Z 1 Z QZ 0 Z 2 2 2 D (QZ 0 Z 1 Z ) D (QZ 0 Z 1 Z ) 2 S i (Z )

¦P

i

ª 1 S i (Z ) « 2 2 4 Q 1 «¬D (QZ 0 Z 1 Z ) 1 1 2 2 2 D (QZ 0 Z1 Z ) D (QZ 0 Z1 Z ) 2 f

(10)

where TC - time-constant of damping of correlation links For electric receivers (ER) with different operation modes [1] n

D - damping factor of correlation function; Z 1 - proper frequency of correlation function. Equations (4)-(6) are presented by the following continuous components of spectral densities of non-linear load currents, obtained according to (2), (3) D[ D f 2 ª 1 S i (Z ) I mQ « 2 ¦ 2 2 Q 1 «¬D (QZ 0 Z ) 2

D 1 TC ,

(6)

where D[ - a variance of random process [ (t ) ;

phase are given in [3]. Table 1 summarizes the averaged ranges of changes of the above mentioned parameters. During the group operation of the welding aggregates the correlation functions are approximated with equations (4) and (5) with the latter (equation (5)) used in case of availability of powerful welding machines [1]. The damping factor of correlation function is

where I rms.i and I av .i - are rms and average currents of one ER.

I av.i

s nom k l k o ; U nom

(16)


snom kl ko , U nom

I rms.i

(17)

where s nom - nominal power of ER; k l - coefficient of load; U nom - nominal voltage. Table 1 gives the limits of variations of the damping factor D for different types of welding machines when using correlation function, as it appears in (4). Correlation functions of load

can be approximated by equations (5), (6). The load variance is determined as 2 2 I rms I av ,

D

(18)

where I rms and I av - rms and average currents, defined by the diagram of the rolling mill load. Table 1 shows mean values of parameters of correlation functions, (5) (in numerator) and (6) (in denominator), of some types of rolling mills.

currents for different rolling mills during operation TABLE 1. Parameters of correlation functions Load

Variance D[ , 2

Damping factor D , -1

Proper frequency Z1 , -1

1,2103y3,1103 8103y104 1,3104y4104

0,36y1,75 1,69y3,74 1,16y3,26

0,49y2,87 2,035y4,73 1,36y6

0,33y10

0,08y2

0,0067y0,2

0,19 y 0,43 0,41 y 0,76 0,33 y 0,41 0,38 y 0,63 0,036 y 0,074 0,064 y 0,096

0,59 y 1,27 0,62 y 1,31

Correlation function

AF:

AF-20 AF-100 AF-200 Welding machines: - single-spot, relief and multi-spot; - joint welds and butt welds; - arc welding aggregates Rolling mills:

K [ (W )

K [ (W )

- rolling mill 950

K [ (W ) K [ (W )

- blooming 1150

D[ e

D[ e

D[ e D W

D W

D[ e

D W

cos Z 1W

¦ n

D W

2 I rms .i

D[

2 I av .i

i 1

cos Z 1W

(cos Z 1W

D sin Z 1 W ) Z1

D[

2 2 I rms I av

- rolling mill 250

0,98 y 1,31 1,05 y 1,43 0,87 y 1,11 0,13 y 0,21

f

Random changing of non-linear load current i (t ) is, as a rule, non-periodical. However, if it is considered within a certain comparatively long interval T where random values i (t ) and i (t T ) are independent, the random process i (t ) may be interpreted as a periodic function with a T period. As T period it is advisable to use damping time of correlation, which can be determined by the diagram of random process correlation function of non-linear load current. In this case T period is proportional to correlation interval. T

nW c ,

where W c - correlation interval

(19)

³ K (W ) dW i

Wc

0

K i (0)

,

(20)

where K i (W ) - correlation function, corresponding to continuous component of spectral density K i (W )

K [ (W )

f

¦ Q 1

I m2 Q cosQZ 0W ; 2

(21)

K i (0) - value of correlation function at W 0 which in all types of correlation function K [ (W ) is

determined by equation f

K i (0)

D[

¦ Q 1

I m2 Q . 2

(22)

Thus, having represented random process as a Section 3. Power Quality Measurements: Techniques, Instruments, Results

255

periodic function, we may determine its amplitude spectrum as [4]

2 I(Z ) Gi (Z ) , (23) T where G i (Z ) - is spectral function of random process i (t ) G i (Z )

T

³ i (t ) e

jZt

dt .

(24)

Spectral density or energy spectrum of random process i (t ) is connected with spectral function with the following equation Gi2 (Z ) , T

(25)

where Gi (Z ) - is a module of the spectral function. Then using (23) and (25) the module of the interharmonics amplitude spectrum is: I (Z )

2

S i (Z ) . T

The loads which are interharmonics sources can be regarded as independent. In this case current density of a EPS point with several interharmonics sources represent the sum of spectral densities of corresponding non-linear loads. To determine interharmonics voltage spectrum a system of nodal equations is compiled which has the following view in matrix form: Yb (Z ) U (Z ) I (Z ) ,

0

S i (Z )

load current from each source individually.

(27)

where Yb (Z ) - bus admittance matrix ; U(Z ) - module matrix of interharmonics voltage spectra modules in EPS points; I(Z ) - module matrix of interharmonics current spectra modules in EPS points (module matrix of interharmonics currents sources. Matrix nodal equation (27) is solved relative to the matrix of bus voltages spectra U (Z )

Z b (Z )I (Z ) ,

(28)

where Z b (Z ) - matrix of network bus impedance (26) Z b (Z ) Yb (Z ) 1 .

The proposed method for computing interharmonics amplitude spectrum allows to extend the existing techniques of calculation of higher harmonics currents and voltages in the points of industrial EPS (electric power system) to calculation of interharmonics, using the method of nodal voltages [5]. Here, if a number of interharmonics sources are connected to the EPS point, the random process of non-linear current variations is a total of random processes caused by

(29)

Example: Let us consider the proposed algorithm of interharmonics voltage spectrum calculation as applied to the scheme in Fig.1. The corresponding equivalent circuit for interharmonics is given in Fig.2.

Fig. 1. Power supply of industrial enterprise Interharmonics sources in the scheme under 256

consideration are AF-100, AF-200 and 12-pulse Electrical Power Quality and Utilisation

thyristor converter for power supply of the rolling mill-650 engine. To determine the modules of mean values of higher harmonics levels given in [5] for different types of AF for the heating time were used.

Fig. 2. Equivalent circuit for interharmonics This is caused by the difficulties involved in performing precise analytical calculation of higher Harmonics levels generated by AF. Currents

interharmonics currents, I 1 (Z ) and I 2 (Z ) , the sources of which are AF-100 and AF-200 the Variances and correlation function parameters are taken from Table 1. Rolling mill current variance is derived from its load changing, as is the correlation function, which is well approximated by equation (5) with the following parameters: D =0,6 -1, Z =1,96 -1. Calculation algorithm for interharmonics voltage spectra in the points of the given scheme was realized with the help of Mathcad. Fig 3 shows the diagrams of interharmonics voltage spectra in points 5-7. Interharmonics levels in points 1-4 are identical with interharmonics levels in points 5 and 6, as low impedance values of the corresponding scheme elements do not significantly influence their flow.

)

b)

) Fig. 3. Interharmonics voltages spectra in points 5(a), 6(b) and 7(c). ¬.ª.: ` # Analysis of the diagrams shows that interharmonics resonance occurs at the frequency " of the harmonic of 5th order. . IV {" # REFERENCES 1. ¨# .ª. , .«. , ªX _ ..: _ # " . - .: , 1992. 2. .., ..,

'{ “|' { { { {} .” // ® `¯°. – ®X`°, 2000. – .33-36. 3. ` # " / ! . .«. , # .. – .: , 1975.


257

4. .. , ¬.ª.: > " . "# # - '. // #. – X`°, 2000. – .136-137. 5. ..: ` " . – .: , 2000.

Prof. Igor V. Zhezhelenko was born in 1930 in Mariupol, Ukraine. He received Ph.D. and D.Sc. degrees from Novocherkask Polytechnical Institute. Presently, he is rector of Priazovskii State Technical University, Member of Academy of Science of high Education of Ukraine. His areas of interest include electric power quality and electromagnetic compatibility.

Mailing address: Yuri L. Sayenko Priazovskii State Technical University Universitetska Str. 7 87500 Mariupol UKRAINE phone: +38-0629-528599, fax: +38-0629-529924 e-mail: [email protected] Tatiana K. Baranenko was born in 1967 in Mariupol, Ukraine. Post-graduate student, assistant at department of Industrial Power Supply at Priazovskii State Technical University. Graduated from PSTU in 1989. Main research area – Quality of electric power. Mailing address: Tatiana K. Baranenko Priazovskii State Technical University Universitetska Str. 7 87500 Mariupol UKRAINE phone: +38-0629-336023, fax: +38-0629-529924

Mailing address: Igor V. Zhezhelenko Priazovskii State Technical University Universitetska Str. 7 87500 Mariupol UKRAINE phone: +38-0629- 332108, fax: +38-0629-529924 e-mail: [email protected] Prof. Yuri L. Sayenko was born in 1962 in Mariupol, Ukraine. He received Ph.D. degree from Institute of electrodynamics of Ukraine National Academy of Science. D.Sc. degree he received from Silesia Polytechnical Institute of Gliwice. Presently, he is Professor of Priazovskii State Technical University, Member IEEE. His areas of interest include electric power quality and electromagnetic compatibility.

258





THE HIGH-SPEED OVERLOADING OF MEASURING TRANSDUCERS OF DISTORTED ELECTRICAL SIGNALS Krzysztof PACHOLSKI Technical Uniwersity of Lodz Lodz (Poland) Abstract - The new phenomena of the overload at input of voltage, current and electrical power transducers is described in the paper. The physics of the phenomena and the influence of the overload on the accuracy of these transducers are presented too. 1. INTRODUCTION The current and voltage of the electric energy consumer supplied by the static converters have been deformed and a frequency of the basic harmonic of these signals belonged to the acoustic band. Such signals are more than a reason of the overloading of input systems of electronic power transducers and of electronic rms-value and average value voltage transducers. Produced today measuring transducers of electrical value realised are on base of an electronic circuits using the operational amplifiers. Reason of the overload of these circuits can be: too high a peak value of a measuring signal and to fast a slope edge of this signal. The first reason causes so-called the amplitude overloading of transducers. Such overload one can avoid by definition so-called the limit value of a peak factor of the signal. Information

about the limit value of a peak factor allows on correct choice of a measuring range of the transducers. The choice of the range, according to the peak value of the measuring signal, does not eliminate the second reason of the overload of the transducers. Reasons of the high-speed overload of the transducers one explained below.

2. THE HIGH-SPEED OVERLOADING OF TRANSDUCERS Application of measuring transducers to work with deformed input signals qualify proprieties socalled. an alternating-current stage (AC- stage) – Fig. 1. This name embraces input part of the transducer, in which the operations on alternating signals are realised. In analogous transducers the ACstage determine circuits previous integrator. The AC-stage of hybrid and sample transducers is analogous part of the circuit of these transducers. w

up

x

UW

F(x)

Fig. 1. Block diagram of an AC-stage of transducers ( w, x - are vectors of input signals of respective block: w [u, i ], x [ xu , xi ] - of power transducers, w ui , x xu - of voltage transducers, F(x) - non-linear operational block (not used in sample transducers): Fx xu - of aver-

age voltage transducers, F x xu 2 - of rms-voltage transducers, Fx xu xi - of power transducers) At distorted measuring signals being characterised with high rate the output signal of amplifiers changes with limit rate S, called Slew Rate. The high-speed overloading occur when the rate of an input signal of transducers has value greater from the limit value S d [5] and is more than once not noticed by the users. Only excessive error of the measurement informs about occurrences of this


259

overload. The component of the error reason which is the high-speed overloading to can attain values from several to a dozen or so percentage. The new component of an error the author called as the overloading error 3. THE OVERLOADING ERROR The definition of the overloading error is given by [5]: df Yop Yo (1) Gp Yo In the above relation Yop marks the output signal of the transducer which is overloaded, Yo is the output signal of the transducer working without overload. Values Yop and Yo of the output signal marked have to be at maintenance of the constant of the measured value by the examined transducer. To occurrences of overloading error indispensable is increase of the rate S um of the input signal of the transducer over the limit value S d . The increase of the parameter S um can come into being in two chances. In first chance reason of the increase of the parameter S um is the increase of a frequency of the input signal with maintenance of his shape. In second chance while, the increase value of the parameter S um qualifies change of a shape of the signal, without changes of the frequency of his basic harmonic. To explanations of the influence exchanged of factors on value of the overloading error the author used an AC-stage of the idealised average value transducers. This circuit creates serial connection of the non-inverting amplifier about amplification equal to 2 and the ideal full wave rectifier - Fig. 2. In aim of picturing influence of the increase of the frequency of an input signal on value of the overloading error author accepted that the examined transducer working with triangular input signals: symmetrical and asymmetrical (Fig. 3). u

non-inverting amplifier

uo

uo

up

ideal full wave rectifier

Fig. 3. Time-domain response of an amplifier (overloaded and working without overloading) with symmetrical (a) and asymmetrical (b) triangular input signals If frequency of these signals has value at which S um ! S d then the output signal of the amplifier has course marked on Fig. 3 with the continuous line. The overloading error of the transducers with the symmetrical input signal is given by:

Gp

U op U o Uo

I

Sd 1 Su

(2

where U om , U olm marks the peak value of the output signal of the overloaded amplifier and the peak value of the output signal of the amplifier working without overloading. The symmetrical triangular signals, about rate S u t S d , cause overloading of an amplifier during all of one's own period. In spite such of an overload the value of the error is less from units. The overloading error of the transducer with asymmetrical triangular input signal (Fig. 3b) is given by: Gp

Fig. 2. The AC-stage of the idealised average value transducer

U om U olm U olm

ª § U 0.5«1 ¨ om « ¨ U olm «¬ ©

· ¸ ¸ ¹

2 º 1

S u Sd

» 1 » S u »¼ 1 S u

(4)

u, u o , u ol

260 U om

uo

U olm


where S u and S u marks the rate of the leading and the rate of the trailing slopes of the triangular signal. From dependence (4) results, that value of the overloading error of an examined circuit, is dependent on from difference of the rate of lead and trail slope of the input signal, ie from. asymmetry of this signal. The high-speed overload of transducers to occur also at distorted input signals which course described continuous functions. Such signals, similarly as ideal triangular signals will be reason of overloading of transducers if the course of the rate of these signals is asymmetrical. In distinction from triangular signals, with reason of increase of the rate of continuous signals, over the limit value S d , is not only change them frequencies but also suitable change of them shape without changes of the basic harmonic's frequency. In aim of explanation of reasons of the overloading error which caused is change of the shape of signals the author accepted, that the average-value transducer from Fig. 2 working with the distorted input signal similar to the symmetrical triangular signal (Fig. 3a). This input signal creates sum first and third harmonics Fig. 4. c S um u

M3

S

M3

S u f t at growth and during falls of this signal in the half-period is asymmetrical – Fig. 5. ui , Su

S um Su

T 2

ui

t

0

t0

S um

t1

t2

Fig. 5. S u

f t of the distorted signal represented on Fig. 4

Confirmation of considerations is course of frequency characteristics of the overloading error of the multimeter (type Fluke 8842A [5]) which represents of Fig. 6. The characteristics from Fig. 6 the author marked for the signal deformed by third harmonic which participation was properly equal to 0.2 and 0.4. At marking of these characteristics reason of the overloading error was change of a phase of the third harmonic from 0 rad on S rad. 0.16

0

Gp

0.14

0

0.12

t S um

0.1

h3

0.4

h3

0.2

0.08

\

0.06

Fig. 4. Waveform of deformed input signal 0.04

Indispensable to occurrences of the overloading error change of a shape above-mentioned. of the signal to obtain one can by change of the phase third harmonics from 0 rad on S rad. . Besides the frequency of the basic harmonic of the signal is determined by the condition:: c S um ! S d ! S um

0.02

f 1 [kHz]

0 -0.02 0

5

10

15

20

25

30

35

Fig. 5. Overloading error as a function of frequency for the multimeter type 8842A.

(5)

where S um is the maximum rate of the signal, c is which harmonics have peaceable phases. S um the maximum rate of the signal which the third harmonic has the phase equal to S rad. Reason of the overloading error of transducers is not only change of a phase of the third harmonic of the signal represented on Fig.. 4 from 0 rad on S rad. This error occur because course of a rate

The bandwith of the mulimeter 8842A is given by frequency 100kHz. From course of characteristics represented on Fig. 6 results that signals distorted harmonics about not large participation which basic harmonics frequency belonging to central range of the pass band, can be reason of overloading error about large values. Similarly as average-value transducers, on changes of phases of harmonics of the input signal, react the rms-value transducers and. the power transducers. At marking the overloading error of transduc-


261

ers one should remember, that changes of phases of harmonics do not have influence on rms-value of the input signal of a device. Besides overloading error independently from rules of activity of power transducer marked has to be separately for the voltage channel and for the current channel.

PhD Krzysztof Pacholski Was born in 1953 in ³czyca, Poland. He has received M.S.c. and Ph.D. degress from the Technical University of ód¦ . Presently, he is researcher in this University. His field of interest is the measurement power and the voltage transducers of distorted electrical signals.

4. CONCLUSIONS

Mailing address: Krzysztof Pacholski Technical University of ód¦ Institute of Elektrotechnology, Metrology and Material Science Stefanowskiego 18/22, 90-924 ód¦ POLAND phone (+48)(0-42) 631-2524 fax (+48)(0-42) 636 22 81 e-mail: [email protected]

In the paper, one introduced the new additional error of measurement transducers of electric distorted signals which author called as overloading error. This error occur at input signals which rate has asymmetric course, and the maximum value of the rate is greater from the limit value S d . Will avoid of the overloading producers of transducers should give value of this parameter. 5. REFERENCES 1. Titze U., Schanke CH.: Uk~ady pó~przewodnikowe, WNT, Warszawa 1987. 2. Pacholski K.: B~d przesterowania przetworników napiciowych sygna~ów impulsowych, Kwartalnik Elektroniki i Telekomunikacji z.1, 1995 pp. 69-87. 3. Pacholski K.: The High-Speed Overloading Error of Voltage Transducers, Measurement 1995 nr 3, vol. 15, pp 165-168. 4. Pacholski K, Marks-Wojciechowska Z..: The Mathematical Model of Electronic Measuring Transducers for Processing Distorted Signals, 9th International Symposium IMEKO 1997, pp. 177180, Glasgow 1997. 5. Pacholski K.: Wp~yw przesterowania szyb kociowego na w~aciwoci metrologiczne przetworników pomiarowych wielkoci elektrycznych (monografia przygotowywana do druku).

262


Section 4 Methods of Power Quality Improvement: Filters, Power Compensation, Phase Balancing, etc. 4.1. BRENNA M., FARANDA R., VALADÈ I., BALLOCCHI G.: Flicker Reduction by Using Distributed Generation and Active Power Compensators (Italy) ........................................................265 4.2. BAJSZCZAK G.: Voltage Distortion Compensation in Transmission Networks by Series Converter Filter (Poland).....................................................................................................................273 4.3. RUSISKI J., KOT E., BENYSEK G.: Active Power Filter’s Behaviour in Non-Periodic Conditions (Poland) .............................................................................................................................279 4.4. DZIEA J.A.: Input-Output Decoupling in Active Power Filter Control (Poland).............................285 4.5. MIENSKI R., PAWEEK R., WASIAK I.: Application of SVC for Load Balancing (Poland).........291 4.6. VARETSKY Y.: Exploitative Characteristics of SVC Filter Circuits (Ukraine)................................297 4.7. KLIMASH V.: Transformer and Thyristor Based Compensator of Voltage Deviations and VAR with Four-Quadrant Control (Russia) .................................................................................................303 4.8. TIRONI E., VALADÈ I., LOPES G., UBEZIO G.: Voltage Quality Improvement Using Superconducting Magnet Energy Storage (SMES) Devices in LV System With Neutral Conductor (Italy)..................................................................................................................................309 4.9. FARANDA R., TIRONI E., VALADÈ I., UBEZIO G.: Comparison Between UPS Line Interactive Devices Designed to Solve Power Quality Problems (Italy) .............................................317 4.10. POSTOLATI V.M., BICOVA E.V., KUZNETSOV V.G.: Characteristics of Controlled Electrical Transmission Lines of an Alternating Current of the Increased Capability and of Their Application for Improvement of Quality of Parameters Modes Power System and Increase of Reliability of Electro Supply (Ukraine, Moldavia) ..........................................................................327 4.11. SAKKOS T., SARV V., JÄRVIK J.: Power Quality Improvement in Diode Rectifiers Using Ripple-Power Re-Rectification and Multifunctional Filter Elements (Estonia) ..................................333 4.12. ABREU J.P.G., BERNARDES D.F.: Converter Transformers Phase-Shift - A Useful Approach - (Brazil) .............................................................................................................................339 4.13. RUSINARU D., MIRCEA I.: Aspects Regarding Load Current Symmetrization in Unbalanced Power Systems (Romania) ...................................................................................................................347 4.14. WALCZAK J., GRABOWSKI D.: Neural Networks for Optimization of Power Systems (Poland)................................................................................................................................................353

Section 4. Methods of Power Quality Improvement:Filters, Power Compensation, ...

263

264




FLICKER REDUCTION BY USING DISTRIBUTED GENERATION AND ACTIVE POWER COMPENSATORS

Morris BRENNA, Roberto FARANDA, Ivan VALADÈ Dipartimento di Elettrotecnica Politecnico di Milano, Italy

Abstract – This paper deals with flicker in distribution network and two countermeasures, which are able to mitigate voltage fluctuations, are proposed. In the presence of high voltage fluctuation it is possibile to reduce its amplitude by using both Distributed Generation, at present growing in importance in distribution network, able to increase short-circuit power [1], and Active Compensation Device capable of quickly exchanging variable amount of non-active power with the network [2]. In this work the use of active compensation devices as preventive countermeasure to limit flicker has not been taken under consideration. Theoretical study results have been verified simulating the behaviour of MV test network in two different layout: radial and meshed. Keywords – Flicker, Distributed Generation, Active Compensation Devices. 1. INTRODUCTION The matter of electrical supply quality is becoming more and more important, mainly because of the stronger and stronger relations towards manufacturing activities and industrial services. Correct operations of industrial devices and machines, from computers to drives, depend on absence of significant disturbances of sinusoidal waveform supply voltage. The same devices and machines can even themselves cause waveform disturbances and increase neutral current flow up to damage other equipment. In particular, the following disturbances in supply voltage can be identified:

x x x x

Giorgio BALLOCCHI Davide Campari Milano S.p.A. Milano, Italy

Harmonics, mainly due to voltage drops caused by harmonic currents inlet in non linear loads; Unbalancing, due to unbalanced current absorption; Voltage sags, due to faults, motors start-up, etc.; Fluctuations which are broadly described in the following.

Voltage fluctuations can be caused by variable loads which change their absorption repeatedly with periods ranging from one tenth to some tens of seconds, by loads which generate occasional power transients (i.e. start up of large induction motors), by the presence of interharmonic frequencies in the net, and by non-linear and timevarying loads which use electric-arc (i.e. arc furnaces, welding-machines). Even slight values of voltage fluctuation whose frequency ranges between 0.5 and 35 Hz can cause flicker. This phenomenon creates an impression of unsteadiness of visual sensation induced by a ligth stimulus whose luminance or spectral distribution fluctuates with time. The European standard EN 61000-3-3 states the limits to these variations according to the repetition rate; especially at a frequency of 8.8 Hz – at which human eye presents its maximum sensibility – the allowed value of 'V/V is 0.27%. At present the countermeasures for limiting voltage fluctuation consist of connecting intermittent loads to a high short-circuit power point of the net, or installing Static Voltage Compensators (SVC) for reactive power [3].

Section 4. Methods of Power Quality Improvement: Filters, ....

265

2. VOLTAGE FLUCTUATIONS – THEORETICAL ANALYSIS –

A PCC

Let us consider the elementary network shown in Fig. 1.a. It’s clear that even if we deal with a particular network layout, the analysis’ results can be generally applied; indeed the interaction between a disturbing load and any other network’s point can be studied with reference to an equivalent circuit similar to that shown in Fig. 1.a. The electrical power network is composed by: x An HV network, supposed of infinity power; x An HV/MV transformer station, whose shortcircuit power at output terminals is indicated with APCC; x A variable load U1 – source of voltage variations – connected to a point whose shortcircuit power is Acc1; x A load U2 – on which voltage variations are evaluated – connected to a point with shortcircuit power of Acc2; HV network

HV/MV transformer PCC MV/LV Substation

MV/LV Substation

U1

U2

(a) Z1

I1

Z2

I2

ZM

E

~

VM

V2

V1

(b) Fig. 1. Single-line diagram (a) and single-phase equivalent electric circuit (b) of the system under study in theoretical analysis.

If we suppose the three-phase system is perfectly balanced, the diagram in Fig. 1.a can be represented by the equivalent circuit shown in Fig. 1.b; using Kirchhoff law, we obtain voltage V 2 at U2 terminals: V 2 E Z M I1 Z M Z2 I 2 (1) In order to evaluate flicker, voltage values referred to nominal voltage and short-circuit power should be taken under considerations. Let us determine short-circuit power referred to one phase at Point of Common Coupling (PCC) and at connecting terminals of U2. These are respectively:

266

A cc 2

Vn2 APCC e jM ZM Vn2 A e jM Z M Z 2 cc 2 PCC

cc 2

Equation (1) can be re-written as follows: V V V2 E (2) n I1 n I2 A cc 2 Vn Vn A PCC In order to compute the variation of V2 when loads’ absorption changes, let’s differentiate the equation (2) considering E voltage as a constant – e.m.f. of the equivalent generator of HV network – : §V 2 · V V d ¨¨ ¸¸ n d I 1 n d I 2 (3) A PCC Acc 2 © Vn ¹ Let us assume the loads have a constant power factor, so that the differential of I 1 and I 2 phasors can be reduced to the differential of their modules. Because the phase angles between voltages of different power network’s nodes are usually small, we can take the projection of phasors’ difference – in the direction of V 2 – instead of the modules’ difference. If we indicate impedance angles of respectively U1 and U2 loads using M1 and M2, then the equation (3) can be approximated by: §V · cosM PCC M 1 d ¨¨ 2 ¸¸ Vn dI 1 APCC © Vn ¹ (4) cosM cc 2 M 2 Vn dI 2 Acc 2 Equation (4) allows us to make some considerations about the voltage sensitivity versus loads’ absorption and network’s characteristics. Let us start by saying that cosine’s values are always positive in usual inductive loads. Then we first observe that V2 voltage sensitivity to distortion loads decreases when PCC short-circuit power increases. So from this point of view, it is profitable to connect distortion loads to high shortcircuit power terminals. Generally speaking, an increase in short-circuit power of the net reduces voltage variations – due to loads changes – in all nodes. It can be obtained, for example, by setting up new generators or strengthening electric lines, but – mainly due to the rise of fault currents – there are limits to the application of these measures. Another relevant factor for voltage stability is the type of loads connected to the network. A load that diminishes inlet current flow as the applied supply voltage diminishes – i.e. a constant impedance – is inclined to limit voltage oscillations; in fact it reduces its contribution to line voltage drops. Vice versa a load that offers an opposite behaviour, such as a large induction motor, is inclined to amplify voltage variations.


3. MEASURES FOR FLICKER REDUCTION In order to evaluate measures for reducing or eliminating flicker, the 15 kV distribution network shown in Fig. 2. – in a complete meshed configuration – has been chosen. Tab. 1. indicates the main data used for lines, transformers, generator and loads. Let us assume that the load connected to node # 2 has a time varying absorption and causes flicker, while the other loads connected to nodes # 1,4,5,6 are constant impedances. HV Network

Busbar I

improve quality to other nodes; network layout modification could seem a simple process to undertake, but means setting up new lines and solving some problems in particular about protection systems and faults locating. Tab. 1. Data used for 15 kV distribution network under study

Name L1 L2 L3 L4 Name

Ts Busbar S

L1

L6

Ts T2 T3

Busbar 5

Busbar 1

Name U1

U5

L8

G3

Busbar 6

L2

U6 L4

Busbar 2

L7

Name L3 L5

Busbar 3

Busbar 4

U1 U2

T3

T2 Busbar F

U2

U4

3.1. Distributed Generation (DG)

Busbar G

G~

Lines R [:] X [:] Name R [:] X [:] 1.6 1.0 L5 1.2 0.6 0.8 0.5 L6 1.2 0.6 0.8 0.5 L7 1.0 0.5 0.4 0.2 L8 0.8 0.5 Transformers An V1n V2n vcc% Connection [MVA] [kV] [kV] 16 220 15 13 YN/Y 5 15 5 5 D/YN 5 15 5 5 Y/D Generator An Vn xd x’d x”d [MVA] [kV] [p.u.] [p.u.] [p.u.] 5 5 1 0.3 0.2 Loads An An [kVA] cosM Name cosM [kVA] 500 0.85 U4 2000 1 3000 cost 0.9 U5 1000 0.85 2000 var 0.9 U6 500 0.85

G3

Fig. 2. Single-line diagram of 15 kV distribution network

Let us assume also that U4 load is greatly sensitive to voltage variations. Indeed in today’s electrical systems there are appliances – such as computer, instrumentation and communication equipment [5] – much more sensitive to voltage fluctuations than lighting devices; in this case U4 could be an electronic data processing centre which can operate only within a narrow band of supply voltage. Among all the means that reduce flicker this study takes under consideration the use of Distributed Generation (DG), the use of Active Compensation Device (ACD) and the modification of network layout. However each of these has relevant technical and economical implications. DG can cause excessive rise of short-circuit current and inappropriate operations of faultprotection device; ACD locally reduces voltage fluctuations – at the critical node – but doesn’t

Let us consider a traditional type of generation, set up by synchronous generators directly connected to the 50 Hz network – it is the case, for example, of co-generation power plant which, due to the exigency to be positioned close to thermal appliances, could have a small size and could be directly connected to the electric distribution network –. Other systems of generations, such as those who use renewable energy fonts or fuel cells, microturbines etc., aren’t dealt with, because even if they are connected to the electrical distribution network, this connection takes place by means of electronic converters, and another kind of analysis is required. As mentioned before, the advantage a traditional DG brings to a distribution network in order to limit voltage variations is given by the increased short-circuit power; indeed both electro-magnetic and electro-mechanic generators’ transients, both voltage regulators’ time constants are too slow to succeed in following the fast variations which origin flicker.


267

Normally generators whose size is lower than 10 MVA don’t take part to voltage regulation, and therefore they are operated at constant power. Another case applies if DG is connected by means of electronic converters. Indeed it is possible a fast and independent control of active and reactive power, but in this case a deeper analysis, which should consider power availability – variable for renewable fonts – and time constants of storage systems too, is required. 3.2. Active Compensation Devices (ACD) There are some active compensation devices [6] able to reduce flicker of loads’ supply voltage; among them we mention the following: x UPS (Uninterruptible Power Systems) with double conversion stage; this device presents high costs, large dimensions, and lower efficiency if compared with other compensators; x DVR (Dynamic Voltage Restorer); this device exchanges only reactive power with the net, thus presenting the limits described in [7]; x Shunt converter with link reactance [8]; this device varies its power factor varying load amount and supply voltage; x Today’s UPQC (Unified Power Quality Conditioner) [9]; this device can stabilise the load supply voltage, can offer a unitary power factor, and allows for a very profitable dimensioning of converters. In this study the use of active compensation devices as preventive countermeasure – i.e. to limit flicker sources – has not been taken under consideration; as preventive countermeasures STATCOM (Static Compensators), or still UPQC working as fast reactive power compensators can be employed. Let us assume that there are no availability of energy storage devices, so that the only way to limit flicker is to exchange reactive power with the net. The device proposed and analysed in [2] has been chosen. It is able to stabilise load voltage supply to its nominal value and to offer a unitary power factor during line undervoltages: thus – to the extends of the present job – the results that follow are similar to those which could have been obtained connecting a UPQC. An electronic variable reactance in series to the power supply line make it possibile to compensate a range of voltages closely dependent on the power factor of the load [7]. In order to vary the output terminals’ power factor of the series unit, a shunt unit is required.

268

If compared with an UPQC the limits of this compensator are converters’ dimensioning and the impossibility of obtaining a unitary power factor in the event of over-voltages compensations. 4. SIMULATION NETWORK

RESULTS

ON

TEST

The type of DG technology here considered is a 5 MVA gas turbine driven synchronous generator connected to node # 3. The IEEE ST1 exciter model has been used to represent potential-source and controlled rectifier excitation system [11] – even if, due to the speed of the transients, the type of excitation system doesn’t affect the behaviour of DG in relation to flicker aspects –. The gas turbine model and the model of the speed regulator are GT type described in [12]. For simplicity the models used for compensator’s series and shunt units are respectively an ideal voltage and an ideal current generator; in such a case the dynamics is faster than in reality. The flicker is generated by U2 load, that is assumed to be constituted by two amounts: the first a constant load of 3 MVA and the latter of a varying load factor – from 1 to 0.8 at a frequency of 5 Hz – of 2 MVA. 4.1. Case 1: Radial Network At the beginning, a radial configuration, obtained by the network shown in Fig. 2. after opening L2 and L3 lines, has been considered. In some network’s nodes – among them the one connected to U4 sensitive load – the limits stated by the in-force standard are exceeded, making it necessary to use circuits’ countermeasure (Fig. 3.). Fig. 4.a points out clearly: x 5 Hz fluctuations of the rms supply voltage at U4 sensitive load; x partial reduction operated by DG; x almost complete compensation operated by the compensator. As well both of the two measures bring the rms voltage’s value near to its nominal value of 8660 V, but the consequences at the other nodes of the net are different (Fig. 4.b). Indeed DG reduces voltage fluctuations in each node of the net – because of the increased short-circuit power –while compensator stabilises voltage to its nominal value in the sensitive load, but tends to increase oscillations in the other nodes – because a voltage reduction increases current absorption thus increasing network’s voltage drops.


101 'U/U (%)

100 Norm With DG

With Comp

10-1 10-1

10

-0

10

1

10

2

103

min-1

104

Fig. 3. Value of voltage fluctuation in node # 4 in the case of radial network (case 1).

The meshed network that is taken under consideration is the one shown in Fig. 2., with L2 and L3 lines connected. With reference to the previous Case 1, the values of voltage fluctuations at the different network’s node are more similar between them and the variations are narrower (Fig. 5.). Here the use of DG improves slightly the quality of voltage fluctuations, while the use of compensator reduces greatly the flicker at the sensitive load and lets it substantially the same at the other nodes of the network. RMS Voltage on critical load (U4) Case 2

RMS Voltage on critical load (U4) Case 1

9000 With Comp With DG Norm

9000 With Comp With DG Norm 8500

Vrms [V]

Vrms [V]

8500

8000 8000

7500 7500

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

1

t [s]

1

t [s]

(a)

(a) Voltage fluctuation Case 2

Voltage fluctuation Case 1 0,8 DeltaV/Vn%

DeltaV/Vn%

0,8 0,6 0,4 0,2

0,6 0,4 0,2 0 1

0 1

2

4

5

6

2

4

5

6

Load

Load Norm Norm

With DG

With DG

With Comp

With Comp

(b) Fig. 4. Simulation’s results in the case of radial network: (a) rms supply voltage value at sensitive load, (b) supply voltage fluctuations at all loads.

4.2. Case 2: Meshed Network For simplicity distribution networks are managed using radial layouts, even if normally there are possibilities of forming meshes. Hence the measure of changing the layout of the net is normally feasible, but environmental constraints in building new lines, and set up and tuning of the accordingly modified protection system must be taken in the right consideration.

(b) Fig. 5. Simulation’s results in the case of meshed network: (a) rms supply voltage value at sensitive load, (b) supply voltage fluctuations at all loads.

The meshed configuration allow to get higher and more uniform distribution of short-circuit power at each node of the network. The consequences of DG are less evident because of the shorter percentage rise in short-circuit power due to its use: indeed flicker reduction in comparison to normal configuration changes from 17% in Case 1 (radial network) to 12% in Case 2 (meshed network).


269

This paper has dealt with the flicker problem through computer simulations on a 10 buses test network. Theoretical analysis showed that an increase in short-circuit power of the net reduces voltage variation, and the fluctuation is very affected by the load type. Among all the means that reduce fliker, Distributed Generation, an active compensation device and network layout modification have been considered. The first can mitigate flicker in all buses increasing short-circuit power in the whole network, in proportion to installed capacity. The second is able to greatly compensate voltage fluctuation on sensitive load, but tends to increase oscillations in the other nodes in case of only reactive power exchange. Even active power exchange does not allow to considerably limit flicker in all the network. Moreover the great importance of network layout, i.e. radial or meshed, has been noticed. In future investigations DG connection through power electronic converter will be considered. A new control strategy able to limit disturbances due to compensator operation will be developed too, also making use of an energy storage device in the compensator.

memoria presentata al Convegno "La qualità del prodotto elettricità. Interfacciamento distributore – utente", Verona 1993. [4] P. Ashmole: Quality of supply – voltage fluctuations. Power Engineering Journal, June 2000. [5] IEEE std 519 1992 IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems. [6] R. Faranda, E. Tironi, G. Ubezio, I. Valadè: Comparison between some UPS line interactive devices able to solve power quality problems. 6th International Conference on Electrical Power Quality and Utilisation, September 19-21, 2001, Cracow, Poland. [7] G.T. Heydt, T. LaRose, M. Negley, W. Tan: Simulation and analysis of series voltage boost technology for power quality enhancement. IEEE Transactions on Power Delivery, Vol. 13, No. 4, October 1998. [8] S.J. Huang, H.L. Jou, J.C. Wu: Electricpower-quality improvement using parallel active-power conditioners. IEE Proc.-Gener. Transm. Distrib., Vol. 145, No 5, September 1998. [9] M. Aredes, K. Heumann, E.H. Watanabe: An Universal Active Power Line Conditioner. IEEE Transactions on Power Delivery,Vol. 13, No. 2, April 1998. [10] G.J.W. Dudgeon, F.V. Edwards, W.E. Leithead, J.R. McDonald: Dynamics of Distribution Networks with Distributed generation. Power Engineering Society Summer Meeting, 2000. IEEE, vol. 2, 2000, pp. 1032-1037. [11] IEEE std 421.5 1992, IEEE Recommended Practice for Excitation System Models for Power System Stability Studies. [12] Computer user guide, ETAP Electrical Transient Pnalyzer Program, Operation Technology Inc.

REFERENCES

BIOGRAPHIES

[1] P. Bauhofer, H. Netzer: Small Power Plants Connected With MV-Networks. TIWAGTiroler Wasserkraftwerke Aktiengesellschaft. [2] R. Faranda, E. Tironi, I. Valadè, D. Zaninelli: Power Quality Improvement Using Series Electronic Reactor and Shunt Power Conditioner. International Symposium and Exhibition on "Electric Power Engineerring at the beginnig of the Third Millenium", Capri (Naples), 12-18 may 2000. [3] S. Bargiacchi, L. Bisiach, F. Concilio, A. Invernizzi, S. Ramponi: Struttura e comportamento della rete di distribuzione.

Giorgio Ballocchi received his Dr. degree in Electrical Engineering from the Politecnico di Milano, Milano, Italy in 1994. His employment experiences include design engineer at Tenax S.p.A., application engineer at GE-Fanuc Automation Italy, automation engineer at Control Techniques, Drive Centre Milano, and Engineering for Campari Group. He is collaborating with the University of Politecnico di Milano carrying out numerical and laboratory exercises. His main research interests include power transmission by high voltage ac cable lines and power quality. Mailing address: Giorgio Ballocchi

Nevertheless the higher short-circuit power due to mesh configuration decreases disturbances generated by compensator in the other nodes of the net. It is interesting to point out that the introduction of new lines can increase voltage fluctuations in some nodes because of the more direct connection between disturbing loads. It occurs at node #1 where the percentage voltage drop rises from 0.14% in the Case 1 to 0.30% in the Case 2, and it remains substantially constant even after DG or compensator connection at U4. 5. CONCLUSIONS

270


Davide Campari Milano S.p.A. Via Campari 7/9, Sesto San Giovanni, ITALY phone: (+39)(02)24950.267 fax: (+39)(02)24950.270 e-mail: [email protected] Morris Brenna received the M.S. degree in Electrical Engineering from the Politecnico di Milano, Italy, in 1999. He is now working toward the Ph.D. degree at Dipartimento di Elettrotecnica of the Politecnico di Milano. His current research interests include power electronics and distributed generation. Mailing address: Morris Brenna Politecnico di Milano Dipartimento di Elettrotecnica Piazza Leonardo da Vinci 32 20133 Milano, ITALY phone: (+39)(02) 23993752 fax: (+39)(02) 23993703 e-mail: [email protected] Roberto Faranda received the Ph.D. degree in Electrical Engineering from the Politecnico di Milano, in 1998 and he is now Assistant Professor in the Electrical Engineering Department of the Politecnico di Milano. His areas of research include power system

harmonics, and power system analysis. Dr. Faranda is a member of AEI. Mailing address: Roberto Faranda Politecnico di Milano Dipartimento di Elettrotecnica Piazza Leonardo da Vinci 32 20133 Milano, ITALY phone: (+39)(02) 23993793 fax: (+39)(02) 23993703 e-mail: [email protected] Ivan Valadè received the M.S. degree in Electrical Engineering from the Politecnico di Milano, Italy, in 1999. He is now working toward the Ph.D. degree at Dipartimento di Elettrotecnica of the Politecnico di Milano. His research interests are power electronics and power quality. Mailing address: Ivan Valadè Politecnico di Milano Dipartimento di Elettrotecnica Piazza Leonardo da Vinci 32 20133 Milano, ITALY phone: (+39)(02) 23993752 fax: (+39)(02) 23993703 e-mail: [email protected]


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ACTIVE POWER FILTER'S BEHAVIOUR IN NON-PERIODIC CONDITIONS Jacek RUSISKI, Emil KOT, Grzegorz BENYSEK Technical University of Zielona Gora Zielona Gora (Poland)

Abstract - Identification and compensation of the non-periodic components is much more difficult than conventional periodic. All this is coming from the "nature" of them because nonperiodic phenomena can be repeated with a fixed with a variable frequency, or this could be a random process. Using Active Power Filters (APF) controlled on the base of instantaneous power theory to compensate non-periodic currents there is need to select very carefully the cut off frequency of the high-pass filter. This paper presents a discussion about use of the Extended Kalman Filters for the compensation of the non-periodic currents in three-phase circuits. APF controlled by the Extended Kalman Filter (EKF) is resistant on the measurement and systems errors (EKF treats non-periodic currents as disturbances which are statistically compensated). Also dynamic properties of the APF with Extended Kalman Filters are better in comparison with instantaneous power theory. 1. INTRODUCTION Some industrial loads like adjustable speed drives feed from frequency inverters or cycloconverters deteriorate supply quality in distribution systems by generating non-periodic currents. There is no question that those components (non-periodic components) have to be eliminated from systems because in other case we will occur some problems with measurement, control of the processes and in general with quality of the energy in supply systems. Well-grounded from practical and specially from economical point of view is that non-periodic components should be eliminated together with “classical” components (higher harmonics, reactive power, negative sequence

current etc.) by the same systems which are used to compensation all the "basic" disturbances. Identification and compensation of such components is much more difficult than conventional periodic components. All this is coming from the "nature" of them. There could be some components which only by a tiny fraction can differ from frequency 50 Hz, besides duration of those components can be considered as a single or as a few cycles. Non-periodic phenomena can be repeated with a fixed or with a variable frequency, or this could be a random process. Compensation of the non-periodic components is not the main task of the APF but they become more common and persistent components of the line currents in distribution systems and they have to be compensated. Reference currents (compensating currents) can be obtained e.g., by using the instantaneous power theory [2], which is based on complex voltage and current transforms and their reverse transforms. If the voltage source is distorted by harmonics, the instantaneous theory does not provide accurate basis for APF. Besides if APF have to compensate not only harmonics, reactive power etc., but also non-periodic components there is very important the proper value of the high-pass filter cut-off frequency. But in spite of that in some cases filter with faster response will track changed load but will not avoid power fluctuation in the source. The purpose of this paper is to introduce an efficient method to obtain the reference current for the active power filter. This method is based on extracting the fundamental sinusoid from distorted current so the reference current can be obtained by subtracting this current from the measured load current. To extract the fundamental sinusoid from distorted current an Extended Kalman Filter was used.


279

2. PROPERTIES OF THE NON-PERIODIC COMPONENTS When switching converters are working synchronously with the line voltage the harmonic generated by them are integer multiple of the line frequency. And there is no problem to handle with them. For example harmonics generated by the sixpulse converter are 5th, 7th, 11th. However this situations is true just only in some cases, in many causes non-periodic currents are also generated. For example when power electronic converters are not perfectly synchronized with the line frequency or the loads vary with frequency which is different to the line frequency. For example the controlled three-phase rectifier with modulated firing angle or rectifier (with constant five angle) with modulated output power. Non-periodic currents are also “generated” by the cycloconverters or adjustable speed drives. Fast control of energy flow may contribute to an increase of non-periodic currents, they could be also generated in faults of the devices. As it was told earlier non-periodic current can increase power loss and disturb some power equipment, they could also contribute to the supply quality deterioration in distribution systems. So there is great interest in identification and compensation of them. On figures (1y4) there are presented example nonperiodic waves and spectrums of them.

permanent component). Non-periodic phenomena could be also repeated with fixed and with variable frequency or it can be a random process. But in most cases non-periodic components of voltages and currents are transient, a kind of a disturbance which is usually much smaller than harmonics. So they can only in slight degree disturb periodicity, such components can be called quasi-periodic [1]. Spectrum of a quasi-periodic component is lumped around harmonic order frequencies nZ1 (Z1 main frequency) with so called interharmonic noise which changes in general with frequency, and band is changed with harmonic order. But we have to remember that spectrum of the quasi-periodic components depends on the “nature” of them. For example, when amplitude of the currents (voltages) slowly changes then spectrum around frequencies nZ1, broadens, with only little contribution to the interharmonic noise. The different situation is when disturbances are short or we do have noncoperiodic components of a random frequency. Those components contribute to the interharmonic noise which can be a low frequency, below the fundamental frequency Z1, a noise in a harmonic band frequency and a high frequency noise.

Fig.2. Non-periodic current and spectrum of it: 1 i50Hz(t)+0.2 i(50y55y50)Hz(t). Fig.1. Non-periodic current and spectrum of it: 1 i50Hz(t)+0.2 i52Hz(t). The duration of the non-periodic component could only in slight degree differ from 50 Hz cycle. It could be just one on few cycles (as well as the 280

If in system with main frequency 50Hz in current there are two components one of 50Hz and the second one of 52Hz: i (t ) i50 (t ) i52 (t ) (1) then current i(t) is non-periodic (so called noncoperiodic [1]) . Electrical Power Quality and Utilisation

3. COMPENSATION OF THE NONPERIODIC CURRENTS USING THE p-q THEORY

Fig.3. Non-periodic current and spectrum of it when current is turned on and off.

Fig.4. Non-periodic current and spectrum of it: (1y1.2y0.9y1) i50Hz(t). On the base of that what was told earlier we see that there is very different to handle with nonperiodic phenomena in distribution systems from regard of complexity of the problem. The choice of a measurement and a digital signal processing procedure as well as choice of a control algorithm for compensators has, a crucial importance.

Most of the active power filters, hybrid filters etc., which have found applications in industry to generate reference signals are using instantaneous power theory. The p-q theory is based on D-E-0 transformation. One of the first advantages of using that transformation is separation of zerosequence components into the zero-sequence axis. In D-E-0 components there are defined instantaneous p and q powers [2]. In so defined p-q powers basic (50Hz) component of current (voltage) is shifted to 0Hz and all higher harmonics are shifted to higher frequencies. So to get the reference signals there is only need to extract from powers p and q averaged values of them. This can be done by using simple high-pass filters which cut-off frequency is equal to: fc=1/T1, where T1 period of the line frequency. But problem of the right choosing cut-off frequency is more complicated in case when APF has to work under non-periodic conditions. Let's consider a situation when dc current of the three-phase rectifier is switched on and off (for example the duration of the off state is equal to period of the line frequency). This kind of wave can be considered as non-periodic [1]. Let's choose two periods in which the average values of powers p, q are calculated. The first one is tending to infinity and the second one is equal to the on-off period. In first case if the average real power actually changes, the control of the compensator can't see it for a long time because average powers are constant for all time. For pulsed real power, if the source power is constant, the APF has to be able to absorb or generate large amount of energy. This can be done by increasing the capacitor. In second case the averaged powers are pulsed as the current, and we will not avoid problem of power fluctuation in the source. If APF are working under non-periodic conditions there is possible to compensate the ac component however one should pay move attention in designing the cut-off frequency of the high-pass filter. There are many APF applications in some complex distribution systems where p-q theory was used and some good results wave obtained. But now when signal processing is so developed there is possibly to use to control APF for example algorithms based on Extended Kalman Filters (EKF) which can better adjust to changed conditions of distribution systems and do behave better in transient situations. Besides proposed


281

control algorithm does not have disadvantages of the p-q based algorithms because is resistant on source voltage distortions. 4. UTILIZATION OF THE EXTENDED KALMAN FILTERS IN APF Extended Kalman Filters are similar to the Luenberger observers [6], but do have few advantages over them. The effectiveness of Luenberger observer depends on the exact setting of the parameters, exact measurement of the output vector. But in real (system), no one from those critters is true (of measurement disturbances, internal noises), and the Luenberger observer can not work any more. Kalman filter incorporates all information that can be provided to it. It process all available measurements, regardless of their precision, to estimate the current value of the states with use of (1) knowledge of the system and measurement device dynamics, (2) the statistical description of the system noises, measurement errors, and uncertainty in the dynamics models, and (3) any available information about initial conditions. In other words Kalman filter combines all available measurement data, and knowledge of the system and measuring devices, to produce an estimate of the desired variable in such manner that the error is minimized statistically. So Kalman filter has a major advantage over other adaptive filters, because provides a solution that directly cares for the effect of the disturbance noises and parameters errors which are handled as noise. The system equations for the Kalman filter are following [6]: x

x y

Ax Bu r Cx 9

A KC ˆx Bu Ky

(3)

But we have to keep in mind that K matrix is settled on the base of covariance of the noises, and determines how the state vector of the Kalman Filter ( xˆ ) is modified after the output of the model is compared with the output of the system. On the following equations there is presented implementation in matrix form of the Extended Kalman Filter:

K (n 1)u (n) u (n) K (n 1)u ( n) Q M

g ( n)

(2)

(4)

T

y ( n) u T (n) wˆ (n) e( n ) d ( n ) y ( n ) wˆ ( n 1) wˆ ( n) e( n) g ( n)

(5) (6) (7)

K (n 1) g (n)u T ( n) K ( n 1) Q P

K ( n)

(8)

where: n - the current algorithm iteration; u(n) - input signal at step n; K(n) - correlation matrix of he state estimation error; g(n) - vector of Kalman gains at step n; wˆ ( n) - vector of filter-tap estimates at step n; y(n) - filtered output at step n; e(n) - estimation error at step n; d(n) - desired response at step n; - correlation matrix of the measurement Qm. noise; QP - correlation matrix of the process noise; isa

where r and ] are the system and measurement noises, y is output of the system and x is vector of the states. We assume, that those noises are stationary, white and Gauss noises. Whiteness pertains to time of frequency relationships of a noise. Gaussianness has to do with its amplitude. Thus, at any single point in time, the probability density of Gaussian noises amplitude takes on the shape of a normal bell-shaped curve. This is very important in case when Kalman filter has to handle with non–periodic currents (voltages), because some of them could be treated as white and Gauss noises. (“Whiteness" means that the noise value is not correlated in time). The overall structure of the Kalman filter is the same as that of the Luenberger observer:

282

ˆx

ila

Va Non linear load

Vb Vc

ika

Lfa Lfb Lfc

APF

`

VDC

ica PWM Follow-up Modulator

icb icc

EKF

Va Vb Vc V*DC

Fig.5. The active power filter circuit topology. The fundamental purpose of the APF is to compensate all the disturbances introduced by loads. This could be done by injecting in opposite


compensating currents (voltages) which are determined by the control algorithm. On Fig.5. there is introduced operation scheme of the shunt active power filter. The desired current that the APF should produce is as follows: iC(t)=iL(t)-iS1(t)

a) 5A/div

(9)

where iL is the distorted load current, and iS1 is the fundamental current. So the reference current iC of the APF can be determined by subtracting the fundamental component from the load current. In our case this is the purpose of the Extended Kalman Filter. In this paper we do not consider the realization of the APF, but it is assumed to be an ideal current generator. All the simulations are realized in Matlab environment. On the following figures there is presented performance of the EKF based APF in case of different non-periodic waves.

0.2s/div b) 5A/div

0.2s/div c)

a) 5A/div

5A/div

0.1s/div b) 5A/div

0.2s/div Fig.7. Simulation waveforms: a) load current iL= 1 i50Hz(t)+0.2 i52Hz(t), b) reference current iC, c) source current iS. a) 5A/div

0.1s/div c) 5A/div

0.2s/div 0

b)

02

04

06

08

1

1A/div

0.1s/div Fig.6. Simulation waveforms: a) load current iL= 1 i50Hz(t)+0.2 i2Hz(t), b) reference current iC, c) source current iS.


0.2s/div

283

c) 5A/div 2.

3. 0.2s/div Fig.8. Simulation waveforms: a) load current iL= b) reference 1 i50Hz(t)+0.2 i(51y55)Hz(t), current iC, c) source current iS.

4.

5dB/div 5. 0dB 6. 100Hz/div Fig.9. Instantaneous frequency response of the EKF. On the base of the presented simulation waveforms we can say that implemented Extended Kalman Filter can very well handle with distorted as well as non-periodic currents. In Fig.9., a typical instantaneous frequency response of the EKF is presented. The filter structure has adopted to a bandpass response with unity gain on the instantaneous fundamental frequency. Attenuation of harmonic frequencies as well as the dc component, is considerable. 5. CONCLUSIONS

In the paper to compensate distortions as well as non-periodic components from currents an Extended Kalman Filter was used. Filter is capable of extracting the fundamental sinusoid from strong disturbanced waves, even if there are considerable frequency and amplitude variations. Besides, practically no knowledge about the network and load characteristics is necessary for the generation of the compensating currents. Taking into consideration fact of development in signal processing we think that there is possibility in utilization of the EKF in APF applications. 6. REFERENCES

1. Czarnecki L.S.: Non-periodic currents: their properties, identification and compensation

284

fundamentals. 2000 IEEE PES Summer Meeting, pp.3-8, Seatle, USA. Akagi H., Kanazawa Y., Nabae A.: Generalized theory of the instantaneous reactive power in three phase circuits. IEPC'83 - International Power Electronics Conference, Tokyo, Japan, 1983, pp.1375-1386. Akagi H.: Active filters and energy storage systems operated under non-periodic conditions. 2000 IEEE PES Summer Meeting, pp.9-14, Seatle, USA. Strzelecki R., Frµckowiak L., Benysek G.: Hybrid filtration in conditions of asymmetric nonlinear load current pulsation. Conf. Proc. EPE'97, Trondheim, 1997, Vol.1, pp.1.8921.897. Strzelecki R., Frµckowiak L., Rusiski J., Benysek G.: Influence of the nonlinear load current pulsation on hybrid filter behavior. Conference proceedings. PEMC'98, Czech Republic, Prague 1998, Vol.2, pp.1344-1350. Maybeck P.S.: Stochastic models, estimation and control. Academic Press, London 1979.

Dr. Grzegorz Benysek was born in 1968 in Sulechów, Poland. He received the M.Sc. and Ph.D. degrees from the Technical University of Zielona Gora. At present he is Researcher in the Technical University of Zielona Gora. His field of interest is in Active Power Filter compensation. Mailing address: Grzegorz Benysek Technical University of Zielona Gora. Institute of Electrical Engineering 50 Podgórna Av., 65-246 Zielona Góra phone:(+48)(0-68) 3282417, fax:(+48)(0-68) 3254615 e-mail: [email protected] Dipl.- Ing. Jacek Rusiski was born in 1970 in Zielona Góra, Poland. He received the M.Sc. degree from the Technical University of Zielona Gora. At present he is Researcher in the Technical University of Zielona Gora.. His field of interest is in Series-Parallel Active Power Filters. Mailing address: Jacek Rusiski Technical University of Zielona Gora. Institute of Electrical Engineering 50 Podgórna Av., 65-246 Zielona Góra phone:(+48)(0-68) 3282568, fax:(+48)(0-68) 3254615 e-mail: [email protected] Dipl.- Ing. Emil Kot was born in 1974 in Ko¶uchów, Poland. He received the M.Sc. degree from the Technical University of Zielona Gora. At present he is Researcher in the Technical University of Zielona Gora.. His field of interest is in Active Power Filter compensation. Mailing address: Emil Kot Technical University of Zielona Gora. Institute of Electrical Engineering 50 Podgórna Av., 65-246 Zielona Góra phone:(+48)(0-68) 3282253, fax:(+48)(0-68) 3254615 e-mail: [email protected]




APPLICATION OF SVC FOR LOAD BALANCING Rozmysaw MIESKI

Ryszard PAWEEK

Irena WASIAK

Technical University of Lodz Lodz (Poland) Abstract- A model of a Static Var Compensator (SVC) for digital simulation of transients in power systems is discussed in the paper. The SVC system of FC/TCR type was designed for load balancing. Theoretical basis for compensating load unbalance is presented. The model was elaborated by means of the EMTP and a control circuit has been constructed with standard TACS elements. Simulation of the compensator performance was done for a selected network and some results of it are presented in the paper. 1. INTRODUCTION In recent years Static Var Compensators (SVC) have been widely used in distribution power networks of many countries. Providing fast reactive power compensation, they perform the task of securing adequate power quality in points of common coupling (PCC) from which various disturbing load devices are supplied. SVC can maintain constant voltage on loads’ buses and reduce voltage flicker, keep good and stable power factor and balance the reactive power consumption. Different SVC configurations are applied but the FC/TCR system is used most often which employs fixed capacitor (FC) and thyristor controlled reactor (TCR). Simulation of transients plays an important role in analysis of power system behaviour. It is usually carried out by standard computer programs; the EMTP is one which is used most often. It offers accurate representation of power system equipment like sources, transformers, lines, switches, etc. and allows to determine current, voltage and other electrical quantities in any electrical network and power system. The SVC built-in model is not available in the EMTP, therefore there is a need to work it out by a user. Some various concepts of modelling have been elaborated and presented in the literature [3, 4, 5]. They differ mainly in solution of a

compensator control circuit, resulting thyristor action. The simple model of the FC/TCR system was also presented in [6] with the control system designed for voltage stabilisation. This paper proposes an extension of the model and presents a method of control for compensating unbalanced loads. 2. THEORETICAL BACKGROUND The aim of load balancing is obtaining identical values of an active power in each phase of the supplying network. It needs negative and zero sequence symmetrical components of phase currents to be reduced or eliminated. In MV 3phase, 3-wire networks a zero sequence component does not exist thus balancing amounts to reduction negative sequence currents only. Network active powers or currents can be balanced by connecting shunt asymmetrical device employing reactive elements. The principle of balancing is illustrated below with the example of a single-phase load which is connected between phases B and C, as shown in Fig 1. UA

A

IC C

R

IC

IBC

B

UAB

IB

UCA

IB

UC

UB IBC

UBC

Fig. 1. Unbalanced load device and the phasor diagram of load voltages and currents The effect of balancing is obtained while connected reactive elements in parallel to the load, as presented in Fig. 2a. The capacitor and reactor reactances should fulfil the relationship:


XC

XL

3R

(1)

291

a)

UAB

b)

c)

A

UAB

UA

UA

IAB

IA

IAB

IA

XL ICA

XC R

C

IC IBC I BC

UBC

UBC

ICA

B UC

IB UB

UC

UB

IB

IC

UCA

UCA

Fig. 2. Balancing the single-phase load by means of a capacitor and a reactor. a) circuit diagram, b) phasor diagram of delta currents, c) phasor diagram of network currents

In Fig. 2b the phasor diagram of delta currents is shown. The network currents, shown in Fig. 2c, are determined from the Kirchoff’s low:

IA

I AB I CA

IB IC

I BC I AB I CA I BC

(2)

It is easy to notice that the phase currents are symmetrical and active. Thus, applying reactive elements in parallel to the load allows obtaining balancing effect and reactive power compensation at the same time. This principle of balancing is also applied by SVCs. The compensator generates in each phase reactive currents which added to the load currents give the required balanced system. The compensator feature of fast changing the injecting currents makes it possible to apply it with frequently time-varying loads. The 3-phase SVC elements have to be connected in delta [7]. The SVC performance is described by formulas: Q AB( SVC )

Q A QB QC

QBC ( SVC )

Q A QB QC

QBC ( SVC )

Q A QB QC

(3)

For each phase the system includes two fixed capacitate branches composed of a LC filters and a reactor connected in series with a bi-directional pair of thyristors, in parallel. In a three-phase system the TCR and FC branches are connected in delta. The LC filters are design to remove the 3rd and 5th harmonic current. The 3rd harmonics appears in the system when firing angles are not equal in each phase. The thyristor valves were modelled as grid controlled switches with a grid signal provided by a control system dependently on the required firing angle D. Configuration of the SVC system being modelled is shown in Fig. 3. load i(t) Qref

u(t)

measuring block Y/y regulator block

firing block

where: QA, QB, QC are phase reactive powers of the load. These formulas refer to an ideal compensation, i.e. when total QS in each phase equals to 0. In more general case one can assume some level of uncompensation when QS z 0. It results in decreasing every SVC powers. 3. SVC MODELLING 292

Fig. 3. SVC configuration The control system was designed to balance the active power consumption of an unbalance load. Electrical Power Quality and Utilisation

ª di1q ( t ) º u( t )» SIGN « «¬ dt »¼

In each phase the system consists of the following blocks: reactive power measurement, PI regulator and firing pulse block. The task of the measurement block is determination of SVC response on the basis of measurement of phase reactive powers of the supplying network. The input signals are phase voltages and currents for each phase. The fundamental harmonics of voltages and currents are filtered from the measured signals and instantaneous power is calculated according to the formula: p1( t ) u1( t )i1( t ) (4)

power ª di1q ( t ) º SIGN « u( t )» = «¬ dt »¼

U1RMS I1RMS

P1

P12RMS

4. EMTP SIMULATION To illustrate usefulness of the presented algorithm and balancing operation of the SVC model proposed, some results of simulation are presented below for the network shown in Fig. 5. Transformer T1 supplies a substation of 15 kV. An unbalanced single-phase load devices connected to the E(G) node are represented by the resistances supplied with a line voltage. Equivalent supply network is represented by 3phase symmetrical voltage source. When neglecting a source resistance, a reactance is determined from the short-circuit power given on the high voltage side of the supply transformer. The models of transformers taken under consideration are based on the built-in EMTP model (BCTRAN procedure [1]). Its parameters are calculated using the results of excitation and short-circuit tests during energisation of the transformer with positive and zero sequence voltages. It is possible to apply saturation characteristics.

Q1

(5)

1 2 S1 2

(6)

S12 P12

(7)

To recognise the inductive or capacitive character of the reactive power the current signal is decomposed into two components: active i1p(t) and reactive i1q(t) [2]: i1 p ( t )

P1 2 U1RMS

u1( t )

(8)

i1q ( t )

i1( t ) i1 p ( t )

(9)

-1 for the capacitive one.

The output of the measurement block is the required reactive power of the compensator. The diagram of the measurement block is presented in Fig. 4. The other blocks of the control systems i.e. regulator and firing were described in [6].

The apparent, active and reactive powers are calculated after determination of RMS values of the three signals u1(t), i1(t), p1(t): S1

= 1 for the inductive reactive

Then a derivative of the reactive component is multiplied by signal u1(t) and a sign of such an expression is examined:

u(t)

Low-pass filter

u1(t)

RMS (50 Hz)

P1 u1( t ) U12RMS

U1RMS

i1p(t) i1(t)

i(t)

Low-pass filter

i1(t)

S1

RMS (50 Hz)

u

I1RMS

u p1(t)

RMS (100 Hz)

P12RMS

P1RM

1 2 S1 2

>

@

d i1q ( t )

i1q(t) S12 P12

u1(t)

|Q1|

dt

u i’1q(t)

SIGN u Q1

P1

Fig. 4. Measurement block


293

bar E. After 0,5 s the same load was switched on between B and C phases, what changed the unbalance character. Load currents for the one and the two devices switched on are presented in Fig. 6. Fig. 7 shows the supplying network currents and illustrate the balancing effect of the SVC. Phase power consumption for the case is shown in Fig. 8.

The following data was assumed for the network elements: short-circuit power on the B bus-bars S = 0,5 GVA; transformer T1 - Sn = 16 MVA; transformer T2 - Sn = 3,15 MVA; SVC - a regulated range of the reactive power is from capacitive to inductive Q = 3 MVAr. The following conditions were simulated. First, one single-phase resistive load device of 1,8 MW was connected to A and B phases of the bus-

G A

C

B

~

D

E

Load K

T1 (Y/y)

Control system

T2 (Y/y)

SVC Fig. 5. Network for study

500 375

b)

a)

250 125 0 -125 -250 -375

2 5 0 . 0

2 5 0 . 0

1 8 7 . 5

1 8 7 . 5

1 2 5 . 0

1 2 5 . 0

6 2 . 5

6 2 . 5

0 . 0

0 . 0

- 6 2 . 5

- 6 2 . 5

- 1 2 5 . 0

- 1 2 5 . 0

- 1 8 7 . 5

- 1 8 7 . 5

- 2 5 0 . 0

- 2 5 0 . 0

0 . 4 5

0 . 4 6

0 . 4 7

-500 0.0 0.2 (file C om p1.pl4; x-var t) c :EA ( file

C

o m

p 1 . p l4 ;

x - v a r

t )

c : E

A

- E

E

A

c : E

0 . 4 8 B

- E

E

0 . 8 5

B

-EEA

( file

0.4 c :EB

C

o m

-EEB

0 . 8 6

p 1 . p l4 ;

x - v a r

t )

0.6 c :EC

c : E

0 . 8 7 A

- E

E

A

c : E

0 . 8 8 B

- E

E

B

0.8

1.0

-EEC

Fig. 6. Load phase currents. a) one single-phase load switched on, b) two single-phase loads switched on

294


500 375

b)

a)

250 125 0 -125 -250 -375

2 5 0 . 0

2 5 0 . 0

1 8 7 . 5

1 8 7 . 5

1 2 5 . 0

1 2 5 . 0

6 2 . 5

6 2 . 5

0 . 0

0 . 0

- 6 2 . 5

- 6 2 . 5

- 1 2 5 . 0

- 1 2 5 . 0

- 1 8 7 . 5

- 1 8 7 . 5

- 2 5 0 . 0

- 2 5 0 . 0

-500 0.0 0.2 (file Com p1.pl4; x-var t) t: IA1H ( file

C

0 . 4 5 o m p 1 . p l4 ;

x - v a r

0 . 4 6 t ) t : IA

1 H

t :

IB

0 . 4 7 1 H

t :

0 . 4 8 IC

1 H

( file

0.4 t: IB1H t: IC1H

C

0 . 8 5 o m p 1 . p l4 ;

x - v a r

0 . 8 6 t ) t : IA

0.6

1 H

t :

IB

0 . 8 7 1 H

0 . 8 8 t :

IC

1 H

0.8

1.0

Fig. 7. Supply network phase currents. a) one single-phase load switched on, b) two single-phase loads switched on

7000

6000

5000

4000

3000

2000

1000

0 0.0 0.2 (file Com p1.pl4; x-var t) t: PA

t: PB

0.4 t: PC

0.6

0.8

1.0

Fig. 8. Phase active powers of the supply network

4. CONCLUSIONS Section 4. Methods of Power Quality Improvement: Filters, ....

295

Reactive power compensation with load balancing is one of the tasks for the SVC systems applied in distribution networks and co-operated with nonlinear and unbalanced loads. For examination of the system performance the method of simulation can be applied effectively and EMTP has been recognised as a good tool for it. Using EMTP requires representation of power system elements. Some element models are available in the program in the form of built-in procedures others, like SVC systems, have to be worked out by an user. This paper presents the way for modelling the SVC of a FC/TCR type with the special attention drawn to the control system designed for compensating the reactive power and load balancing. To examine the presented algorithm and illustrate operation of the system an unbalanced load devices have been selected. The proposed model may be useful for studies and transient analysis of power networks with nonlinear and unbalanced loads. 5. ACKNOWLEDGEMENT This work has been supported by the State Committee for Scientific Research under Contract No. 1459/T10/2000/18 REFERENCES [1]. EMTP Rule Book, Bonneville Power Administration, 1987. [2]. S. Fryze, Moc rzeczywista, urojona i pozorna w obwodach elektrycznych o przebiegach odksztaconych prµdu i napi³cia, Przegld Elektrotechniczny Nr 7 i 8, 1931. [3]. R.H. Lasseter, S.Y. Lee, Digital simulation of static var system transients, IEEE Transactions on Power Apparatus and Systems, vol. PAS-101, no. 10, 1982. [4]. S.Y. Lee, S. Bhattacharya, T. Lejonberg, A. Hammad., S. Lefebvre, Detailed modelling of static var compensators using the electromagnetic transients program (EMTP), IEEE Transactions on Power Delivery, vol. 7, no. 2, 1992.

296

[5]. S. Lefebvre, L. Gerin-Lajoie, A static compensator model for the EMTP. IEEE Transactions on Power Systems, vol. 7, no. 2, 1992. [6]. R. Pawelek, I. Wasiak, Modelling and symulation of SVC performance by means of EMTP programme. Proc. 4th International Conference on Electrical Power Quality and Utilisation, Cracow, Poland, 1997. [7]. S. Piróg, Sterowanie trójfazowymi symetryzujµcymi energoelektronicznymi kompensatorami mocy biernej, ZN AGH, No 14, 1989

Rozmysaw Mieski received M.Sc. and Ph.D. degrees from Technical University of Lodz. At present he is a senior lecturer at the Institute of Electrical Power Engineering of Technical University of Lodz. His area of interest is power quality and AC/DC power network simulator. e-mail: [email protected] Ryszard Paweek was born in 1952 in Chocz, Poland. He received M.Sc. and Ph.D. degrees from Technical University of Lodz. At present he is a senior lecturer at the Institute of Electrical Power Engineering of Technical University of Lodz. He is a secretary of the Editorial Board of Polish periodical “Electrical Power Quality and Utilisation”. His field of interest is power quality. e-mail: [email protected] Irena Wasiak graduated from the Technical University of Lodz, Poland. There she received the Ph.D. degree in electrical power engineering. Presently she is a senior lecturer at the Institute of Electrical Power Engineering, Technical University of Lodz. She is a secretary of the Program Board and a member of the Editorial Board of Polish periodical “Electrical Power Quality and Utilisation”. Her area of interest includes modelling and simulation of transients in power systems, and power supply quality. e-mail: [email protected]




EXPLOITATIVE CHARACTERISTICS OF SVC FILTER CIRCUITS Yurij VARETSKY Lviv Polytechnic National University Lviv (Ukraine)

Abstract The paper describes different exploitative transient phenomena bearing to SVC power filters. Power filters design problem for SVC is related to proper value estimations on transient voltages and currents. The switching transient values for normal operation performances play a significant role in the problem. On the other hand, outside transients impact on the design solutions. This paper discusses nature and features of the transient values for characteristic cases and operational conditions where the consideration are required. 1. INTRODUCTION Growing the static var compensators (SVC) applications in industrial power systems has observed over a few past decades. This trend has been driven by the wide use of arc furnaces, arc welders, adjustable drives, rolling mills, traction loads etc., which have varying loading and generally produce harmonic generation. Some loads, such as arc furnaces, have harmonic generation with stochastically varying unbalanced even and odd harmonics. The SVC's, designed for control of such load performances, include filter circuits (FC) to absorb their own harmonics and harmonics generated by the nonlinear loads. As known from experience [1,2], switching transients in some industrial power systems inclusive filter circuits can result in damages of the FC components frequently. The common practice in FC design is to select a reactor/capacitor combination that allows to limit the harmonic distortion to a specified level and to generate the required reactive power. So, the purpose of this paper is to describe transient events in a power supply system with SVC and to designate the impact of accompanying overvoltages and overcurrents on the FC design. Additional

consideration has been paid to duration of the transients and the number of their occurrences because these factors are important for design of the FC components. It should be noted that the consideration of switching transient conditions can result in a higher rating of the FC capacitors and reactors. The transient events study shown below was carried out by Electromagnetic Transients Software (ETS)[3] for some chosen power supply system examples but includes many common features of other industrial power systems. It is expected that the approaches to the power supply system analysis will be useful for FC application planning and improve FC operation reliability. An example of power supply system is shown in Fig.1. The system involves 220 kV bus supplying 35 kV bus by means of step down wye-delta connected transformer of 160 MVA with the primary neutral solidly grounded. Couple of 50 MVA arc furnace units are connected to the 35 kV bus. Four single-tuned filters and thyristor controlled reactor unit are connected to the bus through the appropriate breakers. The individual filters are sized to supply 25, 30, 17 and 20 MVAR for the 2nd, 3rd, 5th, and HP harmonic filters respectively. The allocation of total design capacity between the various filters is based on the percentage of total harmonic current each filter will carry. 2. OPERATING TRANSIENTS The electrical power supply system exploitation is associated with some types of switching events. Unlike failure switchings, those take place more frequent. The next switching events and faults for the system can be emphasized from point of view on how these ones affect filter circuits:


297

220 kV , Ss.c. = 2190 MVA

TS

160 MVA, 11% 35 kV

Q3

Q2

Q1

Q4

Q5

H

TCR

T1 T2

F2

F3

F

EAF

Fig.1. Arc furnace supply system with SVC

x energizing filters; x clearing filters; x energizing transformer connected to the bus; x faults initiation and clearing on the bus; filter breaker restriking during interruption. These events cause overvoltages across the individual components of the filters that exceed the voltage at the bus. Transient currents oscillating in lightly damped filter circuits can likewise last for a few seconds. Traditional overvoltage protection, such as surge arresters located at the bus, may be inadequate unless the individual filter components are specially rated to handle the extra stress. Metal oxide arresters are sometimes applied to filter components to reduce the overvoltage. They are not however effective in all cases. If they are applied, the maximum continuous operating voltage (MCOV), temporary overvoltage (TOV) and energy dissipation requirements of the arresters must be carefully determined in order to avoid adverse affects on the filter reliability. Not only the stress magnitude is important but the transient duration, the repetitive frequency of the events and in some cases the interval between events can be critical. Filter reactors may be subjected to fast rates of rise of voltage as well as switching overvoltages of long duration. Fast rates of voltage rise can increase the non-linearity of the voltage distribution in the windings and if not accounted for the design can result in overstressing some of the conductor-to-conductor insulation system. Long duration switching overvoltages can result in possible flashover of the external insulation. Short time current overloads, which may last many minutes, involve heating effects. Overcurrents, lasting up to a few minutes, do not involve thermal stresses. Instead, mechanical stress is the key factor under the circumstance. The mechanical stresses

298

can be lower then those associated with short circuit currents but usually they are significantly larger than those associated with normal operation conditions and even momentary overloads. If these overloads occur oftentimes during the reactor lifetime they may result in eventual mechanical fatigue. The frequency of occurence of overcurrents and overvoltages is significant factor for proper selecting filter reactors [1]. For instance, dynamic overcurrents and overvoltages associated with transformer energizing in the analysed power supply system circuits may occur thousands of times a year, leading to mechanical aging of the conductor insulation system and its mechanical breakdown, anticipating its erosion and degradation. The overvoltages which are impressed upon the filter capacitors under the switchings may be very high, short duration magnitude, which can cause internal insulation failures or external flashover. Transient overcurrents are generally not a concern since capacitor units are designed to handle much higher currents than result from filter capacitor switching. 2.1. Energizing filters Switching ungrounded wye filter from a predominantly inductive source can result in a transient overvoltage approaching 1.5...1.7 pu on the station bus. In most cases circuit damping limits overvoltages on filter capacitor banks to the range of 2.0 to 2.8 pu. In general, however, the overvoltages, associated with normal filters energization in the presented system, are do not dangerous for the filter equipment and do not usually endanger substation equipment at the bus location. The peak currents in the filters is a few times higher than steady-state levels. The transient voltage peaks recorded from the simulations are shown in Table 1. The energization of a filter generates steep fronted voltage waves on filter reactor which can result in high local overvoltages along reactor winding length. As a consequence of this phenomenon the adequate measures to prevent the reactor insulation dielectric failure must be provide. Some of the measures will be consider bellow. Considering the transients in filter capacitor bank and filter reactor designs may increase the electrical rating of the filter. These considerations depend on the particular filter arrangement and supply system data.


TABLE 1. Transient overvoltages under filter switching for the power supply system Switching conditions Switching breaker FC breakers state Q2 Q3 , Q4 - on Q2 Q3 , Q4 - off Q4 Q2 , Q3 - on Q4 Q2 , Q3 - off Q3 Q2 , Q4 - on Q3 Q2 , Q4 - off

Maximum overvoltage, pu Substation bus 1 FC capacitor 2 FC reactor 2 1,49 2,86 1,53 1,35 2,80 1,52 3 1,68 1,78 0,883 3 1,65 1,69 0,723 1,34 1,89 1,51 1,31 1,83 1,36

Note: 1 Phase-to-phase voltage is based to the crest nominal value; 2 base is the crest nominal phase-to ground value; 3 for 5th filter.

2.2. Arc furnace transformer energization A special case of transient is transformer energization. When transformer is energized inrush current can be high in magnitude and harmonic content and of long duration (lasting several seconds). The harmonics can excite resonance in filter which extends the duration of the transient and resonance. Fig. 2 shows the voltages and current across the reactor and 0.53 k

2nd filter current

2.35 k 41.4 kV

2nd filter capacitor voltage

110.7 kV 10.2 kV

2nd filter reactor voltage

83.8 kV

Fig. 2. Filter voltages and current resulting from the arc furnace transformer energizing

capacitor of the second filter in the Fig.1 circuit resulting from simulation arc furnace transformer energization[4]. Transformer energization is a regular occurence in arc furnace supply systems. In the evaluating the effect on the equipment it is important to account for the relative frequency of such occurences, which can achieve up to 20,000 times a year for the presented supply system. Descriptor of dynamic overvoltage or overcurrent is their duration, lasting in the range of tens of

milliseconds to seconds. For air core filter reactor short time current overloads, which last many minutes, cause heating effects. Dynamic overcurrents do not last long enough to cause thermal stresses. As it was considered above, mechanical stress plays the defining role in process of determining reactor rating alike short circuit current. Although the mechanical stresses are lower then short circuit currents stresses but they cause significant repeating impacts on filter reactor arrangement, larger than even short-time overcurrents. It is important to understand that short circuit current as filter reactor design datum implies that such currents are a rare event over the lifetime of the reactor and the occurence of infrequent high mechanical loading on structural materials only requires that stress levels be kept within the capabilities of the materials employed. However, dynamic overcurrents, while often lower in magnitude than short circuits, can occur many more times during the reactor lifetime and may result in mechanical fatigue. Dynamic overvoltage will result in momentarily higher operating voltage stress externally across the surface of the reactor and internally through the conductor insulation system. If dynamic overvoltages are enough to produce partial discharge in conductor insulation system, the high repetition rates can result in erosion and degradation of the conductor insulation system resulting in dielectric failure. So, the frequency of occurence of overcurrents and overvoltages is a significant factor that must be considered in reactor designing. 2.3. Filter circuit clearing If there is a successful interruption of the filter current at the first zero crossing and the switching device withstands the transient recovery voltage there are no significant transients on clearing a


299

filter. The occurence of reignitions during breaker opening is consequence of non-compliance with the conditions. Restrikes on the other hands, will produce transient voltages and currents significantly larger in magnitude than those occurring during closing. Since restrikes can occur when there is a charge remaining on the filter capacitor bank it is possible for restrikes to generate transient overvoltages that are much higher in magnitude than on closing. Fig.3 shows phase 3rd filter voltages and current when circuit breaker interrupts the filter currents with restriking. bus voltage

presence of harmonic current will also result in greater surge. The maximum values of transient voltages on filters under restriking are shown in

Umax / U nom

5 4

>

3

2

`

1

I2 / I 1, %

43.8 kV

0

25

50

75

100

Fig. 4. Residual voltages on 2nd filter phases vs 2nd harmonic content filter current

6.5 kA filter capacitor voltage

Table 2. The restriking surges depend on the order of the filter and the harmonic current. Since restriking surge is great it is necessary to evaluate its relationship with withstand impulse voltages. Therefore, if the restriking is observed for the filter circuit breaker, can be necessary to install the surge protective devices. TABLE 2. Maximum filters overvoltages under restriking

138.3 filter reactor voltage

122.6 kV

Harmonic content, % 0 25 50 100

2 5,08 5,34 5,71 6,38

Order of filter 3 4,51 4,63 4,82 5,18

5 4,28 4,32 4,39 4,56

Fig.3. 3rd filter voltages and current under two restrikings (phase A)

Note: Base is the crest value of phase-to-ground voltage

The next switching problem in the investigated circuit is harmonic current flowing through a filter during interrupting. During the charge smelting process in the arc furnace the harmonic current in filter circuit can be a large amount. The behaviour of the current interrupting phenomenon when the current contains harmonic component will be different from that when there is no harmonic component. Fig. 4 shows residual voltages on 2nd filter phases vs harmonic content in interrupted currents (phase A is first interrupted). Base voltage is crest value of nominal phase-to-ground voltage. When the interrupted current contains harmonic component, the maximum recovery voltage of filter breaker increases giving rise to the possibility of restriking. When compared with the interruption of current having no harmonic component the

Generally, SVC is connected to 6-35 kV substation bus supplied from an ungrounded transformer winding. Ground faults in the network connected to the bus may cause considerable overvoltages across the substation equipment. The maximum overvoltages are arisen during restrikings of the faults. The transient overvoltages from the restriking ground arc faults may be dangerous for filter reactor. As it was observed from the field experience, maximum overvoltages in the network under ground arc faults are arisen under definite conditions. These conditions accidental in grain have been simulated[5,6] according to theory proposed by [7]. Fig.5 shows simulated transient voltages across 3rd filter equipment during

300

2.4. Ground faults in connected network


98.0 kV >

29.4 kV

f / f0 = 19 `

UK bus voltages 33.6 kV

`

>

Restriking ground arc fault on the substation bus, directly in the vicinity of the filters, generates steep-fronted voltage waves on filter reactors. Fast rates of rise of the voltages can increase nonlinearity of the voltage distribution in the reactor winding and must be accounted for in the designing. A result of example study of the voltage distribution after last fault at the bus ground arc restriking is presented in Fig.6.

uF

uF1 uF2 uF3

filter capacitor voltages

50 Pc

67.0 kV 4.2 kV 32.9 kV filter reactor voltage in phase A 16.5 kV

Fig. 6. Voltage distribution along the filter reactor winding: uF - terminal voltage; uFn - first three winding model portions voltages (n = 5)

filter reactor voltage in phase B 16.5 kV

filter reactor voltage in phase >

Fig. 5. 3rd filter voltages under restrikings ground fault in phase A restriking ground arc fault on the phase A at a cable connected to the bus.

The bus high frequency transient voltage component determine filter reactor overvoltage magnitude. Frequency f of the component depends on the bus short circuit current Is.c. and the network single-phase ground fault current Ig :

f

f 0 I s.c. / I g ,

(1)

where f0 is system supply frequency. On the other hand, filter reactor high frequency transient voltage component depends on the filter tuning frequency fF and can be estimated from the following expression:

U FR

f 2 U K (1 ( F ) 2 ) 1 , 3 f

(2)

where UK is voltage displacement at the last fault during restriking (see Fig.5).

In modern dry type reactors, conductor to conductor voltage stress levels for the transient overvoltages are minimized through the use of grading techniques and judicious use of additional layers of insulation[1]. Dry type reactors also have an inherent characteristics that mitigates transient voltage effects. Since the stray capacitances to ground are small the winding voltage distribution under transient voltage conditions has a nonlinearity factor of typically less than 2.5. 3. CONCLUSION

Due to their ability to rapidly vary the reactive power output, SVCs are widely used in power supply systems for compensation of changing loads impact. By presented example, the paper examines exploitative filter circuits transients features and shows that consideration of switching transients must be included in the filter design and protection selecting procedures to maintain high filter reliability. 4. REFERENCES

1. Bonner J.A. et al. Selecting ratings for capacitors and reactors in applications involving multiple single-tuned filters. IEEE Trans. on Power Delivery, vol.10, no.1, 1995, pp. 547-555. 2. Harder T.E. AC filter arrester application. IEEE Trans. on Power Delivery, vol.11, no.3, 1996, pp. 1355-1360. 3. Ravlyk A., Gretchyn T. Digital complex for modelling of transient processes in electric


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4.

5.

6.

7.

circuits. Proc. of III Int. Symp. "Metody matematyczne w elektroenergetyce", Zakopane, 1993, pp. 17-20. Varetsky J., Jurahivski A. Transformer energizing in industrial power system with SVC. Proc. of 5-th Int. Conf. "Electrical power quality and utilization", Cracow, 1999, pp.453458. Varetski J., Bachor Z. Overvoltage computer analysis in the network with static VAR compensator. Proc. of III Int. Symp "Metody matematyczne w elektroenergetyce", Zakopane, 1993, pp. 87-92. Varetski J., Bachor Z., Ravlyk A. Transients in 10-35 kV electric networks with ungrounded neutrals under earth faults. Proc. of VII Int. Symp. "Short Circuit Currents in Power Systems", Warsaw, 1996, pp. 1.20.1-1.20.4. Beliakov N.N. Voltages analysis during ground arcing fauls in 6-10 kV ungrounded networks. Elektrichestvo, no.5, 1957, pp. 31-36.(in rus.)

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D.Sc. Yurij Varetsky was born in 1952 in Lviv, Ukraine. He received the Ph.D. degree from Power Research Institute (Moscow, Russia) in 1982 and the D.Sc. degree from Lviv Polytechnic National University in 1999, both in Electrical Engineering. Presently, he is Associate Professor within Chair of Electrical Networks and Systems at Lviv Polytechnic National University. His field of interest includes electric power quality and power systems computer analysis. Mailing address: Yurij Varetsky Lviv Polytechnic National University Chair of Electrical Networks and Systems 16a/2 Dovbush str., 79008, Lviv, UKRAINE phone (+38) (0322) 753318, E-mail: [email protected]




TRANSFORMER AND THYRISTOR BASED COMPENSATOR OF VOLTAGE DEVIATIONS AND VAR WITH FOUR-QUADRANT CONTROL Vladimir KLIMASH State Technical University of Komsomolsk on Amur Komsomolsk on Amur (Russia) Abstract - In this paper the compensator of VAR and voltage deviations is proposed, which main features are improved time response, better shape of the output voltage and simplicity of realization of the closed-loop automatic regulation system in both start-up and static modes. The compensator is based on the principle of independent pulse-width regulation of the transverse component of the booster voltage as a function of network's VAR and the longitudinal component of the booster voltage as a function of load voltage deviation.

requirements of power quality and its economy. The solution of this problem acquires particular urgency for power supply systems with extended power transmission lines and variable nature of the load. This can be attributed to all the industrially developed countries with vast territories and considerable distances between consumers and power supply centers. Unlike devices known in the world practice in the proposed device two complementary functions are combined: complete compensation of reactive power and stabilization of voltage independently of supply network external characteristic slope and the value and the character of the load. The compensator (Fig. 1) consists of two booster transformers and two three-phase voltage source inverters with a shared three-phase recuperative

The necessity to proliferate means of consumers voltage stabilization and supply network reactive power compensation is clearly defined with

10 1

2 3

4

5

15

16 G1

7

17

G2 6

8

V3

9

14

18

12

13 11

Figure 1. Functional diagram of the compensator: 1,2 - input terminals; 3, 4 - the first and the second booster transformers with ratios kT1 > kT2 ; 5 - bank of capacitors; 6, 8 - the first and the second three-phase voltage source inverters with control systems; 7, 9; 10 - synchronization unit; 11 - three-phase recuperative rectifier with controlled valve unit; 12, input three-phase circuit breaker; 13 and output filter; 14; 15 - network’s reactive power sensor; 16 - load voltage deviation sensor; 17 - switch; 18 - timer.


303

rectifier. It also includes sensors of network's VAR and load voltage deviation, a bank of capacitors for power-factor correction, a filter, a three-phase circuit breaker, a start-up switch and a timer. The transformer- and thyristor-based compensator of voltage deviations and VAR operates as follows. is a sum of the Output voltage of the device V 2 and the additional voltage network’s voltage V 1

, which is regulated in magnitude and phase V d with independent regulation of magnitudes of the first harmonics of its transverse V and d1 components. It is determined by longitudinal V d2

the following term:

V 2

V V 1 d

V V . V 1 d1 d2

(1)

The components of the additional voltage

V d1 V

d2

2

2

(2)

b) ' V1 > 0 M2 = 0

a) 'V1 < 0 M2 > 0

. V . V

.

.

.

V2

.I .I

.I .I .I

2

f

V 2

V k ¸ j ·1 k ¸ j · 2 , V 1 2 ¬1 1 ¬2 2

or considering network voltage deviation r 'V 1 , and voltage loss in transformers 'V K

V 2

r 'V 'V V 1 1 K

1 k ¬1 G1 j D1 k T 2 G 2 j D2

Here 'V K

z K1 z K2 I1 ;

.

(3)

zK1, zK2 - short circuit

impedances of booster transformers; I1 - the current consumpted from the network which matches by phase with network voltage and is a sum of the currents of the load, the three-phase recuperative rectifier and the bank of capacitors.

I 1

304

I I I . 2 f c

(4)

1

C

f

.I

2

1

.I

C

d2

d

V1

2

where kT1 , kT2 are ratios of the first and the second three-phase booster transformers (3 and 4); G1, G2 - voltage ratios of the first and the second inverters (6 and 8), which are determined by the ratio of the pulse longitude to the commutation period and are inversely proportional to on-off time ratio; D1 - the phase of the first three-phase inverter 6, which depending on reactive power polarity takes one of the -900 and 900 values; D2 the phase of the second three-phase inverter 8, which depending on load voltage deviation polarity takes one of the 00 and 1800 values. From (1) and (2) we have:

.V V.

Vd2

. V

.

Vd1

V1

.

d

d1

G exp( jD ); k ¬1 V 2 1 1 k V G exp( jD ), ¬2

The angle between the load current and the network voltage is controlled with the output voltage phase depending on the load current phase, and also on the ignition angle and the way of commutation of three-phase recuperative rectifier thyristors 11. From expression (3) and from vector diagrams (Fig.2) one can see that magnitude and phase of vector can be controlled by variation voltage V 2 of ratios G1 and G2 of the first and the second threephase voltage source inverters.

.I

2

.I

C

Figure 2. Vector diagrams of compensator const, M1 0 ). operation modes ( V 2 In the device variation of G1 is done as a function of power quotient deviation from unity, and variation of G2 is done as a function of load voltage . deviation from the previously set level V Reverse of the transverse and the longitudinal components of the booster voltage is done by changing the phase D1 and phase D2 respectively by 180 degrees. During transition from reactive power consumption mode to reactive power generation mode value and polarity of feedback signal from the output of reactive power sensor 15 are changing. According to the polarity of the signal the control system 7 of the first three-phase voltage source inverter 6 changes the algorithm of thyristors commutation and changes the polarity of the transverse component of the booster voltage to the opposite one, and according to the value V d1 of this signal the regulation of the vector magnitude is done to compensate the phase of V d1 the input current (Fig. 3). The signal proportional to the load voltage deviation is taken from the outElectrical Power Quality and Utilisation

a2

b c

22

b2

24

c2

to 20

21

a1

23 c

b to 21

b1 25

10

output for 9

0 a 20

19

A B C

output voltage and compensation of network’s reactive power are done regardless of value and type of the load, and also regardless of network’s voltage fluctuations and deviations. Switching-on the booster transformers with no constant components of magnetizing current and magnetic flux can be done in the following two ways: 1- connecting the secondary winding of the booster transformer to the voltage with 90o phase shift comparing to the network’s voltage; 2 shunting the secondary winding of the booster transformer. In the proposed device both ways were used. The proposed device as a more perfect one, based on the principle of direct compensation, is a good substitution for known compensators of reactive power and stabilizers of three-phase voltage in electric energy supply systems. Four- quadrant forming of the additional voltage vector in the orthogonal coordinate system (Fig. 2) has simple realization and better shape of the output voltage. Besides, this way in comparison to vector forming in polar coordinates also has a disadvantage. In case of orthogonal coordinates two booster transformers are used, and their total power is proportional to the arithmetical sum of the square triangle’s sides (Fig. 2), and at the same time in case of polar coordinates one transformer is used (Fig. 4), and its power is proportional to the geometrical sum of the same square triangle’s sides.

output for 7

input

put of the sensor of load voltage deviation 16. According to the polarity of this signal the control system 9 of the second three-phase voltage source inverter 8 changes the polarity of the longitudinal , and depending on the value of component V d2 this signal regulates the magnitude of this component.

c1

Figure 3. Synchronization unit: 19 - three-phase transformer synchronization; 20, 21 - active filters; 22, 23 - devices shifting phase by 120O; 24, 25 – adders. As a result of such a transverse-longitudinal regulation of the booster voltage, stabilization of the 10 1

2 4 15

5

16 SD

D 6

26

V3

17 8

18

27

E 14

12

13 11

Figure 4. Functional diagram of the compensator with four-quadrant control in polar coordinates: 1,2 - input terminals; 4 - booster transformer with ratios kT ; 5 - bank of capacitors; 6, 8 - the first and the second threephase voltage source inverters with control system 26; 10 - synchronization unit; 11 - three-phase recuperative rectifier with controlled valve unit 12, input three-phase circuit breaker 13 and output filter 14; 15 - network’s reactive power sensor; 16 - load voltage deviation sensor; 17 - switch; 18 – timer; 27 – synchronization pulses phase regulation unit. Section 4. Methods of Power Quality Improvement: Filters, ....

305

This fact enables decreasing mass and sizes of the transformer equipment of the device when using polar coordinates. As for complexity of inverters control and the compensator regulation unit, the following should be mentioned: in case of orthogonal coordinates the inverters control system is more complex, and in case of polar coordinates so does the unit of the whole device regulation. Control system 26 performs an operation of shifting of control pulses’ phases on outputs. Control pulses on 1st and 2nd outputs are shifted respectively by angles D and S-D in regard to initial phase E, which is regulated by unit 27 regarding network’s voltage. The first and the second three-phase inverters 6 and 8 convert the rectified voltage to two AC voltages. The vectors of the first harmonics of these voltages are shifted by common initial phase E in regard to network’s voltage. One of these vectors is regulated in phase by angle D, and the other is regulated by angle S-D. V d1 V d2

e jE e j D . V d

Due to the fact that inverters 6 and 8 are connected with secondary windings of the transformer 4 on both sides, to the windings is applied the difference between the inverters’ output voltages

V d

V 2

V c Z I V 1 d K 1

2

is done by changing D up and down regarding V 1 from 0 to S radians, E being equal to 0 or S radians.

V1+'V1 E

Vd V1

V2

RL-load

V2

R-load IC

M2>0

V V d1 d2

2k V jE V 1 ¬ d e cos D Z K I1 ,

Z c is the short circuit impedwhere Z K Z 1 2 ance of the transformer. It can be inferred from the last expression and the vector diagrams (Fig. 5) that magnitude and phase can be regulated by changing D of the vector V 2 in magnitude and E. Specifically, regulation of V

Vd

e jE e j D , V d

e jE e j ( S D ) V d

ble-supply machine which has a load in its stator’s circuit [1] ) enable making an equivalent circuit of the reactive power compensator (Fig. 2a) and its simplified variant (Fig. 2b) (where the magnetizing current is neglected: I0 | 0 ) which enables to calculate the voltage on the load 2 with sufficient accuracy:

I1

I2+If

or the sum of conjugate complex vectors with phase D, which are presented in a complex plane rotated by angle E regarding networks’ voltage V d

e jD V e j D ) e jE . (V d d

(5)

If to take into consideration the Euler transforms, the voltage of the secondary winding V d

cos D) e jE (2 V d

and its value reduced to the load voltage c V d

cos D) e jE , k ¬ (2 V d

(6)

where kT is the transformer’s ratio. Further reductions of all the parameters of the secondary circuit to the corresponding ones of the primary circuit ( by analogy with the braked dou-

306

Figure 5. Vector diagram of voltages and currents in polar coordinates for the compensator. V1 and 'V1 – network’s voltage and its deviation from the nominal level; V2 and V2’ – voltages on RL- and R- load respectively; Vd and Vd’ – additional voltages of the transformer under RL- and R- load; M2 and M2’ – the phase of the load current under RLand R- load; E - the phase of the additional voltage; I1 = I2 + If + IC – the network’s current and its components (the current of the load, the current of the rectifier and the current of bank of capacitors).


Regulation of phase is done also by changing D, but E in this case is equal approximately to S/2 in regard radians, phase leading regulation of V 2 to V is done by changing D from 0 to S/2 radians,

2.

1

and phase lagging regulation is done by changing D from S/2 to S radians. In the proposed device changing of D is done as a function of network 1’s reactive power deviation from zero, and changing of E is done as a function of load 2 voltage deviation from preset level. When the compensator generating (consuming) reactive power has RL-load, the signal from the output of reactive power sensor 15 goes to the control input of the control system 26 of inverters 6 and 8 and increases (decreases) the effective c by increasing value of the additional voltage V d (decreasing) of the firing angle D regarding the initial phase E and correspondingly increases (decreases) which the phase of the output voltage vector V 2 advances in phase the network’s voltage vector . In this case the sensor 16 of load voltage deV 1 viation gives signal to the control input of synchronization pulses’ phase regulation unit 27 which regulates the phase of additional voltage c and the effective value of the output voltage V d by changing the angle E in regard to the netV 2

work’s voltage. As a result of such influence on the output voltages of the 1st and the 2nd three-phase inverters 6 and 8 by magnitude and phase, the ad c forms its magnitude ditional voltage vector V d and phase in such a way that the load voltage vec is a radius of a previously set circle. tor V 2 In case of R - load the compensator operates the same way, but in this case forming of the output voltage is done in the area of lagging the network’s voltage. During the process of stabilization of the output voltage, when the output voltage is lower (higher) than the previously set (for example, nominal) level, the recuperative rectifier 11 operates in rectifier (inverter) mode and enables the transformer 4 and the whole device to operate in voltage adding (voltage subtracting) mode with consumption of additional energy from the network (with recuperation energy to the network).

3.

4.

5.

6.

7.

Works Collection “Teoriya i raschet elektrooborudovaniya”, Khabarovsk, Khabarovsk Politechnical Institute, 1987, pp. 114-118. Klimash V.S., Simonenko I.G.: Razrabotka i ispitaniya kompensatora otklonenii napryazheniya i reaktivnoi moschnosti. Inter-college Scientifical Works Collection “Povishenie effektivnosti i nadezhnosti sistem elektrooborudovaniya”, Khabarovsk, Far East State University of Railroads, 1999, pp. 113-118. Klimash V.S.: Sposobi i skhemi regulirovaniya amplitudi i fazi napryazheniya v sistemakh elektropitaniya. Proc. of 2nd Siberian Science and Technology Conference, Krasnoyarsk, USSR, 1990, pp. 139-140. Klimash V.S.: Transformatorno-tiristornie preobrazovateli napryazheniya s impulsnim, amplitudnim i fazovim regulirovaniem. Komsomolsk on Amur, Komsomolsk on Amur State Technical University, 1998. Patent RU 2056692, 6 H 02 J 3/18. Transformatorno-tiristornii kompensator reaktivnoi moschnosti / Klimash V.S. // Otkritiya. Izobreteniya. – 1996.- No. 8 – p. 263. Patent RU 2154333, 7 H 02 J 3/18. Kompensator reaktivnoi moschnosti / Klimash V.S., Simonenko I.G. // 10.08.2000.- Bulletin No. 22. Patent RU 2158953, 7 H 02 J 3/18. Transformatorno-tiristornii kompensator otklonenii napryazheniya i reaktivnoi moschnosti / Klimash V.S., Simonenko I.G. //10.11.2000.- Bulletin No. 31.

Mr. Vladimir Klimash was born in 1953 in Komsomolsk on Amur, Russia. He received Candidate of Science degree in electrical engineering from Sanct-Petersburg Mining Institute “G.V.Plekhanov” in Sanct-Petersburg. At present he is Assistant Professor in the Department of Industrial Electronics of Komsomolsk on Amur State Technical University. His area of interests includes power electronics and computer-aided simulation of power electronics’ devices.

Mailing address: Vladimir Klimash State Technical University 27 Lenina Av, Komsomolsk on Amur phone:(42172) 3-60-09 fax:(42172) 3-61-50 email: [email protected]

REFERENCES 1. Klimash V.S.:Voltodobavochnii transformator s tiristornim upravleniem kak mashina dvoinogo pitaniya. Inter-college Scientifical Section 4. Methods of Power Quality Improvement: Filters, ....

307

308




VOLTAGE QUALITY IMPROVEMENT USING SUPERCONDUCTING MAGNET ENERGY STORAGE (SMES) DEVICES IN LV SYSTEM WITH NEUTRAL CONDUCTOR Enrico TIRONI, Ivan VALADÈ Politecnico di Milano Milano, Italy

Giovanni LOPES Schneider Electric Stezzano (Bg), Italy

Giovanni UBEZIO SIEL Trezzano Rosa (Mi), Italy

Keywords: SMES, compensation devices, PWM control, voltage sags.

absorption by the network with a low harmonic content and a high power factor. Some studies have shown that voltage sags and brief outages lasting less than one second are the commonest cause of the malfunctioning of manufacturing processes [1], and that it is therefore necessary to install compensation devices, including a form of energy storage able to supply the load under such conditions. Conventional batteries are not indicated for these applications, since their design is unsuitable when large amounts of power are required for periods of the order of a few seconds. Of the systems that are most suitable for the purpose, those using one or more superconducting magnets as a form of energy storage, known as SMES (Superconducting Magnetic Energy Storage) [2][3], have become increasingly popular.

1. INTRODUCTION

2. CHOICE OF CIRCUIT CONFIGURATION

The ever more widespread use of electronic devices that are highly sensitive to power supply disturbances has called for continual research aimed at improving the quality of the power supply. However, it should not be forgotten that such devices are also one of the causes of deterioration in the quality of the power supplied to utilities: the presence of plants that absorb currents with a high harmonic distortion causes voltage drops on line that also distort the voltage supplied to the other utilities. A system aimed at improving power supply quality is installed in order to protect individual loads against disturbances present in the network, but it has to be designed so as to ensure current

Of the possible circuit configurations suitable for improving the quality of the power supplying a load, consideration has been given to three [4] that have been found significant inasmuch as they are associated with various systems currently in use. Configuration a) consists of a rectifier, an energy storage unit, and an inverter: the 3 units have in common a DC section in which the smoothing capacitors are installed. In any given supply situation, the system disconnects the supply from the load. The active power required by the latter is drawn from the network via the rectifier and, should this not be sufficient, is topped up with the power that can be taken from the energy storage system.

Abstract - The paper analyzes a device that uses a superconducting magnet as a form of energy storage capable of improving the quality both of the power supply to a sensitive load and that of the current absorbed by the network. Research on various circuit configurations has included one that allows for full disconnection of the power supply from the load through an intermediate direct current section: this section is interfaced with the storage section via a new converter capable of reducing losses more effectively than those suggested in the literature. The whole system and the control logics of each converter were checked with numerical simulations.


309

a)

DC

AC

AC

DC

Storage

b) AC DC

Storage

connected in series to the power supply line, injects an e.m.f. in such a way as to stabilize the voltage on the load: in the case of a voltage within contractual limits, there is no active power exchange between the storage system and the load because the voltage injected is in quadrature with the line current. In the case of events that do not allow the current to circulate in the power supply line, a further device would be needed to re-close the currents: this and other limits evidenced in [6] affect the possibility of application in the area of the disturbances dealt with. Table 1 gives the results achievable through configurations a) and b); this paper develops the first of the two configurations which, using the appropriate circuit structure and control of the rectifier, makes it possible to draw a sinusoidal current with a unitary power factor.

c) 3. STRUCTURE OF STORAGE UNIT AC DC

Storage

Fig. 1.1 Circuit configurations Configuration b) consists of a converter, on whose DC section the energy storage is interfaced, and a link reactance. The converter, set in parallel to the load, acts as a voltage source that keeps the voltage RMS constant, varying the voltage phase on the load in such a way as to absorb from the network the total active power required: this can be done up to a limit value in respect of the supply voltage below which the converter has also to supply active power. This voltage limit is associated with the converter power rating, the load power factor, and the value of the series reactance [5][6]. In configuration c), the compensation device consists of a booster transformer supplied by an AC/DC converter, on whose DC section the energy storage system is located. The device, being

The storage system consists of the superconducting magnet, which is an impressed-direct-current energy accumulator, and a converter for interfacing with the direct voltage section defined. Three different operating conditions are provided for: - the charging phase, in which energy is stored in the superconducting coil; - the discharge phase, in which part of the energy stored is used to supply the sensitive load; - the stand-by phase, in which a small fraction of the power drawn from the network compensates for the Joule losses in the non-superconducting parts of the energy storage circuit and for the losses in the magnet due to hysteresis and eddy currents. The storage system is characterized by the level of the voltage applied to the magnet in the discharge phase and by the quality of the same voltage in the charging and stand-by phases. The first parameter is linked with the energy that can be drawn from the magnet, which increases with the voltage applied during discharge, because the power required by the load can also be supplied with lower residual currents. The second parameter is linked with the losses in the magnet due to hysteresis and eddy currents: indeed, these depend

TABLE 1. Comparison between circuit configurations a) and b) in Fig. 1.1 Device Configuration a)

Configuration b)

x x x x x x

310

Characteristics Sinusoidal and stabilized load voltage, without frequency or phase variations. Sinusoidal line current with unity power factor (with suitable circuit structure and rectifier control). High investment and operating costs. Sinusoidal and stabilized load voltage, although sensitive to phase variations corresponding to network voltage disturbances and to network frequency. Network power factor depending on reactance value and supply voltage. Investment cost linked with series reactance value; moderate operating costs.


on the ripple of the circulating current, which is held in check with suitable applied voltage waveshapes [7][8]. D1

G2

cycle of G2. The power lost PP is thus compensated for by applying to the magnet a voltage waveshape consisting of voltage impulses whose amplitude is Vd, and whose average value V SMES , calculated during the switching period of G2, is equal to:

LSMES iSMES Vd

vSMES G1

i SMES

D2

Fig 3.1 A possible circuit configuration for the storage system In the storage system shown in Fig. 3.1 [9], the reversible converter, consisting of controlled valves G1 and G2 and diodes D1 and D2, makes it possible to charge the magnet and to draw off energy during sags in the network voltage. Vd indicates the voltage of the DC section, while iSMES represents the current circulating in the superconducting magnet. Table 2 gives the state of the valves during the operating phases: during discharge, they alternate between a and b, and during charging and stand-by between c and d. TABLE 2 State of valves operating stage G1 discharge a b charging c stand-by d

PP

VSMES

OFF ON ON ON

valve G2 OFF OFF ON OFF

G Vd

in which G is the duty-cycle of G2 and iSMES is the instantaneous value of the current circulating in the magnet, which may be considered constant due to the smallness of the ripple compared with the average value. By reducing the amplitude of the voltage impulse and increasing G, we can compensate for PP with smaller current ripples and losses in the magnet: with G equal to 1 and an amplitude equal to VSMES , this ripple might even be cancelled out. For this reason, a new circuit configuration was studied, as shown in Fig. 3.2, being obtained by adding a step-down chopper upstream of valve G2, consisting of valves D3 and G3, inductor LLOW and capacitor CLOW. D1 LSMES iSMES G2

D1

D2

ON OFF OFF OFF

ON ON OFF ON

During the discharge phase, the voltage vSMES applied to the magnet alternates with values of -Vd (state a) and zero (state b): by taking action on the duty-cycle of valve G1, it is possible to regulate the average applied voltage value in such a way as to draw the required power from the magnet. During charging, the voltage vSMES takes on alternately the values Vd (state c) and zero (state d): in this case, the required charging power is obtained by taking action on the duty-cycle of G2. State b, and similarly d, could be replaced by the state OFF-ON-ON-OFF, or else alternate with it in such a way as to distribute the switching losses between the two valves. In the stand-by phase, too, states c and d are alternated in such a way as to absorb the power needed for compensating for the Joule losses in the non-superconducting parts and for the losses in the magnet due to hysteresis and eddy currents: operation in the stand-by phase differs from that in the charging phase only with regard to the duty-

LLOW

G3

vSMES G1

D2

vLOW

CLOW

D3

Vd

Fig. 3.2 Circuit configuration of suggested storage system In the various operating phases, the state of valves G1 and G2 is identical to that shown in Table 2; valve G3 of the chopper is controlled in such a way as to keep the voltage vLOW at approximately the desired value. This solution makes it possible to improve the quality of the voltage applied to the magnet during the stand-by phase, which consists of impulses whose amplitude vLOW is lower and whose duty-cycle is higher than the circuit in Fig. 3.1. During the discharging phase, on the other hand, the total voltage Vd, is applied to the magnet, thus making it possible to draw from the storage system the same amount of energy as that in the previous circuit. 4. SYSTEM CONTROL The system consists of three units that have in common the DC section with the Vd voltage


311

defined. The control logics of each unit will now be dealt with separately.

active power from flowing from the device to the network.

x Rectifying unit

x The energy storage unit

This unit, whose circuit configuration is shown in Fig. 4.1, draws from the network the power required for stabilizing, at the rated value, the voltage on capacitors CINV of the DC section. The unit consists of the converter valves, of three inductances Ll, and of the filter Lf-Cf for the harmonics at the switching frequency and its multiples.

The control logic of this unit has to provide for stabilization of the voltage Vd, when the total power required by the load has not been drawn from the network, and for maintaining the magnet's state of charge. The first function is obtained by controlling the valve G1, through signal fireG1, in accordance with the logic indicated in Fig. 4.3.a, in which an hysteresis controller processes the error eVd of the voltage Vd(t) in relation to the reference value Vdref : the voltage is kept within a band centered on the reference value and having an amplitude equal to 2'Vd, G1 being opened when the lower limit is exceed and closed when the upper limit is exceeded.

i a a i b b c ic

Lf

Cf

Ll ir_a ir_b ir_c

CINV Vd CINV

vf_c vf_b vf_a

N

Fig. 4.1 Sinusoidal-absorption rectifying unit

Vdref + eVd

AC/DC conversion is effected by absorbing from the line a current with a sinusoidal waveshape and a power factor approximating to the unit. The instantaneous reference phase current values are calculated by the control logic in Fig. 4.2, which is based on the negative feedback of signal Vd(t): the error in relation to the reference value Vdref is handled by a controller that returns the parameter TVd, which afterwards, being multiplied by the phase voltage vf, provides the current reference i’ref; this signal is kept at a value lower than the maximum value Ir_max in order to limit the current absorbed by the network during supply voltage sags. vf_a X

TVd_max Vdref + eVd

-

controller TVd 0

Vd(t)

i’ref_a

iref_a

i’ref_b

iref_b

i’ref_c

iref_c

vf_b X

vf_c X

Fig. 4.2 The rectifier control logic The technique for controlling the valves is of the fixed-band hysteresis kind [10], which makes it possible to keep the current absorbed ir within a band centered on the reference signal iref. The controller is of the proportional-integral type, in order to cancel out the voltage error in steadystate operation, with positive output to prevent

312

fireG1 on

-

Vd(t) eVd

Switching on G1 'Vd maintenance of Vd previous state 'Vd ref Switching off G1

fireG1

off

Vd(t)

4.3.a

t

4.3.b Fig. 4.3 Control of valve G1 The qualitative trend in respect of voltage on the DC section is shown in Fig. 4.3.b. Control of valves G2 and G3 makes it possible to charge the magnet and maintain the state of charge in the stand-by phase. The reference power Pref to be drawn from the DC section is determined on the basis of the current iSMES circulating in the magnet by means of the control shown in Fig. 4.4, in which iSMES_n is the magnet's rated current. The output of the controller, which is of the proportional-integral type, is limited to the maximum value Pcharge_max, which indicates the power it is desired to absorb from the network during the magnet's charging phase. Pcharge

iSMES_n

+

eiSMES

controller

ma

Pref

iSMES(t)

0

Fig. 4.4 Definition of the charging power Transfer to the magnet of the desired power is obtained by causing valve G2 to conduct for a period TonG2, depending on the instantaneous


value of the voltage vLOW and of the current iSMES, in accordance with the formula (4.1):

TonG 2

1

Pref

(4.1)

f G 2 v LOW i SMES

vL_a(t)

in which fG2 is the switching frequency of valve G2. The voltage vLOW is stabilized around the rated value by taking action on the duty-cycle of G3, which is determined by the control loop in Fig. 4.5 [11]. VLOW_ref + eVLOW

controller

controllers (Rd, Rq, Ro), which return the 3 Park components of the reference.

TVLOW

vL_d(t)--

ed +

Rd

vL_b(t)

vL_q(t) - - eq u (t) Rq q T(T Z*t) + uo(t) vL_o(t) - - eo vL_c(t) +

ua(t)

ud(t)

Ro

TT (T=Z*t)

ub(t) uc(t)

vref_o=0 vref_q vref_d

fireG3 comparator

Fig.4.7 The inverter control logic

vLOW(t)

VLOW

t

State of G3 ON

t

OFF

Fig. 4.5 Control of valve G3 x Inversion unit The inversion unit, shown in Fig. 4.6, activates the DC/AC conversion between the imposed voltage section Vd and the load. CINV Vd CINV

iinv_a Lout iinv_b iinv_c Cout

iL_a iL_b iL_c vL_c vL_b vL_a

This paper presents only the principles on which the system control is based; the setting of the parameters of the controllers was effected using the Bode and Nyquist stability criteria, after having designed the passive components [12] (CINV, inductance Ll, inductances and capacitances of the filters L-C, inductances and capacitances of the storage unit) and after constructing a mathematical model of the system. The analytical model was obtained by ignoring the dynamics associated with the switching frequency of the valves and linearizing around the steady-state condition, and therefore is of limited validity: to be able to check the dynamic behaviour of the system, even when faced with disturbances that are not small, it was necessary to make a numerical model, using EMTP software, that would simulate the actual operation of the device. 5. CHECKING BY MEANS OF SIMULATION ON THE CALCULATING-MACHINE

The simulations were carried out with a load of 500 kW, power factor of 0.9lag, a 4H magnet with a rated current of 1150A.

Fig. 4.6 Inversion unit

5.1 Symmetrical voltage sag

This consists of a converter with valves controlled in accordance with PWM technology and an output filter Lout-Cout for the harmonics at the switching frequency and its multiples. The reference waveshape is defined by the logic in Fig. 4.7, which employs the Park transformation on axes rotating to the pulsation of the voltage supplying the load T(T). The control logic is based on the negative feedback of the 3 Park components of the voltage on the load (vL_d(t), vL_q(t), vL_o(t)): each of these is compared with the respective reference value, and the 3 resulting errors are the inputs of as many

In the case of a symmetrical voltage sag of a depth equal to 50% and a duration of 50 ms, whose waveshapes are given in Fig. 5.1, the device is able to stabilize the load's supply voltage without any significant transients. The current absorbed by the network presents stepped variations both when set up and at the end of the voltage sag, due to the fact that the current reference is obtained by using the instantaneous network phase voltage. At the end of the disturbance, current thresholds, set at 1500A, are reached and followed by a damped transient lasting about 5 cycles, during which the parameter TVd in


313

Fig. 4.2 attains the steady-state value, which also makes it possible to absorb from the line the power necessary for charging the magnet.

different gradients are linked with the different powers exchanged by the magnet in the two conditions.

line current (A)

voltage on load (V)

network voltage (V)

5.2 Dissymmetrical voltage sag

Checking behaviour in dissymmetrical conditions is necessary, because one of the assumptions that has led to the calibration of the control system consisted in regarding the three-phase system as symmetrical and balanced, and therefore there is nothing to be said regarding behaviour in dissymmetrical conditions: during the simulations, the case of a 50% voltage sag on one phase was studied. The presence of a homopolar component in the current and in the network voltage leads the device to absorb from the line a power consisting of an average value and a component that fluctuates at a frequency twice that of the network. During the sub-period in which a power greater than that absorbed by the inverter comes from the rectifier, the voltage of the DC section increases, while, during the sub-period in which a smaller comes in, such voltage decreases, thus causing the storage system to intervene: this sets off the stepped trend of the current circulating in the magnet. The waveshapes of the currents absorbed by the network are very similar to the previous case; it will merely be noted that, correspondingly to the fault, there is an increase in absorption on the sound phases due to the increase in the parameter TVd, which leads to intervention by the current thresholds.

coil current (A)

6. CONCLUSIONS

Fig. 5.1 Simulation relating to a symmetrical voltage sag The decrease, of about 4A, in the current circulating in the superconducting magnet during the voltage sag is due to the intervention of the storage system, while the subsequent growth occurs during the recharging of the magnet: the 314

The suggested device is able to supply correctly a load whose characteristics are similar to that examined, even when there are disturbances from the electricity network, due to the speed with which the storage system studied intervenes. Absorption of current by the electricity network has a low harmonic pollution rate, and the power factor is very close to the unit, thus making it possible to contain the deterioration in voltage quality attributable to this device. The interfacing converter of the superconducting magnet has shown itself to be effective, even during the charging and stand-by phases of the magnet, thus making it possible to supply the amount of power required without inducing any significant current harmonic components in the superconducting coil.


network voltage (V) voltage on load (V)

line current (A) coil current (A)

Fig. 5.2 Simulation relating to a dissymmetrical voltage sag REFERENCES

[1] L. Fellin: I disturbi negli impianti elettrici utilizzatori. Paper discussed at conference “La qualità del prodotto elettricità. Interfacciamento distributore-utente”, Verona 25-26 novembre 1993, (in Italian).

[2] S. Zannella: I superconduttori ceramici Proprietà e applicazioni nei sistemi elettrici. AEI Vol. 81, No. 11 November 1994 (in Italian). [3] W.E. Buckles, M.A. Daugherty, B.R. Weber, E.L. Kostecki: The SSD: a commercial application of magnetic energy storage. Presented at the 1992 Applied Superconductivity Conference, Chicago. [4] S.J. Huang, J.C. Wu, H.L. Jou: Electricpower-quality improvement using parallel active-power conditioners. IEE Proc.-Gener. Transm. Distrib., Vol. 145, No 5, September 1998. [5] D. Lauria, E. Tironi: Some Considerations on Active Compensation Devices. ETEP Vol. 3, No 3, May/June 1993. [6] G. Lopes, I. Valadè: Miglioramento della qualità della tensione attraverso l’impiego di sistemi con accumulo energetico in magneti superconduttivi (SMES). Degree thesis at Dipartimento di Elettrotecnica of the Politecnico di Milano, A.A. 1998-1999, Supervisor Prof. E. Tironi (in Italian). [7] I.J. Iglesias, J. Acero, A. Bautista: Comparative Study and Simulation of Optimal Converter Topologies for SMES Systems. IEEE Transactions on applied superconductivity, vol- 5, n° 2, June 1995, pg. 257. [8] C. Levillain, P.G. Thérond: Minimal Performances of High Tc Wires for Cost Effective SMES compared with Low Tc’s. IEEE Trans. On Magnetics, Vol. 32, 4, p. 2308, 1996. [9] V. Karasik, K. Dixon, C. Weber, B. Batschelder, G. Campbell, P. Ribeiro: SMES for power quality applications: a review of technical and cost considerations. 1998 Applied Superconductivity Conference, Palm Desert CA, 13-18 settembre 1998. [10] J.M.D. Murphy, F.G. Turnbull: Power Electronic Control of AC Motors. Oxford: Pergamon, 1988. [11] N.Mohan, T.M. Undeland, W.P. Robbins: Power Electronics: Converters, Applications and Design. New York ,John Wiley & sons inc, 1989. [12] G. Superti Furga, E. Tironi, P. Zanotti: Confronto tecnico-economico fra sistemi di compensazione di buchi di tensione e interruzioni di breve durata. Paper discussed at conference “La qualità dell’ elettricità e la compatibilità con gli impianti utilizzatori di media e bassa tensione”, FAST, Milano 12 dicembre 1995 (in Italian).


315

BIOGRAPHIES Giovanni Lopes graduated in Electrical Engineering at the Politecnico di Milano, Italy, in 1999. He is now working in Schneider Electric. His research interests are power electronics and power quality. Mailing address: Giovanni Lopes e-mail: [email protected] Enrico Tironi graduated in Electrical Engineering at the Politecnico di Milano, Italy, in 1972, in the same year joining the Politecnico's Electrical Engineering Department, where he is now full professor. His areas of research include power electronics and power system harmonics. He is a member of the Electrical Power System Group of the Italian National Research Council (C.N.R.) and of AEI. Mailing address: Enrico Tironi Politecnico di Milano Dipartimento di Elettrotecnica Piazza Leonardo da Vinci 32, 21133 Milano, Italy phone:(+39)(02) 23993713, fax:(+39)(02) 23993703 e-mail: [email protected]

316

Giovanni Ubezio graduated in Electronic Engineering at the Politecnico di Milano, Italy, in 1977, subsequently joining the Siel Company, where he is R&D manager. His areas of research include power electronics and power system harmonics. He is a member of the UPS Group of C.E.I. and of AEI. Mailing address: Giovanni Ubezio SIEL SpA Trezzano Rosa (Mi), Italy phone:(+39)(02) 90986256 e-mail: [email protected] Ivan Valadè graduated in Electrical Engineering at the Politecnico di Milano, Italy, in 1999. He is now working for a second, post-graduate degree at the Politecnico's Electrical Engineering Department. His research interests are power electronics and power quality. Mailing address: Ivan Valadè Politecnico di Milano Dipartimento di Elettrotecnica Piazza Leonardo da Vinci 32, 21133 Milano, Italy phone:(+39)(02) 23993752, fax:(+39)(02) 23993703 e-mail: [email protected]




COMPARISON BETWEEN UPS LINE INTERACTIVE DEVICES DESIGNED TO SOLVE POWER QUALITY PROBLEMS Roberto FARANDA, Enrico TIRONI, Ivan VALADE’ Politecnico di Milano Milano (Italy) Abstract - The paper compares certain UPS line interactive devices, taking into consideration converter power ratings in steady-state conditions and device functions (such as load voltage stabilization, filtering and symmetrization, line current filtering and balancing, and power factor improvement). A new UPS line interactive topology is introduced, consisting of two shunt converters with a common d.c. section and an inductance between them on the a.c. side; this makes for voltage stabilization and improvement of the power factor and input line current waveforms, with a low converter power rating. After a brief steady-state analysis, the behaviour of the device during transients is verified by means of numerical simulations. Keywords: power quality, voltage sags, UPS line interactive device 1. INTRODUCTION The recent, ever more widespread use of powerelectronic devices has increased the degree of reliability to be expected of electrical systems. Special importance attaches to the reduction in voltage sags and outages, which have in the past been the cause of frequent malfunctioning in industrial processes [1]. For some time now, with a view to improving electric power quality, static continuity units (UPS, Uninterruptible Power Systems) have been in use. Effective comparison between the different types of UPS has to be carried out from the load supply point of view, bearing in mind: - Stability, waveshape and load voltage frequency; - Supply continuity and from the network side, taking into account: - The power factor at the foundamental harmonic; - The waveshape of the current absorbed.

Giovanni UBEZIO SIEL Trezzano Rosa (Mi) (Italy)

The double-conversion static continuity groups make it possible to obtain a stable voltage on the load that is continuous and has a very limited harmonic content. This is the only solution that can provide the load with a voltage irrespective of network frequency (asynchronous coupling). By applying suitable controls and circuital topologies (Pulse Width Modulation rectifier [2]), it is possible to obtain sinusoidal unit-power-factor absorption. The disadvantages of this configuration- that is, its high cost, the volume it occupies, and the low efficiency [3], are gradually decreasing with improvement in the performance of the semi-conducting materials. The UPS currently being developed are of the lineinteractive type, which make it possible to limit plant and operating costs, as against the loss of some of the functions listed above. The load is supplied with conditioned power via a parallel connection of the a.c. network and the UPS inverter [4]: thus, the frequency of the voltage on the load of necessity depends on the network frequency (synchronous coupling). The next section will illustrate the main characteristics of two typical UPS line-interactive devices, while the third will introduce a new type that appears to be promising. 2. UPS LINE INTERACTIVE-DEVICES: TWO MATURE SOLUTIONS Of the possible UPS line interactive devices already suggested in the bibliography [3][5][6], this section deals with the performances and limitations of two whose basic layouts are shown in Fig.s 2 and 2.1. Three operating modes are presented, depending on the origin of the power required by the load: - normal mode: entirely from the network; - partial mode: part from the network and part from the energy storage; - islanding mode: all from energy storage.


317

x Parallel converter with coupling inductor The circuital diagram, indicated by the initials S1, is shown in Fig. 2.1: the shunted converter acts as a voltage generator in parallel to the load.

~

~

~ ~ ~

Link reactance

Static by-pass

Static switch CA

The value of the reactance suggested in literature [7], taking into consideration the converter's power rating and the value of the short-circuit current upstream, is in the 0.4-0.5 range in p.u. in relation to the rated load impedance. x Parallel converter with series coupling converter The power circuit, indicated by the initials S2, is shown in Fig. 2.2.

CC

Series transformer Energy storage

Fig. 2.1 Circuital structure of UPS with coupling inductor (S1)

~

~ ~ ~

~

Static by-pass

Static switch

CA

CA CC

Compensation takes place by causing the shunt converter to exchange only reactive power up to a line voltage limit, beyond which the inverter also exchanges active power. The limit voltage is linked with the converter rating, the series reactance value, and the load characteristics. To be able to operate in islanding, the converter has to be designed for at least the total power of the load. The load voltage is stable as RMS value, with low harmonic content and slight dissymmetry; the changeover between the different operating modes takes place without a break because the parallel converter is always active. The current absorbed by the network may be unbalanced or distorted if the network voltage is dissymmetrical or distorted [7]. The chief problem with this solution lies in the power input factor, which varies considerably with the load power factor, with the power absorbed by the user, and with the reactance value chosen: for low loads and with a line voltage lower than that on the user it is possible to have capacitive performance. Another problem is the severity of the operation of the static by-pass for the phase shift between the network and the load voltages [8]. Performance is high and the cost limited due to the presence of only one converter through which, in normal conditions a fraction of the rated load current flows. The presence of the coupling reactance naturally limits the current in the event of a short circuit upstream of the device, thus preventing the converter from operating in such a way as to limit the current and consequently cause the voltage on the load to deteriorate.

318

CC

Energy storage

Fig. 2.2 Circuital structure of the UPS with a series converter (S2). If the shunt converter is controlled as a voltage generator there are no transients in the voltage on the load during the changeover between different operating modes; in this case, the series unit is controlled as a current generator [5]. The functions performed by the voltage and current generators may also be exchanged between the two units [6]. This device is able to provide the load with a sinusoidal, symmetrical and constant RMS voltage and to absorb a network current with a unit power factor that is both sinusoidal and balanced. This too achieves synchronous coupling with the network. In normal operating conditions, the two units exchange with the network both reactive and active power [9]: the active power exchanged is equal to and opposite so as not to vary the state of charge of the storage system. Compensation with a unit power factor may be achieved in different ways by varying the active power passing through the common d.c. section. Some special operating conditions make it possible to achieve one of the following further objectives: a) Nil phase shift between the voltage on the load and that on the network (in order to avoid heavy duty for the bypass commutator and in order to minimize the voltage injected by the series converter); b) Minimal losses (by minimizing the current flowing through the converters).


The power rating of the shunt converter must be at least equal to the load's rated power; that of the series converter depends largely on the control strategy adopted. The efficiency of the device also depends on the control strategy and is always greater than that of the double-conversion UPS: indeed, the current circulating in the parallel unit is a fraction of the load total, while that in the series unit is low compared with the line current, depending on the transformation ratio of the booster transformer. When a short circuit occurs in the network, no part of the circuit is able naturally to limit the growth of the current up to the instant at which the static switch opens: the parallel converter, which also feeds the fault, can therefore function in such a way as to limit the current by causing an alteration in the voltage on the load. 3. A NEW COMPENSATOR DEVICE The circuital diagram of the innovative compensator proposed in [8] is shown in Fig. 3.1 and may be regarded as an improvement on Solution S1. It consists of a static input switch, a link inductance between the network and the load, and two converters, a main and a shunt, in parallel to the load and the network respectively and with a d.c. section in common. There is also an energy storage system.

A synchronous connection is set up between the network and the load, which makes it impossible to stabilize the frequency on the load. x Normal operating mode In the normal operating mode, the main inverter has the job of stabilizing, symmetrizing, and filtering the voltage on the load. The shunt converter enables the system to obtain a higher input power factor (active power factor correction through reactive power exchange). Co-ordination between the two inverters makes it possible for the control strategy to limit the phasing angle · between the load voltage and that of the network to a maximum value compatible with the requirements of the static by-pass. Indeed, as long as all the power required by the load can pass through the link reactance without the limit value · being reached, there is no active power exchange between the two converters and the main inverter operates in the same way as the converter in S1 solution. When, on the other hand, the control logic limits the phasing angle, the link reactance is no longer able to channel all the power required by the load, and therefore the remainder is made to pass through the d.c. section common to the two converters, thus setting up the active power flow marked P* in Fig. 3.2. 1

1-P*

1

Link reactance

P*

~

~

Link reactance

~ ~

~

CC

CC

Static by-pass

Static switch

Shunt inverter

CA

CA

CA

CA CC

Normal mode of UPS operation

CC

Energy storage

Main inverter

Energy storage

Islanding operation Bypass mode of operation

Fig. 3.1. Circuital diagram of innovative compensator The shunt inverter acts as a current generator. The main inverter acts as a voltage generator, in such a way that the changeover between the different operating modes can take place without any significant transients of the voltage on the load. Even though having the same number of converters as the solution with the series unit, the new device is more reliable, because any fault in the shunt inverter still enables it to work as a parallel device with link reactance.

Fig. 3.2. Active power flow in operation with · limited. P* indicates the power that the two converters exchange through the d.c. section. In the event of the network voltage being distorted or dissymmetrical, a distorted or unbalanced current, which is filtered by the shunt inverter, flows through the inductor. In general, the two converters exchange power that is active (when operating with the · limited), reactive and distorting: all these have to be considered at the design stage. x Islanding operation If the RMS voltage or the frequency departs from the tolerances laid down at the design stage (0.91.1Vn was assumed in respect of voltage), the device switches operation mode to islanding by opening the static input switch. The changeover


319

a: normal mode of operation 1.25

1.00

power [p.u.]

from one operating mode to the other takes place without any significant transients in the voltage on the load because the main inverter continues to stabilize, symmetrize and filter the load voltage. One of the innovative aspects of this configuration lies in the fact that, with this operating mode, the load is supplied by both converters, thus making it possible to contain their power ratings: this aspect will be more fully dealt with later on in this section.

0.75

0.50

0.25

0

0.7lag

0.8lag

0.9lag 1 0.9lead load power factor

0.8lead

0.7lead

0.8lead

0.7lead

0.8lead

0.7lead

b: islanding operation 1.25

1.00

power [p.u.]

In this configuration, as in S1 solution, the coupling reactance value affects the electrical features of the system. It was chosen in such a way as not to cause limitation of the current in the main inverter due to a short circuit upstream of the device: in particular, wishing to limit the shortcircuit current to double the rated value of the load, 0.5 p.u. was obtained for the link reactance in respect of load impedance.

0.75

0.50

0.25

0

0.7lag

0.8lag


c: All possible operation mode 1.25

320

1.00

power [p.u.]

x Some considerations about power converter rating In order to compare this configuration with the others, some considerations were advanced on the active and reactive power alone; the distorting power, connected mainly with the harmonic components of the current in any distorted load, has the same effect on the various configurations examined, thus calling for oversizing of the converter in parallel to the load. The magnitudes in p.u. are expressed on the basis of the load apparent power and the rated load voltage. In [8] calculations were made of the apparent powers exchanged by the 2 converters in the normal operating mode as the load power factor varies between 0.7lag. and 0.7lead., observing the constraint on the input power factor 0.9lag. < cos()network < 1 and assuming for the angle · the range of variation indicated hereunder: -20° < < 0° in which the limit of 20° is a compromise between the need to protect the static by-pass and the need to control the size of the shunt inverter. The curve in Fig. 3.3.a relating to the main inverter shows, as a function of the load power factor, the maximum power that the converter has to exchange in the operating conditions that may occur in the normal operating mode: that is, taking the network voltage as variable between 0.9 and 1.1 p.u. and the load factor as variable between 0 and 1.

0.75

0.50

0.25

0

0.7lag

0.8lag


Power flowing through main inverter Power flowing through shunt inverter Sum of the powers flowing through the main and shunt inverters

Fig. 3.3. Converter power rating The curve identified by the circles has the same meaning and relates to the shunt inverter, while the curve identified by the squares gives the two previous curves and illustrates the quantity of semi-conducting material required. When operating in islanding mode, which is referred to in Fig. 3.3b, the apparent power required by the load may be subdivided into different modes between the two converters. In advancing these considerations with regard to the powers exchanged, it was decided to adopt the following strategy: - Minimize the current flowing in the two converters by causing the inverters to exchange the currents in phase with the load current; - Make the main converter work up to the maximum power determined in the normal operating mode as a function of the load power factor: any missing current component is supplied by the shunt inverter. Electrical Power Quality and Utilisation

TABLE 3.1. Comparison between the three line interactive UPS considered Compensator

Shunt + Link reactance

Shunt + Series

UPS – Shunt device

Functions Load voltage -stabilization -elimination of harmonics/dissymmetries -continuity -stabilization -elimination of harmonics / dissymmetries -continuity -stabilization -elimination of harmonics / dissymmetries -continuity

Network current -Waveshape connected with the network voltage

Converter power rating

AshunttAload

Efficiency

Limits

-Input power factor -Static switch operation -Frequency unregulated

High

- Depends on -Sinusoidal and control strategy; AshunttAload balanced than Aserie depends on higher -unit power double control mode UPS factor conversion

- Irregular operation in the event of short-circuit on network side - Frequency unregulated

-Sinusoidal and balanced -power factor required

-Frequency unregulated

The curves in Fig. 3.3.b represent, as a function of the load power factor, the maximum power that the converters have to exchange in the possible operating conditions in islanding mode, that is as the network voltage varies between 0.9 p.u. and 0, and the load factor between 1 and 0. For the control strategy chosen, the curve relating to the main inverter is the same as that obtained in the normal operating mode. Fig. 3.3.c represents the maximum values of the powers exchanged by the converters and their sum, taking into consideration both the operating modes: this figure enables considerations to be advanced regarding the power ratings of the two inverters. For the loads taken into consideration, the device proposed calls for very small overall power rating of the converters – less than 1.15 p.u., reached in respect of loads with a power factor of 0.7lag. The minimum value is recorded for loads with power factors approximating to 0.9lead, and is slightly lower than unity because part of the reactive power is absorbed by the coupling reactance. Bearing in mind that the two converters exchange limited powers, compensator efficiency is high and presents the maximum for loads with a power factor of 0.95lead., corresponding to the minimum for the total power passing through in the normal operating mode. In order to optimize compensator rating and efficiency, it would be possible to install capacitors on the output side centering at about 0.95lead. the range of variation of the load power factor. Table 3.1 summarizes the performances and limits of the 3 solutions analyzed.

(Amain+Ashunt) 1.15*Aload

High

4. CONTROL LOGIC The control logic of the compensator is now explained with reference to single-phase operation. The control of the main inverter is shown in Fig. 4.1. Vbat

quartz reference

Commutator

vline From network detectore vOUT

Ibat_main

Block B

PLL

vOUTref Block A

Main inverter valves control

Block A = generates control signals for main inverter Block B = definition of angle ·

Fig. 4.1 Main inverter control In the normal operating mode, the control makes possible both the stabilization of the RMS value of the load voltage and regulation of the angle ·. The reference signal of the output voltage vOUTref is in a module equal to 1p.u., while the frequency and the phase are defined by the Phase Locked Loop (PLL). The PLL generates a sinusoid whose frequency is equal to that of the network (synchronous hookup) and whose phase is shifted in relation to that of the angle · defined by Block B.


321

Block B Vbat

controller PI

limiter ’

Ibat

Fig. 4.2 Block B defining angle ·. Block B, as shown in Fig. 4.2, contains a PI controller, whose input signal is the power that the main inverter takes from the d.c. section and a limiter to ensure a relatively low phase shift · . The controller acts in such a way as to cancel out the power exchanged, by varying the output signal ·’: a limiter then limits this signal in the 0° to –20° range, thus supplying the signal · compatible with the performance of the static switch. Thus, as long as the limiter does not intervene, the main inverter does not exchange power with the d.c. section. Operation in islanding mode works the same way, the only difference being that the reference sinusoid for the voltage on the load is defined by an internal quartz reference. Block A stabilizes the voltage on the load at the reference value by means of a closed-loop control: the error between the RMS voltage vOUT and the reference voltage vOUTref is processed by a PI controller that supplies the reference value used in PWM control of the main inverter. Fig. 4.3 represents the control of the shunt inverter. cos()net_lim iline I Block D Sr vline

5. TEST AND RESULTS ³

Block F iOUT

Divider block

vbat ibat

Block E

+ +

iS_ref_is

iS_ref_n

Commut. I Change S_ref ref. iS

Block C

ISa

Vbat_ref iS From network detector Block C = generates control signals for shunt inverter Block D = control of input power factor Block E = control of power flow in d.c. section Block F = generates synchronous reference

Fig. 4.3 Shunt inverter control

322

Block C, which is similar to the previous Block A, and the firing circuit, not shown in the figure, ensure that the current actually exchanged by the shunt inverter follows the reference iS_ref calculated. Depending on the state of the network, the reference current is selected by the commutator between a signal used in the normal operating mode and one in the islanding operating mode. In the normal operating mode the reference current iS_ref_n consists of two contributions: ISr in quadrature in relation to the network voltage, and ISa in phase. The former, the size of which is defined by Block D, performs the power factor correction of the system between 1 and the value cos()rete_lim, made equal to 0.9lag. The latter enables the power flow in the d.c. section to be managed and is calculated by stabilizing the value of the voltage on the batteries by means of a feedback loop contained in Block E. Block F defines a sinusoidal reference linked with the network voltage in order to be able to transform the signals ISr and ISa into sinusoidal waveshapes. When operating in the islanding mode, the control logic for the shunt inverter must be changed, because, on the one hand, power factor correction is not called for, and, on the other, the power flow in the d.c. section is managed differently from the normal operating mode. Contrary to the explanation given in Section 3, the current reference for the shunt inverter iS_ref_is is defined by the divider block as a fixed percentage of the load current: the fraction was defined in such a way that the rated power, shown in Fig. 3.3c to be equal to about 0.55p.u., would not be exceeded in any load situation.

The performance of the single-phase device in accordance with the control logic proposed was checked by means of numerical simulations on the computer in various operating conditions. For the sake of simplicity, the main and shunt inverters were modelled as a controlled voltage and current generator respectively, while the battery was modelled as a voltage generator with a very high capacitance in series, whose slow discharge represents that of the battery. No consideration was given to the network frequency variations, so that it was possible to ignore the PLL: to define the voltage vOUTref, use was made of a reference signal created by a 50Hz sinusoidal generator, whose phase is then shifted by the angle ·, calculated by the control logic. The load considered has the apparent rated power of 20kVA, a power factor of 0.8lag., supplied at Electrical Power Quality and Utilisation

the voltage of 230 V RMS at the frequency of 50 Hz. In accordance with the explanation given in Section 3, the link inductance was made equal to 4.2mH. We now describe simulations of a number of cases of special interest.

to the active power absorbed by the load, which is equal to 16kW. a: load and line voltage

5.1. Undervoltage without changeover to islanding operation Fig. 5.1 shows the waveshapes of the more significant variables relating to undervoltage to the lower limit of 0.9p.u. (equal to 207V), beginning at instant 7.4s. The voltage on the load (fig. 5.1.a) undergoes no substantial change either in amplitude or in phase: since, prior to the disturbance, the angle · already had the limit value of 20°, there is no phase change. For the same reason, there are values other than zero in respect of the active powers supplied by the two converters (Fig. 5.1.b), both before and after the disturbance. When the undervoltage takes place, · being constant, the active power flowing through the link is diminished and, consequently, the power exchanged by the converters is increased. Since the device remains in the normal operating mode, and therefore without drawing on power from the batteries, the algebraic sum of the active power supplied to the line by the two converters is nil, except during brief transients.

Vnetwork Vload b: active power supplied by the converters to the network Pmain Pshunt

c: current RMS values

Imain Ishunt

5.2. Voltage sag with changeover to islanding operation In accordance with this simulation, starting from rated operating conditions, at instant 9s a permanent voltage sag is created, the depth of which is such that the network detector causes changeover to operation in islanding mode: the trend in respect of the more significant variables is shown in Fig. 5.2. The delay in changing the operating mode, shown in Fig. 5.2a, is linked with the time the network detector takes to update the RMS voltage value; faster systems of detection operating on instant voltage values could be set up. The same figure shows that the voltage on the load presents no significant disturbances. Fig. 5.2b shows that, before the voltage sag there is a power flow in the d.c. section common to the two converters, but that no power is taken from the storage system. On the other hand, during the disturbance the active power supplied by the converters are both positive, so that both contribute towards supplying the load, and their sum is equal

Iload

Fig. 5.1 Trend in respect of some electrical variables in the event of undervoltage to the lower limit of 0.9p.u. According to the logic adopted, during operation in islanding mode, the inverters exchange currents in phase with the load current; so that the sum of their RMS values is equal to that of the load current. 6. CONCLUSIONS The central aspect of this research lies in the proposal for a new UPS line interactive device able to improve the supply voltage of a sensitive load (stabilization, elimination of harmonics and dissymmetries, continuity) and the current absorbed from the network (power factor required, and sinusoidal, balanced waveshape). The device proposed may be regarded as a development of the shunt converter with link reactance solution that solves the latter's problem of the network-side


323

power factor and the static by-pass. The new proposal consists of two shunt converters with a link reactance, which have low power ratings, especially given the fact that, when operating in islanding mode, the load is supplied by both inverters, so that the inverter in parallel to the load may have a power rating lower than 1 p.u.

x Introduction of a more detailed converter model that makes it possible to consider features not analyzed in this paper. x Creation of a low-power experimental prototype. 7. REFERENCES

a: load and line voltage

Vnetwork Vload b:active power exchanged by the converters

Pmain Pshunt

c: current RMS values

Imain Ishunt Iload

Fig. 5.2. Trend in respect of some electrical varables in the event of a voltage sag with changeover to islanding operation. After the theoretical study, a control logic was proposed that was eventually checked by means of numerical simulations in various operating conditions. Since the device was found to be promising, research will probably be developed along the following lines: x Extension of the normal operating mode to a wider range of network voltages. x Extension of the control logic to the three-phase situation.

324

[1] W. E. Reid: Power Quality Issues - Standards and Guidelines. IEEE Transactions on Industry Applications, Vol. 32, NO. 3, May/June 1996. [2] A. Von Jouanne, P. N. Enjeti, B. Banerjee: Assessment of Ride-Through Alternatives for Adjustable-Speed Drives. IEEE Transactions on Industry Applications, Vol. 35, NO. 4, July/August 1999. [3] S.-J. Huang, J.-C. Wu, H.-L. Jou: Electricpower-quality improvement using parallel active-power conditioners. IEE Proc.-Gener. Transm. Distrib., Vol. 145, No 5, September 1998. [4] ENV 50091-3 Standard, Uninterruptible Power Systems (UPS), Part 3: Performance requirements and test methods; [5] S. Rathmann, H. A. Warner: New generation UPS technology, the delta-conversion principle. Industry Applications Conference IAS 96, Conference Record of the 1996 IEEE, Vol. 4, pages. 2389-2395. [6] M. Aredes,K. Heumann, E.H. Watanabe: An Universal Active Power Line Conditioner. IEEE Transactions on Power Delivery,Vol. 13, No. 2, April 1998. [7] D. Lauria, E. Tironi: Some Consideration on Active Compensation Devices. ETEP Vol. 3, No. 3, May/June 1993. [8] A. Lionetti, M.L. Passera: UPS-Shunt. Una nuova topologia per gruppi statici di continuità. Degree thesis at Dipartimento di Elettrotecnica of the Politecnico di Milano, A.A. 1999-2000, Supervisor Prof. E. Tironi (in Italian). [9] G.T. Heydt, W.Tan, T. LaRose, M. Negley: Simulation and analysis of series voltage boost technology for power quality enhancement. IEEE Transactions on Power Delivery, Vol. 13, No. 4, October 1998.


BIOGRAPHIES Roberto Faranda graduated in Electrical Engineering at the Politecnico di Milano in 1998 and is now Assistant Professor at the Politecnico's Electrical Engineering Department. His areas of research include power system harmonics and power system analysis. Dr. Faranda is a member of AEI. Mailing address: Roberto Faranda Politecnico di Milano Dipartimento di Elettrotecnica Piazza Leonardo da Vinci 32, 21133 Milano, Italy phone:(+39)(02) 23993793, fax:(+39)(02) 23993703 e-mail: [email protected] Enrico Tironi graduated in Electrical Engineering at the Politecnico di Milano, Italy, in 1972, in the same year joining the Politecnico's Electrical Engineering Department, where he is now full professor. His areas of research include power electronics and power system harmonics. He is a member of the Electrical Power System Group of the Italian National Research Council (C.N.R.) and of AEI. Mailing address: Enrico Tironi Politecnico di Milano Dipartimento di Elettrotecnica Piazza Leonardo da Vinci 32, 21133 Milano, Italy phone:(+39)(02) 23993713, fax:(+39)(02) 23993703 e-mail: [email protected]

Giovanni Ubezio graduated in Electronic Engineering at the Politecnico di Milano, Italy, in 1977, subsequently joining the Siel Company, where he is R&D manager. His areas of research include power electronics and power system harmonics. He is a member of the UPS Group of C.E.I. and of AEI. Mailing address: Giovanni Ubezio SIEL SpA Trezzano Rosa (Mi), Italy phone:(+39)(02) 90986256 e-mail: [email protected] Ivan Valadè graduated in Electrical Engineering at the Politecnico di Milano, Italy, in 1999. He is now working for a second, post-graduate degree at the Politecnico's Electrical Engineering Department. His research interests are power electronics and power quality. Mailing address: Ivan Valadè Politecnico di Milano Dipartimento di Elettrotecnica Piazza Leonardo da Vinci 32, 21133 Milano, Italy phone:(+39)(02) 23993752, fax:(+39)(02) 23993703 e-mail: [email protected]


325

326




CHARACTERISTICS OF CONTROLLED ELECTRICAL TRANSMISSION LINES OF AN ALTERNATING CURRENT OF THE INCREASED CAPABILITY AND OF THEIR APPLICATION FOR IMPROVEMENT OF QUALITY OF PARAMETERS MODES POWER SYSTEM AND INCREASE OF RELIABILITY OF ELECTRO SUPPLY

VITALY M. POSTOLATI, ELENA V. BICOVA

Vladimir G. KUZNETSOV

Academy of Sciences of Republic of Moldova Kishinev (Moldavia)

Ukrainian Academy of Sciences Kiev (Ukraine)

Abstract - The theoretical ground and design realization of alternating current power lines of the new type - Controllable Flexible A.C. Transmission Systems (CFACTS) are considered. Earlier lines of such type had been known as Controlled Self - Compensating High Voltage Transmission Power Lines (CSHVL), as well as Controlled Compact Transmission Power Lines. The CFACTS design principles and the basic CFACTS design directions are presented. The design version, connection diagram and control methods are described. The basic characteristics of CFACTS (10, 35, 110 kV) which already built and maintained long as well as CFACTS (220-1150kV) parameters, obtained on the basic of the design are given. It to has shown that the basic characteristics of CFACTS are better than those for the usual power lines. CFACTS provide natural power of the line increase by 20 to 50 per cent, 2-3 times larger total power flow density in a line cross-section, parameters and power flow controllability, ecological effect decrease, economy of capital expenses by 10 to 30 per cent and adduced ones expenditure by 10 to 20 per cent per transmitted power unit. 1. INTRODUCTION The prospects of the development of electric power engineering are associated with the further increase of the electrification level. This approach requires the growth of power flows and further extension of the electric power network and an increase in the

number of generating plants, the utilization of new power sources, including remote ones, and long and super - long power transmission lines construction. In order to succeed in solving the problems mentioned above, it is necessary to study the questions dealing with the increase of high voltage transmission line capability, operating conditions control, the improvement of reliability and the technical and economic parameters of all of the power-supply system. Under the actual conditions of the scale of electric power line construction, especially of high and ultra-high voltage power lines, the problems concerned with the reduction of the ecological effect of those kinds of lines and the decrease of the area of plough used for the lines construction become, of great importance. New types of power lines and other equipment, aimed to overcome the main problems, mentioned above. The concept of CFACTS [1-3] is one of novel approaches in this field. At present, on the basis of investigations and design carried out by a number of institutes the CFACTS 10, 35, 110 kV pilot models are constructed and their electrical tests are performed. Besides, the mechanical and electrical tests of CFACTS 110, 220 and 500 kV are owed out. Some versions of CFACTS 500 kV are designed, technical and economic estimates for CFACTS 750 and 1150kV are performed, and the system efficiency of CFACTS designed for various voltages and aimed at the possible application to further development of the networks is estimated.


327

2. BASIC RESULTS As to their design realization, CFACTS are power lines formed by some three - phase single - circuit transmission lines which are separated by a minimum permissible space, Fig. 1.

The CFACTS include phase-shifting devices (providing the control of shift angle of three-phase voltage systems for different circuits as well as compensating devices) required for the additional control of the normal and emergency operating conditions, Fig. 2, 3. Owing to the new technical improvements, the specific conditions are provided under which the enhanced electromagnetic mutual effect of single-circuit power lines involved arises. This effect lea do to the change of the primary parameters by 20 to 40 per cent. If the sign of the effect is chosen in the right way, the power line capability increases. The control of the shift angle between the voltage systems, corresponding to different circuits, and, as a consequence of the equivalent parameters values is performed by means of phase-shifting transformers (PST) or switching networks. In conjunction with the application of the compensating devices, this method provides the wide range control of the parameters, describing both a power line and a power network as a whole, Fig. 4a, 4b, 5, 6.

328

The studies showed that the reduction in air gap between CFACTS circuits can be made through the different circuit phases arrangement at the minimum permissible phase-to-phase distance, the phases being associated in pairs for the double circuit CFACTS and in triplets for the triple circuit ones, etc. The phases design can be different. They own attain single conductors or be split, the split phase components being arranged in the circular way or otherwise, forming a plane in accordance with the requirement of the component charge balance [3,4]. The reduced phase-to-phase distance, chosen on the basis of the phase-to-phase limit voltage – e. g. in the case of the double circuit CFACTS a double phase operating voltage must be accepted - and switching and lightning surges magnitudes can be equal to d = (0.2 - 0.4) D, where D is a phase -to-phase distance, used in practice for single-circuit power lines and multi-circuit ones now. Besides, it is necessary to take the tower elements out of the gap between the phases with reduced phase-to-phase distance. In some cases the insulating spacers must be installed between the phases which reduced clearance in the span. Mechanical tests of the CFACTS at 10, 35, 110, 220, 500 kV confirmed the reliability and efficiency of the present design. The different circuit phases - e. g. in the case of CFACTS 500 kV are located on the tower at the distance of 5.0 m, but the span of the phase-to-phase gap is reduced, using insulating elements - lightened insulator chains, plastic insulating structures, etc.and equal to 4.0 m [5]. The investigations showed that for the split phases their mechanical stability can be provided without


the use of insulating phase-to-phase structures by means of the proper choice of different distances between the components along the spans. This approach makes the equipment lighter. The other overall dimensions of the line in question are considered to be equal to those for usual power lines.

The versions of CFACTS circuit diagrams are given in Fig. 2, 3. They represent the different

methods for the choice and control of the phase shift between voltage systems, corresponding to different circuits, as well as the methods of compensating devices connection. The "anti phase" operating conditions for voltage systems (º = 180 el. d.) can be realized through the application of power transformers with the different vector groups, the change of the phase shift between the different voltage systems can be obtained by switching - over the phase, corresponding to one of the circuits. The phase-shifting devices, connected to the line terminals and the intermediate nodes, make it possible to perform the wide range control. The connection of compensating devices can be usual, i.e. according to "phase-ground" arrangement, but sometimes it can be made between the phases with reduced phase-to-phase gap, corresponding to the different circuits. In the latter case the voltage difference, applied to these phases, changes within wide range. The application of the circuit described diagram, increases the line parameters control range. The Fig. 5 displays the characteristic curves, representing CFACTS capability as a function of phase shift between the voltage systems, corresponding to the different circuits. The same dependencies are correct for CFACTS charging power as well. The power line, characterized by wide control range is able to have a very important effect on a complex power system, because this kind of line can control active and reactive power both under the normal and emergency operating conditions. It follows from some investigations that without phase control it is impossible to obtain the required distribution of power flows in complex electric power networks and provide the economical and reliable work of the system in question. The phase control not only allows the changing of CFACTS parameters, due to phase shift between the voltage systems, corresponding to the different circuits (below we will call it


329

quadrature - axis phase shift- º), but results in the change of the shift angle for node-to node voltage (i.e. direct-axis phase shift, denoted as- ¸). Thus, the various application of phase -shifting devices is possible. The diagram of a network section, including CFACTS and phase - shifting transformers (PST), is shown in Fig. 4. The dependence of double-circuit CFACTS-500 kV limit power (Pm) on angles º and ¸, mentioned above, is represented in Fig. 6. The PST are able to control of the parameters of the lines and power system operating conditions. If the PST are equipped with high-speed control systems, it is possible to control transient phenomena in power systems (e. g. electromechanical ones). The study of PST and CFACTS continues. Besides, concrete steps have been taken to make practical use of the basic ideas of CFACTS. The designs, carried out, make it possible to construct CFACTS at the level up to 500 kV, and shift angle º = 120 el. deg. between the voltage systems as well as to estimate the other version of CFACTS of various voltage classes. On fig. 7 is exhibited the double circuits CFACTS by the voltage 110 kV, which is constructed also long time is in operation in the power system of Republic of Moldova. 10 to 1150kV parameters, obtained on the basis of performance design and pre-design estimates are listed in tables 1 and 2.

3. CONCLUSION. By comparison with the traditional double circuit' or two Single - circuit lines the CFACTS in question provide the following technical and economical advantages. 1. Economy of capital expenses by 10 to 30 per cent and of adduced expenses by 10 to 20 percent transmitted power unit. 2. Increase by 2-4 time of total power flow density within the line cross-section, formed by tower height and width. The basic technical and economic parameters of CFACTS versions are determined and compared with those for the usual power lines The CFACTS 3. The line natural load increase by 20 to 50 per cent, accompanied by transmission line capability growth. 4. The decrease of ecological effect, due to the lowering of the maximum electric field intensity below the line by 15 to 40 per cent and almost by double the reduction of the domain, formed by equipotent surfaces at ground level. 5. The decrease of ploughed land area, used for the line construction, by 30-35 per cent. 5. Nearly 2-fold reduction of magnetic field intensity in the neighborhood of the power lines which is accompanied by less effect on communications and other facilities. 6. The CFACTS, equipped with the facilities, designed for the control of phase shift between voltage systems, corresponding to the different circuits, enabling the control of operating conditions and power flows in the system. System reliability and controllability improvement.

Fig.7. The double circuit CFACTS of voltage 110 kV. 330


Technical parameters of the constructed pilot double circuit controlled high-voltage lines (10, 35, 110 kV) (in Republic of Moldova) TABLE

1.

Parameters

Measurement unit

Length of lines Calculated load Conductors

km

Voltage, kV 10 35 9,5 8,7

MW

3

12

70

-70

-90

Phase shift between voltage systems corresponding to the different circuits Distance between the phases with reduced phase-tophase clearance, corresponding to the different circuits Inductive resistance (at º=120 el. deg.)

el. deg.

120

120

150 0-120

m

0,3

0,6

1,1

Ohm/k m

0,308

0,34

110 34

0,37

4. REFERENCES. 1. Certificate of authorship N 566288 (USSR). Alternating current transmission / V.M. Postolati, V.A. Venikov, Yu.N. Astakhov, G.V. Chaliy. L.P.Kalinin. Accepted on 21/3/74, N 2006496. Published in IB 1977, N 27. 2. Alternating current transmission / V.M. Postolati, V.A. Venikov, Yu.M. Astakhov, G.V. Chaliy, L.P. Kalinin. US patent N 4001672, 1977; GDR patent N 116990; France patent N 7508749, 1977; GB patent N 148844, 1978; Sweden patent N 75032268,1978; Canada patent N 10380291978: FRG patent N 2511928,1979; Japan patent N 1096176,1982. 3. Yu.N. Astakhov, V.M.Postolatiy, I.T. Komendant, G.V. Chaliy. Controlled Power Lines / Edited by V.A. Venikov, Kishinev, "Shtlintsa", 1984, 296 pp.

TABLE 2. Technical parameters of some version of double circuit CFACTS at the 220, 330, 500, 750, 1150 kV ParaMeameters surement unit Length km of lines

330

200400

300600

3xAS -300

5xAS AS-300 -300

13xA S-300

m

100 300 2x AS300 2,2

3,0

4,0

6,0

10,0

Ohm

375

367

338

342

338

Ohm

222

214

183

191

209

Ohm

206

199

170

177

195

W

258

592

1478

3282

7820

W

416

965

2567

5742

12146

W

468

1090

2929

6334

13580

Conductors Distantbetween the phases with reduced phasetophase clearance Wave impedance of: at º=0 el. deg. at º=120 el. deg. at º=180 el. deg. Natural electrical power of: at º=0 el. deg. at º=120 el. deg. at º=180 el. deg.

Votage, kV 500 750

220

400800

1150

5003000

, 4. G.N. Alexandrov, V.V. Ershevich, S.V. Krilov at al. UHV Power Lines Design / Edited by G.N. Alexandrov and L.L. Peterson, Leningrad, "Energoatomizdat", Leningrad department, 1983, 368 pp. 5. V.M. Postolati, V.A. Martinov, V.M. Lavrentiev, Yu.M. Koriagin. "Line insulation


331

test using an experimental section of a 500 kV Double Circuit Controlled Self -compensating High-Voltage Transmission Line". Very high voltage networks Symposium, 1995, 31may - 2 June, Sibiu - Romania, Proceedings, Vol. 2, p.p. 20-23. Dr. Vitaly Postolati was born in 1937 on the Ukraine. Has completed in 1961 Institute of electrification of an agriculture and has received a speciality of the engineer - electrician. He received his Ph. D. degrees in the Moscow Power Engineering Institute in 1968 and D.Sc. in the Institute of Electrodynamics of the Ukrainian Academy of Sciences in 1988. In 1994 he was elected Corresponding - Member of the Academy of Sciences of Republic of Moldova. In period with 1981 for 1994 there was by the director of the Institute of Power Engineering of the Academy of Sciences of the Republic of Moldova. In present time he is the head of the Controlled Transmission Lines Laboratory. His areas of interests include High Voltage Transmission Lines and Electrical systems, an analysis of modes of power systems, power safety and common problems of a power engineering. Mailing address: Vitaly Postolati, Institute of Power Engineering, 5, Academy str., Chishinau MOLDOVA2028, Phone (+373 2) 73-53-88; 72-06-31 (home), Fax: (+373 2) 24-55-33, E-mail: [email protected] [email protected]

Prof. Vladimir Kuznetsov, Corresponding - Member of the Academy of Sciences of Ukraine are working the deputy director of the Institute of Electrodynamics of the Ukrainian Academy of Sciences. He is the famous scientist in area of power engineering, of the electrodynamics and electrical engineering. His scientific interests are coupled to the large systems of a power engineering, researches of quality of electrical energy, systems of conversion of electrical energy and various electro technical devices, common problems of a power engineering and power safety. He is a term of a number of international organizations and societies, carries on the large operation on development of international cooperation.

Mailing address: Vladimir Kuznetsov, The Institute of Electrodynamics of the Ukrainian Academy of Sciences, 56, Pobedy pr., Kiev-57, UKRAINE, 252680 Phone (+044) 446-01-51; 446-23-41; Fax: (+044) 446-94-94

Mrs. Elena Bicova was born in 1961 on the Republic of Moldova. She received degree of the Electrical Engineering in 1985 in the Moscow Power Institute of Russia. In 1995 she finished the Pedagogic University and received degree of the Professor of mathematics. She are working in the Institute of Power Engineering of the Academy of Sciences of the Republic of Moldova as the scientific employee. Her scientific interests are coupled to researches of transmission lines of high power and modes of electrical systems, problems of ecology, power safety, mathematical simulation of electrical systems and common problems of a power engineering. Mailing address: Elena Bicova, Institute of Power Engineering, 5, Academy str., Chishinau MOLDOVA ,2028 Phone (+373 2) 73-53-88; 72-06-31 (home), Fax: (+373 2) 24-55-33, E-mail: [email protected] [email protected]

332




CONVERTER TRANSFORMER PHASE-SHIFT - A USEFUL APPROACH J. POLICARPO G. Abreu

DELVIO F. Bernardes

Itajuba Federal School of Engineering Itajuba, MG (Brazil) Abstract - This paper deals mainly with problems related to converter transformer phase-shift. Due to the ever increasing number of non-linear loads, mitigation techniques for reducing harmonic contents is undoubtedly a topic in which a great deal of study has been invested during the last years. Among other techniques, different strategies for making higher the number of pulses of a converter [1-4, 9-10] have been used in order to improve the quality of power. As a natural consequence, large quantities of energy are saved, so that the need of investment in generation and transmission is considerably lower. Actually, this paper presents a useful approach to the analysis of the most usual three-phase transformer connections in order to check which factors affect its phase-shift. The importance of such a issue resides in learning how to obtain a certain phase-shift necessary to achieve a desired number of pulses for a converter. Key words: Converter Transformer Phase-shift, Power Quality. 1. INTRODUCTION

On the other hand, it is also known that an ordinary three-phase system has intrinsically six voltages 60o displaced one another. The voltages are: RS, RT, ST, SR, TR, and TS as shown in figure 1. From now on, this system will be named sixvoltage system. UTS URS U TS UTR 60

o

URT USR UST

Fig. 1. Six-voltage system

So, it can be concluded that it is fully inevitable that the output voltage ud of a typical converter has only six pulses, as seen from figure 2. From now on, this ordinary six-pulse converter will be named valve.

1.1. Input voltage phase-shift a) Basic converter number of pulses On one hand, it is well known that a converter number of pulses is given by: p = 360o / T

ud

ud

(1)

where: p – number of pulses of the output voltage; T - phase-shift between input voltages.


Fig. 2a. valve

339

120

ud

S 3 0

-120

Zt

Fig. 4a. A 12-pulse converter 0

60

120

180

240

300

360

Fig. 2b. Valve output voltage waveform

ud 120

b) Obtaining number of pulses higher than six. On the other hand, it can be concluded that for obtaining a number of pulses higher than six, it is required: a) a number of valves at least equal to the desired number of pulses divided by six; b) a number of input voltages equal to the number of pulses and displaced T degrees each other. In other words, ns sets of six-voltage systems, displaced T degrees one another.

(2)

where: nv - number of valves; ns - number of sets of six-voltage systems. For instance, a 12-pulse converter is feasible if two sets of six-voltage systems, 30o displaced one another, are applied to two valves. See figures 3 and 4.

Fig. 3. Two sets of six-voltage systems In other words, in figure 4(a), T1 and T2 should be connected in order to provide D minus E equal to 30o. In being so, this converter would have a 12pulse output voltage as shown in figure 4(b). 340

0

Zt

-120 0

60

120

180

240

300

360

Fig. 4b. 12-pulse converter output voltage 1.2. Different ways of obtaining T

Equation (2) makes it clear: nv = ns = p/6

6

There are different ways of obtaining a desired T. For instance, T equal to 30o can be obtained by placing D equal to 0o and E equal to r30o. This can be achieved by using either a set of two two-circuit transformers, Y / Y and Y / ' connected [1], or one three-circuit transformer, Y / 8 / ' (' / ' / Y) connected [2]. Although less usual, another way also used is through a set of two two-circuit transformers, 8 / Extended ' connected. In this latter case, T equal to 30o is obtained by placing D equal to 15o and E equal to 15º, as proposed by Carlsson [4]. For a 24-pulse converter, where a 15o phase-shift is needed, two arrangements could be done: either a set of four two-circuit transformers, ' / ' (0o), ' / Z (15o), where Z stands for Zig-zag, ' / Y (30o), and ' / Z (45o), or a set of four two-circuit transformers, ' / Z (22.5o), ' / Z (7.5o), ' / Z (7.5o), and ' / Z (22.5o). Undoubtedly, there would be several ways of achieving the desired T. The point is how to arrange transformer connections to aim at this goal? In order words, what one is expected to do with a given connection to achieve a desired phaseshift? How this really works? Can transformers provide any phase-shift? Electrical Power Quality and Utilisation

Moreover, different arrangements might cause unlike impedance values to come up, so that noncharacteristic harmonics could appear leading to the increase of apparent power and worsening system power factor. This is to say that this harm would be provoked by both imbalance and harmonics.

voltages are positioned with respect one another. Figure 5 shows both cases.

1.3. Problems related to power Another problem is the one related to system occupation by delivered power. Shortly thereafter converters firstly appeared problems related to power definitions have been raised. Besides that, recently, some researchers have shown the greater is the system imbalance the larger is the harmonic contents [5]. There are several studies about new definitions for electric power. One of them [6] states that a more appropriated equation for apparent power is:

Figure 5 – Coil polarity 2.2. Ordinary Connection Voltage Relationships

Delta Connection: Line voltage and phase voltage are related as: .

UL

S

P Q D A 2

2

2

UL

(3)

where: S - apparent power; P - active power Q - displacement power; D - distortion power; A - imbalance power. Taking into account that the power factor is defined as: P PF (4) S

(5)

Wye Connection: In this case, line voltage and phase voltage are related as: .

2

.

UF

.

3 U F e r j30

(6)

where: () signal is for positive phase sequence; () signal is for negative phase sequence.

Zig-zag Connection: For this connection two voltage equations can be shown: .

.

UF

3 U B e r j30

(7)

and .

UL

.

3 U F e r j30

(8)

it can clearly be seen that the lesser are D and A the greater will be PF. In order words, appropriate transformer connection arrangements can lead to less imbalance, therefore less harmonic contents, and then better usage of the apparent power drawn from the system.

Equations (5) through (8) show that a transformer ' / 8 or 8 / ' connected has an intrinsic phaseshift of 30º, for instance. Exactly the same would be for a transformer Y / Z connected. On the other hand, a transformer ' / = connection is expected to have either a 0º or a 60º phase-shift. It is, however, too early to show the whole scenario., for other affecting factors are to come as it will be seen.

2. BASIC FACTORS AFFECTING TRANSFORMER PHASE-SHIFT

3. OTHER FACTORS AFFECTING TRANSFORMER PHASE-SHIFT

In fact two basic factors contribute to define a transformer phase-sift value, namely: its polarity and its connection arrangement [7].

Besides coil polarity itself and connection arrangements, some other factors can contribute decisively to change a transformer phase-shift [7,8]. They are:

2.1. Transformer polarity 3.1. Ways of assembling Delta Connection A coil polarity determines the instantaneous direction of its induced voltage. Comparing two coils they may have either the same polarity or opposite ones, depending on how their induced

There are two basic forms of assembling Delta connection. Both are shown in figure 6. Its important to remark from the analysis of figure 7


341

that the two forms of assembling the delta connection lead to a 60º displacement between the two sets of voltage phasors of either of them.

basic zig-zag connections have every coil with the same number of turns. Conclusions drawn thereafter will be based upon this premise. Figure 11 shows both related phasor diagrams, from where it can be seen that both are 60º displaced.

Fig. 6. Delta Connection assembly UA1A2

UC1C2

UC1C2

Fig. 10. Zig-zag Connection assembly UB1B2

UA1A2 (a)

UB1B2 (b)

Fig. 7. Phasor Diagrams for Delta Connection 3.2. Ways of Assembling Wye Connection Figure 8 shows the two ways of forming Wye connection. Figure 9 shows the related phasor diagrams, respectively. In this case the two sets of voltage phasors are displaced 180º one another.

(a) C

A = B 2

1

4

A

A = B = C 3

C = A 2

3

1

3

4

B = C 2

B

4

1

Fig. 8. Wye Connection assembly (b)

UC1C2

UB1B2 UA1A2

Fig. 11. Phasor Diagrams for Zig-zag Connection 3.4. Phase Sequence Change

UA1A2 UB1B2

UC1C2 (a)

(b)

Fig. 9. Phasor Diagrams for Wye Connection 3.3. Ways of Assembling Zig-Zag Connection Figure 10 shows two ways of assembling Zig-zag connection. It is important to remark that these 342

In figure 12 it can be seen a delta connection fed with negative phase sequence. Corresponding to this, figure 13 shows the related phasor diagram. Comparing phasor diagrams shown in figure 7a with this last one, it can be seen that both sets of voltages are fully overlapped. This can be understood as if negative phase sequence, in some way, could compensate the effect of the assembling change of a delta connection. This implies that a change in phase sequence provokes a change in the phase-shift of 60°. Electrical Power Quality and Utilisation

R

From what has been exposed so far it might be concluded that the various combinations of these three ordinary connections would lead to some phase-shifts perfectly expected from the general equations seen in item 2.2, as shown in Table 1.

S T A

B

1

C

1

1

TABLE 1 A

B

2

C

2

CONNECTIONS

2

Fig.. 12. Delta Connection fed with negative phase sequence UC C 1

UA A 1

2

UB B

2

1

P H A S ES H I F T

0o 30o 60o 120o 150o 180o 210o 240o 300o 330o

8/8 X

8/'

8/=

X

X

'/' X

'/8

'/= X

X X X

X

X

X

X X X

X X

X

X X

X X X

X

X X X

2

Fig. 13. Phasor Diagrams for Delta Connection in Fig. 12 Zig-zag connection presents a phenomenon alike, as can be seen from figures 10 and 14 and 11a and 15.

Fig. 14. Zig-zag Connection fed with negative phase sequence

On the other hand, some modifications to two of these basic connections, namely ' / Z, could lead to different values for the phase-shift.

3.5. Number of turns Alteration Zig-zag, as well as extended delta connection [6], can have its phase-shift modified by altering the number of turns of its coils conveniently. Figures 16 and 17 show, for zig-zag connection, how it is possible to obtain a new phase-shift by changing appropriately the number of turns, without changing the terminal voltage, though. Delta extended connection allows this kind of procedure for obtaining a new phase-shift. However, differently from zig-zag, its terminal voltage varies by making this. R S

C1

T A1

B1

C1

A '2

B'2

C '2

A4 = C2

B4 = A2

A3 = B3 = C3

A3

B1

A1

A '4

C4 = B2

Fig. 15. Phasor Diagram for Zig-zag Connection in Fig. 14

B3

B'4

C3

C '4

Fig. 16. Modified Zig-zag connection.


343

6 - REFERENCES

Fig. 17. One-Phase Phasor Diagram for Modified Zig-zag connection 4 - CONCLUSION It can be concluded that the obtainment of a transformer phase-shift comply with certain rules imposed by firstly the nature of the connections themselves and secondly how they are assembled. In short, it does not present a random behavior at all. It has been seen that the coil polarity, the way of assembling a connection, the phase sequence, and even, in certain cases, the number of turns can lead to a change in the transformer phase-shift. The following statements can be derived from what have been exposed formerly: 1 - It is impossible to obtain transformer phaseshifts lesser than 30º by using usual connection arrangements; 2 - So, for converters with up to 12 pulses, conventional transformers, with Y, ', or Z connections can be used; 3 - But, for converters with 18 pulses or higher, special arrangements for ' and Z connections or others, are needed. So, it is important to remark that for obtaining a converter with a certain number of pulses, there is a need of knowing: a) which arrangement of connections can lead to the related phase-shift? and b) what are the differences about the performance of unlike connections, which leads to the same transformer phase-shift? This paper is therefore helpful to providing responses for these basic questions.

344

1. Pelly, B. R. - Thyristor Phase-Controlled Converters and CycloConverters; John Wiley and Sons, Inc. 1971; 2. Power Converter Handbook - Theory, Design, and Application - Canadian General Electric Company Ltd, Ontario, Canada, 1975; 3. Yacamini and J.C. Oliveira, “Harmonics in Multiple Convertor Systems: A Generalized Approach”, IEE Proceedings-B, Vol. 127, no 2, pp. 96-106, March 1980; 4. Carlsson, L.; “Extended Delta Converter Transformer for 12-Pulse Operation in HVDC Projects”, Seminar on HVDC Transmission – Rio de Janeiro, Brazil, 1986; 5. Rashid M.H. and Maswood A.I.: Analysis Of Three-Phase AC-DC Converters Under Unbalanced Supply Conditions. IEEE Transaction on Industrial Applications, Vol. 24, No. 3, pp. 449 – 455, May/June 1988; 6. Bernardes, D.F., Guimarães, C.A.M., Hermeto, A.E., Abreu, J.P.G., Arango, H., “Pitfalls on Power Definitions in Electrical Systems with Neutral”, IEEE Power Tech ’99 Conference, Budapest Hungary – 1999. 7. Abreu, J.P.G., Rezek, A.J.J. & Candido, A.; “Modelling and Implementation of 48-Pulse Multiconverter”, Proceedings of 6th ICHPS, Bologna, Italy, 1994, pp. 50-54; 8. Abreu, J.P.G., Nascimento, J.G.A.; “Considerações sobre Defasagem Angular e Operação em Paralelo de Transformadores” (In Portuguese); 1st International Seminar on Distribution, Belo Horizonte, Brazil, 1986; 9. Cengelci, E., Sulistijo, S.U., Woo, B.º, Enjeti, P., Teodorescu, R. and Blaabjerg, F.; “A New Medium-Voltage PWM Inverter Topology for Adjustable-Speed Drives”, IEEE Transactions on Industry Applications, Vol. 35, No. 3, May/June 1999; 10. Hammond, P.W., “A New Approach to Enhance Power Quality for Medium Voltage AC Drives”, IEEE Transactions on Industry Applications, Vol. 33, no 1, January/February 1997;


Dr. José Policarpo G. Abreu

Delvio Franco Bernardes

was born in 1952 in Madeira Island, Portugal. He received a Ph.D. degree in Electrical Engineering from the University of Campinas (Brazil) in 1991. He is Full Professor and the Power Quality Study Group Coordinator at Itajuba Federal School of Engineering (EFEI), Brazil. He has been nominated for the Chairmanship of the 10th IEEE ICHQP - International Conference on Harmonics and Quality of Power, to be held in Rio de Janeiro, Brazil in 2002. Currently, his research interests include power quality issues, power definitions under abnormal conditions, induction motors and transformers, and electric drives.

was born in 1954 in Sao Paulo, Brazil. He received his M.Sc. degree in Electrical Engineering in 1991 from Itajuba Federal School of Engineering - EFEI, where he is currently pursuing his Ph.D.. Presently, he is professor of the Institute of Electrical Engineering of the EFEI. His areas of interest include power quality, electric machines and drives.

Mailing address: José Policarpo G. Abreu Itajuba Federal School of Engineering - EFEI Power Quality Study Group - GQEE P. O. Box # 50 37500-903 Itajuba - MG BRAZIL Phone: (+55)(35) 3629-1243, fax: (+55)(35) 3629-1187 E-mail: [email protected]

Mailing address: Delvio Franco Bernardes Itajuba Federal School of Engineering - EFEI P. O. Box # 50 37500-903 Itajuba - MG BRAZIL Phone: (+55)(35) 3629-1172 E-mail: [email protected]


345

346




ASPECTS REGARDING LOAD CURRENT SYMMETRIZATION IN UNBALANCED POWER SYSTEMS Denisa RUSINARU Ion MIRCEA University of Craiova Craiova (Romania)

Abstract - The present paper is dealing with one of the multiple aspects of the energy quality. It is about the unsymmetrical operation of the power systems determined by supplying of the unbalanced industrial customers. There are proposed different solutions for the load current symmetrization by using SVCs. The variation laws of the SVCs' parameters are determined for different operation requirements. The situations that assumed only the load symmetrization or those that regard other aspects of electrical energy quality are considered. On the other hand it is approached the influence of the symmetrization SVCs presence on the electrical quality indices.

2. LOAD CURRENTS SYMMETRIZATION For symmetrization of the load currents flowing through an unbalanced tri-phase consumer one/two SVC(s) can be coupled in the consumer node in order to inject different reactive power on each phase [1], according to the measurements. A proper design of the SVC’s should follow other aspects of the energy quality as well: i.e. reactive power compensation, harmonics level. In these compensators presence the load looked from the supplying systems appears as balanced. Supplying system ZNL

1. INTRODUCTION The unfavourable effects of unbalanced loads connecting in the power system have generated a great number of approaches. Their purpose was the explanation of those phenomena regarding power flows in these systems. The final result was finding of proper power definitions and consequently methods for load balancing. One of the most efficient method for state values symmetrization in an unbalanced power system is SVC’s using. Each phase of these compensators can be independently controlled. Therefore, the reactive power compensation can be achieved different on each phase. The present paper includes the equations for determination of compensators’ parameters, including the results of the simulation programs (MATLAB 5.) designed for this purpose.

IaS

ZaL

IbS

ZbL

IcS

ZcL

YiC jBi Y

Y config. SVC

SVC’s control

Consumer

jBik' ' config. SVC

Figure 1. Supplying of an unbalanced consumer in the presence of SVC’s for load symmetrization


347

Considering the voltages at the load terminals as symmetrical, the load currents is written in symmetrical components as following [2]: ª I C0 º ªY C 0 0 º « C» « 0 » C «I » « 0 Y 0 »U (1) « C» « C» 0 Y «¬ I »¼ ¬ 0 ¼ with the sequence admittance given by the equation: ªY C º ªY a º « C» » 1 « (2) «Y » [ A] «Y b » «Y C » «¬Y c »¼ ¬ 0¼ with [A]-1 the inverse of Fortescue matrix 1º ª1 1 1« 1 [ A] 1 a a 2 »» (3) « 3 2 «¬1 a a »¼

and Y i Gi jBi , i=a, b, c – phase load admittances. Therefore, the symmetrical components of the compensators’ currents are: for Y- SVC: ª I Y º ª jBaY º « Y» Y» 1 « (4) « I » [ A] « jBb »U «I Y » « jBcY » ¬ ¼ ¬ 0¼ Y with jBi – phase admittances of the compensator; for '-SVC: ' ª I ' º º ª jBab « '» « '» (5) « I » [ A' ]« jBbc »U ' «I ' » « jBca » ¼ ¬ ¬ 0¼ ' with jBik – phase admittances of the compensator (i,k – a, b, c); [A’] – component transformation matrix for 'compensator currents definition ª 1 1 1º (6) [ A' ] ««a 2 1 a »» «¬ 0 0 0 »¼ Dimensioning of the compensators’ parameters should aim the cancellation of zero or/and negative current components. Mathematically speaking, these issues can be represented by the following equations: C Y ' e I e I e I 0

mI eI mI mI

C

Y 0

348

mI mI eI 0 mI 0 mI mI

Y 0

C

Y

'

0

C 0

The solution of equation system (7) gives the susceptances of the two compensators (eq.8) that could balance on-line the load according to the state values of the system recorded in the supplying node of the load. [B]=[T][M] (8) where: ª BaY º « Y» « Bb » not « B Y » [ B] « c' » - the column vector of the SVC’s « Bab » «B' » « bc' » «¬ Bca »¼ susceptances; ª PaC /(U aC ) 2 º « C C 2» « Pb /(U b ) » not « P C /(U C ) 2 » c [ M ] « cC » - the column vector of the C 2 «Qa /(U a ) » «Q C /(U C ) 2 » b « bC » «¬Qc /(U cC ) 2 »¼ measured state values in the load connection node; [T] – susceptances / state values link matrix. The singularity of the system (7) is given only by introducing a supplementary condition, given by the special request on system operation. This condition results if it is desired: 2.1. The elimination of imaginary part of the positive component of the load current by one compensator (the other one does not produce any percentage of this component) [1]

-

supplementary condition: ' ' Bbc' Bca Bab link matrix:

-

ª 1/ 3 0 1/ 3 « 1 / 3 0 1/ 3 « « 1/ 3 1/ 3 0 [T ] « 2 / 3 3 2 / 3 3 0 « « 0 2/3 3 2/3 3 « 0 2/3 3 «¬ 2 / 3 3

-

'

0 1 0 0 0 0

0º » 0» 1» » 0» 0» » 0»¼

(9)

supplementary condition [1]:

c

B B 3B 3B 3B º»¼U

ª BY «¬ a

2

Y 2 b

Y 2 c

' 2 ab

' 2 bc

' 2 ca

(10)

C 0 Y

1 0 0 0 0 0

(8)

2.2. Power loss minimization

def

(7)

0

0

c min

dc dB aY

0

(10’)


2

-

link matrix:

ª 1/ 3 3 / 4 0 1/ 3 1/ 4 1/ 4 º « » 1 / 3 0 1 / 3 1 / 4 3 / 4 1 / 4 » (11) « « 1/ 3 1/ 3 3/ 4» 0 1/ 4 1/ 4 [T ] « » 2 / 3 3 2 / 3 3 0 1 / 18 1 / 18 1 / 18» « « 0 2 / 3 3 2 / 3 3 1 / 18 1 / 18 1 / 18» « » 0 2 / 3 3 1 / 18 1 / 18 1 / 18»¼ «¬ 2 / 3 3

2.3. Voltage adjusting

-

suplementary result: maintain the voltage at the consumer end of the line at the same value as that at the beginning end of the line; dimensioning hypothesis: compensators – consumer group behaves as a balanced load with conductance given by:

-

2.5. Simple symmetrization, without reactive power compensation

The equation system (7) from that the last equation was removed is an undetermined one. Even so, considering one of previous conditions (& 2.2, 2.3) and one variable as parameter in a certain variation domain can obtain a solution. Power loss minimization - independent variable considered: Bbc' - link matrix:

[T ]

def

Ge

1 C Ga GbC GcC 3

(12)

-

phase susceptance of the compensator: § § 2 Rt G e · ·¸ ¸ Be ¨¨ X t r X t2 X t2 Rt2 ¨¨ 2 2 2 2¸¸ ¨ R G X G t e t e © ¹ ¸¹ ©

/ Rt2 X t2

f (line length) (13)

2.4. One compensator use for symmetrization

x ' configuration SVC - result: annulling the negative load current component and the imaginary part of its positive component; -

link matrix: ª 3 3 1« [T ] 3 « 0 9« 3 0 ¬

2 2 1 º » 3 1 2 2» 3 2 1 2» ¼ (14) 0

x Y configuration SVC - result: annulling the zero load current component and the imaginary part of its positive component; -

link matrix: ª 0 1/ 3 « [T ] « 1 / 3 0 « 1/ 3 1/ 3 ¬

1/ 3 1 0 1/ 3 0 1 0

0

0

0º » 0» 1» ¼ (15)

-

ª 0 1 / 3 1 / 3 3 / 4 « 0 1/ 3 5/8 « 1 / 3 « 1 / 3 1 / 3 0 5/8 « «2 / 3 3 1 / 3 3 1 / 3 3 2 / 3 «2 / 3 3 1 / 3 3 1 / 3 3 2 / 3 ¬

1/ 4 7/8 1/ 8 0 2/3

1/ 4 1/ 8 7/8 2/3 0

0º » 0» 0» » 1» 1»¼

(16) modified column vector of state variable: ª PaC /(U aC ) 2 º « C C 2» « Pb /(U b ) » « P C /(U C ) 2 » c « cC » [ B ] «Qa /(U aC ) 2 » (17) «Q C /(U C ) 2 » b « bC » «Qc /(U cC ) 2 » « » Bbc' ¬ ¼

Similarly, the SVC’s parameter expressions can be obtained for the other mentioned conditions. 3. RESULTS OF SIMULATION PROGRAMMS AND CONCLUSIONS

For each of the situations presented in section 2 an algorithm set was achieved by using MATLAB 5.2. Program Package [3]. By using these applications the variation curves of the SVC’s parameters were plotted according to the mentioned situations (Appendix fig. A.2…5). The algorithms were applied for the case of unbalanced consumer electric railway traction. The load evolution in its feeding node (railway traction substation) was registered on a limited period of time (1/4 of a day) – Appendix fig. A.1. In addition the algorithms include a section for active power losses determination on the load feeder (Appendix fig. A.6). They can be extended by subroutines for power factor calculation. The curves for SVC’s susceptances representation can serve for on-line adjusting strategy of the compensators’ parameters for any dynamic load with estimated variation. The results of simulation were summarised in Appendix -Table 1. 4. REFERENCES


349

Denisa Rusinaru University of Craiova Electrotechnics Faculty, Energy Department 5, Lapus Str., 1100-Craiova,ROMANIA phone:(+4)(051) 43-64-47, fax:(+4)(051) 43-64-47 e-mail: [email protected]

1. Lee S.Y., Wu C.J.: On-line reactive power compensation schemes for unbalanced three/phase four wire distribution feeders. IEEE Trans. Power Delivery, vol. 8, no. 4, Oct. 1993, pp. 1958-1965. 2. Antoniu I.S. Electrotehnics Fundamentals (in Romanian). EDP, Bucharest 1974. 3. Rusinaru D. Considerations regarding the influence of the unbalanced operation of the consumers on the power system (in Romanian). Paper Ph.D. stage, Craiova 2000.

Ion Mircea was born in 1947 in Islaz, Romania. He received his Ph.D. at University of Craiova. At present he is professor in the Electrotehnics Faculty of Craiova. His field of interest is in nonconventional energy sources, transmission and distribution networks, energy markets structures. Mailing address: Ion Mircea University of Craiova Electrotechnics Faculty, Energy Department 5, Lapus Str., 1100-Craiova,ROMANIA phone:(+4)(051) 43-64-47, fax:(+4)(051) 43-64-47 e-mail: [email protected]

BIOGRAPHIES Denisa RUSINARU was born in 1968 in Craiova, Romania. In 1993 she graduated Electromechanical Engineering at University of Craiova. At present she is lecturer in the Electrotehnics Faculty of Craiova. Her field of interest is in energy quality and distribution networks. Mailing address:

APPENDIX

TABLE 1. Simulation results Interest values I+C [A] I-C/ I1C I0C/ I1C Line power [kW/km]

losses

Case 2.1

'P

Susceptances SVC-Y [mS] BaY BbY BcY

Susceptances SVC-'[mS] Bab' Bbc' Bca' Representation

350

Case 2.2

Case 2.3

Case 2.4 'Y

Case 2.5

19,12..62,8 0 0 0,45..5,3

0 0 0,25..2,3

0 0 25..600 [kW] for 10...100km

0 0,004..0,01 0,35..3,25

0,99..1,01 0 0,35...3,8

0 0 1..4,5

0,04..0,33 0,04..0,33 0,42..0,92

-0,68..-0,18 0,06..0,28 0,06..0,28

0,0005...0,00 6 for10...100 km

0 0 0

-2,3...-0,6 0,1...0,35 0,1...0,35

-1,75..-0,6 -0,08..-0,01 0,5..1,6

0,15..0,4 -0,6..-1,8 0,2..2,2

0,05..0,13 -0,27..-0,07 0...0,07

-0,12..0,03 -0,24..-0,08 -0,24..-0,08

0 0 0

-2..0,3 -1,1..0,9 -1,8..0,25

fig.A2.a fig.A7.a

fig.A2.b fig.A7.b

0,0001...0,00 2 for 10...100km fig.A3 fig.A7.c

fig.A4.a fig.A7.e

fig.A4.b fig.A7.f

fig.A5 fig.A7.d


Fig. A1. Railway traction consumer a.

b.

characteristics in supplying node: a. phase voltages; b. phase active powers; c. phase reactive powers; d. phase apparent powers; e. current asymmetry coefficients; f. active power losses on the line

c.

d.

e.

f.

a.

Fig. A3. SVC’s parameters: complete symmetrization case, with voltage adjusting (case 2.3) b. Fig. A2. SVC’s parameters: a. complete symmetrization case, without economic efficiency condition (case 2.1); b. complete symmetrization case, with economic efficiency condition (case 2.2)

a. b. Fig. A4. SVC’s parameters for load current symmetrization: a. compensation with SVC-'; b.compensation with SVC-Y (case 2.4)


351

a.

b. Fig. A5.

SVC’s parameters for load

current symmetrization (without reactive c. power compensation) - case v: a, b, c SVC-Y susceptances; d, e - SVC - ' susceptances( Bbc'¼parameter)

d.

a.

e.

b.

d.

c

e.

f.

Fig. A7. Active power losses on the line: a. corresponding to case 2.1; b. corresp. case 2.2; c. corresp.case 2.3 (parameter: line length L); d.corresp. case 2.5.; e, f – corresp. Case 2.4

352




NEURAL NETWORKS FOR OPTMIZATION OF POWER SYSTEMS Janusz WALCZAK Dariusz GRABOWSKI Silesian University of Technology Gliwice (Poland)

Abstract - This paper is a first step in research concerning application of systems based on neural networks in optimization of power systems. The main task consists in symmetrization of phase currents as well as phase powers of the load. The problem may be solved by means of two neural networks. The first one is used for identification of parameters of a non-stationary linear or nonlinear system source-load on the base of currents and voltages measured in a measurement section. The second one is applied for solution of the symmetrization problems and results in the determination of the symmetrizator parameters. The paper contains the formalization of the optimization problem and is focused on building of the neural network that makes possible identification of the power system. The theoretical consideration has been illustrated by an example. 1. INTRODUCTION A lot of scientific works have been devoted to solution to problems of symmetrization and optimization of multiphase systems [1], [5], [9], [10]. These systems may be regarded as models of power systems with different degree of complexity [1], [5]. Their analysis was carried out assuming that the waveforms were either sinusoidal [5] or nonsinusoidal [1], [8]. The optimization of working conditions of these systems (Fig. 1) may concern the following problems: the minimization of root-mean-square (RMS) values of currents delivered to the load, the minimization of distortion of current and voltage waveforms, the minimization of power losses during the transmission of energy from the source to the load,

the maximization of power factor of the system, the minimization of reactive power delivered from the source to the load, the symmetrization of phase active powers delivered to the load, as well as the composition of the problems mentioned above. The optimization of the systems may be also based on many other quantities, e.g. apparent power or economic indexes. i1

i1' SOURCE

u 1' u 2' u 3'

i2' i3'

TRANS. LINE

u1 u2 u3

i2 i3

LOAD

Fig. 1. The simplified model of a power system Regardless of requirements arising from specific technical conditions, the quality functions of the optimization problems should be formulated on the base of unambiguously defined and having physical interpretation electrical quantities. The realisation of this goal is not an easy task. The reactive power in three-phase systems with sinusoidal waveforms is not a measure of energy oscillations between sources and loads in these systems [11]. Moreover, the apparent power is defined in a few different ways [6]. It results in conclusion that even in three-phase systems with sinusoidal waveforms it is impossible to define power factor in unambiguous way [6].


353

The problem is even more serious in systems with nonsinusoidal waveforms, for which a lot of definitions of reactive, apparent and distortion power are known. Unfortunately, they have no physical interpretation [4]. The introduction of orthogonal decompositions of source currents [3] a few years ago and its application to construction of some power quantities proved to be an illusion. The questionable physical sense of these decompositions is limited at the very most to the class of systems containing ideal sources, which do not exist in engineering, and is useless in the case of systems with non-zero source impedance [8], [12]. So one can come to a conclusion that it is reasonable to formulate the task of power systems optimization taking advantages of functionals based on unambiguous and having evident interpretation quantities, i.e.: instantaneous power, active power, root-mean-square (RMS) values of currents and voltages. The formulation and solution to the optimization problems for systems with nonsinusoidal waveforms may be carried out in different ways [8] depending on models and goal functions that are taken into account.

i3' e1

e2

e3

EQUIVALENT SOURCE

u1 u2 u3

i1 i2 P1 i3 P2 P3

2.1. The model of the system It has been assumed that during the observation period T0 the model of the system (Fig. 2) consists of: ideal three-phase source of nonsinusoidal voltages e1(t), e2(t) and e3(t), representing the influence of nonlinearity and unstability of loads on occurrence of nonsinusoidal waveforms, passive SLS multi-port representing impedances of the source and the transmission line, multi-port which is a model of the load and receives given phase active powers PD (D = 1, 2, 3) that are not necessary equal each other. As a result the model of the source obtained during identification is described by the following formula:

eD

uD

¦ >ZDE i E @t , 3

D 1,2,3 ,

where: uD - voltage across the measurement section, i.e. source terminals (Fig. 2), ZDE - stationary and linear impedance operator given by:

measurement section

>Z DE i E @t ³ zDE (t W )i E (W )dW . 2.2. The optimization problem

The problem of optimization of the system shown in the Fig. 2 may be formulated as follows: l

The system with time-varying parameters, which is shown in the Fig. 2, has been considered in the paper. The identification of parameters of the model of the system, solution to the optimization task formulated in the next section and determination of the symmetrizator parameters should be performed on-line and in accordance with the above sequence. For that purpose the recurrent neural networks [2], [7] have been applied in the paper.

(2)

0

Fig. 2. The considered model of the system

min {iD }

3

1 Uk T 1

¦¦

k 0D

ª d k iD « k ¬ dt 0«

T

³

2

º » dt , »¼

U k R , (3)

while delivering given active power PD to the load: T

PD

1 uD (t )iD (t ) dt T

³

e PD

Z PD ,

(4)

0

where: T

e PD

1 eD (t )iD (t ) dt , T

³ 0

354

(1)

E 1

t

EQUIVALENT LOAD

i2'

SOURCE IMPEDANCE

i1'

2. THE STATEMENT OF THE PROBLEM


(5)

u1

Threephase source

u2

i1 i2 i3

P1 P2 P3

u3 i1 i2 i3

measurement section

NN for identification of system parameters

u1 u2 u3

Load

NN for determination of symmetrizator parameters

Symmetrizator

Fig. 3. Complex neural network system for identification, optimization and symmetrization

1 T

Z PD

º ªt « zDE (t W )i E (W )dW »iD (t ) dt . (6) »¼ 10 « ¬0

3 T

¦³ ³ E

It may be proved that solution to the problem (3) with the equality constraint (4) is given by phase currents aiD (D = 1, 2, 3), which: have minimum RMS value in the sense of the formula: l

3

¦¦

ai

k 0D

2

T 1 ª d k a iD º Uk » dt , « T ¬« dt k »¼ 1 0

³

(7)

have minimum distortion, i.e. the first harmonic is the main component of these currents, enable symmetrization of the system, i.e. equal active power PD delivered to each phase of the load in given time period: PD

P1

P2

P3

const .

(8)

The optimum phase currents of the source, obtained in the consequence of solution to the optimization problem (3), are the input of the control system of the symmetrizator. The parameters of the symmetrizator should be in turn selected in order to achieve the minimum, in the sense of the norm, difference between currents delivered by the source and the currents obtained as a result of optimization. The optimum working conditions stay unchanged only if the system parameters do not vary in time. In the case of timevarying systems it is necessary to perform on-line: identification of the model parameters (Fig. 2) by solution to the optimization problem (3), follow-up control of the symmetrizator parameters.

Two individual neural networks may be applied for realization of each of these aims (Fig. 3). It is beyond any doubt that optimization followed by symmetrization requires preceding identification of the model parameters of the threephase system (Fig. 2). This problem has been considered in the following part of the paper. 3. IDENTIFICATION OF THE MODEL

The equations of the model of the source (1) in time domain are very complicated and identification of the voltages eD () and impulse response zDE () in discrete time domain leads to solution of a very high order equations. By this reason it is profitably to perform identification of the model, i.e. voltages eD () and impulse response zDE (), in frequency domain. The frequency representation of the operator (1) is expressed by the formula: Eh ZhIh

Uh , h N ,

(9)

where: Eh = [E1h, E2h, E3h]T - the vector of complex RMS values of source voltages e1(t), e2(t), e3(t) for h-harmonic: Eh

1Eh

j 2Eh ,

(10)

Ih = [I1h, I2h, I3h]T - the vector of complex RMS values of phase currents i1(t), i2(t), i3(t) across the measurement section (Fig. 2) for h-harmonic: Ih

A h jB h ,

(11)

Uh = [U1h, U2h, U3h]T - the vector of complex RMS values of phase voltages


355

u1(t), u2(t), u3(t) across the measurement section (Fig. 2) for h-harmonic: Uh

C h jD h ,

ª1 « «1 «1 « «¬1

(12)

Zh - the 3x3 matrix of complex source impedances for h-harmonic: Zh

R h jX h .

where: $

h

>aij @ ,

Ih ,

>xi @,

?h

AB (h2) AB (h3) AB (h4)

(13)

The problem of identification consists in determination of the vectors Eh and matrices Zh for finite number of harmonics hmax assuming that sets of voltage and current harmonics {Uh, Ih} in the measurement section are given. The problem of identification may be brought to solution of a system of n 24 linear equations for each harmonic: $ h? h

AB (h1)

AB (hk )

ª A ( k ) h « (k ) «¬ B h

A (hk )

BA (hk )

ª B ( k ) « ( kh) «¬ A h

B (hk )

x

B (hk )

A (hk )

A (hk ) º », B (hk ) »¼

(16)

B (hk ) º », A (hk ) »¼

(17)

Rih, Xih - vectors created from i-th row of matrices Rh and Xh, i {1, 2, 3}:

>bi @ .

Taking into account formulae (9) - (13) the system of equations (14) may be expressed as follows:

a11

ª C (1) º « h(1) » « Dh » «C ( 2 ) » « h » «D (h2) » « (3) » , (15) «C h » « D ( 3) » « (h4) » «C h » «D ( 4) » ¬ h ¼

where:

(14) Ih

ª 1Eh º « E » «2 h» (1) º « R 1h » BA h » ( 2) » « BA h » « R 2 h » BA (h3) » « R 3h » »« » BA (h4) »¼ « X1h » «X » « 2h » ¬« X 3h ¼»

a11

x

a21

x

R ih

>Ri1h

Ri 2 h

Ri 3 h @ T ,

(18)

X ih

>X i1h

X i 2h

X i 3h @ T .

(19)

b1 a12

x

+ +

-

6

+ +

+

a1n

an1

6

P

³

P

³

Pn

³

x1

+

an1

x

x

x

a1n

x

a2n

x

x2

bn an2

x

+ +

-

6 +

ann

x

+ +

6 +

ann

x

Fig. 4. The bloc diagram of the recurrent neural network for identification of system parameters

356


xn

measurement section

Fig. 5. The power system identified in the example

The upper index (k) in equations (15) - (17) indicates the number of measuring experiment, k {1, 2, 3, 4}.

In the paper the Hopfield neural network [7] has been applied for solution to the systems of equations (14). This approach requires formulation of the problem in optimization way. The searched vectors ;h may be find by means of minimization of the following goal function:

1 ( A h X h b h )T ( A h X h b h ) . 2

(20)

It can be proved that necessary conditions for this quadratic optimization problem are always fulfilled [2]. The dynamic of the neural network in this case is described by the following differential equation:

dX h dt

PF ( X h ) PA Th ( A h X h b h ) ,

P - the vector of learning rate. The block diagram of the network has been shown in the Fig. 4.

4. HOPFIELD NEURAL NETWORK

F( h )

where:

5. THE EXAMPLE The system shown in the Fig. 5 has been analyzed in order to verify the proposed method of identification of power system parameters. The simulations of the neural network have been carried out by means of the program NeuroSolutions [13], which proved to be a very powerful tool in research concerning neural networks (Fig. 6). The input data for the neural network consists of currents and voltages across the measurement section (Fig. 3 and 5). The complex RMS values of these currents and voltages, measured four times with constant interval, have been brought together in the Tab. 1.

(21)

Fig. 6. The realization of the neural network (Fig. 4) for identification in NeuroSolutions TABLE 1. The input data of the neural network Section 4. Methods of Power Quality Improvement: Filters, ....

357

#

1

2

3

values of currents and voltages across the measurement section (Tab. 1). Both the matrix $ and the vector Ehave been presented in the Tab. 2. The obtained results (Fig. 7 and 8) are very close to parameters of the system under inspection (Fig. 5). The final values of the parameters determined by the network have been put into the Tab. 3. The analysis of the Fig. 9 leads to the conclusion that the only thing, which could be improved, is the rate of convergence of the neural network. It may be done by implementation of the conjugate gradient method in spite of the steepest descent method [2].

4

U11 [V]

214.3-j 3.6

217.3-j2.4

218.6-j1.1

219.1-j0.75

U21 [V]

- 110.3-j183.7

-110.7-j187.1

-110.3-j188.7

-110.1-j189.2

U31 [V]

- 104.0-j187.4

-106.6+j189.4

-108.4+j189.8

-107.8+j189.6

I11 [A]

194.0-j64.6

105.9-j17.8

51.7-j12.4

35.0-j7.4

I21 [A]

-152.9-j135.7

-68.35-j82.8

-36.6-j38.6

-30.8-j28.13

I31 [A]

-41.1+j200.3

-37.5+j100.6

-15.1+j51.0

-21.7+j67.8

The matrix $ and the vector E (14) have been determined on the base of the complex RMS TABLE 2. The matrix $ and the vector E for the system shown in the Fig. 5 $ 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

E

1

1

0

0

0

0

0

-194

0

0

-194

0

0

-194

0

0

-65

0

0

-65

0

0

-65

0

0

214

2

0

1

0

0

0

0

0

153

0

0

153

0

0

153

0

0

-136

0

0

-136

0

0

-136

0

-110

3

0

0

1

0

0

0

0

0

41

0

0

41

0

0

41

0

0

200

0

0

200

0

0

4

0

0

0

1

0

0

-65

0

0

-65

0

0

-65

0

0

194

0

0

194

0

0

194

0

0

4

5

0

0

0

0

1

0

0

-136

0

0

-136

0

0

-136

0

0

-153

0

0

-153

0

0

-153

0

184

6

0

0

0

0

0

1

0

0

200

0

0

200

0

0

200

0

0

-41

0

0

-41

0

0

-41

-187

7

1

0

0

0

0

0

-106

0

0

-106

0

0

-106

0

0

-18

0

0

-18

0

0

-18

0

0

217

8

0

1

0

0

0

0

0

68

0

0

68

0

0

68

0

0

-83

0

0

-83

0

0

-83

0

-111

9

0

0

1

0

0

0

0

0

38

0

0

38

0

0

38

0

0

101

0

0

101

0

0

10

0

0

0

1

0

0

-18

0

0

-18

0

0

-18

0

0

106

0

0

106

0

0

106

0

0

2

11

0

0

0

0

1

0

0

-83

0

0

-83

0

0

-83

0

0

-68

0

0

-68

0

0

-68

0

187

12

0

0

0

0

0

1

0

0

101

0

0

101

0

0

101

0

0

-38

0

0

-38

0

0

-38

-189

13

1

0

0

0

0

0

-52

0

0

-52

0

0

-52

0

0

-12

0

0

-12

0

0

-12

0

0

219

14

0

1

0

0

0

0

0

37

0

0

37

0

0

37

0

0

-39

0

0

-39

0

0

-39

0

-110

15

0

0

1

0

0

0

0

0

15

0

0

15

0

0

15

0

0

51

0

0

51

0

0

51

-108

16

0

0

0

1

0

0

-12

0

0

-12

0

0

-12

0

0

52

0

0

52

0

0

52

0

0

1

17

0

0

0

0

1

0

0

-39

0

0

-39

0

0

-39

0

0

-37

0

0

-37

0

0

-37

0

189

18

0

0

0

0

0

1

0

0

51

0

0

51

0

0

51

0

0

-15

0

0

-15

0

0

-15

-190

19

1

0

0

0

0

0

-35

0

0

-35

0

0

-35

0

0

-7

0

0

-7

0

0

-7

0

0

219

20

0

1

0

0

0

0

0

31

0

0

31

0

0

31

0

0

-28

0

0

-28

0

0

-28

0

-110

21

0

0

1

0

0

0

0

0

22

0

0

22

0

0

22

0

0

68

0

0

68

0

0

68

-108

22

0

0

0

1

0

0

-7

0

0

-7

0

0

-7

0

0

35

0

0

35

0

0

35

0

0

1

23

0

0

0

0

1

0

0

-28

0

0

-28

0

0

-28

0

0

-31

0

0

-31

0

0

-31

0

189

24

0

0

0

0

0

1

0

0

68

0

0

68

0

0

68

0

0

-22

0

0

-22

0

0

-22

-190

358

200 -104

101 -107


E11

E21

E31

E12

E22

E32

250 E [V] 200 150 100 50 iteration [-] 0 0

3

6

9

12

15

-50 -100 -150 -200 -250

Fig. 7. The output of the neural network - voltage parameters of the model

R1

R2

R3

X1

X2

X3

40 R, X [m:] 20 iteration [-] 0 0

100

200

300

400

500

600

700

800

900

1000

-20

-40

-60

-80

-100

Fig. 8. The output of the neural network - impedance parameters of the model TABLE 3. The results of identification by the neural network after 2000 iterations 1E1

1E2

1E3

2E1

2E2

2E3

[V]

[V]

[V]

[V]

[V]

[V]

219.97 -110.04 -109.97

-0.01

190.52 -190.42

R1

R2

R3

X1

X2

X3

[m:]

[m:]

[m:]

[m:]

[m:]

[m:]

20.78

21.22

20.61

25.77

25.48

25.92

6. CONCLUSION The proposed neural approach proved to be an effective way of identification of the model parameters for three-phase time-varying systems. The investigations have confirmed that identification of source voltages requires a few steps of learning while identification of source impedances needs a few hundreds of steps assuming zero initial conditions. The global convergence of the algorithm of identification is the advantage of the applied neural network.


359

100000 F [V2] 10000

1000

100

10

1

0,1

0,01 iteration [-] 0,001 0

200

400

600

800

1000

1200

1400

1600

1800

2000

Fig. 9. The goal function during learning process REFERENCES 1. Arrilaga J., Bradley D.A., Bodger P.S.: Power system harmonics. J.Wiley, New York 1985. 2. Cichocki A., Unbehauen R.: Neural networks for optimization and signal processing. J.Wiley, New York 1993. 3. Czarnecki L.S.: Orthogonal decomposition of currents of nonsinusoidal voltage source supplying asymmetric three-phase load. Proc. of X SPETO, Wisla 1987, pp. 137-141. 4. Czarnecki L.S.: What is wrong with the Budeanu concept of reactive and distortion power and why it should be abandoned. IEEE Trans. Inst. Meas., vol . 38, 1989. 5. Gonen T.: Modern power systems analysis. J.Wiley, New York 1987. 6. IEEE Trial - use standard definitions for the measurement of electric power quantities under sinusoidal, nonsinusoidal, balanced or unbalanced conditions. IEEE 1459-2000. 7. Madziuk J.: Hopfield neural networks. Theory and applications. Exit, Warsaw 2000. 8. Pasko M., Walczak J.: Optimization of power quality in electrical systems with nonsinusoidal waveforms. Monograph, ZN Pol. Sl. Elektryka, Z. 150, Gliwice 1996. 9. Piróg S.: Power electronics. Negative influence of power electronics systems on energy sources and some methods of its reduction. Wyd. AGH, Cracov 1988. 10. Piróg S.: Symmetrization of three-phase electric loads. AGH, Nr 1122, Cracov 1988. 11. Walczak J., Pasko M.: Power and oscillations of energy in one-phase systems with sinusoidal

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waveforms. Proc. of IX Conf. PPEE, Wisla 2000, pp.261-265. 12. Walczak J.: Optimization of energy-quality properties of electrical systems in the Hilbert spaces. ZN Pol. Sl. Elektryka, Z. 125, Gliwice 1992. 13. NeuroSolutions v.3.0 - Users Guide. Gainesville 2000. Janusz Walczak, Ph.D., D.Sc., Eng. He obtained a doctorate in 1986. In 1993 he became an associated professor in Silesian University of Technology. He has been active in the areas of analysis and synthesis, signal processing, power theory in systems with nonsinusoidal waveforms. Mailing address: Janusz Walczak Silesian University of Technology, Electrical Depart. ul. Akademicka 10, 44-100 Gliwice, Poland phone: (+48)(32) 237-12-29, fax: (+48)(32) 237-12-58 e-mail: [email protected] Dariusz Grabowski, Ph.D., Eng. He graduated in 1993 and joined the Electrical Department in Silesian University of Technology. In 2000 he obtained a doctorate. His research interests are in fields of nonlinear systems synthesis, neural networks and application of optimization in electrical engineering. Mailing address: Dariusz Grabowski Silesian University of Technology, Electrical Depart. ul. Akademicka 10, 44-100 Gliwice, Poland e-mail: [email protected]


Section 5 Power Quality in Competitive Electricity Markets. Economic Aspects of Power Quality and Costs of Supply

5.1. MIELCZARSKI W., WASILUK-HASSA M., SAMOTYJ M.J.: Power Quality in Electricity Markets (Poland, USA)........................................................................................................................363 5.2. GOMES R.J.R., BRASIL D.O.C., MEDEIROS J.R.: Power Quality Management as a Goal of ONS (Operador Nacional do Sistema Eletrico) the Brazilian Transmission ISO (Brazil) ..................369 5.3. EGUIA P., TORRES E., FERNANDEZ E., SAENZ J.R.: The New Quality of Supply Regulatory Framework in Spain. Will it Benefit the Network User? (Spain) ......................................377 5.4. HOWARD M.W.: Life Cycle Cost Analysis for End Use Power Quality Mitigation with Advanced Energy Storage Technologies: A Case Study (USA) ..........................................................383 5.5. BAJSZCZAK G.: Cost Evaluation of Non-Active Power Compensation in Transmission and Distribution Networks (Poland) ...........................................................................................................393 5.6. ARANGO H., ABREU J.P.G., DOMINIGUES E.G., PAULILLO G.: Using the Pricing Theory of Financial Derivatives to Predict Payments of Electric Energy Revenues (Brazil)..........................401 5.7. SZKUTNIK J.: The Quality of Distribution as a Factor Decreasing Costs of Energy (Poland).........407 5.8. GRINKRUG M., TKACHEVA Y.: Improvement in Power Voltage Quality and Reduction in Power Losses at City Low Voltage Transformer Substations (Russia)................................................411

Section 5. Power Quality in Competitive Electricity Markets. Economic Aspects ...

361

362





POWER QUALITY IN ELECTRICITY MARKETS

Wadysaw MIELCZARSKI Technical University of Lodz Lodz (Poland)

Magdalena WASILUK-HASSA Polish Power Grid Company Warsaw (Poland)

Abstract. The presented paper analyses the key international power quality standards and addresses some elements in current legal documents. The paper also provides brief explanation of such elements, and also presents the leading PQ standard descriptions.

1. INTRODUCTION Growing competition within the power industry, coupled with the increasing use of sensitive electronics in both domestic and industrial equipment, has drawn attention to the issue of power quality.

Marek J. SAMOTYJ EPRI Palo Alto (California, USA)

for the supply quality undetermined. More experience comes from the countries where electricity markets have been introduced some time ago. A good example is the Australian state Victoria, where the Electricity Distribution Code [2] as the first ever written document splits responsibility for quality supply between a distributor and a customer. Another example can be found in South Africa where Eskom, the South African power utility - fifth biggest in the world, conducted very broad research and analysis on power quality. During last six years very broad, Eskom implemented program called: “Quality of Supply”. This program was given the highest priority. It combines the quality of grid operation with salaries for employees of executive levels. The program development was stimulated by customers’ needs for higher quality and by changes in South African industry.

The Polish electric power sector alike in other countries of Eastern Europe is facing problems: how to ensure power delivery to customers with guaranteed power quality parameters. To address There is no doubt that the Polish electricity sector such a problem, we will need to prepare not only is already driven by the market rules, where quality coherent legal regulations but also to make and reliability of supply are of the greatest organizational changes, to ensure adequate importance. Consumers, who have TPA can select education of technical staff and activate some their suppliers so it is obvious the they want to get mechanisms which are conductive to power better prices and services, especially those related quality. While starting the changes, it is worth to to quality. It also well known that if a customer follow experiences of electric utilities in other requires the power delivery with appropriate countries, which already achieved significant level (described in a contract) parameters of quality, it is in power delivery quality and introduced the necessary to install measurement equipment, which electricity markets. Certainly the United States, will allow the monitoring such parameters. That all where programs related to power quality are means, that means expenditures to cover the cost broadly spread, can be numbered among these of installation and services. The main parameters countries. However, in the US the vertically of the supply quality for the Polish power system integrated monopolies create the regulations, are prescribed by the Bill (the secondary law which are not clear for customers. Moreover, the ordinance) issued by the Ministry of Economy. US regulation such as IEEE Standard determine the technical conditions leaving the responsibility In September 1996 IEC (TC77) decided to create a new 2. WHAT IS POWER QUALITY? working group aiming at the development of technical reports and standards relating to power quality for low Section 5. Power Quality in Competitive Electricity Markets. Economic Aspects ...

363

and medium voltage supply networks and points of common coupling. The group has approved the following definition of power quality: “Power Quality – set of parameters defining the properties of the power supply as delivered to the user under normal operating conditions in terms of continuity of supply and characteristics of voltage (symmetry, frequency, magnitude, waveform).” However, it is not the only one definition which is in use. In Poland and in other countries, the uniform definition of power quality is still under discussion. Unfortunately, there is also a lack of uniformed, common definitions of particular parameters, which describe power quality. Generally, power quality has been characterized by voltage (in this: oscillations, dips and flicker), frequency, distortion of voltage and current curves, asymmetry of supply. 3. LEGISLATION ON POWER QUALITY It is obvious that power quality becomes one of the factors, which allows utilities to be competitive on the electricity market. Naturally, it can only be achieved if power quality parameters can be comparable, so we need standards, and other legislation. In Poland, the Bill concerning specific conditions required to allow the connection of the qualified party to energy network, recovery of the connection costs, revenue requirements, provision of network services, network’s operation and maintenance, and quality standards in services provided to all customers is an obligatory document which describes some power quality obligations. The Bill introduced in 1998 was amended in 2000. The new version better squares with possibilities of supply served by distribution companies and its now more unified with the standard EN 50160. It states that parameters described in the Bill need to be kept, unless another commitments are set up in contract between supplier and customer. All other documents (standards, recommendations) are not obligatory. Even the standard EN 50160, which was translated into Polish and signed up, is not mandatory.

364

How does it look in other countries? In the United States for example, the state of New York adopted electric service standards for reliability and power quality in 1991. The reliability standards address sustained interruptions lasting five minutes or more. There are specific reliability targets each utility must meet in order to provide what is considered acceptable service reliability. All other disturbances fall under the category of power quality. The power quality standards adopted in New York are programmatic or “soft”. No numerical performance indices were established. Each utility is required to consider power quality in the design of its distribution power-delivery systems components and to file a power quality program with the New York Commission that included its procedures, specifications, and goals for performance. The programs are designed to respond promptly to customer reports of power quality problems and to avoid, mitigate, or resolve power quality problems, they are required to have the capability to do so. The utilities are also required to file an annual power quality report. Is it not paradise...? The standards’ requirements regarding power quality programs recognize the inherent limitations of the subject area. The limitations include lack of fully accepted definitions, lack of data base, the costs and limits of technical devices presently available for diagnostic and analytic work, as well as solutions to power quality problems. Sounds similar to Polish, and probably other countries’, reality. With increased political favor in Europe for the idea that electricity is a “product” rather than a “service”, there is a corresponding increase in IEC standards activity that may help define the specific characteristics of this product. In response, many power producers view voltage as the critical indicator of product quality. To cover abnormal conditions of voltage or “out-of-spec” conditions of electricity, several new international standards are emerging. In 1993, CENELEC published “Voltage Characteristics of Electricity Supplied by the Public Distribution Systems,” BTTF 68-6. This standard defines abnormal voltage conditions and states requirements for the percentage of time that the voltage will be within normal ranges. In 1995, the International Union of Producers and Distributors of Electrical Energy (UNIPEDE) published its “Measurement Guide for Voltage Characteristics”, which provides an approach to measurement and compliance verification for voltage. Even the Union Internationale D’Electrothermie (UIE) is preparing a guide on power quality that covers voltage dips, short interruptions, harmonics, and unbalances. In 1995, IEEE published Standard 1159, “Recommended Practice for Electrical Power Quality and Utilisation

Monitoring Power Quality”. All of these standards describe the various kinds of abnormal voltage conditions, including fluctuations, dips, interruptions, transient overvoltages, unbalances, and voltage harmonics. The standards are also coming from the power producer’s viewpoint and, therefore, tend to define power quality in terms of voltage quality. Currently in progress are several new standards that address power quality more in terms of compatibility than voltage quality. They tend to define compatibility levels between end-use equipment and the electric power supply. In the IEC, these standards are found in the series 1000-3-X “Basic compatibility Limits for Immunity and Emissions”. Developed under Technical Committee 77, these are standards to achieve EMC. 4. POWER QUALITY - WHIM OR NECESSITY? There are three factors influencing an increased need for solving and preventing power quality problems. They are the increasing use of power quality sensitive equipment, increasing use of equipment that generates power quality problems, and the deregulation of the power industry. All these factors influence the utilities and their customers’ competitiveness. Also the electronics revolution has opened a whole new chapter in standards development activities by dramatically changing the nature and requirements for powering electrical equipment. While treating electricity as a “product”, it is necessary to look into its quality, what is naturally by any other product. It can also be stated that the quality is even of greater importance in electricity, because more and more equipment, sensitive to disturbances in supply, has been produced. It is valid not only for industry factories (technological lines or particular devices) but also for commercial customers. Simultaneously, along with technical progress (mostly electronics), more devices, which cause disturbances and worsen quality of power, are produced. Paradoxically, customers, who require to be supplied with high power quality, can make it lower.

To have “tools” to affirm the possible go back on the quality of supply and to estimate loses caused by that it is first necessary to establish standard values for particular parameters. A lot of organization and trade associations all around the world are working on establishing power quality standards. Power quality becomes to be of UNIPEDE and UIE interest. The significance of power quality is growing worldwide and it is caused by necessity of setting contracts for power delivery and one of the contracts’ elements are power quality parameters values. Unfortunately, at the moment one cannot say that we have complete set of power quality standards. It was also stated during the Conference: “Power Quality in power girds in Poland” organized by Polish Power Transmission and Distribution Association and held in Pozna on November 9th –10th 2000. The best example of estimation of financial loses caused by not sufficient power quality parameters is the United States, where – it is common knowledge – all can be “converted” into money and everything has its value. It has been estimated that low power quality costs customers in the US billions of dollars annually. Customers are equipped in devices (for example: speed drives, computers, robots, and others) which are very sensitive for power quality. Additionally customers, who are acting in conditions of competition count their loses not only as break in production, but also as loss of “chance”. Because of above-mentioned and driven by growing competition, power utilities offer to their clients new contracts and services. They also cooperate with customers to find an “economic answer” to financial loses, which arise as consequence of low power quality. And there is no wonder, because deregulation opened new market worth 210 billions of dollars. Power utilities are fighting to get the biggest share of the market and what follows – the biggest profit. They started to offer new services, also on power quality, to keep “old” and to win new customers. Even though power delivery interruptions are rather rare in European Union’s countries, in the US and Japan, there are a lot of examples, where offered power quality parameters can be not sufficient to supply clients’ sensitive processes. That problem became to be even more important, since in 1997 the European Union, in aim to counteract disturbance effects which occur in distribution grids, introduced its new regulations concerning electromagnetic compatibility (EMC). Influential organizations, such as for example: ABB High Voltage Technologies and Siemens Power Transmission & Distribution (EV), response to mentioned above regulations and to deregulations, and in the aftermath its consequence liberalization of power markets in Europe was offering spectrum of measurements to ensure


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uninterruptible power delivery to the customers, who require or need it. It is also essential that also customers, not only power utilities, are more and more familiar with power quality issues and they are demanding still higher quality of power. Growing concern with power quality associate broadening of devices, which operation depends on control systems based on microprocessors and on electronic equipment and the latter are sensitive for power quality [1]. Another factor which influences growing interest in power quality is continuous looking for higher global productivity of industrial processes. It led to installation of high-efficient equipment, such as speed drives with regulated speed of revolutions and devices phase factor correctors. After-effect of that was increase of harmonics injected to the regional power system and growing worry with its impact on entire power system. 5.

ADDRESSING QUALITY OF SUPPLY IN THE ELECTRICITY DISTRIBU-TION CODE IN VICTORIA.

5.1. Supply frequency The Electricity Distribution Code states that National Electricity Market Management Company (NEMMCO) is responsible for the frequency of each distributor's distribution system, having an obligation under the National Electricity Code to use reasonable endeavours to maintain system frequency at 50 Hz, subject to the allowable variations set out in that Code so a distributor has no obligation in respect of the frequency of its distribution system. 5.2.

Voltage deviations

It has been decided that subject to the variation determined in Table 1, a distributor must maintain a nominal voltage level at the point of supply to the customer's electrical installation in accordance with the Electricity Safety (Network Assets) Regulations 1997 or, if these regulations do not apply to the distributor, at one of the following standard nominal voltages: (a) 240 V; (b) 415 V; (c) 480 V; (d) 6.6 kV; (e) 11 kV; (f) 22 kV; or (g) 66 kV. The variations from the relevant standard nominal voltage 366

listed in clause 3.2.1 may occur in accordance with Table 1. A distributor must use its best endeavours to minimise the frequency of voltage variations as allowed (Table 1) for periods of less than 1 minute. TABLE 1 The allowed voltage variations

5.3. Signals sent on wires The Electricity Distribution Code allows to use distribution lines to sent ripple control and other communication signal stating that a distributor may send, in accordance with IEC 1000-2-2, signals for the following: (a) ripple control systems; or (b) mediumfrequency power-line carrier systems; or (c) radiofrequency power-line carrier systems. 5.4. Harmonics Harmonic level in distribution networks are function of mutual interaction of customers’ equipment and supply network. Some harmonics generated by equipment with nonlinear characteristics are boosted by network resonance so a critical point becomes who is responsible for harmonic distortion. The Electricity Distribution Code splits responsibility between a distributor and a customers imposing the obligation on voltage harmonics on a distributor while a customer is responsible for current distortion. Despite the solution is not perfect it keeps the responsibility balance between a distributor and a customer. When a dispute arises, the Office of the Regulator-General can employ experts to judge the case. The Electricity Distribution Code states that a distributor must ensure that the harmonic levels in the voltage at point of common coupling nearest to a customer's point of supply comply with the levels specified in Table 2. The current harmonics allowed are determined using IEEE Standard. TABLE 2. The allowed voltage harmonics.


The Electricity Distribution Code imposes the obligation on a customer who must keep harmonic currents below the limits specified in Table 3 and otherwise comply at its nearest point of common coupling with the IEEE Standard 519-1992 'Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems'.

period s of less than 2 minutes: (a) up to10% for a standard nominal voltage up to 1 kV; and (b) up to 4% for a standard nominal voltage above 1 kV. 6.

CONCLUSION

The main goals of electricity markets is to lower electricity prices and increase the standard of services. Supply quality is a part of such defined services.

TABLE 3. The allowed current harmonics. The introduction of electricity markets is a new challenge for engineers and legislators. The current technical regulations have to be determined as a set of laws to be obeyed by both sides: distributors and customers. The responsibility for quality of supply must be clearly defined as it imposes the cost of keeping such standards. 7. 5.5. Negative sequence voltage and load balance A level of negative sequence components in distribution networks depends on balance of customers load and symmetry of supply lines. A main parameters used of unbalance analysis is a level of a negative sequence. Such a parameter is difficult to understand for most customers and in particular they are not familiar with measures that should be taken to keep such a parameter under the prescribed values. The Electricity Distribution Code splits responsibility imposing the obligation on a distributor to keep negative sequence voltages on the allowed level while a customer should keep balance in his load. The Electricity Distribution Code states that a distributor must maintain the negative sequence voltage at the point of common coupling to a customer's three phase electrical installation at a level at or less than 1 %. However, the negative sequence voltage may vary above 1% of an applicable voltage level, but not beyond 2% for a total of 5 minutes in every 30 minute period.

REFERENCES

1. Mielczarski W.: Quality of Electricity Supply and Management of Network Losses. Energy Optimal Solutions, Melbourne, 1997 2. Office of the Regulator-General, Victoria: “Electricity Distribution Code”, Melbourne, April, 1999

Wadysaw Mielczarski received MSc, PhD, and DSc degrees in power system engineering from Technical University of ód¦ in Poland in 1973, 1978 and 1987, respectively. He has over 25 years of experience as an engineer, a consultant and an academic. W. Mielczarski has published 6 technical books (including a book on “Competitive Electricity Markets” published by Nova Science Publishers, New York, 1998) and over 150 journal and conference papers. While working at Monash University in Melbourne he assisted the Australian Government agencies (the Office of the Regulator-General and the Electricity Industry Ombudsman) and the market participants as an Adviser since 1992. He is involved in several international projects for Ontario Hydro Technologies, Brazilian Regulatory, and the Polish electricity supply industry. Currently he is a Professor of Technical University of ód¦, being employed by the Institute of Electrical Power Engineering.

The Electricity Distribution Code prescribes that a customer must ensure that the current in each phase of a three phase electrical installation does not deviate from the average of the three phase currents: (a) by more than 5% for a standard nominal voltage up to 1 kV; and (b) by more than 2% for a standard nominal voltage above 1 kV. However the deviations are permissible for Section 5. Power Quality in Competitive Electricity Markets. Economic Aspects ...

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Mailing address. Technical University of ód¦ Institute of Electric Power Engineering 90-924 ód¦ ul. Stefanowskiego 18/22 Tel: (0-42) 631 25 96 Fax: (0-42) 631 26 06 Email: [email protected] a.pl Magdalena Wasiluk-Hassa was born in 1965 in Warsaw, Poland. She received MS degree (1988) and Ph.D. degree (1996) from Technical University in Warsaw, Poland. Since 1997, Dr. Wasiluk-Hassa has been working for Polish Power Grid Company. She is a Task Area Manager in Corporate Strategy Office. Her main scope of activities includes: development and implementation of new technologies in energy sector, cooperation with EPRI and power quality. Previously, she dealt with technical and legal standards’ coordination between Polish utility industry and European Community, participation in working group (under auspices the Ministry of the Economies) for preparation of the Polish electric power sector to the negotiation process of Poland’s integration with the European Union and power quality issues. Ms. Wasiluk-Hassa is a member of Institute of Electrical and Electronics Engineers, Inc. (IEEE), CIGRE and Polish Electrical Engineers’ Association (SEP). Mailing address: Magdalena Wasiluk-Hassa Polish Power Grid Company Mysia 2, St., 00-496 Warsaw

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Poland phone: (+48, 22) 693 22 62, fax: (+48, 22) 693 12 68 e-mail: [email protected] Marek J. Samotyj received his B.S. (1969) and M.S. (1971) degrees in Electrical Engineering from Silesian Polytechnical University in Poland. He received an M.A. (1976) in Communication from the Jagiellonian University in Cracow, Poland, and an M.S. in Engineering-Economic Systems from Stanford University in 1985. From 1981 to 1982, he was a Fulbright Senior Scholar, and a Fellow of the Professional Journalism Program at Stanford University. He is the program manager for the newly created Consortium for Electric Infrastructure to Support a Digital Society – CEIDS. He joined the Institute in 1985. As a project manager and then a team leader and Business Area manager, he was responsible for the applications and field testing of adjustable speed drives and conducted research in the power quality area. Before coming to EPRI, Mr. Samotyj was a Research Assistant to the Energy Modeling Forum at Stanford University (19821985). He is a member of the IEEE and a member of CIGRE, (International Conference on Large High Voltage Electric Systems). In the past, he served as Secretary and ViceChairman of the IEEE Power Quality Standards Coordinating Committee 22. He also actively participates in CIGRE Working Groups on adjustable speed drives and on power quality. He is the editor of the EPRI Power Quality newsletter, Signature. Mailing address: Marek J. Samotyj EPRI 3412 Hillview Ave., Palo Alto CA 94304-1395 USA phone: (+1, 650) 855 29 80, fax: (+1, 650) 855 85 75 e-mail: [email protected]




POWER QUALITY MANAGEMENT AS A GOAL OF ONS (OPERADOR NACIONAL DO SISTEMA ELÉTRICO) THE BRAZILIAN TRANSMISSION ISO Roberto J. R. GOMES Dalton O. C. BRASIL José R. MEDEIROS Operador Nacional do Sistema Elétrico Rio de Janeiro (Brasil)

Abstract - The Brazilian energy sector’s recently adopted deverticalized model imposes new rules that must be established in such a way as to satisfy the consumer market. In this context, the quality of the electric energy itself has become one of the most important issues to be attended. This article begins by presenting a simplified overview of the new scenario and includes the diverse agents and their main working relationships. Considering the ample institutional experience in Brazil in this area, ONS has been working diligently to bring into the new model all the previous concern with respect to the power quality. To this end, ONS has created an adaptation process that is as transparent as possible. With the support of ANEEL, ONS has been stimulating a broad spectrum debate including all possible questions of interest to the many participating agents that require access to the Basic Grid and government organs, universities and research centers. This article presents the main aspects of the rules and procedures being established in the system and offers some reflections and questions regarding the guarantee of quality of the energy provided in the Brazilian energy sector’s new environment. 1. INTRODUCTION A model that has been functionally deverticalized characterizes the new structure of the Brazilian electric sector. This implies a segregation of the activities involving generation, transport, which includes transmission and distribution, and commercialization. The model features three very important institutions, namely: ANEEL, ASMAE and ONS.

ANEEL (Agência Nacional de Energia Elétrica), the national electric energy agency. This is a government organ playing the role of the sector regulator in the Brazilian system whose mission it is, to proportion the most favorable conditions so that the energy market will be assured continuous growth and operation while providing a certain balance between the interests of the participating agents and the benefits to society as a whole. ASMAE (Administradora de Serviços do Mercado Atacadista de Energia Elétrica), a private enterprise institution created by federal law whose main purpose is to coordinate the activities implicit in the commercialization of energy. Such commercialization is done through the wholesale market agent called MAE or through bilateral agreements for the purchase and sale of power among the generation agents, distributors, free consumers and retail agents. ONS (Operador Nacional do Sistema Elétrico), the exclusive independent system operator in Brazil is a privately owned entity created by federal law in 1998. The ISO is responsible for the following prime activities: x x

x

x

the planning and programming of operation which includes the centralized generation dispatch; the supervision and control of the interconnected electric energy systems in the country as well as the international interconnections with Brazil's neighbors; the contracting and administration of the transmission services which include the respective procedures for access to the national Basic Grid and ancillary services; the elaboration and presentation of proposals to ANEEL regarding the future expansion of


369

x

the installations as part of the Basic Transmission Grid and for reinforcements to the present system (All proposals presented in either case would have to be authorized and/or tendered by ANEEL); the defining of rules and procedures for conducting the operation of the Basic Transmission Grid. These rules in turn must also be approved by ANEEL.

In Brazil, the basic network is composed of a series of high voltage installations equal or superior to 230kV. In Figure 1 the dimensions of the area covered by the basic network are shown.

In the rules mentioned above, those pertaining to the preservation of the power quality are highlighted. ANEEL is responsible for the regulation and fiscalization of the quality indicators established. In the new model, the agents interact through a body of legal instruments created for this purpose. To name a few: x x x

commercial agreements and grid procedure guidelines administered by ONS and the Basic Transmission Grid; resolutions , decisions and norms established by ANEEL; other agreements and procedures guidelines regarding the distribution grid.

The basic objective of these instruments is to clearly establish the individual responsibilities of the participating agents to assure that the needs of the market are being cared for in an adequate and satisfactory way. It is worth remembering that in former times, the concept of quality was regulamented within the scope of a verticalized federal government controlled model. Internationally, there are some studies that use the focus of the deverticalized system principally where the de-regulation has been in effect for some time, as in [1]. This article will treat the theme of power quality in the Brazilian Transmission System Basic Grid under the following aspects: x agreements that permit the treatment of power quality; x responsible actions by the playing agents, including ONS, with a view to guaranteeing the maintenance of power quality all through the process; x performance indicators to be considered: definitions, basic premises for their establishment, methods to be used to identify the party responsible for quality, ONS or otherwise, etc.

Fig. 1 TABLE 1 below, shows some of the basic facts related to the Basic Grid in Brazil as of 1999. TABLE 1 Transmission Lines Extension (km) 230 31,052 345 8,927 440 6,036 500 15,716 600(*) 1,612 750 2,114 (*) – HVDC System Voltage Level (kV)

Installed Capacity Substations (MVA) 35,146 34,480 15,437 49,538 --16,750

3. THE AGREEMENTS In Figure 2 a typical but simplified form is shown. This represents the main agents that are part of the physical structure in the existing system model. The contracts are necessary to establish the rights and obligations of all parties involved in the day to day business of the transmission sector. Below the figure in question is a legend to help understand the contracted commercial activities and the resulting cash flow that may be in play among the participating agents.

2. THE BASIC TRANSMISSION GRID

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conduct of all agents in the Basic Grid insofar as power quality is concerned. CCT

G

CCT

T

CCD

D

CL CUSD

CUST

CUST

CPST

ONS

Other contracts used in the system but outside the jurisdiction of ONS complete the menu of documents that set the rules for the relations among agents and these, with ANEEL. To cite a few; x x

Fig. 2 ¾ AGENTS: x ONS - Operador Nacional do Sistema, the Brazilian ISO; x T - Transmission Agents; x G - Generator Agents; x D - Distribution Agents; x CL - Free Consumers. ¾ CONTRACT TYPES: x CCT- Contrato de Conexão da Transmissão (TCA–Transmission Connection Agreement); x CCD - Contrato de Conexão da Distribuição (DCA-Distribution Connection Agreement); x CPST - Contrato de Prestação de Serviços da Transmissão (TSA-Transmission Service Agreement); x CUST – Contrato de Uso da Transmissão (TUOSA-Transmission Use of System Agreement); x CUSD - Contrato de Uso da Distribuição (DUOSA-Distribution Use of System Agreement). Besides regulating all aspects of price and supply conditions, the contracts also establish the rules for defending power quality, principally those rules associated with the responsibilities assumed by each of the players in the new model. These rules, implicit in the contracts, "CUST" (Contract for the Use of the Transmission System), and the "CPST" (Contract for the Rendering of Transmission Services), are in accordance with the Grid Procedures Manual elaborated by ONS where the power quality indicators and standards of the Basic Grid are detailed. In the near future there will also be contracts to establish the rules for the commercializing of ancillary services. In this way, the contracts and the Grid Procedures Manual together with the resolutions and other releases from ANEEL, form the body of legal instruments which establish the standards of performance and

x

contracts for market concessions firmed between ANEEL and market agents; bilateral buy/sell contracts signed by generators and distribution agents or free consumers; buy/sell contracts closed by agents through the wholesale marketplace - MAE.

4. GRID PROCEDURES These procedures are composed of a set documents being elaborated by ONS together with the participating agents in the power system whose final version must be approved by ANEEL. They establish the procedures and technical requirements for the implementation, use and operation of the transmission systems and define the responsibilities of ONS and all other users of the system. The Grid Procedures, still under elaboration at this time, adopt as a reference source in regard to power quality, all the documentation existent on this subject as was written for the old verticalized model [2], [3]. For this reason, the present concept of power quality is still being defined in view of the new business scenario and different market dynamics which demand finer detailing as to the placement of responsibilities. 4.1. Determining Responsibility The need to attribute responsibilities in the new model is a prime requirement in the determining of an indicator. It is necessary to have the means to identify any violation of standards, which affect performance or quality, and in second instance, to adopt sanctions and other corrective measures to eliminate such occurrences. After having registered such a violation, an adequation control process must be initiated starting with the identification of the agent or agents responsible by to the event, in question. Based on this premise, two concepts establish themselves, these being "indicator/global performance standard" and "indicator/individual performance standard". These two concepts are presented below in a synthetic manner:


371

x

Indicator/Global Performance Standard - A way to verify the global performance of the Basic Grid from a power quality viewpoint.

Such standards should be understood to be levels of compatibility among diverse indicators that could compromise the performance of the power system as a whole. They should permit the users the ability to define the requirements of their own systems with adequate levels of immunity. The violation of such standards, after investigation by ONS, could bring ONS to adopt measures such as; proposals for expansions or reinforcements in the Basic Grid, adoption of operational measures, etc. x

Indicator/Individual performance Standard - A method for verifying the individual performance of an agent in the system so as to minimize the possibility of said agent to provoke a disturbance that would deteriorate in a major or minor way, the global performance of the system.

The two modules in the Grid Procedures Manual [4], which treat these aspects and may be seen under the titles: "Performance Standards and Minimum Requirements for Installations in the Basic Grid " which deals with global indicators and standards and " Access to the Transmission System " which deals with individual indicators and standards. 4.2. The Monitoring of Performance Indicators ONS together with the other agents have evaluated the best strategy for accompanying the Basic Grid performance indicators and have chosen the following alternatives : x x x

the continuous monitoring of the indicator; the evaluation of performance by means of periodic measurement campaigns, and the monitoring or campaigning as a result of a complaint or accusation by or against an agent in the system.

Another aspect equally important to this work has been the defining of the protocols and minimum requirements to be used as parameters regardless of the measurement strategies to be finally adopted. It is evident that the results of the measurements made by the diverse agents should be accepted by all involved parties and sanctioned by ANEEL. 4.3. Proposed Indicators The indicators that are being adopted to best reflect the global performance of the Basic Grid are: continuity (sustained interruption of service), steady state voltage variation, frequency variation,

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voltage fluctuation, voltage imbalance, harmonic distortion and short-duration voltage variation. The indicators being used to demonstrate the individual performance of the accessing agents in the Basic Grid are : voltage fluctuation, voltage imbalance, harmonic distortion and short-duration voltage variation. In reference to the Basic Grid installations, the Grid Procedures Manual establishes indicators to evaluate, among other aspects, the performance level in the face of harmonic distortions, equipment failure rates and average repair times . Figure 3 illustrates the scope of coverage of each indicator: B a sic G rid – G lobal Indicators TL PS

P ower S ubstation

T ransm ission L ine

T ran sm issio n S ystem In stallatio n s P erform ance m inim um R equirem ents

Indiv idual Indicators O th er Ag en ts In stallatio n

Fig. 3

5. ANALYSIS OF THE INDICATORS This section presents, in a general way, some comments and reflections regarding the indicators which have been the object of great interest by the agents during the elaboration of the Grid Procedures Manual. Specifically these topics include ; continuity in the Basic Grid, voltage fluctuation, harmonic distortion and short duration voltage variation. 5.1. The Monitoring of Performance Indicators Continuity in the Basic Grid In the old verticalized model of the Brazilian energy sector, the responsibility for the continuity of energy supply was attributed to the supply entity whose function it was, to generate and transmit electric energy. In the new deverticalized model, the responsibilities have been desegregated . The responsibility for the power generation adequacy now lies with the generator agent and ONS while the safety of the transmission lies with the transmission agent and ONS.


In the case of Brazil, and in view of the complexity created by the segregation of these responsibilities among the agents, the assurance of continuity of energy delivery to the final consumer is being considered by the examination of the following: x Sanctions can be adopted to penalize distribution agents in the case of any violation of continuity standards. They are the unique responsible for the assurance of continuity of energy delivery to the final consumer. x Income derived from the transmission agents, from penalties guaranteed by contract, due to the unavailability of their respective installations. These funds will be shared among the affected agents in the Basic Grid; x Analysis of the continuity indicators at the installations of the Basic Grid that interface with the generators, distributors, free consumers and commercial agents. These areas are denominated control points. The analysis of the continuity indicators at the control points, will subsidize the information necessary to the planning activities of the distributor companies as well as help identify the eventual breaks in performance levels of the grid. This is possible when comparisons are made between the new information gathered at the control points and the historical data registered from the same sources. To evaluate the continuity, the following indicators will be used: Duration of Disconnection at the Control Point, Frequency of Disconnection at the Control Point, Duration (Maximum), of Disconnection at the Control Point. 5.2. Voltage Fluctuation The phenomena of voltage fluctuation includes among it´s effects an occurrence called scintillation or flicker. Such an effect corresponds to the human perception of the changes in luminosity. This leads to a differentiated treatment from the point of view of the definition of the indicators as well as the standards to be adopted. In spite of the fact that voltage fluctuation is a phenomenon with a broad spectrum, the scintillation effect was considered as a determinant factor based on opinions from the diverse agents consulted during the process of the Grid Procedures Manual elaboration. The application of the Pst and Plt standards applied directly to the busses in the Basic Grid, and whose values are equal or larger than 230 kV, has been questioned, given the fact that this effect was measured using incandescent lights with 220V.

For this reason, the Grid Procedures have adopted the scintillation standard of 220V. This then raises the question of how to define a standard of voltage level in the grid that corresponds to the standard established as being 220V. Such a relation depends fundamentally on the damping factor between the phenomena of fluctuation at the busses in the Basic Grid and the points located in the distribution grid at the 220V level. Besides this, the standards adopted in low voltage must be adapted to evaluate the scintillation disturbances in distribution networks operating at 120V. At this time, measurements are being taken at the grid busses and at distribution points near the nonlinear loads that include arc oven operations. The purpose of this is to evaluate the possibility of adopting such damping factors. Additionally, the possibility of adopting maximum and minimum global limits, is being studied to assist in the evaluation of global performance in the Basic Grid based on the following items : x x

x

an indicator value below the minimum limit indicates adequate performance; an indicator value within the acceptable limits indicates that the performance should be observed. In this case any action to adequate the performance will depend on complaints from the users in the Basic Grid; an indicator value above the limits of performance indicates that measures to adequate the performance will be taken, after identifying the responsibilities for such occurrences,.

The method to be used for identifying the contribution of each agent in the global performance and the way to compare these contributions with the individual standards has not yet been established. 5.3. Harmonic Distortion The global indicator for harmonics corresponds to the total harmonic voltage distortion (THVD), obtained from the square root of the quadratic sum of harmonic voltages from the second to the fiftieth order. Table 2 presents the standards adopted for this indicator at the busses in the Basic Grid. TABLE 2 Harmonic Distortion – Global Standards odd values even values order value (%) order value (%) 3,5,7 2 2,4,6 1 9,11,13 1,5 0,5 t8


373

15 a 25 t 27

1 0,5 THD = 3 %

The same indicator is utilized to determine the individual performance of each agent and free consumer. However, there is the possibility that an indicator associated with the harmonic current distortion could be used as an indicator by the accessing agent in the Basic Grid. At this point , no method for evaluation has yet been established independent of the indicator to be chosen. In spite of the contributions made by the large consumers in influencing the harmonic distortion index, it is known that the proliferation of smaller devices both domestic and commercial such as computers and no-breaks of small capacity as well as compact fluorescent lamps etc., can in some cases, be a determining factor in the global harmonic distortion index. It is therefore, essential that the manufacturers of electrically powered equipment be committed to minimizing the negative effects of their products on the power system which ultimately affect the power quality of the Basic Grid as a whole. 5.4. Short-Duration Voltage Variation In our concept, the short-duration voltage variations are events whose effective voltage values (rms), in any phase, is situated temporarily below 0.9 pu or above 1.10 pu, relative to the nominal voltage, during an interval of 1 cycle (16.68 ms), to 1 minute. An analysis of the voltage variations involves the evaluation of the performance of an expressive number of busses in the Basic Grid whose variations occur at nominal voltage. The strategy used by ONS in relation to this indicator consists of: x

x

x

a first stage searching for ways to diagnose the behavior of such indicators at the control points by the analysis of the results of measurement readings already taken, those being taken now and those to be taken in the future; to set up this diagnostic pattern on a national level and incorporate international standards. For this end, ONS will participate in the “International Project for Assessment of Transmission System Reliability and Voltage Sag Performance” being developed by Electrotek; based on the above actions, proceed to develop a national standard.

So far, the question of power quality guarantees in a deverticalized environment, as is the case in the new Brazilian model, is a question hardly touched from a national viewpoint. This issue simply has not yet been amply discussed. Given the technoeconomic repercussions of such a theme, it is possible to conclude that it is of the utmost importance for all agents in the National Grid to participate in this debate about constructing the rules that will ultimately determine the quality of electric energy in the coming years and the guidelines for contractual relations which will result from the interaction of theses same agents as the market grows. Other aspects of this question are still under discussion and it is clear the work will continue to seek formation of a quality ambient for the Brazilian power market to operate in. Some of the other items in this question yet to be resolved are: x differential treatment of loads between the loads traditionally present and those being implemented under the new model; x ways to evaluate performance of future loads when new requests for access to the transmission and distribution systems in the operational phase are made; x creation of a present performance standard which includes the possibility for evaluating the possible exceptions to the rule and the creation of a gradual process of evolution to bring the performance standard up to a new level once future objectives have been determined; x the possibility of evaluating a violation of individual performance by indirect means when direct measurements are not possible; x evaluating the possibility of regionalized standards, given the importance of a specific industrial plant in the social and economic development of an area and its possible impact on the Basic Grid in generating disturbances, etc. 7. ACKNOWLEDGMENTS The authors gratefully acknowledge the contributions of Luiz Roberto Bezerra, Dário de A. C. Gueiros, Álvaro J. P. Ramos and the other member of the Quality of Energy Working Group of ONS.

8. REFERENCE 1. M.Mc Granaghan, B.W. Kennedy, M.Samotyl : Power Quality Contracts in a Competitive Electric Utility Industry. 8th ICHQP – Atenas, 1998. 2. CGOI/SCEL and GCPS/CTST: Criterios e Procedidmentos para o Atendimento a

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Consumidores com Cargas Especiais. (novembro/97). 3. CGOI/SCEL and GCPS/CTST: Procedimentos de Medição para Aferição da Qualidade da Onda de Tensão Quanto ao Aspecto de Conformidade (distorção harmonica, flutuação e desequilibrio de tensão)., (novembro/97). 4. Grid Procedure of ONS – Module 2: Performance Standards and Minimum Requirements for Installations in the Basic Grid and Module 3: Access to the Transmission System. 5. Roberto J. R. Gomes, Dalton O. C. Brasil, José R. Medeiros, Roberto Bezerra, Dário de A. C. Gueiros, Álvaro J. P. Ramos: Reflexões sobre a Garantia da Qualidade de Energia no novo Ambiente Competitivo. Reunión Internacional Geración y Transmisión CIER – Medellín ,1999

Dalton O. C. Brasil was born in São Paulo, Brazil on May 27, 1949. He obtained his Electrical Engineering degree from the Escola Politécnica da Universidade de São Paulo, Brazil in1972 and his Msc degree from Universidade Federal de Pernambuco, Brazil in 1996. At this time he works for ONS as Transmission Administration Manager. Mailing address: Dalton O. C. Brasil Operador Nacional do Sistema Elétrico GAT - Gerência de Administração da Transmissão Rua da Quitanda, 196, 20o andar, 20091-000, Centro, Rio de Janeiro, RJ BRASIL phone:(+55)(21) 2039694, fax:(+55)(21) 2039420 e-mail: [email protected] José R. Medeiros was born in Rio de Janeiro, Brazil on March 19, 1953. He obtained his Electrical Engineering degree from the Instituto Militar de Engenharia do Rio de Janeiro, Brazil in 1976 and his MSc degree from COPPE/UFRJ, Brazil in 1991. At this moment he works for ONS as a consultant engineer in the Power Quality area.

9. BIOGRAPHIES Roberto J. R. Gomes was born in Recife, Brazil on August 8, 1948. He obtained his Electrical Engineering degree from the Universidade Federal de Pernambuco, Brazil in 1971 and a post graduation from the Universidade Federal de Paraíba in 1972. At this time Mr. Gomes is the Transmission Services Administration Director at ONS.

Mailing address: José R. Medeiros Operador Nacional do Sistema Elétrico GAT - Gerência de Administração da Transmissão Rua da Quitanda, 196, 19o andar, 20091-000, Centro, Rio de Janeiro, RJ BRASIL phone:(+55)(21) 2039656, fax:(+55)(21) 2039420 e-mail: [email protected]

Mailing address: Roberto J. R. Gomes Operador Nacional do Sistema Elétrico DAT - Diretoria de Administração dos Serviços de Transmissão Rua da Quitanda, 196, 23o andar, 20091-000, Centro, Rio de Janeiro, RJ BRASIL phone:(+55)(21) 2039699, fax:(+55)(21) 2039423 e-mail: [email protected]


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THE NEW QUALITY OF SUPPLY REGULATORY FRAMEWORK IN SPAIN. WILL IT BENEFIT THE NETWORK USER? Pablo EGUIA Esther TORRES School of Engineering of Bilbao University of the Basque Country Bilbao (Spain)

Elvira FERNANDEZ José Ramón SAENZ School of Engineering of Bilbao University of the Basque Country Bilbao (Spain)

Abstract –The quality of supply that distribution utilities serve to their customers is a group of technical and economical characteristics, inherent to the electricity supply and required by every customer, that can be classified into three different aspects depending on its content: continuity of supply, which is related with the number and duration of service interruptions to network customers; power quality, which is related with the different disturbances affecting the voltage waveform; and satisfaction to customer, which is related with the treatment given to the customer by the utility about service complaints, billing complaints, etc. In this paper, the new quality of supply regulatory framework recently approved in Spain, which takes into account the three aspects of quality mentioned above, is exposed as well as a reflection about if this new framework will benefit the customer or, on the contrary, the perception the user has about the quality of supply he receives will be as usual.

- Satisfaction to customer: it is related with the treatment given to the customer by the utility about service complaints, billing complaints, etc. The importance placed by the customer on this three aspects within the quality of supply has varied through time. At first, as the electricity networks were being developed, the most important aspect of the quality of supply was the continuity of supply. As the networks became more robust, the aspect related with power quality took more importance, mainly because of the introduction of load devices that are sensitive to waveform disturbances. Finally, with the deregulation and restructuring processes taking place nowadays, the aspect related with the satisfaction to customer is acquiring more importance due to the possibility of choosing the supplier, possibility that is recognized to every customer in the new regulatory frameworks. In this paper, the new quality of supply regulatory framework recently approved in Spain, which takes into account the three aspects of quality mentioned above, is exposed as well as a reflection about if this new framework will benefit the customer or, on the contrary, the perception the user has about the quality of supply he receives will be as usual, that is to say, nothing will change.

1. INTRODUCTION The quality of supply that distribution utilities serve to their customers is a group of technical and economical characteristics, inherent to electricity supply and required by every customer, that can be classified into three different aspects depending on its content: - Continuity of supply: it is related with the number and duration of service interruptions to network customers. - Power quality: it is related with the different disturbances affecting the voltage waveform.

2. THE USER IN THE OLD REGULATORY FRAMEWORK 2.1. Electricity industry regulation Previously to the approval of the new Electricity Law [3] and its Bylaws [4,5], the electricity industry in Spain was regulated under a norm


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known as the Marco Legal Estable (Regular Legal Framework). The main characteristics of the industry structure and operation were the following ones: - Mixed property. The sector was operated by public and private utilities. 4 utilities owned 95% of the market. - Horizontal concentration. Two big utilities owned around 80% of the generation and distribution businesses. - Vertical integration. The utilities were vertically integrated, they generated, distributed and sold the electricity. - Centralized planning. Generation and transmission planning was subjected to Parliament and Government approval. - Centrally dispatched. Generation was dispatched by an independent operator based on a minimisation of generation and transmission costs. As it can be observed from above, electricity industry was characterised by a huge concentration and a closed operation. In this framework, the customer was a passive subject, as he could only buy electricity at a regulated price and from the utility that supplied in his geographic area. 2.2. Quality of supply regulation In relation to the quality of supply, the regulatory framework was a traditional one, that is to say, the main aspect of quality of supply was supply interruptions but there were no regulated limits to the number and duration of interruptions as well as no regulated economic penalties assigned. This does not mean that when an interruption occurred the customer was defenceless in front of the distribution company but he could sue the company to achieve economic compensation for the damages caused. The problem was that the customer had to prove the relationship between the power interruption and the economic losses, i.e., he had to monitor his connection to the distribution network to prove that a fault had occurred. As the costs to monitor the connection were high, only big customers could afford it, so the majority of customers, residential and commercial, were defenceless. The quality of supply economic equilibrium, low investment and maintenance costs for the distribution company against good quality of supply for the customer, was clearly displaced in favour of the distribution company. As for the two other aspects within quality of supply, power quality was only regulated in relation to low voltage variations, which were fixed to r 7% of nominal voltage, and the

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satisfaction to customer aspect was very poor as the customer could not choose among different suppliers, the supply market was captive. In short, the quality of supply regulatory framework was badly established and the network user, the customer, was, in general, defenceless against bad quality performance. 3. THE USER IN THE NEW REGULATORY FRAMEWORK 3.1. Electricity industry reforms In January the 1st of 1998, a new regulation started to apply to the electricity industry as a new Electricity Law was passed. This new regulatory model represents a revolution in the organization and operation of the electricity industry and is in pace with the electricity sector reforms taken place all over the world. The main characteristics of the industry structure and operation are the following: - Privately owned. The sector is operated by the same 4 utilities but the public one was privatised before the new regulation started. - Horizontal concentration. This question is still unresolved and it will be up to the private companies to decide whether to de-concentrate or not. - Vertical de-integration. The utilities must separated its regulated activities (transmission and distribution) from its un-regulated ones (generation and supply). - Decentralised generation planning. There is freedom for any generation company to install new capacity of the type and location preferred. - Power pool. The electricity is traded by buyers and sellers in a power pool operated by an independent market operator. The dispatch is based on a price merit order, the cheaper offers are accepted till the demand is covered up. - Customer eligibility. Any customer is free to choose his electricity supplier based on an eligibility calendar. Nowadays, any customer connected to the distribution network at a voltage higher than 1 kV can choose his supplier. From the year 2003 every customer will be able to choose his supplier. - Free access to the network. Access to transmission and distribution networks is permitted to any user under a regulated Third Party Access scheme. As it can be observed, comparing the new regulatory framework with the old one outlined in 2.1., there has been an important change in the way the industry operates.


In the new framework the customer is no longer a captive consumer, as he has the power to choose its supplier. This fact implies that quality of supply take a more important role than in the old framework as the quality served to the customer becomes an element of discrimination among suppliers as important as the energy price offered. 3.2. Transmission, distribution and supply Bylaw Due to the facts stated in the previous section, a new quality of supply regulatory framework was needed as the old one was completely inadequate to the needs of the new electricity industry organizational and operational framework. This new quality of supply regulatory framework was introduced last year in the Transmission, Distribution and Supply Bylaw [2]. This framework regulates quality of supply in transmission and distribution networks but as only a few customers are connected to the transmission network only distribution system quality of supply regulatory framework will be outlined and discussed in the next section. 4. DISTRIBUTION SYSTEM QUALITY OF SUPPLY REGULATIONS 4.1. Quality classifications and indexes The distribution system quality of supply regulatory framework establishes the quality conditions that the service provided by the distribution company must comply with. The established conditions are a minimum and the customer can pact with the distribution company higher quality levels paying a plus over the regulated network tariff, which is calculated taking into account inversion and operation costs withstood by the distribution company to provide the service with the minimum regulated quality levels. The Bylaw gathers the three different aspects of quality of supply mentioned in the introduction, and defines a group of indexes for each one: - Continuity of supply: it is related with the number and duration of service interruptions to network customers. An interruption occurs when the voltage in the supply point falls below 1 % of its nominal value. The interruptions are classified into short interruption, lasts less than 3 minutes, and long interruptions, more than 3 minutes. Continuity of supply is measured using four indexes TIEPI, measures the mean duration of an interruption affecting several customers;

NIEPI, measures the number of times an interruption to a customer has occurred; TIEPI80, measures the deviation in the duration of an interruption to several customers from the mean value measured by the TIEPI index. - Power quality: it is related with the different disturbances affecting the voltage waveform. The Bylaw refers to the European Norm EN 50.160 for types of disturbances and limits. - Satisfaction to customer: it is related with the treatment given to the customer by the utility about service complaints, billing complaints, measurement issues, and other issues derived from the contract subscribed between the customer and the distribution company. Satisfaction to customer is measured using the following indexes: maximum time for elaborating a proposal for a new connection (estimate and point of connection); maximum time to carry out works in the network necessary for the new connection; maximum time for connection and inspection of energy meter; maximum time to attend a billing or measurement claim; maximum time for reconnection after disconnection for nonpayment. As the quality of supply in a distribution network depends on the geographical location and on the kind of customers connected, as the structure of the network is different and so, the quality supplied, the new framework takes into account this fact and classifies the quality of supply into: - Individual quality: it is of contractual nature and refers to each customer. - Zonal quality: it is related with the geographical zone covered by the distribution company. It is classified into four different zones: - Urban zone: areas with more than 20.000 supplies. - Suburban zone: areas between 2.000 and 20.000 supplies. - Rural concentrated zone: areas between 200 and 2.000 supplies. - Rural dispersed zone: areas bellow 200 supplies. The new regulatory framework establishes for each of the indexes stated above a limit that can not be surpassed. There is a different limit for individual quality and for zonal quality. If one or more limits are exceeded the distribution company will have an economic penalty. It is in this aspect where the new quality framework excels the old one. The distribution company becomes accountable for the quality performance of its network, so when the performance is poor,


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bellow regulated levels, the company is automatically penalised economically. If the limits exceeded are in individual quality, the company will have to refund the customer part of the money billed, and if the limits exceeded are in zonal quality, the company will be obliged by the administration to make investments in the zone until the quality performance returns to regulated levels. The regulated limits established as well as the economic penalties attached to them are shown in the next two sections. 4.2. Individual quality regulated limits and economic penalties The limits established for the continuity of supply are different depending on the nominal voltage of the connection to the network. If the customer is connected to the network at a high voltage level (Un t 36 kV) the limits established are shown in table 1. TABLE 1. Continuity of supply regulated limits. High voltage connection Zone All

Interruption duration (Hours/year) 4

Nº of interruptions (per year) 8

If the customer is connected to the network at a medium voltage level (1 kV < Un < 36 kV) the limits established are shown in table 2. TABLE 2. Continuity of supply regulated limits. Medium voltage connection Zone Urban Suburban Rural Conc. Rural Disp.

Interruption duration (Hours/year) 4 8 12 16

Nº of interruptions (per year) 8 12 15 20

If the customer is connected to the network at a low voltage level (Un < 1 kV) the limits established are shown in table 3. TABLE 3. Continuity of supply regulated limits. Low voltage connection Zone Urban Suburban Rural Conc. Rural Disp.

Interruption duration (Hours/year) 6 10 15 20

Nº of interruptions (per year) 12 15 18 24

If the distribution company exceeds the interruption duration limit it will have to refund the customer in the next billing a quantity equivalent to the cost of the energy consume of the mean annual power billed times the difference between 380

the number of interrupted hours and the regulated limit, valued at 5 times the energy tariff charged to the customer. The economic penalty obtained can not exceed the 10% of the annual billing of the customer. This limit is established to protect the distribution company from excessive economic losses. If the distribution company exceeds the number of interruptions limit it will have to refund the customer in the next billing a quantity equivalent to the cost of the energy consume of the mean annual power billed times the interrupted hours valued at the energy tariff charged to the customer, times the difference between the number interruptions and the regulated limit divided by eight. The economic penalty obtained can not exceed the 10% of the annual billing of the customer. If the two limits are exceeded the penalty most favourable to the customer will be taken. For the second aspect of the quality of supply, i.e., power quality, the are no regulated limits and no sanctions as this aspect is still unregulated and will be the subject of a future bylaw (there is no expected date, by now, for the approval of this bylaw). For the last aspect of quality of supply, satisfaction to customer, the limits regulated are the following: - The maximum time for elaborating a proposal for a new connection varies between 5 days for low voltage supplies and 60 days for high voltage supplies. - The maximum time to carry out works in the network necessary for the new connection varies from 5 days for low voltage supplies and 80 days for high voltage supplies. - The maximum time for connection and inspection of energy meter is established at 5 days. - The maximum time to attend a billing or measurement claim is established at 5 days for supplies under 15 kW of contracted power and 15 days for the rest. - The maximum time for reconnection after disconnection for non-payment is established at 1 day. If this limits are exceeded the distribution company will have to refund the customer 5.000 pts (30 €) per limit exceeded with a maximum of 20.000 pts (120 €). 4.3. Zonal quality regulated limits and economic penalties For zonal quality the only limits established are related with the continuity of supply, and depend


on the classification of the geographical area supplied. The limits established are shown in table 4.

developed the quality measurement network needed in Spain. 7. CONCLUSIONS

TABLE 4. Continuity of supply regulated limits. Zonal quality Zone Urban Suburban Rural Conc. Rural Disp.

TIEPI (Hours) 2 4 8 12

TIEPI80 (Hours) 3 6 12 18

NIEPI (per year) 4 6 10 15

If the distribution company zonal quality performance is poor, i.e., the limits are exceeded, the company will be obliged by the administration to make investments in the zone until the quality performance returns to regulated levels. 6. QUALITY MEASUREMENT In the previous section the main aspects of the new quality of supply regulatory framework have been exposed. The new framework defines a group of indexes to measure the quality performance and fixes limits to its values so if the limits are exceeded the distribution company is economically penalised. If this new system is going to work correctly, it will be necessary to measure the value of the different indexes so they can be known and available to the distribution companies, the customers and the administration. So it is a must to install and operate a quality measurement network. At present, this network will have to be developed and maintain by the distribution companies because the cost of doing it by the customers will be unaffordable for the majority of them. Part of this network is already established because distribution companies monitor the quality performance of their networks but the problem is that the methodologies used and the indexes monitored vary through companies so it is not possible to established good quality performance comparisons among different geographical areas of Spain. It can be concluded from the situation exposed that until a new and homogeneous quality measurement network is established by the distribution companies the new quality of supply regulatory framework introduced in Spain will not work properly. Several countries have developed quality measurement studies to obtain information about quality performance and economic costs associated to quality performance levels, the USA, Argentina, Canada and Norway are among the most advanced. This studies [1, 5, 7, 8, 10, 11] can be used to

In this paper the new quality of supply regulatory framework established in Spain as a consequence of the deregulation process started in 1998 has been exposed. This framework represents a great advance in comparison with the old quality framework but there is still a lot of work needed to be done, mainly, as it has been stated in the previous section, in the development of quality measurement network. Until this network is developed and the first measurements are analysed, the customer user will not benefit from the new regulatory framework. As a minimum time of three years is needed to develop and install the network (2 years) and two evaluate the quality performance initial situation (1 year of measurement), the network user will start to notice the changes in the quality regulatory framework in the medium term. 8. REFERENCES 1. Billinton R., Tollefson G., Wacker G., Chan E., Aweya J.: A Canadian customer survey to assess power system reliability worth. IEEE Transaction on Power Systems, vol. 9, no. 1, February 1994, pp. 443-450. 2. Boletín Oficial del Estado Nº 310: Real Decreto 1955/2000, de 1 de diciembre, por el que se regulan las actividades de transporte, distribución, comercialización, suministro y procedimientos de autorización de instalaciones de energía eléctrica. http://www.boe.es 3. Comisión Nacional del Sistema Eléctrico: Ley del Sector Eléctrico, Centro de publicaciones de la CNE, Madrid 1998. 4. Comisión Nacional del Sistema Eléctrico: Desarrollo normativo de la Ley del Sector Eléctrico, Centro de publicaciones de la CNE, Madrid 1998. 5. Gunther E.W., Metha H., Savage N.: A survey of distribution system power qualityPreliminary results. IEEE Transaction on Power Delivery, vol. 10, no. 1, January 1998, pp. 322-329. 6. Heydt G.T.: Electric power quality: A tutorial introduction, IEEE Computer Applications in Power, vol. 11, no. 1, January 1998, pp. 15-19. 7. Kariuki K.K., Allan R.N.: Factors affecting customer outage costs due to electric service


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interruptions. IEE Proceedings Generation, Transmission and Distribution, vol. 143, no. 6, November 1996, pp. 521-528. 8. Kjolle G., Rolfsen L., Dahl L.: The economic aspects of reliability in distribution system planning. IEEE Transaction on Power Delivery, vol. 5, no. 2, April 1990, pp. 1153-1157. 9. López V.: Sistema Eléctrico Nacional. Ley de ordenación y normas complementarias. Tecnos, Madrid 1995. 10.Sanghvi A.P., Balu N.J., Lauby M.G.: Power system reliability planning practices in North America. IEEE Transaction on Power Systems, vol. 6, no. 4, November 1991, pp. 1485-1492. 11.Sullivan M.J., Vardell T. Johnson M.: Power interruption costs to industrial and commercial consumers of electricity. IEEE Transactions on Industry Applications, vol. 33, no. 3, November 1997, pp. 1448-1458.

Pablo Eguia was born in 1973 in Bilbao, Spain. He received his M.Sc. degree in electrical engineering from the University of the Basque Country. At present he is Associate Professor at the Department of Electrical Engineering at the University of the Basque Country. His fields of interest include power system restructuring and deregulation and power transmission and distribution systems. Mailing address: Pablo Eguia Electrical Engineering Department University of the Basque Country School of Engineering of Bilbao Alda. Urquijo s/n, 48013 Bilbao SAPIN phone:(+34)94-601-7225, fax:(+34)94-601-4200 e-mail: [email protected] Esther Torres was born in 1972 in Bilbao, Spain. She received her M.Sc. degree in electrical engineering from the University of the Basque Country. At present she is Associate Professor at the Department of Electrical Engineering at the University of the Basque Country. Her fields of interest include power system restructuring

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and deregulation, transmission planning and distribution systems. Mailing address: Esther Torres Electrical Engineering Department University of the Basque Country School of Engineering of Bilbao Alda. Urquijo s/n, 48013 Bilbao SAPIN phone:(+34)94-601-7225, fax:(+34)94-601-4200 e-mail: [email protected] Elvira Fernandez was born in 1974 in Galdacano, Spain. She received her M.Sc. degree in physics and electronic engineering from the University of the Basque Country. At present she is Associate Professor at the Department of Electrical Engineering at the University of the Basque Country. Her fields of interest include load forecasting and power system studies. Mailing address: Elvira Fernandez Electrical Engineering Department University of the Basque Country School of Engineering of Bilbao Alda. Urquijo s/n, 48013 Bilbao SAPIN phone:(+34)94-601-7225, fax:(+34)94-601-4200 e-mail: [email protected] Prof. José Ramón Saenz was born in 1959 in Tolosa, Spain. He received his M.Sc. degree and his Ph.D in electrical engineering from the University of Navarra in Spain. Presently, he is Full Professor and Head of the Electrical Engineering Department at the University of the Basque Country. His areas of interest include wind energy, electric power quality, deregulated electric markets and electrical machines Mailing address: José Ramón Saenz Electrical Engineering Department University of the Basque Country School of Engineering of Bilbao Alda. Urquijo s/n, 48013 Bilbao SAPIN phone:(+34)94-601-4031, fax:(+34)94-601-4200 e-mail: [email protected]




LIFE CYCLE COST ANALYSIS FOR END USE POWER QUALITY MITIGATION WITH ADVANCED ENERGY STORAGE TECHNOLOGIES: A CASE STUDY Michael W. HOWARD EPRI PEAC Corporation Knoxville, Tennessee (USA) Abstract - Many electrical power quality mitigation systems using advanced energy storage technologies have been planned, designed, produced, and operated with very little concern for their life cycle cost (LCC). Although different facets of cost have been considered in the development of these new systems, the costs have often been viewed in a fragmented manner. The estimated costs associated with development, design, testing, production, construction, operations, and support activities have been isolated and not viewed on an integrated life cycle basis.

Experience has indicated that a large portion of the total cost for many electrical power quality mitigation systems using advanced energy storage technologies is the direct result of activities associated with the operation and support of these systems, while the commitment of these costs is based on decisions made in the early stages of the system life cycle. Further, the various costs associated with the different phases of the life cycle are all interrelated. Thus, in addressing the economic aspects of an electrical power quality mitigation system using advanced energy storage technology, one must look at total cost in the context of the overall life cycle, particularly during the early stages of conceptual design and advanced system planning. Life cycle cost, when included as a parameter in the systems engineering process, provides the opportunity to design for economic feasibility. This paper will discuss life cycle cost analysis and use an example to demonstrate the project balance method as one useful technique in analyzing the selection of an advance power quality mitigation system.

1. INTRODUCTION To determine the total life cycle cost of a power quality mitigation solution, the cost of power quality must be established. A power quality cost program provides justification to the business proposition for a power quality program and provides cost justification for the corrective actions. Power quality cost measurements provide guidance to the power quality program, much as the cost accounting system does for general management. It defines and quantifies those costs that are directly affected by a power quality improvement program, thus allowing power quality to be managed more effectively. Simply stated, power quality costs are a measure of the costs specifically associated with the achievement of product or service qualityincluding all product or service requirements established by the company and its contracts with customers and society. The recent combination of economic trends, increased cost growth experienced for many systems and products, the continuing reduction in buying power, budget limitations, increased competition, and so on, has created an immediate awareness and interest in the total power quality mitigation system life cycle cost. The total power quality mitigation system cost is often not visible during the early stages of system design, particularly those costs associated with system operation and support. The essence of a disciplined life cycle cost evaluation and analysis program is designed to “flush” out the hidden costs, those costs that are typically found hidden below the surface. Life cycle costing is also employed in the evaluation of alternative power quality mitigation system design configurations. The analysis constitutes a systematic approach employing life


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cycle costs to arrive at a cost-effective solution. Reasons for the popularity of life cycle costing are primarily due to increased budget limitations, increased cost awareness among users, increased competition, costly products, and increasing maintenance costs. Life cycle cost analyses is also important in establishing, reducing, and controlling cost. This paper presents an example of the project balance method as the preferred method in performing a life cycle cost analysis for power quality mitigation equipment. 2. COST OF POWER QUALITY

The real value of a power quality program is determined by its ability to contribute to customer satisfaction and company profits. The costs of power quality are those costs associated with the pursuit of power quality improvements. To develop the concept of power quality costs, it is necessary to establish a clear picture of the difference between power quality costs and the cost of power quality. Fundamentally, every time work is redone due to a power quality event, the cost of power quality function increases. Obvious examples are the reworking of a manufactured item, the retesting of an assembly, the rebuilding of a tool, or the correction of a bank statement. Other examples may be less obvious, such as the repurchasing of defective material, response to customer complaints, or the redesign of a faulty component. In service organizations, obvious examples include the reworking of a service, such as the reprocessing of a loan operation, and the replacement of a food order in a restaurant. In short, any cost that would not have been expended if power quality were perfect, contributes to the cost of power quality. For many companies, a formal power quality improvement program is a direct result of the realization that power quality plays a major factor in maintaining and increasing the all-important customer base. A comprehensive power quality improvement program starts with management's understanding and support. Whether for a manufacturing or a service company, a power quality program includes establishment of performance standards in each area of the operation, monitoring of actual power quality performance, corrective action as required, and continuous power quality improvement. A power quality cost program provides justification to the business proposition for a power quality program and provides cost justification for the corrective actions demanded. Power quality cost measurements provide guidance to the power

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quality program, much as the cost accounting system does for general management. It defines and quantifies those costs that are directly affected by a power quality improvement program, thus allowing power quality to be managed more effectively. Simply stated, power quality costs are a measure of the costs specifically associated with the achievement of product or service qualityincluding all product or service requirements established by the company and its contracts with customers and society. Requirements include marketing specifications, end-product and process specifications, purchase orders, engineering drawings, company procedures, operating instructions, professional or industry standards, government regulations, and any other document or customer needs that can affect the definition of product or service. More specifically, power quality costs are the total of the cost incurred by (a) investing in the prevention of non-conformances to requirements, (b) appraising a product or service for conformance to requirements, and (c) failing to meet requirements. Power quality costs represent the difference between the actual cost of a product or service and what the reduced cost would be if there were no possibility of substandard service, failure of products, or defects in their manufacture, which is associated with a power quality event. Although it is rare that a company would go so far as to identify power quality costs down to the level of a secretary correcting a letter containing a mistake. Every company lives with significant elements of costs that fit this description. Unfortunately, significant chunks of power quality costs are normally overlooked or unrecognized simply because most accounting systems are not designed to identify them. It is not too difficult to understand why most company’s top management is more sensitive to overall cost and schedule than to power quality. The interrelationship of power quality, schedule, and cost without attention to the contrary is likely to be unbalanced in favor of schedule and costand often unwittingly at the expense of power quality. This imbalance will continue to exist as long as the real cost of power quality remains hidden among total costs. In fact, such a condition can easily set the stage for a still greater imbalance whenever the rising, but hidden, true cost of power quality grows to a magnitude that can significantly affect a company's competitive position. When the cost of power quality rises without constraint, or is tolerated at too high a level, failure to expose the condition will ultimately become a sign of ineffective management, yet it is entirely Electrical Power Quality and Utilisation

possible for this condition to exist without top management's awareness. A power quality cost program can provide specific warning against dangerous, quality-related situations. On the premise that any dollar expenditure that could have been avoided will have a direct but negative effect on profits, the value of clearly identifying the cost of power quality should be obvious. Achieving this clarity of identification, however, is more easily said than done. A real danger lies in finding and collecting only a small portion of the costs involved and having it represented as the total. There are many ways of hiding costs in industry, as there are people with imagination. This is an all too natural phenomenon in organizations that are never fully charged with all inefficienciesbecause some inefficiencies are hidden and not measuredand thus are able to maintain an illusion of effective management. In this kind of industrial organization, departments that cause inefficiencies in areas besides their own, frequently get off scot-free because the problems they create and their responsibility for them are never properly identified. The costs of handling such problems are buried in the same way that other real power quality costs are buriedas an accepted cost of doing business. If top management had all the facts, it would demand the measurement and control of significant power quality costs. Each identified power quality problem carries with it a tangible recovery cost, which can be assigned a value. This is the essence of power quality cost measurement. In a certain percentage of cases, however, the value of the intangible costs entailed may transcend the pure economics of the situation. For example, what is the cost of missing an important milestone in a schedule? Power quality problems are more often at fault here than other problems. But the most important of all intangible power quality costs is the impact of power quality problems and schedule delays on the company's performance image in the eyes of its customers, with all of its implications for the profit picture and the company's future. The effect of intangible power quality costs, often called "hidden power quality costs," is difficult, if not impossible, to place a dollar value on. Some companies, however, have found a "multiplier effect" between measured failure costs and true failure costs. A large semiconductor company, for example, reported that its "experience indicates that a multiplier effect of at least three or four is directly related to such hidden effects of power quality failure." Figure 1 compares true failure costs to an iceberg with the more commonly measured failure costs as just the "tip of the

iceberg." The bulk of failure costs is “hidden” below the surface and is usually responsible for "sinking the ship."

Figure 1. Hidden Costs of Power Quality and the Multiplier Effect The negative effect on profits, resulting from product or service of less than acceptable power quality or from ineffective power quality management, is usually dynamic. Once started, it continues to mushroom until ultimately the company finds itself in serious financial difficulties due to the two-pronged impact of an unheeded increase in power quality costs coupled with a declining performance image. Management that clearly understands this understands the economics of power quality. Fortunately, a ready-made prescription awaits its decision-effective use of a forceful quality management and improvement program, fully supported by a power quality cost system. The most costly condition occurs when a customer finds defects. Had the manufacturer or service organization found the defects (through much inspection, testing, and checking), a less costly condition would have resulted. If the manufacturing or service organization's power quality program had been geared toward defect prevention and continuous power quality improvement, defects and their resulting costs would have been minimized-obviously, the most desirable condition. The goal of any power quality cost system, therefore, is to facilitate power quality improvement efforts that will lead to operating cost reduction opportunities. The strategy for using power quality costs is quite simple: (1) take direct attack on failure costs in an attempt to drive them to zero; (2) invest in the "right" prevention activities to bring about improvement; (3) reduce appraisal costs according to results achieved; and (4) continuously evaluate and redirect prevention efforts to gain further improvement. In a practical sense, real power quality costs can be measured and then reduced through the proper analysis of cause and effect. As failures are revealed through appraisal actions or customer


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complaints, they are examined for root causes and eliminated through corrective action. The elimination of power quality root causes means the permanent removal. The further along in the operating process that a failure is discoveredthat is, the nearer to product or service uses by the customerthe more expensive it is to correct. The concept applies to service as well. Usually, as failure costs are reduced, appraisal efforts can also be reduced in a statistically sound manner. The knowledge gained from this improvement can then be applied, through prevention activities or disciplines, to all new work. As straightforward as this approach may appear, it cannot work unless there is first a basic power quality measurement system that clearly identifies the correctable elements of performance failures, which represent the best potential for cost improvement. Such a system is designed to use the data from inspections, tests, process control measurements or evaluations, and power quality audits. All power quality costs can be categorized into three groups: prevention, appraisal, and failure costs. Failure costs are further divided into internal and external failure costs. Prevention costs are the costs of activities specifically designed to prevent poor power quality in products or services. Examples include the costs of new product review, quality planning, supplier capability surveys, process capability evaluations, quality improvement team meetings, power quality improvement projects, power quality education, and training. Appraisal costs are the costs associated with measuring, evaluating, or auditing products or services to assure conformance to power quality standards and performance requirements. These include the costs of incoming and source inspection/test of purchased material; validation, verification, and checking activities; in-process and final inspection/test; product, process, or service audits; calibration of measuring and test equipment; and the costs of associated supplies and materials. Failure costs are the costs resulting from products or services not conforming to requirements or customer/user needsthat is, the costs resulting from poor power quality. Failure costs are divided into internal and external failure cost categories. Internal failure costs occur prior to delivery or shipment of the product, or the furnishing of a service, to the customer. Examples include costs of scrap, rework, reinspection, retesting, material review, and downgrading. External failure costs occur after delivery or shipment of the product, or during or after furnishing of a service, to the 386

customer. Examples include the costs of processing customer complaints including necessary field service, customer returns, warranty claims, and product recalls. 3. LIFE CYCLE COST ANALYSIS

The recent combination of economic trends, increased cost growth experienced for many systems and products, the continuing reduction in buying power, budget limitations, increased competition, and so on, has created an immediate awareness and interest in the total power quality mitigation system cost. Not only are the acquisition costs associated with these new systems rising, but also the costs of operating and maintaining systems already in use are increasing at alarming rates. This new awareness and interest in total power quality mitigation system cost is primarily due to issues such as engineering changes occurring throughout the design and development of the system, supplier changes during the procurement of system components, system production and/or construction changes, changes in the logistic support capability, initial estimating inaccuracies, and numerous other unforeseen problems. It has been noted by several cost analysts that cost growth due to these various causes typically range from 5 to 10 times the rate of inflation. At a time when considerable system cost growth is being experienced, budget allocations for many categories of systems are decreasing from year to year. The net result is that less money is available for acquiring and operating new systems or products and in maintaining and supporting the systems that are already in use. The available funds for projects when inflation and cost growth are considered are decreasing at a rapid rate. The current economic situation is further complicated by additional problems related to the actual determination of system and/or product cost. For example, total system cost is often not visible, particularly those costs associated with system operation and support. The cost visibility problem can be related to the "iceberg effect.” The essence of a disciplined life cycle cost evaluation and analysis program is designed to “flush” out the hidden costs, those costs that are typically found hidden below the surface. The application of life cycle costing methods in system and product design and development is realized through the accomplishment of life cycle cost analyses. A life cycle cost analysis may be defined as a systematic analytical process of evaluating various alternative courses of action


with the objective of choosing the best way to employ scarce resources. Life cycle cost refers to all costs associated with the system or product as applied to the defined life cycle. The life cycle, tailored to the specific system being addressed, forms the basis for life cycle costing. In general, life cycle cost includes the following: Need analysis. Need analysis evaluates the wants or desires for new power quality mitigation systems because of obvious deficiencies or problems that become evident through operational deficiencies. System planning function. System planning function includes such activities as feasibility study; advanced system planning (system selection, specifications and plans, acquisition plan, evaluation plan, system use and logistic support plan); planning review; proposal. Research and development. Research and development activities includes feasibility studies; product research; engineering design; design documentation; software; test and evaluation of engineering models; and associated management functions. Production and construction. Production and construction costs include industrial engineering and operations analysis; manufacturing (fabrication, assembly, and test); facility construction; process development; production operations; quality control; and initial logistic support requirements (e.g., initial consumer support, the manufacture of spare parts, the production of test and support equipment, etc.). Operation and support. Operation and support costs include consumer or user operations of the system or product in the field; product distribution, transportation, and traffic management; and sustaining logistic support throughout the system or product life cycle (e.g., customer service, maintenance activities, supply support, test and support equipment, transportation and handling, technical data, facilities, system modifications, etc.). Retirement and disposal. Retirement and disposal cost include disposal of nonrepairable items throughout the life cycle; system/product retirement; material recycling; and applicable logistic support requirements. Life cycle costing is employed in the evaluation of alternative power quality mitigation system design configurations. The analysis constitutes a step-by-step approach employing life cycle costs to arrive at a cost-effective solution. The analysis process is iterative in nature and can be applied to any phase of the system or product life cycle.

The application of life cycle costing is following an increasing trend in recent years. Reasons for the popularity of life cycle costing is primarily due to increased budget limitations, increased cost effectiveness awareness among users, increased competition, costly products, and increasing maintenance cost. Life cycle cost analyses is also important in establishing, reducing, and controlling cost. Other applications of life cycle costing include: x

Choosing the most beneficial procurement

strategy x x

Determining cost drivers Making strategic decisions and design trade-offs x Selecting among options x Making source selections x Assessing new technology application x Providing objectives for program control x Formulating contractor incentives x Improving comprehension of basic design associated parameters in product design and development x Optimizing training needs x Forecasting future budget needs Experience has shown that a major portion of the projected life cycle cost for a given system or product stems from the consequences of decisions made during early planning and as part of the system conceptual design. Such decisions, made because of a needs analysis or a design feasibility study, actually guide subsequent design and production activities, product distribution functions, and the various aspects of sustaining system support. Thus, if ultimate life cycle costs are to be optimized in designing for economic feasibility, it is essential that a high degree of cost emphasis be applied to the early stages of system/product development. Figure 2 reflects a characteristic life cycle cost trend curve as related to actions occurring during the various phases of the life cycle. The system/product life cycle phases presented in Figure 2 is translated to reflect emphasis on the early planning and design stages of a program. As illustrated, approximately 60% of the projected life cycle cost is committed by the end of the system planning and conceptual design stage, even though actual project expenditures are relatively minimal at this point in time. This curve varies with the individual system; however, it does convey a trend relative to affects of decisions on ultimate life cycle cost.


387

Life cycle cost studies should always consider the after-tax cash flows that a project produces in evaluating the financial performance of the investment. The ATCF represents the amounts of money that a project contributes to (or drains from) the firm. Generally, ATCF is regarded as a better indicator of profitability than NIAT. This is because a firm can go bankrupt while reporting profits, but it will remain solvent as long as its cash (i.e., ATCF) and liquidity positions are strong. Benefits and costs are related to different activities at different points in time over the project life cycle. As a result, a common point of reference must be assumed so that all alternatives can be Figure 2. Actions Affecting Life Cycle Cost compared on an equivalent basis. This point of reference is generally the present time and all 4. AFTER-TAX CASH FLOW ANALYSIS AND future benefits and costs for each year in the life LIFE CYCLE COSTING cycle is discounted to their present equivalent amounts. The present equivalent amount is Life cycle cost analysis typically involves the discounted based on a company’s assigned evaluation of alternative power quality mitigation discount rate or Minimum Attractive Rate of proposals. Each proposal considered in the Return (MARR). evaluation process represents a potential The net present value (NPV) is considered the investment and should be viewed from the standard economic investment measure used to standpoint of anticipated benefits or revenues and equate different points in time. Other common costs that will occur over the designated life cycle economic investment criteria include: future of the investment. value, annual equivalent, internal rate of return, The completion of a life cycle cost analysis serves Solomon's average rate of return, modified internal as an aid in the decision-making process pursuant rate of return, aggregate benefit/cost (B/C) ratio, to the evaluation of alternatives. It also provides a netted B/C ratio, Lorie-Savage ratio, and Project basis for estimating budgetary requirements for a Balance (PB). This paper presents an example of defined system configuration over its life cycle. In the PB method as the preferred method in developing the life cycle costs, the following evaluating life cycle cost analysis investment parameters must be considered: criteria for power quality mitigation equipment. B = gross benefit received or revenues obtained. E = operating expenses plus interest paid for the The NPV is calculated by computing the present use of borrowed capital, both debt and value of the cash flow projections based on a rate equity capital. of interest. The NPV expression is: d = depreciation; this is the recognition of cost N due to loss in the value of assets, such as n NPV (i, n) ¦ Fn 1 i property, buildings, or equipment.. n 0 T = income taxes; these are costs to an organization which amount depends on Where i is the MARR per period, n represents time profits remaining after expenses are paid or and is measured in discrete compounding periods. accounted for. Fn is the project cash flow projection, and N is the t = effective income tax rate used for computing project evaluation period. income taxes. If the NPV is greater than $0, the project generates The relationship between net income before taxes a surplus of funds and should be accepted. Thus, (NIBT) and net income after taxes (NIAT) is the firm should accept any project for which the present value NPV(i, n) is positive, and reject any shown in the following equation: NIBT = B – (E + project for which the NPV(i, n) is negative. The d) = B – (E + d) – t[B – (E + d)] = (1 – t)[B – net present value can also be viewed as the (E + d)] where, NIAT = NIBT – T. cumulative sum of all cash flows generated by the The net income after tax is usually different from project in excess of the investment and discounted the after-tax cash flows that are produced by a at the firms MARR. project. If depreciation deductions are added back The PB is defined as the net equivalent amount of to net income after taxes, a project's after-tax cash investment remaining during the life of the project. flow (ATCF) in year k can be estimated as ATCFk The PB is calculated as PB(i)n = (1 + i)PB(i) n -1 + equals NIATk + dk. F n, where PB(i)0 = F0. If the PB is greater then $0, 388


the project recovers the initial investment plus any interest owed and has a profit at the end of the project life. Therefore, if the PB is greater then $0, the project should be accepted. In addition, the PB provides quantitative information about four important characteristics associated with investment decisions. The project balance shows the future value, discounted payback period, area of negative balance and the area of positive balance.

final selection is estimated to be a $20,000 onetime expense. x Construction Costs: The construction cost associated with the new power quality mitigation equipment is estimated to be $65,000. x Operations and Support Costs: The recurring operational support costs is shown in Table 5.1. These estimated costs include such items as maintenance, repair, staff training, supplies, storage, rent, overhead charges, and other contingencies. x Retirement and Disposal Costs: The retirement and 5. EXAMPLE – PROJECT BALANCE disposal costs for theMETHOD power quality mitigation equipment is estimated to be 8.8% of the initial capital investment at the end of year six or $49,500. As an example of life cycle cost analysis using the x Capital Investment: The initial capital investment project balance method, consider a firm who is required to purchase the new power quality evaluating an investment in a new power quality mitigation equipment is $560,000 and will be mitigation equipment. The problem identified by depreciated for seven years. the firm is frequent voltage sags causing excessive x Marginal Tax Rate: 40% down time, scrap, rework, and general loss x Minimum Attractive Rate of Return (MARR): production. After reviewing the firm’s annual 15%

production report and the anticipated demand for it’s product in the next few years, the firm’s management team has decided to evaluate the investment in a new power quality mitigation device that will help reduce the number of voltage sags. This new power quality mitigation equipment is anticipated to eliminate voltage sags from disrupting the sensitive electronic manufacturing equipment, thereby adversely affecting production operations. The analysis will require one year needs assessment and vendor selection. The decision objective is to determine if the investment in a new power quality mitigation system is feasible and profitable. Details of the project benefits and costs are shown below. x

x

x

x

Needs Analysis Costs: The engineering staff estimates the cost of performing the initial needs analysis costs is $28,000. The needs analysis costs includes labor to review operational performance associated with lost production due to frequent voltage sags causing the firm’s sensitive electronic equipment to malfunction. These costs will be expensed in the year they occur. Benefit: The benefit of the power quality mitigation equipment includes the reduction of scrap and downtime and increase production. The benefit derived by the installation of the equipment is shown in Table 5.1 for years one through six. System Planning Costs: System planning costs is estimated to be $20,000. This involves initial planning on how the new power quality mitigation system will be integrated into the manufacturing electrical system and its impact on overall electrical and mechanical infrastructure. System Procurement Costs: The engineering and procurement staff expenses associated with developing the procurement specification, review the alternative solution offering and negotiating the

The company's six-year benefit and cost statement is shown in Table 1. The net present value of the investment is $443,944. Since the net present value is positive, the project should be considered. The project balance is graphically shown in Figure 3. Again, since the sixth year project balance is positive, the project should be considered. The project balance describes the net equivalent amount of dollars tied up in or committed to the project at each point in time over the life of the project. The project balance is denoted as PB(i)n where i is the opportunity cost rate (MARR) and n is the period computing the PB. If PB(i)N>0, the firm recovers the initial investment plus any interest owed, with a profit at the end of the project. If PB(i)N=0, the firm recovers only the initial investment plus interest owed and breaks even. If PB(i)N0. The net present equivalent value of this project is simply NPV (i, N ) PB (i) Nn 1 i N and the future value is the PB at period N. The discounted break even point for the project is the point where the PB(i)n=0. Thus, using the PB method, one can easily derive several important economic interpretations about the project such as net present value, future value, and break-even point. In addition, one can visually see the amount of capital at risk indicated by the negative area in Figure 3. The project balance shown in Figure 3 indicates that the break-even point is 3.5 years. Initially, the firm has $639,000 of capital exposes to risk. However, in each future year, the benefit is greater than the expense thus reducing the exposed risk.


389

A greater benefit or less expenses in earlier years would have the reverse affect, that is, reducing the amount of exposed capital and thus reducing the breakeven point.

Beginning in year 3.5, the firm recognizes a gain on its capital assuming a 15 percent MARR. A greater MARR would result in extending the breakeven point and exposing more capital to risk.

$1,200,000 $1,000,000

Project Balance

$800,000 $600,000 $400,000

Area of Positive Balance

$200,000 $Area of Negative Balance

$(200,000) $(400,000)

Breakeven Point

$(600,000) $(800,000) 0

1

2

3

4

5

6

Year

Figure 3. Project Balance

TABLE 1. Benefit and Expenses for Project Balance Example Year

0

1

2

3

4

5

6

Benefit

$0

$295,000

$412,000

$495,000

$535,000

$589,000

$550,000

Needs Analysis

$28,000

$0

$0

$0

$0

$0

$0

System Planning R&D

$20,000

$0

$0

$0

$0

$0

$0

Procurement

$20,000

$0

$0

$0

$0

$0

$0

Construction

$65,000

$0

$0

$0

$0

$0

$0

$0

$25,000

$35,000

$95,000

$53,000

$28,000

$36,000

$133,000

$25,000

$35,000

$95,000

$53,000

$28,000

$36,000

Operational Total Expenses Depreciation NIBT

$0

$89,286

$89,286

$89,286

$89,286

$89,286

$89,286

($133,000)

$180,714

$287,714

$310,714

$392,714

$471,714

$424,714

$53,200

($72,286)

($115,086)

($124,286)

($157,086)

($188,686)

($169,886)

NET INCOME

($79,800)

$108,429

$172,629

$186,429

$235,629

$283,029

$254,829

CASH FLOW

0

1

2

3

4

5

6

NET INCOME

($79,800)

$108,429

$172,629

$186,429

$235,629

$283,029

$254,829

Depreciation

$0

$89,286

$89,286

$89,286

$89,286

$89,286

$89,286

($560,000)

$0

$0

$0

$0

$0

$0

$0

$0

$0

$0

$0

$0

$49,500

Income Tax (40%)

Initial Investment Salvage Value Gains Tax AFTER TAX CASH FLOW

390

$0

$0

$0

$0

$0

$0

($19,800)

($639,800)

$197,714

$261,914

$275,714

$324,914

$372,314

$373,814


6. CONCLUSIONS

Many electrical power quality mitigation systems have been planned, designed, produced, and operated with very little consideration for their total life cycle cost. Experience has indicated that a large portion of the total cost for many electrical power quality mitigation systems is the direct result of activities associated with the operation and support of these systems, while the commitment of these costs is based on decisions made in the early stages of the system life cycle. Life cycle cost, when included as a parameter in the systems engineering process, provides the opportunity to design for economic feasibility. The recent combination of economic trends, increased cost growth experienced for many systems and products, the continuing reduction in buying power, budget limitations, increased competition, and so on, has created an immediate awareness and interest in the total power quality mitigation system cost and in the evaluation of alternative power quality mitigation system design configurations. A disciplined analysis methodology constitutes a systematic approach employing life cycle costs to arrive at a cost-effective solution. The project balance method is one technique that can be used to help determine the life cycle cost for power quality mitigation equipment. However, the most important feature of the life cycle cost analysis technique using the project balance method is the structured methodology required to determine the projects economic feasibility along with the additional information provided by the methodology. As shown in the example discussed in this paper, life cycle cost analysis combined with the project balance method offers a technique to help understand and evaluate the economic feasible of a power quality mitigation solution given the anticipated life cycle costs and benefits of the proposed project.

BIOGRAPHY Dr. Michael Howard is president of EPRI PEAC Corporation, as well as an adjunct professor in industrial engineering at The University of Tennessee, Knoxville. Experience He has over 20 years of experience working with companies ranging from entrepreneurial start-ups to large public companies with responsibilities including general business, finance, sales and marketing, research and development, and operations. His current research focus is on the use of dynamic simulation to better understand the complex interactions of large industrial systems and the use of engineering economic analysis principles in the systematic evaluation of the costs and benefits of proposed engineering projects, especially as it relates to the mitigation of electrical disturbances. PROFESSIONAL AFFILIATIONS AND ACTIVITIES Institute of Electrical and Electronics Engineers (IEEE) Registered Professional Engineer American Society for Engineering Management Society of Manufacturing Engineers Institute of Industrial Engineers Institute of Management Accountants EDUCATION Bachelor of Science in Electrical Engineering, The University of Tennessee Master of Science in Business and Engineering Management, The University of Pittsburgh Ph.D. in Industrial Engineering, The University of Tennessee.


391

392




USING THE PRICING THEORY OF FINANCIAL DERIVATIVES TO PREDICT PAYMENTS OF ELECTRIC ENERGY REVENUES Hector ARANGO, J. Policarpo G. ABREU,

Elder G. DOMINGUES,

Gilson PAULILLO

Itajubá Federal School of Engineering Itajubá (Brazil)

Abstract - The object of this paper is to use the price theory of financial derivatives so as to obtain the expected value of the electric energy revenue used for establishing the price of vested contracts and bilateral contracts as well. In this analysis, the established price of the consumed energy is binomial and the concept of marginal costs is used accordingly. The aspects related to uncertainty of demand are taken into account by using overtariffing. The optimum contractual demand can be obtained according to the respective overtariffing as well as the revenue behavior for lower and higher values of the optimal contractual demand.

The regulatory doctrine establishes that the price to be charged for electric supply will be proportional to the increase of it, which will be as follows: electric energy cost (EEC) and investment cost (IC). The investment cost includes costs of expansion of transmission and generation systems. The other cost component, EEC, is associated to the electric consumption and is obtained according the load curve. This second component is also dependent on investment (I), since, the efficiency of the generation and transport systems for a specific consumption (Pt), depends on how large the associated physic system is. For this reason, the electricity cost (C) can be expressed as:

1. INTRODUCTION C

Until the early 1980`s, the Brazilian tariff system was based only on accounting costs. Regardless of month or utilization hour, the prices both for electric demand and consumption were fixed. From 1982 on, the marginal cost concepts have been used. The seasonal tariffs have then been introduced thus originating a more rational electric energy consumption. After the Brazilian electric system was restructured, overtariffing has been applied (TO ) when the registered demand ( R D ) during the period of revenue surpasses the value of the contractual demand (C D ) . This has occurred in the vested contracts and in most bilateral contracts. In the future, overtariffing will be replaced by the spot price, which will increase the uncertainty revenue to be paid in the month as a result of the volatility of the spot prices.

IC ( I ) EEC ( Pt , I )

(1)

According to the regulatory doctrine, the price charged to a client that consumes an increment of the electric product ('Pt), should be based on the impact caused by the addition of this increment and by the suppliers’ commitment to maintain the physical dimension of system. Such dimension should be adapted to the consumption so that the process will be economically feasible and adequate. Hence, the increase of the capital invested in the system ('I) taking into consideration an increment in the product consumed is given by:

'I

wI 'Pt wPt

(2)

Consequently, from (1) we have:

2. REGULATORY DOCTRINE Section 5. Power Quality in Competitive Electricity Markets. Economic Aspects ...

401

'C

wC wC 'Pt 'I wPt wI

2920

By manipulating the equations (2) and (3) we have:

'C

ª wC wI wC º « » 'Pt ¬ wI wPt wPt ¼

(4)

ª wC wI wC º The term « » is known as the ¬ wI wPt wPt ¼ marginal cost of the product, and it expresses how much it costs for the system to increase the consumption of the product in one unit in relation to a certain policy of expansion.

3. THE BINOMIAL ENERGY PRICING In the vested contracts as well the most bilateral contracts, the established price of the consumed energy is binomial. In the analysis below, we can notice that the marginal costs define the consumer market requirements for over the physical system through a single load variable. In practice, the physical dimension of the transmission and the generation systems depend on the time series of the average power, calculated for successive integrated 15-minute intervals. The higher value of the average power of the time series is called demand (D). On the other hand, the generation required energy (E) is dependent on the water volume and the fuel quantity. For this reason, the costs associated to energy are dependent on the expansion of the water reservoir system and on fuel consumption, which is necessary for thermal completion. For the consumer, the marginal cost reflect the demand and energy increments that his consumption brings about when one link of the transport and production network is used. As a result, the electric contract predicts the revenues (REV) based on a binomial structure: REV

D TD E TE

(5)

where TD end TE are the demand and energy tariff that should keep the necessary proportion with the marginal costs. The values of E and D are calculated by: 1 2920 (6) E Pt 4 i1 and

¦

402

D

(3)

MAX Pt i 1

(7)

Where i varies from 1 to 2920 periods of 15 min, which correspond to 730 hours monthly. Nowadays, an electrical contract always has an element of uncertainty. We can add here the uncertainty of the payments as far as demand and energy are concerned. If a contractual demand value (CD) and two tariffs are adopted, the one of demand, TD , and the other of overtariffing TO , the component of revenue related to demand (REVD), for a registered demand RD on the period of revenue is: REV D

TD C D TO MAX ( R D C D ,0)

(8)

where the term TO is obviously established in a much superior value than that of the demand tariff ( TD ). The component related to TO has the same structure of a Lookback Put Option, a type of financial derivative that is much used in the modern finance theory [1,2]. Then, the revenue (REV) is given by:

REV

REV E REV D

E TE TD C D

TO MAX ( R D C D ,0)

(9)

where REV E is the comportment of revenue related to energy. Since E and RD are random variables, the payments (REV) also will present certain randomness. The expected values of the comportments related to energy and demand are: EXP ( REV E )

TE EXP ( E )

(10)

EXP( REVD ) TD C D TO MAX ( R D C D ,0) (11)

To estimate the value of EXP(REVD), it becomes necessary to resort to the historical values of demand registered in the past (RDt) It will be assumed that the time series of RDt , follows a lognormal stationary distribution function. Then, the random variable L ln R Dt is gaussian and the mean ( P L ) and standard deviation ( V L ) can be estimated by:

PL

1 N

N

¦L

(12)

t 1


1 N

VL

N

¦

(L P L ) 2

(13)

t 1

With these estimates it is possible to express the value of EXP(REVD) like: EXP ( REVD ) TD e LC

f

TO 2S V L

³

( e L e LC ) e

correspondent to EXP(REVD) to be paid in the future. The value of LC (and also C D ) that minimizes EXP(REVD) is the one causes the derivate of the equation (14) to be equal to zero, namely:

(LPL )2 2V L2

TD e

dL

f

TO e LC

LC

2S V L

LC

³e

( L P L )2 2V L2

dL

(20)

LC

(14) Where LC

The equation (14) can be rearranged and placed in the standard form, namely: EXP( REVD ) TD e LC TO e TO e LC

1 2S

f

³e

2 · § ¨ P V L ¸ ¸ ¨ L 2 ¹ ©

1 2S

f

³e

2

dz1

dz2

z2

(15) where z1 and z2 are, respectively: z1

f

1 2S

LC

³ eP L

z2 2

dz

1 N ( z)

L

VL

L V L2 P L

VL

(16)

where N(z) is a normal probability density function. Therefore, the value of LC and the optimal contractual demand ( C D* ) are given by, respectively: § T · LC P L V L N 1 ¨¨1 D ¸¸ (22) © TO ¹ §

z2

L PL

VL

(17)

Therefore, the expected value ( EXP(REV ) ) and the variance ( VAR(REV ) ) of the revenue (REV ) can be calculated by: EXP( REV ) EXP( REVE ) EXP( REVD ) VAR( REV )

(21)

z1

z 22 2

and cancel the

VL

exponential, the result will be:

TD TO

z 12

L PL

z

If we substitute

ln C D .

EXP( REV 2 ) EXP 2 ( REV )

(18) (19)

4. OPTIMAL CONTRACTUAL DEMAND

The client should propose a convenient value of contractual demand (CD), but we know that small values lead to high revenue from frequent overtariffing. The values that are too high also burden the revenue because of the excessively high fixed component. Then, it is necessary to choose the appropriate value of CD, according to the consumption rhythm of each client. Since the demand in future periods of revenue are not predictable, we can minimize the value

C D*

DP e

V L N 1 ¨¨ 1 ©

TD TO

· ¸ ¸ ¹

(23)

The D P values corresponds to the logarithm average of the preterit demands.

DP

e

PL

§ ¨¨ ©

1

N

3 t 1

·N R Dt ¸¸ ¹

(24)

5. SIMULATION RESULTS

Table 1 below shows the registered demand history ( R D ) and the respective value of L ln( R D ) used in the simulations. TABLE 1. Demand history and the respective value of L L ln R D RD [kW] 497 6,21859 504 6,22258 511 6,23637 504 6,22258 475 6,16331 461 6,13340


403

502

5800

To/TD=2 To/TD=4

5300

EXP(REVD) - [R$]

The result illustrated in figure 1 shows the values of the optimal contractual demand ( C D* ) for T different values of O . TD

To/TD=6 To/TD=8

4800

To/TD=10

4300 3800

500

3300 Optimal CD - kW

498

2800

496

440

450

460

494

470

480

490

500

510

520

530

540

CD [kW]

492 490

Figure 3 – EXP(REVD) for different values of

488 486 2

3

4

5

6

7

8

9

TO TD

10

To/TD

6. CONCLUSION

Figure 1 – C D* for different values of

TO TD

The graph illustrated in figure 2 shows the T behavior of EXP ( REV D ) versus CD for O 2 . In TD these simulation we consider the tariff of demand (TD) equal R$ 6,00/kW.

3300

EXP(REVD) - [R$]

3250 3200 3150

The work described in this paper can be applied to more complex tariff systems. For instance, when the seasonal tariffs for different classes of consumers and different periods of the year are taken into consideration. Setting the revenue as a function of MAX operator, it is understood as financial derivatives pay-off, which plays a decisive role in electric markets and in models of pricing establishment developed over the last decades. In the future, spot price will substitute the overtariff. This price has a strong volatility which increases the uncertainty present in the revenue future values.

3100 3050 3000

7. REFERENCES

2950 2900 440

450

460

470

480

490

500

510

520

530

540

CD [kW]

Figure 2 – EXP(REVD) for

TO TD

2

1. Wilmott P. Howilson S. Dewynne J.: The mathematical of financial derivatives. Cambridge University Press, New York, 1999. 2. Eldton E. J. Gruber M. J.: Modern Portfolio Theory and Investment Analysis. John Wiley & Sons, New York, 1995

Figure 3 shows the same behavior for the different T values of O . TD It is worth noting that after the optimal contractual demand, the under-estimated values of contractual demand reach much higher EXP(REVD) than the CD over-estimated values. It has occurred mainly T for higher values of O . For this reason, it is TD recommended that the contractual demand be established next to the optimal point. Dr. Hector Arango

404


Received his Ph.D. degree in Electrical Engineering from the University of São Paulo, Brazil, in 1996. He is a full Professor at Itajubá Federal School of Engineering. At present his research interests include power quality issues, electricity legislation, electrical machines and transformers, definitions on nonsinusoidal conditions and risk analysis on the electricity markets. Mailing address: Hector Arango Itajubá Federal School of Engineering - EFEI Institute of Electrical Engineering – IEE Power Quality Study Group - GQEE P. O. Box # 50 37500-903 Itajubá - MG BRAZIL phone: (+55)(35) 3629-1243, fax: (+55)(35) 3629-1187

Prof. Elder G. Domingues Was born in 1971 in Carmo do Paranaíba, Brazil. He received his M.Sc. degree in Electrical Engineering at the Federal University of Uberlândia. At present, he is studying for his D.Sc. degree at Itajubá Federal School of Engineering. He is also an Associate professor of CEFET-GO (Technological and Educational Federal Center of Goiás). His areas of interest include analysis of power system, electric power quality and risk analysis on the electricity markets. Mailing address: Elder Geraldo Domingues Itajubá Federal School of Engineering - EFEI Institute of Electrical Engineering – IEE Power Quality Study Group - GQEE P. O. Box # 50 37500-903 Itajubá - MG BRAZIL phone: (+55)(35) 3629-1243, fax: (+55)(35) 3629-1187 e-mail: [email protected]

Dr. José Policarpo G. Abreu was born in 1952 in Madeira Island, Portugal. He received a Ph.D. degree in Electrical Engineering from the University of Campinas (Brazil) in 1991. He is Full Professor and the Power Quality Study Group Coordinator at Itajubá Federal School of Engineering (EFEI), Brazil. He has been nominated for the Chairmanship of the 10th IEEE ICHQP - International Conference on Harmonics and Quality of Power, to be held in Rio de Janeiro, Brazil in 2002. Currently, his research interests include power quality issues, power definitions under abnormal conditions, induction motors and transformers, and electric dirves. Mailing address: José Policarpo G. Abreu Itajubá Federal School of Engineering - EFEI Institute of Electrical Engineering – IEE Power Quality Study Group - GQEE P. O. Box # 50 37500-903 Itajubá - MG BRAZIL phone: (+55)(35) 3629-1243, fax: (+55)(35) 3629-1187 e-mail: [email protected]

Gilson Paulillo was born in 1967 in Araraquara, Brazil. He received the M.Sc. degree in electrical engineering from Itajubá Federal School of Engineering - EFEI, where he is finishing his Ph.D.. Presently, he is Researcher at Power Quality Study Group – GQEE, of the Institute of Electrical Engineering of the EFEI. His areas of interest include power quality, induction motors and transformers and electric drives, as well as computer analysis of power system. Mailing address: Gilson Paulillo Itajubá Federal School of Engineering - EFEI Institute of Electrical Engineering – IEE Power Quality Study Group - GQEE P. O. Box # 50 37500-903 Itajubá - MG BRAZIL phone: (+55)(35) 3629-1312, fax: (+55)(35) 3629-1326 e-mail: [email protected]


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THE QUALITY OF DISTRIBUTION AS A FACTOR DECREASING COSTS OF ENERGY Jerzy SZKUTNIK Technical University of Cz³stochowa Cz³stochowa (Poland) Abstarct - The report constitutes an attempt to present another look on the quality of energy, which is predominantly described by the voltage level and reliability of the network. The author suggests making an additional evaluation basing on the quality of the energy distribution factor, which is defined as the ratio of the justified technical losses and the real technical losses in the network of a distribution company. The analyses to be presented in the report will be conducted on the basis of calculations in different scenarios with usage of the software STRATY`99. It is planed to make the analyses for individual kind of network ( 110 kV, medium voltage and low voltage ) as well complex analysis for the whole network system. The results of the research will constitute an additional hint how to decrease costs of energy distribution. 1. INTRODUCTION The basic condition of assurance of a good position of a firm on market is not only efficient management, but also undertaking of optimum decisions related to the development and its financing sources. On rightness of these decision depends the perspective competitiveness of companies, their share in the market and possibilities of generating of profits. The development of companies, also belonging to the energy sector, takes place fundamentally across realisation of investments. Investment is a long-term process, it requires collection of capital for financing of first outlays, which will bring effects some time later [1]. We can distinguish the following types of investments:

Reconstruction – replacement of used up or obsolete devices by new ones aiming at preventing of increase of costs connected with the process of getting old of the fixed assets

1. Modernisation – enabling to decrease costs of energy distribution connected with the use of new technology 2. Developments – leading to realisation of strategy of a company The decisions relating to the development of a company have to lean on objective criteria. The main tool, determining the base of undertaking these decision, is profitability of investment. There are a number of methods helpful by undertaking of investment. The most renown are: x time of return and discounted time of return x return on equity x internal rate of return ( IRR) x Net Present Value( NPV) x Breakeven point By usage of the above methods one can estimate which of the development strategies is characterised by the best efficiency. As a supplement to the presented measures of efficiency, it is proposed the new coefficient of efficiency of investment, derived from point of view of energy losses [2]. This coefficient enables to choose such a development strategy of a network, which assures the maximum efficiency of investment. 2. SOFTWARE STRATY`99 AS A BASIC TOOL OF ESTIMATION OF EFFICI-ENCY OF DISTRIBUTION OF ENERGY The problem of efficiency of distribution of electric energy is a basic task of suitable departments of energy boards. The helpful software in this range is software STRATY` 96 and its modernised version STRATY`99 [3,4 ]. This software is designed to analyse of network losses in scale of energy regions and energy boards. Calculations are conducted in the following breakdowns: 1- according to sources their


407

origin, 2- in partition on technical and trade losses, 3- in partition on real and justified losses. At present, the software operates in different versions in 28 energy boards. Moreover, there is a split into real and technical losses. The real are losses which would occur if the existing network devices were used properly and the flow in the network was correct.. The difference between real losses and justified losses are the technical unjustified losses. The presented report refers to the real and justified losses. The analyses of this issue show that among distribution companies there are large differences in amounts of technical real and justified losses. So, the justified losses constitute certain level, which can be reached through an efficient strategy of decreasing of losses. 3. COEFICIENT DISTRIBUTION ENERGY

QUALITIES OF OF ELECTRIC

As mentioned earlier, there are a numebr of coefficients, which can determine estimation of performance of distribution companies – technical, economic or financial coefficients. For the estimation of efficiency of energy distribution, it is proposed the coefficient defined as follows: wqd = 'Etrz / 'Etu where: wqd - coefficient of distribution quality, 'Etrz - level of real technical losses [MWh], 'Etu - level of justified technical losses [MWh].

Generally coefficients of values of approx. 1 indicate good performance of distribution companies. Below are values of coefficient wqd for different kinds of losses calculated by the software STRATY` 99 for one of the distribution companies: 1. charge losses in low voltage network wqdonN = 1, 1868 2. technical losses in low voltage network wqdtnN = 1, 0541 3. charge losses in medium voltage network wqdoSN = 1, 0321 4. technical losses in medium voltage network wqdtSN = 1, 0310 5. charge losses in 110 kV network wqdo110 = 1, 0023 6. technical losses in 110 kV network wqdt110 = 1, 0032 7. technical losses in distributive networks wqdk = 1, 0257 The coefficient represented by the relationship of technical losses in all kind of networks (7distribution network) is simultaneously the coefficient for complex estimation of quality of distribution. In the analysed company the quality of distribution has the lowest level in the low voltage network, and is the highest in 110 kV network. On the final result the main influence have the charge losses in each network, especially in the low voltage network. The Analyses can be conducted also in the scale of energy regions. This issue is shown in the Fig.1, which presents values of coefficient (technical losses in nN and SN) in the analysed company as well as in each of the regions.

1,04 1,02

408

1 0,98

RE5

RE4

RE3

RE2

0,94

RE1

0,96 SD

The presented coefficient can refer to various kinds of losses in network and can constitute a synthetic coefficient of estimation of performance of all energy boards, if all technical losses of a distribution company are taken into account. The value of coefficient wqd is fundamentally contains oneself in the range of 1y 1, 25. It depends on the kind of losses, which are estimated. The value of coefficient below 1 testifies about this, that in range of a given kind of losses, the distribution company shows better efficiency of energy distribution than the average for the group of dozens of distribution companies in the country, which indicate the lowest coefficient of energy losses in the distribution network of 110 kV, SN and nN.

Fig.1. Coefficient qualities of distribution for institution and of regions Regions 4 and 5 have the level of distribution beyond expectations (values below 1), the worst situation is in region 2, the total value wqdtnN + NS = 1, 038 results from performance of all regions. There were conducted also analyses of influence of the increase of line on each of the network levels and of feeding station for the quality of distribution of Electrical Power Quality and Utilisation

energy. The coefficients relating to stations are stable, while the increase of lines causes also the increase of coefficient wqd. Despite the nominal decrease of charge losses in the network, proportions between real and justified losses is slightly growing. It is especially visible in the medium voltage network. Fig.2 depicts the course of losses in this network for increase of length of line of medium voltage 5, 10 and 15 %, Fig.. 3 – course of coefficient wqd. The conclusions resulting from the above presented graphs indicate univocally the necessity of obeying suitable cuts in network because this enables the maintenance of coefficient wqd at the unchanged level. 64000 63000 62000 61000 60000 0%

5%

10%

15%

%

Fig. 2 Course of charge losses In order to do so, one can use computer software, which optimise place of line cut, eg. software PRAO. w 1,08 1,06 1,04 1,02 1 0,98 0%

5%

10%

15%

%

Fig.3. Course of coefficient wqd. Summarising the presented report it should be stressed that the proposed new kind of qualitative estimation of performance of a distributive network in companies can be constitute an additional parameter , which enables to reduce the costs of energy distribution. In the times of searching for costs origins, the coefficient wqd informs about possibilities of decreasing of losses in the given area of energy distribution. The proposed estimation can be commonly used, because the software by which you can calculate the coefficients is commonly used in distribution companies.

4. REFERENCES 1. [Szkutnik J: Strategia rozwoju sieci rozdzielczych w spóce dystrybucyjnej, a mo¶liwoci zapewnienia rodków bud¶etowych na jej realizacj³ Bud¶etowanie Dziaalnoci Jednostek Gospodarczych – Teoria i Praktyka. Wydzia Zarzµdzania AGH, Kraków, czerwiec 2000 2. Szkutnik J: Selection of distribution network development in energy board on the basis of instrument effectiveness ratios from the energy losses point of view, I TH International Scientific Symposium ELECTROENERGETIKA, STARA LESNA, SLOVAKIA, January 2001 3. Szkutnik J., Gawlak A.: Program Straty 96 dla Rejonów i Zakadów Energetycznych, II Konferencja Naukowo-Techniczna Zastosowania Komputerów w Elektrotechnice ,PoznaKiekrz, kwiecie 1997 4. Gawlak A., Szkutnik J.: Strategia identyfikacji strat handlowych przy pomocy programów komputerowych: STRATY`96 oraz STRAHA III Konferencjia Naukowo-Techniczna Zastosowania Komputerów w Elektrotechnice, PoznaKiekrz, kwiecie 1998 r. Jerzy Szkutnik Born he 3rd February 1948. In 1972, he graduated from Technical University of Cz³stochowa and started working ace assistant in Power Engineering Section of Technical University of Cz³stochowa. In 1980 he acquired the rank of Doctor of Technical Science at Silesian Technical University. He is and lecturer in the following subjects: supply and distribution of energy, distribution network operation, Economy of energy distribution. He participates in many researches – over 150 handicap Power Engineering Institute, Government of Poland and directly handicap energy boards - at present, co-operation with 28 energy distributors in Poland. Author and co-author of over 50 scientific publications and papers concerning energy distribution, presented during domestic and international conferences, co-author of and book about supply and distribution of energy. Co-owner of 2 patents. VicePresident of Association of Graduates of Technical University of Cz³stochowa. Mailing address: Jerzy Szkutnik Ph. D. Electrical Department of Technical University of Cz³stochowa Power Engineering Institute Al. Armii Krajowej 17 42-200 Cz³stochowa e-mail: [email protected] tel. 0048343250808 fax. 0048343250803


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IMPROVEMENT IN POWER VOLTAGE QUALITY AND REDUCTION IN POWER LOSSES AT CITY LOW VOLTAGE TRANSFORMER SUBSTATIONS Miron GRINKRUG State Technical University of Komsomolsk on Amure Komsomolsk on Amure (Russia)

Yulia TKACHEVA State Technical University of Komsomolsk on Amure Komsomolsk on Amure (Russia)

Abstract - To cut the losses and to improve the quality of electric power, it is suggested that step-down transformer substations in residential cite districts de supplied with two transformers of different power. The transformers are arranged in such a way that the lower power transformer operates when the load is built up, and both transformers are in operation when the load is close to its peak value. The switching circuit for the transformers has been designed. It ensures the transformers are turned on without voltage loss at the consumers`. The calculations have been performed to work out the two transformers which ensure the yearly lowest power losses. As also follows from the calculations, the maximum voltage drops are reduced, and the voltage at the consumers` is maintained at a steady level. The calculations have been made for different levels of the peak power load and for the variations of various parameters influencing the load diagram

current is 0,125. The load rate ranges from 0,009 to 0,543. The average voltage drop in cable lines does not exceed 0,5% of the rated voltage. The value of the load at the city consumers` varies greatly depending on the time. The load changes are cyclic in nature. Daily, weekly, and yearly cycles of load changes are distinguished. Load fluctuations have been studied by many authors, for example >@. The sources >@ give the standard daily load graphs for typical consumers. (Fig. 1) presents the power loads characteristic of an apartment house for winter extremes. 1,2 1 0,8 0,6 0,4 0,2 0 1

The key factors influencing the voltage drop value and the losses in distributing low voltage lines are the parameters of step-down transformers and lines, as well as the loads at the consumers. The length of the city cable lines is rather short, and the values of voltage drops and losses in these lines are small. Based on the estimates, the value of voltage drop in cable lines makes 1-2% per 1 km at the rated temperaturerise current in them. Actual lengths of cable lines, generally, are significantly less than 1 km. The average length of cable lines in the city of Komcomolsk– on-Amur is 0,52 km. The average cable line load rate calculated for the winter extremes as the ratio of cable current to the rated temperaturerise Section 7. Reliability and Continuity of Supply

3

5

7

9 11 13 15 17 19 21 23

Figure 1. – The graph for power loads at the transformer substation. for a standard apartment house equipped with electric stoves for a standard apartment house equipped with gas stoves Weekly cycles of load changes are explained by seven-day labour organization system. Yearly cycles are caused by seasonal temperature variations on the territory where the power system operates >@. The graph on (Fig.2) shows the relative maximum daily power load plotted against the time during a year.

411

Table 1 1,2 relaf.max.

1 0,8 0,6 0,4 0,2 0 1 5 9 13 17 21 25 29 33 37 41 45 49 53 t, weeks

Figure 2. The graph of the relative maximum daily power load plotted against the time during a year

This function can be approximated by:

Prelaf .

A B sin(C t D)

Pmax .doily Pmax .abs.

The measurements at Komcomolsk-on-Amur power substations have given the authors the like characteristics. For the residential areas the values of A 0,7 y 0,8 , B 0,2 y 0,3 , C 1,26 , and D 0,33 were obtained. Using the above graphs and specifying the parameters of A,B, and Pmax.load we can plot the graph of the expected loads for the year. The actual load diagram is likely to have some deviations from the calculated one. But these deviations are insignificant in terms of their duration and value. The power load of the city residential areas is, practically, active cos M 0,92 y 1 . The value of power losses in the step-down transformers and the voltage drop in them are greatly influenced not only by the power load, but also by the rated power and the parameters of the transformer operating into the load. The relation of the transformer rated power load to the maximum power load is important rather than the former one itself. To provide operation free of damage, in is practiced to install such substation transformers that Prated .transf . Pmax .load . Yearly power losses and voltage drops in step-down substation transformers were calculated for different maximum load at the consumers`. In those calculations, the transformer was chosen out of the power series produced in Russia and stipulated by Prated .transf . Pmax .load . Transformer performance data are given in Table 1.

Section 7. Reliability and Continuity of Supply

P rated kW

No-load losses kW

Short circuit losses

U short circuit %

U rated kV

25 40 50 63 100 160 180 250 320 400 560 630 1000

0,125 0,18 0,35 0,265 0,365 0,54 1 1,05 1,6 1,45 2,5 2,27 3,8

0,6 0,88 1,325 1,28 1,97 2,65 4 3,7 6,07 5,5 9,4 7,6 12,2

4,5 4,5 4,5 4,5 4,5 4,5 5,5 4,5 5,5 4,5 5,5 5,5 5,5

6 6 6 6 6 6 6 6 6 6 6 6 10

The diagram of loads was calculated based on the graphs of Figures 1 and 2. In this case the parameters of A and B varied. The transformer efficiency was determined was determined every hall and hour by the following equation:

K 1

Prated

Qno load .losses E 2 Q short .circuit cos M Qno load .losses E 2 Q short .circuit

where Prated is the transformer rated power, kW, Q no-load is no-load losses, Q short circuit is short circuit losses, E=Pload / Prated transf. is the capacity factor, Pload is the power load, kW, cos M is the power coefficient. The yearly power losses were calculated as the sum of losses for all performances: Q yearly.losses Qlosses . The value of the relalive

¦

yearly losses equal to the ratio of the total yearly losses to the supplied power was calculated as qi (1 ni ) .

¦

The transformer voltage drop was found by the following equation: 'U

E (U a cos M U r sin M )

E 2 (U r cos M U a sin M ) 2 200

, where Ua and Ur – are active and reactive voltage components, V. The maximum voltage drop occurs in the transformers during the maximum load. The results of the calculations for different values of cos M are given in Table 2. It can be assumed from the results of the calculations that:

412

Table 2 Pmax P consu trans. mers kW kW

COS(F)=0,93 dU max V

25 63 100 160 250 250 320 400 400 560 560 560 630

25 50 100 150 200 250 300 350 400 450 500 550 600

13,85 10,19 12,67 11,02 8,97 11,23 13,11 9,54 10,91 10,70 11,89 13,09 11,28

25 63 100 160 250 250 320 400 400 560 560 560 630

25 50 100 150 200 250 300 350 400 450 500 550 600

13,85 10,19 12,67 11,02 8,97 11,23 13,11 9,54 10,91 10,70 11,89 13,09 11,28

1. Voltage drops in the transformers reach the value of 13 V and over. The maximum value of voltage drop depends on cos M . Seasonal load fluctuations, actually, don`t affect the 'U max . 2. The relative yearly power losses

q

COS(F)=0,96

Q Q losses losses k Joule % A=0.8 B=0.2 7 145 738,04 2,53 12 655 685,76 2,24 22 612 547,62 2,00 30 347 490,84 1,79 46 690 050,93 2,07 54 838 426,59 1,94 80 674 117,63 2,38 70 400 927,32 1,78 78 208 068,65 1,73 112 938 581,17 2,22 121 758 395,54 2,16 131 376 457,47 2,12 112 980 516,12 1,67 A=0.7 B=0.3 6 440 297,03 2,61 11 630 917,01 2,36 20 279 573,59 2,06 27 549 745,29 1,86 43 610 024,34 2,21 50 346 208,33 2,04 74 009 264,17 2,50 65 172 425,49 1,89 71 593 858,04 1,82 105 012 028,58 2,37 112 333 951,54 2,28 120 266 850,41 2,22 104 576 354,76 1,77

Qloss equal to the ratio of the total Qsup pled

yearly losses to the total supplied power make (1,6y2,614)%. The value of there losses is affected by the seasonal and daily fluctuations of the load. When the amplitude of seasonal variations rises from 0,2 up to 0,3 the relative value of the losses increases by 1,1y1,2 times. To cut the power losses it was suggested that transformer substations be supplied with two transformers of different power >@. The lower power transformer operates when the load is low. When the load rises the higher power transformer is switched on and the lower power transformer is switched off. And both transformers are in operation when the load is built up significantly. For each pair of Section 7. Reliability and Continuity of Supply

dU max V

Q losses k Joule

Q losses %

12,86 9,31 11,55 9,90 7,98 9,99 11,71 8,43 9,65 9,45 10,51 11,57 9,71

6 938 611,90 12 288 321,00 21 940 512,20 29 442 128,00 45 306 944,28 53 190 443,38 78 282 830,38 68 294 137,02 75 849 239,51 109 609 315,01 118 140 083,05 127 445 737,07 109 568 243,34

2,46 2,18 1,94 1,74 2,01 1,88 2,31 1,73 1,68 2,16 2,09 2,05 1,62

12,86 9,31 11,55 9,90 7,98 9,99 11,71 8,43 9,65 9,45 10,51 11,57 9,71

6 255 792,92 11 298 137,51 19 683 313,88 26 736 344,59 42 333 485,43 48 847 446,70 71 842 498,47 63 241 857,34 69 453 455,03 101 956 883,46 109 034 987,06 116 706 868,19 101 444 648,14

2,54 2,29 2,00 1,81 2,15 1,98 2,43 1,83 1,76 2,30 2,21 2,15 1,72

transformers there exist certain ranges of efficient operation corresponding to the least losses and the load values at which the transformers should be switched over. There has been developed a circuit allowing for no losses of voltage when the transformers are switched oven. It is presented on (Fig. 3). 6

7

A 4

8

2

1 5

9 B

3

Figure 3. Switching circuit A - high voltage side, B – low voltage side, 1 – the lower power transformer, 2 – the higher power transformer, 3, 4, 5, 8, 9 – power circuit breakers with actuators, 6 – the power transforming device, 7 – the microcontroller.

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Table 3 Pmax Transformer Transformer consum 1 2 (kW) (kW) (kW) 25 50 100 150 200 250 300 350 400 450 500 550 600

25 40 63 100 160 160 160 160 160 250 400 400 400

25 100 160 160 160 400 400 630 630 630 630 630 630

25 50 100 150 200 250 300 350 400 450 500 550 600

25 40 63 100 160 160 160 160 160 160 250 400 400

25 100 160 160 160 400 400 400 630 630 630 630 630

25 50 100 150 200 250 300 350 400 450 500 550 600

25 40 63 100 160 160 160 160 160 250 400 400 400

25 100 160 160 160 400 400 630 630 630 630 630 630

25 50 100 150 200 250 300 350 400 450 500 550 600

25 40 63 100 160 160 160 160 160 160 250 400 400

25 100 160 160 160 400 400 400 630 630 630 630 630

COS(M)=0,93 dUmax 2 dU vax 1 (V) (V) A=0.8 B=0.2 6,91 10,38 7,47 6,32 7,12 7,34 7,30 8,03 7,34 7,63 6,42 6,80 6,38 8,17 8,48 6,56 8,51 7,50 6,66 8,44 5,94 7,78 6,54 7,84 7,14 7,76 A=0.7 B=0.3 6,91 10,38 7,47 6,32 7,12 7,34 7,30 8,03 7,34 7,63 6,42 6,80 6,38 8,17 6,42 9,54 8,51 7,50 8,26 8,44 6,50 9,39 6,54 7,84 7,14 7,76 cos (M) = 0,96 A=0.8 B=0.2 6,42 9,64 6,88 5,76 6,51 6,59 6,65 7,21 6,59 6,85 5,76 6,01 5,73 7,22 7,61 5,65 7,64 6,46 5,92 7,27 5,25 6,70 5,78 6,75 6,31 6,68 A=0.7 B=0.3 6,42 9,64 6,88 5,76 6,51 6,59 6,65 7,21 6,59 6,85 5,76 6,01 5,73 7,22 5,76 8,43 7,64 6,46 7,41 7,27 5,78 8,08 5,78 6,75 6,31 6,68


Q losses (kJoule)

Q losses %

6 831 268,40 11 285 289,92 20 015 007,80 28 596 249,78 36 760 882,76 44 948 625,76 53 648 137,74 63 390 399,90 72 068 775,05 80 485 117,93 87 795 832,31 94 788 274,52 101 996 527,5

2,42 2,00 1,77 1,69 1,63 1,59 1,58 1,60 1,60 1,58 1,56 1,53 1,51

6 191 696,62 10 266 562,50 18 024 457,84 25 488 622,07 33 137 090,55 40 331 780,82 47 569 093,56 55 995 773,23 63 553 486,30 72 266 298,02 79 378 777,46 86 000 304,48 91 996 525,16

2,51 2,08 1,83 1,72 1,68 1,64 1,61 1,62 1,61 1,63 1,61 1,59 1,56

6 634 311,31 10 954 568,85 19 418 066,50 27 739 351,44 35 658 821,63 43 594 957,84 52 023 588,88 61 472 421,21 69 881 123,20 78 039 926,04 85 142 297,63 91 915 446,56 98 897 871,06

2,35 1,94 1,72 1,64 1,58 1,54 1,54 1,56 1,55 1,54 1,51 1,48 1,46

6 015 227,29 9 967 868,73 17 490 212,02 24 729 568,69 32 148 989,84 39 123 328,33 46 134 240,45 54 302 632,36 61 629 189,61 70 082 580,12 76 969 544,60 83 407 874,25 89 214 796,54

2,44 2,02 1,77 1,67 1,63 1,59 1,56 1,57 1,56 1,58 1,56 1,54 1,51

414

For each load diagram there is a pair of transformers connected in the above circuit with the least total losses. In this research, there were found the transformer pairs securing the least yearly power losses. The estimatifons were obtained by exhaustion with the help of the special program providing optimization for the switching modes. the estimates are given in Table 3. 1. Comparing Tables 2 and 3 shows that the two transformers which a power substation is equipped with and which operate in power – switch –over mode ensure the 3-6 V reduction in voltage drop at maximum power loads. 2. Due to the optimized operation of the transformers, the relative yearly power losses decrease by 1,06-1,56 times, 1,23 in average. 3. The total power of substations and their cost rise. But the adoption of power regulated transformer substations may be economically expedient/ The payback period estimated on the basis of Russian prices is 7-15 years. REFERENCES 1. I. Tulnich, G. Nudler. Electric power lines for apartment and public buildings. – M. Energoizdat, 1983. –304 pages, ill. 2. V. Timchenko. Fluctuations of power load and exchanged power in power systems. – M. Energy, 1975. –208 pages with illustrations. 3. Hand book of electrical engineering in 3 volumes. V. 3, part I. Production and distribution of electric power. (edited by MEI: professors: I. Orlov (Chief editor and others)M. Energoizdat, 1988. – 880 pages with illustrations. 4. M. Klima. Power systems optimization. Trans from czech. Edited by V. Okoryakova. – M. Vysshaya shk ., 1991. 302 pages with ill. 5. M. Grinkrug, V. Solovjev, Yu. Tkacheva, D. Balagansky. Low voltage transformer substation with two-step power control to cut the losses. Komsomolsk–on–Amur, KnASTU, Bulletin, 2000. –12-14 pages. ill.


Mr. Miron Grinkrug was bon in 1957 in Komsomolsk-on-Amur, Russia. He received M.Sc. degree in technical engineering from Technical University in St.-Petersburg. At present he is professor of Department of Physics in Komsomolsk-onAmur State Technical University. His areas of interest include computer analysis of power system and electric power quality.

Mailing address: Miron Grinkrug Komsomolsk-on-Amur State Technical University 27 Lenina st., Komsomolsk-on-Amur phone: (42172) 3-61-50, fax: (42172) 3-61-50 e-mail: [email protected] Ms. Yulia Tkacheva was bon in 1965 in Vladivostok , Russia. In 1999 has finished KnASTU on speciality the engineer - builder. At present she is teacher of Department of Physics in Komsomolsk-on-Amur State Technical University. Her areas of interest include computer analysis of power system and electric power quality. Mailing address: Yulia Tkacheva Komsomolsk-on-Amur State Technical University 27 Lenina st., Komsomolsk-on-Amur phone: (42172) 3-61-50, fax: (42172) 3-61-50 e-mail: [email protected]

415


416

Section 6 EMC in Electrical and Electrical Power Engineering. Electrical and Exploitative Characteristic of Loads and Electrical Energy Converters

6.1. BECK H.P., RÖSNER J.: Wind Energy Converter with Asynchrounous Machines and 12-pulse AC Controller in Generator Mode (Germany) ....................................................................................419 6.2. PAULILLO G., ABREU J.P.G.: Analysis of 12-Pulse Power Converters Under Unbalanced Voltage Supply A Novel Transformer Topology (Brazil) .....................................................................425 6.3. KOCZARA W., DZIUBA R., LEONARSKI J., AL.-KHAYAT N.: Variable Speed Set for Embedded Power Generation (Poland, UK)........................................................................................433 6.4. STRZELECKI R., SMOLESKI R., KEMPSKI A.: Reduction of the Bearing Current in PWM Motor Drives by Means of Common Mode Voltage Cancellation (Poland) ........................................439 6.5. JANSON K., JÄRVIK J., VINNAL T.: Installed Capacities of Reactive Components and Transformer in Line Frequency Resonant Converters (Estonia) .........................................................445 6.6. MYASOEDOV Y.V., SAVINA N.V.: Evaluation of Influence of Low of the Electric Power Quality on the Account of Electric Power Consumption in Networks with the Tractive Load (Russia) ................................................................................................................................................451 6.7. FUSTIK V., ILIEV A., WEBER H., PRILLWITZ F.: Dynamic Characteristics of the Unit A in HPP Vrutok in Islanded Operation (Macedonia, Germany)................................................................459 6.8. TOROPCHINA L.V., TOROPCHINA S.V.: Quality Parameters of Electric Power and Their Influence on the Work of Electric Receivers (Russia)..........................................................................467 6.9. BARABOI A., ADAM M., PANCU C.: The influence of Supply Voltage Quality to the Actuators Behavior (Romania) ............................................................................................................471 6.10. WIDLOK H.: Energy Quality Aspects of Modernisation of a Large-Power Rolling Mill Drive (Poland) ................................................................................................................................................477

Section 6. EMC in Electrical Power Engineering. Electrical and Exploitative Characteristic ....

417

418




ANALYSIS OF 12-PULSE POWER CONVERTERS UNDER UNBALANCED VOLTAGE SUPPLY A NOVEL TRANSFORMER TOPOLOGY Gilson PAULILLO

José Policarpo G. ABREU

Itajuba Federal School of Engineering Itajuba, MG (Brazil)

Abstract - This paper presents a novel transformer topology to feed power converters under voltage imbalance supply based upon a novel approach of the special transformer ADZ, whose main contribution is non-characteristic harmonics mitigation. Additionally, the impact of voltage unbalance factor – VUF in the power converter harmonic generation is analyzed by addressing VUF, harmonic order and its magnitude, as well as positive voltage sequence, harmonic order and its magnitude. This analysis also contributes to show how powerful the proposed topology is. 1. INTRODUCTION Electric system voltages are not perfectly balanced due to uncompleted overhead transmission lines transposition and unbalanced loads, even 3-phase ones. As a consequence, they absorb unbalanced currents and then, in turn, the voltages become unbalanced. It can be said, therefore, that imbalance is inherent to the electric system, so much so that 2% of voltage imbalance limit in utility transmission and distribution planning studies are widely accepted [1]. The consequences of voltage imbalances in power converters have been widely studied and reported in technical literature. Several troubles come up due to non-characteristic harmonics generated by power converters feeds by an unbalanced voltage supply system. Protection relays malfunction, interference in communication systems, overvoltages and high currents, excessive losses in other rotating machines, high dc distortion, kVAr inductive meters errors, etc.. Since static power converters are nonlinear in nature, under non-ideal

conditions such as unbalanced voltage, they generate both characteristic and non-characteristic harmonics on both ac and dc sides [2]. Several papers have been investigating the performance of power converter under unbalanced supply conditions. Yacamini and Oliveira [3] have described a method of calculating harmonics associated with non-ideal supply conditions. Rashid and Maswood [2] analyzed the anglecontrolled scheme for a three-phase ac-dc power converter and had investigated the effects of the supply imbalance on the power factor, harmonic factor, distortion factor and THD. Sakui and Fujita [4] proposed a practical method for calculating the harmonic currents of a three-phase bridge controlled rectifier under unbalanced supply conditions. Another method to provide the closedform expressions for all harmonics, under balanced and unbalanced operating conditions, has been proposed by Enjeti and Ziogas [5]. Hu [6] described a technique for obtaining the sequence impedance and the equivalent circuit of a power converter at fundamental frequency, whose results can be used in the analysis of power converters under unbalanced supply. Another method, based on asymmetrical firing angle, to cancel the second harmonic at the converter output under unbalanced voltage supply was presented by Ngandui [7]. In all of these mentioned papers the authors deal with the modeling of converters through two different methods, namely time domain analysis and frequency domain analysis. However, it is important to remark that all of them deal mainly with power converter philosophy. This paper, instead, deals with a new proposal of power converter topology based upon a special transformer topology created and implemented by

Section 6. EMC in Electrical and Electrical Power Engineering. ...

425

Abreu, named ADZ [8]. This special transformer arrangement is based upon the ADZ feature of allowing the output voltage phasors module and phase angle to be controlled simultaneously or not, phase by phase. A model of this configuration applied to a 12-pulse power converter has been simulated by using EMTP simulation package Results from simulation showing the harmonic spectra as well as the voltage and current waveforms in ac-dc sides of the converter are also presented. These results show the technical feasibility of the proposed topology, especially when compared to classical configurations, such as Delta/Wye/Delta. Finally, an investigation concerning the impact of VUF on power converter’s harmonic generation addressing VUF, harmonic order and its magnitude is presented. Its results can be also used to show the effectiveness of the proposed topology.

Its conception is based on the proper selection of the proper auxiliary coils of the ADZ independently per phase. The basic idea is to make these phase voltages equal to the balanced portion of the phase voltages at the origin of the feeder. The undesirable portion will be eliminated and output voltage phasors will be balanced [9,10,11]. The voltage unbalance factor (VUF) is obtained by means of symmetrical components analysis considering VA, VB and VC input phase voltages of the compensator. The positive sequence voltage VT represents the balanced portion of the system voltage and will be used to determine the output voltages VX, VY and VZ as shown in figure 1. . VA . VB . VC

Figure 1 – The balanced system.

2. T-ADZ TOPOLOGY

According to [11], the taps and the polarities of the auxiliary coils can be determined as follows:

2.1. The ADZ The phase shift autotransformer ADZ has a complex and variable transformation ratio, enabling it to provide a non-conventional phaseshift between I-O voltage signals. The variations in the output voltages magnitude and/or phase angle allowed by ADZ is achieved through the properly selection of the taps and polarities of the ADZ auxiliary coils. In short, if the input voltage phasor has a module equal to 1.0 p.u. and an angle phase equal to 0q, the output voltage phasor base equation, in p.u., is [8,9,10]: . N N VS 10q K1T1 2 1 120q K 2T2 3 1 120q N1 N1

.

VX

.

V T

N . N . V A K1B T1B 2 V B K 2C T2C 3 V C N1 N1 .

.

VY

.

K 2A T2A .

VZ

(1)

.

VS - output voltage phasor N1 - number of turns of the main coil N2 - number of turns of the 1st. auxiliary coil N3 - number of turns of the 2nd. auxiliary coil K1 - polarity of the signal of the 1st. aux. coil K2 - polarity of the signal of the 2nd. aux. coil T1 - tap of the 1st. auxiliary coil T2 - tap of the 2nd. auxiliary coil

2.2. The LDUC The Line Drop and Unbalance Compensator – LDUC, is an application of the ADZ to mitigate or even eliminate imbalances in the electric system.

(2)

V T 120$ . N3 . N . V A V B K1C T1C 2 V C N1 N1

(3)

.

V T 120$

K1A T1A

where:

426

LDUC

. VX . VY . VZ

. N2 . N . V A K 2B T2B 3 V B V C N1 N1

(4)

where the parameters are the same as used in (1) and V is the module and T is the phase angle of the positive sequence voltage. The exact value of the taps tij can be found through the appropriate calculation of the previous equations. It is important to remark that for each tij two values of Tij are possible once the first tap means an exact value of the second one. In other words, once the system has three phases, the consequent arrangement of all possibilities results in sixty-four possible values for the output voltage. Applying the taps and the polarity values in (2) to (4), the output voltages and the VUF can be obtained. Thus, the actual taps to be selected at LDUC are chosen amongst those sixty-four possibilities. The selected possibility will permit the smallest imbalance level at the output terminals of LDUC. 2.3. A Novel Topology – T-ADZ Electrical Power Quality and Utilisation

In order to feed power converters a special topology was developed to control the module and phase angle of the output voltage, independently per phase. Its application permits to mitigate voltage imbalance and, consequently, to feed power converters under unbalanced voltage supply. This novel topology, presented in Figure 2, uses the following configurations in the primary and secondary coils: x primary: the same topology of ADZ is used; x secondary: two main coils per phase are used. The number of coils in the secondary is defined according to the equipment application. If it is used to feed a 6-pulse power converter then just one main coil will be necessary. Otherwise, from ADZ and LDUC, the proposed transformer is fed through auxiliary coils. This fact has been vital to the development of the special transformer. Besides, as the conception of this topology has been based upon ADZ and LDUC behaviors, the methodology for choosing the taps and the polarities developed in the previous section can also be used. Thereupon, considering the fact that the special transformer uses ADZ connection arrangement, it will be named T-ADZ. Its constructive characteristic is presented in figure 2.

From the previous equations showed for ADZ and LDUC, T-ADZ generic relationship can be obtained through MMF balance. It shows that the taps and the polarities are the only unknown variables in the T-ADZ input currents. In order to carry out this task, these variables will be obtained by processing a special methodology whose initial approach is the presented LDUC methodology by means of an interactive process which has been implemented by using the MatLab software. 3. SIMULATION RESULTS By means of EMTP simulation, a 12-pulse power converter was tested considering an ideal supply system and an unbalanced supply system. In both cases this converter was implemented considering Delta/Wye/Delta and T-ADZ arrangement for the main transformer. The T-ADZ model for the 12pulse converter is presented in figure 3. id

N1= N4= 1 pu N5= 0,577.N1 pu

ia1

xa1

n iA

xd

N4

iB

N5 iC

ed ia2

Xa2

Rd

N1

N 3 N 4

N 2

N 1

N 5

Figure 3 – T-ADZ topology applied to a 12-pulse power converter. In order to compare simulation and laboratory results, the maximum VUF considered was 5.4 % (VA=88,00q; VB=106,0240q; VC=100120q). Following the ac input current and dc output, voltage waveforms are presented, as well as the harmonic spectrum, obtained from this case.

Figure 2 – The novel topology T-ADZ. According to the kind of load to be connected to the secondary coil, the relationship between the number of turns of primary and secondary coils can be modified. As T-ADZ will be used to feed a 12-pulse power converter, where 30 degree phase shift between the converters are necessary, the following assumptions have to be considered: N1 = N4 = 1,0 p.u.; N5 = 0,577 p.u. Wye connection to the 1st secondary coil; Delta connection to the 2nd secondary coil.

3.1. Conventional Topology Figures 4 and 5 present the simulation results of conventional 12-pulse power converter.


427

INPUT CURRENT - PHASE B

Amplitude (pu)

0.2000

0.1500

0.1000

0.0500

0.0000 0.00

6.00

12.00

18.00

24.00

30.00

harmonic generation, considering conventional and T-ADZ topologies. In both cases, two groups of voltage imbalances were applied to the system presented in figure 3. In the first one, eight different voltage imbalance conditions were applied. In the second one, eight different unbalanced voltages with the same VUF. Table 1 and Table 2 present these groups.

Harmonic

Figure 4 – Input current harmonic spectrum – IB.

TABLE 1. Electric supply system condition. Case 1 2 3 4 5 6 7 8

OUTPUT VOLTAGE 200

Voltage (V)

150

100

50

VA 10 10 10 10 10 10 10 10

VB 1 240 1,01 240 1 240 0,863 240 1,06 240 1,1 240 1,13 240 1,1 240

VC 1 120 0,95 120 0,9 120 0,861 120 1,2 120 1,4 120 1,55 120 1,9 120

VUF [%] 0 1,9 3,5 5,0 5,5 10,3 13,5 21,3

0 0

10

20

30

40

50

TABLE 2. Electric supply system – VUF = 6 %.

Time (mS)

Figure 5 – Output voltage. 3.2. T-ADZ Topology Simulation results by using the T-ADZ topology feeding a 12-pulse power converter are presented at figures 6 and 7.

Case 1 2 3 4 5 6 7 8

VA 0,813 0 0,827 0 0,829 0 10 10 1,191 0 1,232 0 1,252 0

VB 0,844 240 0,855 240 1 240 1 227,7 1 240 1 240 1,139 240 1,151 240

VC 0,98 120 1 120 1 120 1 113,9 1 109,7 1 120 1 120 1,02 120

Type 3I¼- UV 2I - UV 1I - UV 2I - A 1I - A 1I - OV 2I - OV 3I - OV

V+ 0,88 0,89 0,94 1,00 1,00 1,06 1,12 1,14

INPUT CURRENT - PHASE B

Amplitude (pu)

0.2000

0.1500

0.1000

0.0500

0.0000 0.00

6.00

12.00

18.00

24.00

30.00

Harmonic

Figure 6 – Input current harmonic spectrum – IB T-ADZ topology. 200.0

150.0

100.0

4.1. Cases of Table 1

Vol tag e 50.0 (V)

0.0

0

10

20

30

40

50

Time (mS)

Figure 7 – Output voltage - T-ADZ topology. 4. ANALYSIS OF POWER CONVERTERS UNDER UNBALANCED VOLTAGE SUPPLY Further analysis has been carried out in order to study how the VUF affects power converters 428

where the voltage imbalance types means: 1I - UV – single phase under voltage imbalance; 2I - UV – two phases under voltage imbalance; 3I - UV – three phases under voltage imbalance; 1I - OV – single phase over-voltage imbalance; 2I - OV – two phases over-voltage imbalance; 3I - OV – three phases over-voltage imbalance; 1I - A – single phase angle displacement; 2I - A – two phases angle displacement;

Figures 8 through 11 present the behavior of noncharacteristics harmonics (3th through 29th) generated by the 12-pulse power converters, excluding characteristics ones (11th, 13th, 23th and 25th). Figures 8 and 9 show the results obtained by applying conventional topology. Results obtained from T-ADZ topology are shown in figures 10 and 11.


example, 3rd harmonic is reduced from 10.9 % to 3.6 % and 15rd harmonic is reduced from 5.3 % to 2.2 %, just changing the power converter topology. The same occurs with all other harmonics; the impact of voltage imbalances in the power converters harmonic generation. In this case, the operation of power converter is not a problem. The strong point are the effects of its operation under voltage imbalances on the power system, specially non-characteristic harmonics; considering this point of view, the T-ADZ is a strongly positive solution. When applied to power converters, the non-characteristic harmonics damages can be minimized.

12 10

Magnitude [%]

8

3th harmonic 5th harmonic 7th harmonic 9th harmonic 15th harmonic

6 4

-

2 0 0

1.9

3.5

5.0

5.5

10.3

13.5

21.35

VUF [%]

Figure 8 – Non-characteristics Harmonics (3th through 15th) – Conventional Topology.

-

4 3.5

Magnitude [%]

3 17th harmonic 19th harmonic 21th harmonic 27th harmonic 29th harmonic

2.5 2 1.5 1 0.5 0 0

1.9

3.5

5.0

5.5

10.3

13.5

21.35

VUF [%]

4.2. Cases of Table 2 These cases have been selected to verify how these different voltage imbalances conditions also affect non-characteristic harmonic generation. This analysis has been carried out related to positive voltage sequence and non-characteristic harmonics magnitude. Figures 12 to 15 show these results.

Figure 9 – Non-characteristics Harmonics (17th through 29th) – Conventional Topology.

4 3.5

4

Magnitude [%]

3 3th harmonic 5th harmonic 7th harmonic 9th harmonic 15th harmonic

2.5 2 1.5

Magnitude [%]

3

3.5

2.5


2 1.5 1 0.5

1

0 0.88

0.5

0.89

0

0.94

1.00

1.00

1.06

1.12

1.14

Positive Voltage Sequence [p.u.]

0

1.9

3.5

5.0

5.5

10.3

13.5

21.35

VUF [%]

Figure 10 – Non-characteristics Harmonics (3th through 15th) – T-ADZ Topology.

Figure 12 – Non-characteristic Harmonics (3th through 15th) – Conventional Topology. 3.5

2.5

3 2

1

Magnitude [%]

Magnitude [%]

2.5 17th harmonic 19th harmonic 21th harmonic 27th harmonic 29th harmonic

1.5


2 1.5 1

0.5

0.5 0 0

1.9

3.5

5.0

5.5

10.3

13.5

21.35

0 0.88

0.89

VUF [%]

Figure 11 – Non-characteristics Harmonics (17th through 29th) – T-ADZ Topology. Comparing these results, it is possible to observe: - the effectiveness of the T-ADZ topology, specially in the low order harmonics. For

0.94

1.00

1.00

1.06

1.12

1.14


Figure 13 – Non-characteristic Harmonics (17th through 29th) – Conventional Topology.


429

0.9 0.8

Man\gnitude [%]

0.7 0.6 3th harmonic 5th harmonic 7th harmonic 9th harmonic 15th harmonic

0.5 0.4 0.3 0.2

need to change the traditional 12-pulse power converter operation. So, the conventional 12-pulse power converter can be connected in an unbalanced supply system without restrictions. However, the T-ADZ topology needs a robust and fast tap change system for its proper operation. 6. REFERENCES

0.1 0 0.88

0.89

0.94

1.00

1.00

1.06

1.12

1.14


Figure 14 – Non-characteristic Harmonics (3th through 15th) – T-ADZ Topology. 0.6

Magnitude [%]

0.5

0.4


0.3

0.2

0.1

0 0.88

0.89

0.94

1.00

1.00

1.06

1.12

1.14


Figure 15 – Non-characteristic Harmonics (17th through 29th) – T-ADZ Topology. Curves from figures 14 and 15 show that the harmonics behavior is independent from positive voltage sequence when T-ADZ topology is used. Consequently, there is no relationship between them. Harmonic magnitudes present almost a constant behavior in all of simulated cases. However, this can not be said about the conventional ones. Once more, the main conclusion from Figure 12 to 15 is the effectiveness of the T-ADZ topology when comparing to conventional ones. For example, in the case 8, with positive voltage sequence values 1.14, the 9th harmonic is reduced from 2.3 % to 0.6 %. 5. CONCLUSIONS This paper presents an analysis of power converters under voltage unbalance, introducing a new transformer topology, T-ADZ, which can mitigate non characteristics harmonics generated by the converter when fed with unbalanced voltages. The results also show the technical feasibility of T-ADZ as an effective option to mitigate voltage imbalance in the electric system. When comparing T-ADZ to other solutions proposed by different authors, there will be no

430

1. GCPS/CTST/GTCP Technical Repport.: Critérios e Procedimentos para o Atendimento a Consumidores com Cargas Especiais. Eletrobrás, Rio de Janeiro, Brazil, Fev. 1993 (in Portuguese). 2. Rashid M.H. and Maswood A.I.: Analysis Of Three-Phase AC-DC Converters Under Unbalanced Supply Conditions. IEEE Transaction on Industrial Applications, Vol. 24, No. 3, pp. 449 – 455, May/June 1988. 3. Yacamini R. and Oliveira J.C.: Harmonics in Multiple Convertor Systems: A Generalized Approach. IEE Proceedings-B, Vol. 127, no 2, pp. 96-106, March 1980. 4. Sakui N.M. and Fujita H.: Calculation of Harmonic Currents in Three-phase Convertor with Unbalanced Power Supply Conditions. IEE Proceedings-B, Vol. 139, no 5, pp. 478484, September 1992. 5. Enjeti P.N. and Ziogas P.D.: Analysis of a Static Power Converter under Unbalance: A Novel Approach. IEEE Transactions on Industrial Electronics, Vol. 37, no 1, pp. 91-93, February 1990. 6. Hu L.: Sequence impedance and equivalent circuit of a converter system. IEE Proceedings - Electrical Power Applications, Vol. 144, no 6, pp. 409-414, November 1997. 7. Ngandui E., Olivier G., April G.E. and Guimarães C.A.M.: DC Harmonic Distortion Minimization of Thyristor Converters under Unbalanced Voltage Supply using Asymmetrical Firing Angle. IEEE Transactions on Power Electronics, Vol. 12, no 2, pp. 332342, March 1997. 8. Abreu J.P.G.: Development and Implementation of a Phasor Controller. Doctorate Thesis, University of Campinas, Campinas, Brazil, 1990 (in Portuguese). 9. Abreu J.P.G., Arango H. and Paulillo G.: Proposal for a Line Drop and Unbalance Compensator. Proceedings of the IEEE - 7th ICHQP - International Conference on Harmonics and Quality of Power, Las Vegas USA, pp. 276-279, 1996. 10. Abreu J.P.G., Guimarães C.A.M. and Paulillo G.: A Power Converter Autotransformer. Proceedings of the IEEE – International Electrical Power Quality and Utilisation

11.

Conference on Harmonics and Quality of Power, Athens – Greece, October, 1998. Paulillo G.: An Electromagnetic Voltage Unbalance Compensator. Master’s Dissertation, EFEI, Itajubá, Brazil, 1996 (in Portuguese).

Gilson Paulillo was born in 1967 in Araraquara, Brazil. He received his M.Sc. degree in electrical engineering from Itajuba Federal School of Engineering - EFEI, where he is finishing his Ph.D. program. Currently, he is a Researcher with the Power Quality Study Group – GQEE, of the Institute of Electrical Engineering of EFEI. He has been nominated for the Executive Secretariat of the 10th IEEE ICHQP - International Conference on Harmonics and Quality of Power, to be held in Rio de Janeiro, Brazil in 2002. His areas of interest include power quality, induction motors and transformers and electric drives, as well as computer analysis of power system. Mailing address: Gilson Paulillo Itajuba Federal School of Engineering - EFEI Institute of Electrical Engineering – IEE Power Quality Study Group - GQEE P. O. Box # 50 37500-903 Itajuba - MG BRAZIL phone: (+55)(35) 3629-1312, fax: (+55)(35) 3629-1326 e-mail: [email protected]

Dr. José Policarpo G. Abreu was born in 1952 in Madeira Island, Portugal. He received a Ph.D. degree in Electrical Engineering from the University of Campinas (Brazil) in 1991. He is Full Professor and the Power Quality Study Group Coordinator at Itajuba Federal School of Engineering (EFEI), Brazil. He has been nominated for the Chairmanship of the 10th IEEE ICHQP - International Conference on Harmonics and Quality of Power, to be held in Rio de Janeiro, Brazil in 2002. Currently, his research interests include power quality issues, power definitions under abnormal conditions, induction motors and transformers, and electric dirves.

Mailing address: José Policarpo G. Abreu Itajuba Federal School of Engineering - EFEI Institute of Electrical Engineering – IEE Power Quality Study Group - GQEE P. O. Box # 50 37500-903 Itajuba - MG BRAZIL phone: (+55)(35) 3629-1243, fax: (+55)(35) 3629-1187 e-mail: [email protected]


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432





VARIABLE SPEED SET FOR EMBEDDED POWER GENERATION Wlodzimierz KOCZARA Robert DZIUBA Jaroslaw LEONARSKI Technical Univesity of Warsaw Warszawa, (Poland)

Abstract – New power generation system of variable speed constant voltage and frequency, three phase four wires is described. The variable speed integrated generator (VSIG) consists of prime-mover, permanent magnet generator and three-phase ac/ac power electronic converter. The VSIG speed is adjusted by load power signal. The system performs high quality output voltage also for unbalanced and non0linear loads. Results of tests confirm the quality of the output ac voltage. 1. INTRODUCTION Deregulated electrical energy market results in development of low cost power sources. Power authority charge end-user not only by costs of electrical energy production but also long distance transmission and distribution. Additionally when the end-user requires increase of demand of a delivered power and it results in need of reinforcing of supply grid then costs of the access to power are very high and again the end-user is charged. As an alternative to grid connection is generation power on site. (Fig. 1) There are on the marked great number of manufactures offering gensets equipped in synchronous generators driven by diesel engines. However, such conventional gensets are large, heavy and noisy. The genset is sized to peak power including non-linear loads. Oversized genset is expensive and its fuel demand is high. Operation with typical load profile with one or two peaks of power causes that the generating unit average load is mostly much below the rated power. Low load operating damages quickly the engine. When per year number of operating is high then to avoid this damage an additional load resistors are used (“dummy load”). These additional loads are obvious source of fuel

Nazar AL-KHAYAT Newage International Stamford, (United Kingdom)

losses and then exploitation costs are high. However, an emerging technique of variable speed power generation [1] – [4] offers new small size, not noisy gensets and hence is very promising for the future power generation. The paper present power quality of the variable speed three-phase generation units developed and produced by Newage International. 2. SYSTEM OVERVIEW The variable speed generating set block diagram is shown in Fig.2. The prime mover PM drives the permanent magnet generator PMG that produces variable frequency and amplitude ac voltage. A power electronic block Co converts variable amplitude and frequency voltage into constant standard 50Hz/3*400V ac voltage. The speed of the engine is adjusted to demanded load power that is sensed by power sensor PS. The reference speed signal Sre, produced by the load power sensor is followed by the speed controller SC that receives additionally signal of an actual speed Sac and initial (no load) reference speed Smin. The speed controller adjusts the fuel injection of the engine. The output voltage, produced by the converter Co is independent of speed of the generator. Therefore, speed is adjusted to get best performances of the engine. The range speed of prime mover depends on used engine and type of load. Usually for diesel engine this range is between 1200rpm to 3000rpm. In practice the industrial diesel engine is not dedicated to operate continuously with 3000rpm. Therefore the maximum speed 3000rpm is related only to peak power that average time is much smaller than time of engine operation. In case of frequent peak power the maximum speed is adequately reduced. It is known that maximum efficiency (minimum fuel consumption) of the currently produced industrial diesel engines is


433

FUEL EXPLORING & PRODUCTION

FUEL DISTRIBUTION

GENERATION

MARKET

TRANSMISSION ON SITE

POWER GENERATION DISTRIBUTION

RETAIL & SERVICES

CONSUMER A CONSUMER B

Fig.1. Conventional and dispersed power supply systems

PMG Generator

Pacv

Co Converter

Pm SS Speed Sensor

Pacs

Lo Load 3Ph-3/4W Pacs

PM PS Power Sensor

Prime-mover Pch FI Fuel Injection

Fu

ES Energy Storage

Fc

Sac

SC Speed Controller

Sre

Srmin Fig.2.Blockdiagram of the Variable Speed Integrated Generator (VSIG). 434


close to 2000 – 2300 rpm. This range speed is related to 60 to 80% of rated load power that in practice is close to average load power. As the output voltage does not depends on the generator voltage then any generator can be applied. The Newage International recently developed a new range of permanent magnet axial flux generators and controllers. Fig. 3 shows an example of such generator. Axial flux generator [3], [4] offers a number of advantages when compared to radial flux generator. The machine configuration allows the construction of a compact machine with high number of poles. Basically, the generator consists of two rotor plates and a slotless toroidal wound stator. The generator rotor plates gets bolted on the engine shaft while it’s stator get fixed to the engine bell housing. The generator is modular in design with magnet width and wire size as the only parameters to adjust to for different power delivery capabilities. Using the axial flux permanent magnet generator we get saving in generator weight, size and price. The generator, integrated to engine, as is shown in Figure 3, is very short and light. Three-phase four wires converter produces three independent ac voltages. A digital signal processor (DSP) operating as system controller is used to keep the three ac instantaneous voltages. The same DSP is applied to calculate output power and then to control speed of the engine.

3. POWER QUALITY To provide a high quality of output power the generating unit has to perform x Constant amplitude and rms of the output sinusoidal voltage x Low total harmonic distortion at both linear and non-linear loads x Balanced and unbalanced load capability x Short circuit and over-current protection x High transient current x Constant frequency x Low DC component in the output voltage (to avoid problems with transformers saturation) The output ac voltage is produced by the dc/ac converter. Multiloop control circuit of the control system of the output voltage is designed to assure all above requirements. The output voltage is corrected every Pwhat is not possible in conventional synchronous generator where voltage is controlled through adjusting excitation current. The three-phase four wires dc/ac converter controls separately every phase voltage. Thus unbalanced three-phase loads are not effecting the quality of the output voltage. Special algorithm has been developed for nonlinear loads. Demanded current with high crest factor can be supported with low output distortion.

Fig. 3. Permanent magnet generator integrated to diesel engine.


435

Transient current depends on the rating of the IGBT and dc link capacity. Frequency of the output voltage is dependent on microprocessor reference and thus may be precisely maintained or adjusted. As the output voltage is produced by synthesis of impulses with PWM method and feedback signals are responsible for measurement errors, a DC component may appear on the output voltage. 4. LOAD TESTS A 19kVA 3 phase 380-415V 50/60Hz unit variable speed integrated generator was built and tested. The VSIG system is significantly lighter in weight (250kg lighter) and smaller in size (1m3 smaller) when compared to a more conventional genset. The permanent magnet generator has a high power density 1.5kW/kg (about ten times a conventional generator). The power electronic stacks are engineered for minimum size. Both (air and liquid) cooling techniques were developed. Special attention was paid to the output waveform quality. The system was tested with different types of load (leading and lagging power factor). The proposed control algorithm was realised by using a high speed DSP. Tests proved that the proposed control algorithm is robust and of sufficiently high bandwidth to achieve desired performance. Fig. 4 shows the output voltage and load current at single from 5 to 12 kW. The ac voltage is effected in very short time and coming back to its initial value. Voltage waveform in case of rated active power load is shown in Fig. 5. The voltage is high quality sinewave. Behaviour of the inverter at nonlinear load is shown in Fig 6. In this case, only one phase of the inverter is loaded by a full-bridge rectifier with an LC filter. The THDV factor for the non-linear load is 2.36% at crest factor =3.6

DC VOLTAGE

AC CURRENT

AC VOLTAGE

Fig. 4. DC voltage, AC current and AC voltage for step of load from 5 kW to 12 kW.

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Fig. 5. Ouptput voltage 240 vrms and load current for a case of phase rated active power.

Fig 6. Voltage waveform for phase non-linear load 2.5 kW crest factor 3.6, THDV =2.36%. Three phase voltages are presented in Fig. 7 and 8. Fig. 7 demonstrates case of step load in phase u whereas the Fig. 8 turn-off the load. In both case the output voltage is stiff and there is no any difference between loaded and unloaded phase. The experimental results proved excellent performance of the inverter. 5. Summary A novel type generating set was presented. The variable speed concept enables a better utilisation of the engine capabilities and the economy of the set is very attractive to the user. The set will out perform the conventional set in many areas. Variable speed generating technology is an emerging technique that can be used to improve technique of power generation and its feasibility is increasing due development of power electronics and DSP control. The experimental results proved excellent performance of the proposed inverter control algorithm. The system works very stable loaded both linear and non-linear. The output voltage is very stable at no load and full load 0.5%and has THDV factor smaller than at nonlinear loads 2.4%. Electrical Power Quality and Utilisation

AC Voltages

Current

(a)

Fig. 7. Thre-phase no-load voltage and unbalanced step load –phase U rated power.

Fig. 8. Three phase voltage and transients when turn-off rated power - phase U.

6. REFERENCES 1. Koczara W., Dziuba R., Leonarski J., Nazar Ak-Khayat: Flexible Speed Power generation as an Emerging Technology of Electrical

Power Generation. Proceedings of International Scientific Conference on “Energy saving in Electrical Engineering. Warsaw, 1416 of May 2001. Poland pp. 223 – 227.


437

2. Grzesiak L., Koczara W., Da Ponte M.: LoadAdaptive Variable Speed Generating System – Behaviour Analyse of Dynamic. Proceedeings of European Power Electronic and Drive Conference EPE’99 Lausanne September 1999. 3. W. Koczara, L. Grzesiak, M. da Ponte: Hybrid Load-Adaptive Variable-Speed Generating Set: New System Topology and Control Strategy. Proceedings of International Conference on Power Generation, Powergen, Orlando, Florida 7-9 December 1998, USA. 4. Grzesiak L., Koczara W. M. da Ponte. Power Quality of the Hygen Autonomous Load – Adaptive Adjustable Speed Generating System, 1999. Applied Power Electronics Conference APEC’99. Dallas, Texas, March 1999. 5. E. Clark, N. Sidell, Jewell, D. Hove: High Temperature Electromagnetic Devices. TRW Automotive Technical Centre. The U.K. Magnetic Society UK. 5 October 2000. Solihull. 6. N. Brown, L. Haydock, E. Spooner: 3 Dimensional Finite Element Analysis of a Toroidal Wound Axial Flux Permanent Magnet Generator. UPEC’99, Leicester. UK 1999.

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Robert DZIUBA, Technical University of Warsaw, Institute of Control and Industrial Electronics ul. Koszykowa 75, 00-662 Warszawa, Poland e-mail: [email protected] Dr. Nazar AL-KHAYAT, Prof. Lawrence Haydoc, Newage International Barnack Road, Stamford PE9 2NB, United Kingdom e-mail: [email protected] Prof. Wlodzimierz KOCZARA Technical University of Warsaw, Institute of Control and Industrial Electronics ul. Koszykowa 75, 00-662 Warszawa, Poland e-mail: [email protected] Jaroslaw LEONARSKI Technical University of Warsaw, Institute of Control and Industrial Electronics ul. Koszykowa 75, 00-662 Warszawa, Poland e-mail: [email protected]





REDUCTION OF THE BEARING CURRENT IN PWM MOTOR DRIVES BY MEANS OF COMMON MODE VOLTAGE CANCELLATION Ryszard STRZELECKI Robert SMOLESKI Adam KEMPSKI Technical University of Zielona Gora Zielona Gora (Poland)

Abstract - In this paper the method of active common mode voltage cancellation using serial active filter is presented. The filter is based on emitter follower using complementary MOSFET transistors. The experimental results show that designed and constructed active filter is capable to significantly reduction of common mode voltage and to whole elimination of bearing currents (EDM Electric Discharge Machining) in bearings of induction motors fed from PWM voltage source inverters. The power dissipation in active filter is analyzed as well. 1. INTRODUCTION The phenomenon of bearing currents in induction motors has been known for decades. It has been reported by Alger [1] in the 1920’s that the basic reason for these currents is asymmetric flux distribution inside of the motor. This problem has been effective solved with modern motor designing and manufacturing practices. However, unexpectedly, the problem has returned since power electronic devices are becoming common in adjustable speed drives (ASD). During the past few years it has recognized that the numbers of prematurely faulted bearings of induction motors fed from PWM inverters has increased significantly [2]. These failures can be easily distinguished from the other due to unique detoriation pattern (flutting on a bearing race). Mainly responsible for the bearing currents in modern ASD’s is unavoidable instantaneous electrical asymmetry at the output of the inverter. 2. COMMON MODE VOLTAGE AND BEARING CURRENTS IN PWM ASD DRIVE

impossible to achieve the balance between phases instantaneously. The average voltage in a neutral point with respect to ground create so called common mode (CM) voltage source of system:

un

u A u B uC 3

(1)

Fig.1. shows the output phase voltage waveforms generated as a series of nearly rectangular pulses of different widths and resulting CM voltage waveform. 500V 0V 500V 0V 500V 0V 500V

Phase A Phase B

Phase C Common mode voltage UN

0V

0

20

40 ms

Fig.1. Common mode voltage generation at the output of PWM inverter.

The CM voltage is a staircase function of amplitude equal to DC bus voltage Ud and the frequency equal to the inverter switching frequency. The steps of CM voltage are r1/3 Ud as it is schematically shown on Fig.1. The experimental waveform of CM voltage is depicted on Fig.2.

When the motor is powered by PWM inverter, it is


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Fig.2. Common mode voltage at the output of inverter. The source of CM voltage at the output of inverter is the cause of a voltage emerging on the shaft because of the distribution of parasitic capacitances inside of the motor. These create an internal capacitive divider and the shaft voltage can be expressed [2] as:

UW UO

S SW

C SW C B CWK

In modern bearings the thickness of lubrication layers is 0.2 Àm – 2 Àm [3]. It enables charge to accumulate on the rotor assembly until the voltage exceeds the dielectric capability of the bearing grease and then can lead to frequently repeated flashover in bearings. This is known as an electric discharge machining (EDM). It is the reason of premature electrical induced bearing failure. Under normal operating conditions it is impossible to observe directly the bearing currents that flow inside the bearing. To detect bearing currents the special treatment has been employed. To break up the EDM current path the bearings have been insulated from the motor frame and thin insulator layers have been bypassed by special measuring wires (Fig.4). Insulation layer

(2)

where : UW -shaft voltage, U -voltage of stator windings neutral point, CSW-capacitance between stator windings and rotor, CB-bearing capacitance, CWK-capacitance of air gap.

Special wire

PE

Fig.4. Measuring arrangement.

The necessary condition for a shaft voltage appearance is the bearing capacitance CB (between balls and races of bearing). It has appeared as a result of a thin hydrodynamic oil film in rotating bearings.In this case the bearing balls are not in electrical contact with races because the grease has an insulating effect. Fig.3 shows common mode voltage at stator windings neutral point and shaft voltage.

The destructive EDM current is a wave-formed impulse with risetime of nanoseconds. A peak of EDM current can reach a value of amperes As we have been proved in previous works [4] the EDM pulses are electrostatic effects and they appear randomly. Their occurrence depends mainly on the level of shaft voltage. The EDM currents are often observed directly coupled to a switching instant of the inverter but in our investigations we have often observed the delay time between reaching the level of shaft voltage which is sufficient to discharge and the moment of breakdown as well (Fig.5).

Fig.3. Common mode voltage at stator windings neutral point and shaft voltage.

Fig.5. Shaft voltage and bearing current events.

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The voltage rise rate du/dt has then a minor influence. Therefore, as we have proved using statistical approach [5], passive methods for common mode current reduction are not effective solutions for bearing currents elimination (histograms on Fig.6 and Fig.7).

executed with the application of the emitter follower (T1,T2). The common mode voltage is detected on Y-connected capacitors (C1) and added to the phase voltages by the common mode transformer (Fig.8). Such technique of elimination can be identified with serial active filter.

Fig.8. Drive system with serial active filter. 3.2. Accuracy of CM voltage compensation in experimental system Fig.6. 3-D histogram of bearing current amplitude and awaiting time to puncture (drive without CM choke).

Fig.7. 3-D histogram of bearing current amplitude and awaiting time to puncture(drive with CM choke)

The original arrangement build by Ogasawara and Akagi was designed using bipolar transistors for 3x200V 60Hz inverter supply. Each of transistors of emitter follower has to work under whole DClink voltage (282 V). In our investigations we have examined the concept of serial active filter for 3x380V 50Hz. Bipolar p-n-p transistors of rating voltage of 600 V are not available as a standard. MOSFET P-channel transistors of required ratings are accessible easier. They are faster than bipolar transistors but require use of gate resistor. The voltage drop on the gate resistor in switching instant of the inverter can cause slight deviation of full compensation, especially in case of very fast switching event. Fig.9. shows CM voltage in neutral point of wye-connected capacitors at the output of inverter and CM voltage at the output of emitter follower.

3. EDM CURRENT ELIMINATION BY MEANS OF CM VOLTAGE CANCELLATION 3.1. Principle of serial active filter The active common mode voltage cancellation method proposed by Ogasawara and Akagi [6] allows mitigating the problem at its source rather than suppressing du/dt by means of conventional passive methods. Natural consequence of the common mode voltage elimination is the elimination of the shaft voltage and bearing currents. The addition common mode voltage of opposite polarity to phase voltages realizes the compensation in the proposed system. It is

Fig.9. CM voltage at neutral point at the output of inverter and CM voltage at the output of emitter follower.


441

As we can see from the Fig.9 output voltage of emitter follower maps common mode voltage waveform almost perfectly with the exception of high frequency oscillations after the fastest switching. Observed strongly damped switching oscillations are caused by high value of voltage drop on the gate resistor in this case. Fig.10 shows the influence of risetime of common mode voltage waveform on voltage drop on the gate resistor and voltage waveform at the output of emitter follower.

Fig.11. Drain-source voltage, drain current and power dissipation in emitter follower; fc = 16kHz.

Fig.10. Voltage drop on the gate resistor and voltage at the output of emitter follower. 3.3. Power dissipation in the emitter follower The magnetizing current is supplied mainly from the emitter follower due to the low output impedance of this circuit. It means that magnetizing current flows in both transistors and power losses in emitter follower depend on inductance of load (CM transformer). The magnetizing current has approximately piecewise triangular waveforms because of rectangular shape of common mode voltage. The amplitude of this current depends on inductance of CM transformer and carrier frequency of inverter. The absolute maximum power dissipation and carrier frequency period are the basis of the selection of magnetizing inductance of CM transformer [7]. Fig.11-15 show the comparison of power dissipation in one of transistors at different carrier frequency of inverter. Additionally, these figures show waveforms of drain-source voltage and drain current, respectively. The total amount of dissipated power in whole emitter follower is approximately two times greater.

442





Fig.16. Shaft voltage and bearing current in drive with serial active filter.

Total power dissipation measured in the same time interval has changed from 13.6 W for carrier frequency 16kHz to 35.2 W for 4kHz.

Note that trigger status on Fig. 16 is set as „normal” and the trigger value is 20 mA only.

4. EFFECT ON EDM CURRENTS Presented serial active common mode filter is capable for elimination of baring current by means of common mode cancellation. Fig.15. shows an effect of serial active filter on CM voltage and shaft voltage waveforms.

Fig.15. Common mode voltage and shaft voltage in drive with serial active filter. The almost perfectly cancellation of CM voltage in our system causes that value of shaft voltage not exceed 3 V. It is to small value to puncture of the grease film in bearing. In such condition bearing currents have not emerged. It has been experimentally proved that no EDM current was observed in long measuring period (Fig.16).

5. CONCLUSIONS The aim of our work has been adapted the concept of common mode voltage cancellation to EDM current elimination in ASD drives fed from mains of rated value obligatory in UE. It was necessary to apply MOSFET transistors in place of bipolar transistors used in original work of Ogasawara and Akagi. In spite of voltage drop on gate resistor our serial active filter is effective solution for CM voltage cancellation and consequently of EDM currents problem. The power dissipation of emitter follower corresponds to a small fraction of rated power of induction motor and strongly depends on inverter carrier frequency and inductance of CM transformer. Applying of high carrier frequency leads to decrease of magnetizing current and allows reducing dissipating power and active filter dimensions. Our current researches concern on the selection of MOSFET transistors and core material of CM transformer to decrease of power dissipation and dimensions of active filter. 6. REFERENCES 1. Alger, Samson: Shaft Currents in Electric Machines, A.I.R.E. Conference, Philadelphia, Pa., February 1924. 2. Macdonald D., Gray W.: PWM drive related bearing failures, IEEE Industry Appl. Magazine July/August, 1999 pp. 41-47. 3. Doyle Busse et al.: System Electrical Parameters and Their Effects on Bearing Currents, IEEE Trans. on Industry Appl., vol. 33, No. 2, March/April 1997 p.577-585.


443

4. Kempski A.: Capacitively Coupled Discharging Currents in Bearings of Induction Motor Fed from PWM (Pulsewidth Modulation) Inverters, Electrostatics 2001 Journal of Electrostatics (in press). 5. Kempski A., Strzelecki R., Smoleski R., Fedyczak Z.: Bearing Current Path and Pulse Rate in PWM-Inverter-Fed Induction Motor PESC’2001 Vancouver, in press. 6. Ogasawara S., Akagi H.: An Active Circuit for Cancellation of Common Mode Voltage Generated by PWM Inverter, PESC’97, 1997, pp.1547-1553. 7. Ogasawara S., Akagi H. Modeling and Damping of High-Frequency Leakage Currents in PWM Inverter-Fed AC Motor Drive System, IEEE Industry Appl. Magazine, September/October, 1996 pp.1105-1113.

M.Sc. Robert Smoleski was born in 1973 in Krosno Odrzaskie, Poland. He received the M.Sc. degree in electrical engineering from Technical University of Zielona Góra. At present he is Researcher in Institute of Electrical Engineering of Technical University of Zielona Góra. His field of interest is Electromagnetic Compatibility in Power Electronics. Mailing address: Robert Smoleski Technical University of Zielona Góra Institute of Electrical Engineering Ul. Podgórna 50 65-246 Zielona Góra phone:(+48)(0-68) 32-82-253, fax:(+48)(0-68)325 46 15 e-mail: [email protected]

Prof. Ryszard Strzelecki was born in 1955 in Bydgoszcz, Poland. He received M.Sc. degree from Technical University of Kiev, Ph.D. and D.Sc. degrees from Institute of Electrodynamics of the Ukrainian Academy of Science. Presently he is Professor of Institute of Electrical Engineering in Technical University of Zielona Góra, Poland. His research interests are in area of modelling, control, design and stability analysis of the power electronics systems. Mailing address: Ryszard Strzelecki Technical University of Zielona Góra Institute of Electrical Engineering Ul. Podgórna 50 65-246 Zielona Góra phone:(+48)(0-68) 32-82-508, fax:(+48)(0-68)325 46 15 e-mail: [email protected]

Dr Adam Kempski was born in 1953 in K³pno, Poland. He received the M.Sc. and Ph.D. degrees in electrical engineering from Technical University of Wrocaw. At present he is Researcher in Institute of Electrical Engineering of Technical University of Zielona Góra. His field of interest is Electromagnetic Compatibility in Power Electronics. Mailing address: Adam Kempski Technical University of Zielona Góra Institute of Electrical Engineering Ul. Podgórna 50 65-246 Zielona Góra phone:(+48)(0-68) 32-82-342, fax:(+48)(0-68)325 46 15 e-mail: [email protected]

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INSTALLED CAPACITIES OF REACTIVE COMPONENTS AND TRANSFORMER IN LINE FREQUENCY RESONANT CONVERTERS Kuno JANSON, Jaan JÄRVIK - Member, IEEE,

Toomas VINNAL

Tallinn Technical University Tallinn (Estonia)

Abstract – This paper is devoted to the comparison of specific capacities of capacitors, inductors and transformers in different types of network frequency resonant converters. For the converter with alternating parallel and series resonance approximate relation between the specific capacities, operating point range and overloading capacity of capacitors and inductors have been developed.

1. INTRODUCTION During recent years there has been increasing interest towards high power factor converters with low THD in line current. Switch-mode converters have been in the focus of this development. But there are also available line frequency resonant converters that are also line friendly. Making use of such converters has been modest, as they include bulky and heavy reactive components. Comparing line frequency converters and switchmode technology some kind of imagination can be obtained by comparing the electronic ballast of fluorescent lamps to the ballast inductor. Although the ballast inductor exceeds the lamp in power rating it is still 2...3 times cheaper than electronic ballast and the dimensions are about the same. Moving towards higher power rates the converter cost of line frequency converters per 1kW will be even much less. The installed capacities of circuit components of network frequency resonant converters are different in different circuits. Besides each circuit includes the possibility to vary the component parameters (inductors, capacitors, transformer voltages) within some range. Therefore the installed capacities of circuit components are different. Minimization of installed capacities is of interest for designers.

2. CAPACITIES OF REACTIVE COMPONENTS IN NETWORK FREQUENCY RESONANT CONVERTERS OF DIFFERENT TYPE

In network frequency resonant converters the load current is passing through capacitors and inductors. These converters usually are capable for shortcircuiting the output and they are used for supplying electrical arc. When supplying arc the short-circuit mode usually lasts for short periods causing overload for some types of converter. Three different types of converters are known: series resonance converter, parallel resonance converter and converter with alternating series and parallel resonance (PSA converter). The series-resonance network frequency converters are known as Boucherot circuits [1]. In these circuits the load impedance is connected in parallel either to the capacitor or inductor of the series resonant circuit (Fig. 1a). The current passing the load resistance R is constant. If the resistance increases the voltage increases up to saturation of the transformer. So the voltage cannot increase any more. The currents in inductor and capacitor increase also when the load resistance increases. Maximum currents and maximum capacity of capacitor and inductor are determined by the transformer parameters. Approximately the specific capacity of the inductor is 1,4...1,8kVA and the capacitor 1,6...2kvar/kW. The inductor and capacitor are loaded most of all at no-load and least at shortcircuit.


445

~

a) L 1.4...1.8 kVA/kW

1.0

I, P IC

1.6...2 kvar/kW C

IL1

0.75

T 1 kVA/kW

Pd

0.5

0.25

R

b)

0

C1 1.4...1.6 kvar/kW L1 0.6...0.8 kVA/kW

L2 0.4...0.6 kVA/kW

~ T 1.03...1.05 kVA/kW wL

L1 0.5...0.6 kVA/kW

0.75

1.0

1.25

Ud

Fig. 2. The current of main inductor (IL), capacitor (IC) and output power of the converter with parallel resonance (Pd)

~

c)

0.5

0.25

wC

C 0.7...0.9 kvar/kW

L2 0.2...0.4 kVA/kW

The output power is the highest in case the output voltage Ud is approximately equal to supply voltage. The current of inductor and capacitor is significantly lower at maximum output power than at short circuit. Rated power of reactive components has to be selected from short circuit mode in this case. Additional inductor L2 increases the voltage and power of capacitor C. Approximately the specific capacity of main inductor L1 is 0,6...0,8kVA/kW, additional inductor L2 0,4...0,6kVA/kW and capacitor 1,4...1,6kvar/kW. In the converter with alternating parallel and series resonance ([3] Fig. 1c) the currents of capacitor C and main inductor L1 are approximately the same from no-load to nominal mode. In nominal mode the converter output power Pd is the highest (Fig. 3). 1.0

I, P IC IL1

0.75 Pd 0.5

Fig. 1. Series resonance (a), parallel resonance (b) and alternating series and parallel resonance (c) line frequency resonant converters The converter with parallel resonance (Fig. 1b) has full parallel resonance when the load circuit is short-circuited. In addition to main inductor L1 an additional inductor L2 has to be used to reduce higher harmonics in network current [2]. This converter has the highest load current in the capacitor and inductor at short circuit mode (Fig.2).

446

0.25

0

0.25

0.5

0.75

1.0

1.25

Ud

Fig. 3. The current of inductor IL1, capacitor IC and the output power Pd of the converter with alternating parallel and series resonance The specific capacity of main inductor of the PSA converter is approximately 0,5...0,6kVA/kW and capacitor 0,7...0,9kvar/kW.


One phase of the converter transformer includes two secondary windings (WL and WC on Fig. 1c). Voltages of these windings can be chosen equal or different. This choice is characterized by the ratio of no-load voltages

KE

U WC 0 , U WL 0

(1)

where UWC0 is no-load voltage of the capacitive branch supply winding; UWL0 is no-load voltage of the inductive branch supply winding. Factor KE could be named as converter voltage distribution factor. One phase of the converter includes two inductors (L1 and L2 on Fig. 1c). Ratio between additional inductor (L2) and main inductor (L1) inductive reactances

KL

xL 2 x L1

(2)

4. MINIMISATION OF REACTIVE COMPONENT CAPACITIES (POWERS?) IN THE PSA CONVERTER First of all the factors, which affect installed capacities have to be found. Let us examine the simplest circuit without additional inductor, where the supply voltages of phase shifting circuits are equal ( UWC UWL ) and the currents of phase shifting circuits are equal ( I CK I LK ). In case the rectifier bridge is ignored, the voltages of capacitor C and inductor L are equal to voltages of phase shifting circuits in the short-circuit mode (Fig. 4a). In nominal mode also the current through the common point of secondary windings could be ignored and an equivalent circuit as in Fig. 4b could be used. T ICK

T ILK

C

L

UWC

UWL

C

L Rn In

Kn•UWL

3. VARIABILITY OF CONVERTER CHARACTERISTICS AND PARAMETER DISTRIBUTION FACTORS IN PSA CONVERTER

are determined, if its circuit and distribution factor values are given. Converters with the same circuit and distribution factors can have different nominal voltages and currents, but they have the same power factor, the shape of output characteristic, the same amount of current higher harmonics and the same specific capacities of reactive components.

Kn•UWC

Comparing the three converter types in Fig. 1 one can see that the lowest unit power has the PSA converter. These converters are of most interest. Still not all the PSA converters are with the same parameters. Inductances, capacitances and voltages of transformer windings could be varied within some range. This is accompanied by changes in characteristics and by changes in the power rates of reactive components of the converter.

UWC + UWL

a)

b)

could be named inductance distribution factor, where xL2 is inductive reactance of additional inductor; xL1 is inductive reactance of main inductor.

Fig. 4. Evaluation of load current and voltage of reactive components: a – in short circuit mode; b – in nominal mode

The reactive load in the transformer secondary windings can in the short-circuit mode compensate mutually either totally or partly. This is characterized by the ratio between reactive powers of the transformer secondary windings:

The currents of reactive components are in nominal mode equal to nominal load current I n . Due to equal reactive impedances of C and L the voltage over Rn is equal to the sum of secondary

KQ

QLK , QCK

(3)

where KQ is reactive power distribution factor for the short-circuit mode; QLK is reactive power of the inductive branch in case of short-circuit; QCK is reactive power of the capacitive branch in case of short-circuit. Values of distribution factors KE, KL and KQ are choosable. This choice affects converter characteristics. The characteristics of the converter

windings voltages. Let us use a factor K n to take into account the difference between the currents of short circuit and nominal mode.

Kn

In ICK

In I LK

(4)

Approximately it could be assumed that the current of reactive components varies linearly when overcoming from nominal mode to short circuit.


447

Two possibilities exist. Either the current of reactive components is maximal at short-circuit ( K n d 1 ) or it is maximal in nominal mode ( K n t 1 ). In case the nominal mode current is higher, the calculated values of reactive power of reactive components (installed capacities) in the form:

QCa

K n2 UWC ICK ,

QLa

K n2 UWL I LK .

(5)

The load power could be calculated as from Fig. 4b in the form Pn K n ICK (UWC UWL ) . (6) Taking

I CK

into

account

that

UWC

UWL

and

I LK the specific capacity of the capacitor

battery qC and the specific capacity of the main inductor qL could be calculated as follows:

qC

qL

0,5K n , kui K n t 1.

(7)

The specific capacity is minimal if K n case of previous simplifications the specific capacity of the capacitor battery inductor are qC min qL min 0,5 kvar/kW.

1. So in minimal and main (8)

The capacity of reactive components is minimal in case the current of reactive components does not vary in the range from short circuit to nominal load. But if the current increases when overcoming to nominal mode the installed capacities also increase. If K n 1 the current of reactive components increases in the short circuit mode. As for nominal mode this is overloading. In such case the capacities of reactive components depend upon their capability to withstand overloads and the duration of overloads. If the reactive components are not capable for overloads the installed capacities have to be calculated from short circuit mode

QCa

UWC ICK ,

QLa

UWL I LK .

(9)

From (6) and (9) we get

qC

qL

1 , if KnW (T )@

1 2 2 2 G TA G TV G Wh . 3

For automated of measurement complexes

1 2 2 2 G TA G TV G Wh G S2 , 3 where

Gc

(7)

G TA , G TV , G Wh , G S - an average value of inaccuracy ¬A, ¬V, counter, devices of the collection and data base communications. According to /1/ under small current loads of the feeder I1* 1,5% and normal quality of the electric power.

W (T ) ¦Wi (T ) ; * r

i 1


453

1 3

Gc

cos* M

2m

where m, l , b, a1 , a2 , b1 , b2 , A, B -

ª z22 l c 2 I12 b A BS L 2 º K 2 n a1 I1b1 a2 b2 I1* * * ¬ * ¼

2

,

(8)

the factors of the approximation which dependent from types ¬A, ¬V, and counters.

For instance, for the feeder with voltage equal 10 kV

Gc

1 3

cos* M

0,34

ª0,254 z1,3 I 0,9 0,5 8,33 103 S 2 º K 0,94 8,74 I 1,25 60,72 0,54 I1* 2* 1* 1* L «¬ »¼

2

.

Within the range of loads 5 y 100% I1* nom

Gc

1 3

2

cos* M

2m

n ª z22 l c 2 I12 b A BS L 2 º ¨§ ¦ ci I1mi ·¸ . * * * ¬ ¼ ©i 1 ¹

(9)

For instance, for the feeder 10 kV:

Gc

1 3

cos* M

0,34

ª0, 254 z1,3 I 0,9 0,5 8,33 103 S 2 º c I 2 c I c 2 , 2* 1* 1 1* 2 1* L 3 «¬ »¼

where

ci

- values of factors for different ranges of the current load depending on the type of the counter. In the table 1 values ci for counters of induction systems are brought;

I1*

I1

- the relative value of current load (multiple of primary current ¬A); I1nom z2 z2* - the relative value of the input resistance of the measurement circuit ¬A; z2nom - the power of the load ¬V; SL cosM Here cos* M , where cos M nom 0,8 - a nominal corner of the load of ¬A; cosM nom TABLE 1. The factors of approximation for analytical dependencies of inaccuracy of counters of induction systems from current of load Class of accuracy 1 1,5 2 3

Small loads

5% d I1* d 7% 1 -0,875 -0,8625 -0,85 -0,625

2 12,625 13,088 13,55 11,835

3 -44,25 -47,475 -50,7 -48,75

7% d I1* d 20% 1 0,0154 0,0231 0,0308 0,0462

Under the low quality of the electric power a current of the load, I1 , can be described by the following mathematical model according to /2/:

I1

454

I av 1 J 2 R(W ) 1 K nsI 1 K 2 I ,

Average and greater loads

(10)

2 -0,542 -0,813 -1,084 -1,626

where I av J R(W ) K nsI K2 I

3 4,292 6,438 8,584 12,876

20% d I1* d 120% 1 -7·10-4 -1,05·10-3 -1,4·10-3 -2,1·10-3

2 0,089 0,1135 0,178 0,267

3 -1,9 -2,85 -3,61 -5,7

an average value of the current of feeder; the factor of variation; the correlation function standardized; the factor of non-sinusoidal of current;

- the factor of the inverse sequence of the current.


The factor of non-sinusoidal of current is defined on the expression:

¦ I Q I Q Q n

1

2

K nsI

I

1

2

2

DI1Q DI 2Q

I 2 V I1 V I 2

.

Herewith the losses of the electric power in lines, supplying substation, are calculated on expressions:

(11) 2

The factor of the inverse sequence of the current inheres on the expression:

'W

* AL

2

WA* WR* RL ; 'WR*L U S2 T

2

2

WA* WR* X L , (13) U S2 T

where

n

K2 I

I2 I1

I 2 ¦ I 2Q Q 2 n

I 1 ¦ I 1Q

.

(12)

Q 2

In

given

expressions

I 1Q , I 2Q , DI1Q , DI 2Q -

accordingly average value and dispersion of currents of direct and inverse sequences of the Q harmonica.

I 1 , I 2 - the average values of currents of direct and K nsI

inverse sequences of the first harmonica (under f=50 Hz). and K 2 I can be determined by experimental

or calculated way. Substituting expressions (11), (12) and (10) in (8) or (9), but then in (5), possible get a true value EPC each output feeder of structured unit under the * low quality of the electric power, ª¬WEPQ (T ) º¼ . Using similar approach, the value of the electric power under the normal quality, ª¬W * (T ) º¼ , corrected on the totality of factors, which are affecting on inaccuracy of the account of the electric power, is possible define. Difference between them and will give the value of EPC of output feeder of the substation, nondiscounted or rediscounted because of low EPQ. For the study of inaccuracy of EPC of the input feeders of substations under the low quality of the electric power were considered following events: electrical supply of substation on the radial scheme; electrical supply of substation on the main scheme. Radial scheme. In this case the corrected value EPC of feeder, which supplies a considered substation from substations (stations), pertaining to more high hierarchical level, is taken of as reliable value.

WA* , WR* - the values of active and reactive electric power of output feeder of power source to considered substations, which is corrected with provision for low EPQ; - voltage of power source; US T - considered of length time; RL , X L - accordingly active and inductive resistance of lines with provision low EPQ. It is here RL defined on /2/, but X L - a well-known way. Then corrected with provision for low EPQ the value of EPC of each input feeder of the substation will is: WA*IN

WA* 'W A*L ; WR*IN

WR* 'WR*L

(14)

Main scheme. In this case for the allocation of non-discounted or rediscounted electric power, because of low EPQ, when the electric power balances are compiling, it is necessary to take losses of the electric power account into parts of main scheme. In this connection on the method of least squares must be corrected on PEPQ, not only values EPC, as well as loss of the electric power on parts of main scheme. But in the equation of the balance must be present total losses of the electric power, including and from low EPQ, the whole main scheme as a whole. The true values of EPC with provision for low EPQ (i.e. positive or negative additive, stipulated by the low quality of the electric power) of input feeders of substations, which are supplied from the main scheme, as well as loss of the electric power in parts of main scheme is defined on expressions:


455

n

G c di di

m

NB ¦ G ci d i ¦ G 'c j d 'j i 1 n

i

i 1

n

¦d ¦d i

* WEPQ (T ) Wi (T ) 'W i

i 1

n m § NB d G G 'c j d 'j ¦ ¦ c i n ¨ i i 1 i 1 ¨ d d G ¦ ci i i n n i 1¨ d d 'j ¦ ¦ i ¨ i 1 i 1 ©

'j

i 1

n m · ª º NB d G G 'c j d 'j » ¦ ¦ c i ¸ m « i i 1 i 1 ¸ ¦ «G 'c j d 'j d 'j » n n ¸ i 1« » d i ¦ d 'j ¦ ¸ «¬ »¼ i 1 i 1 ¹ n

G 'c d 'j d 'j 'W j (T ) 'W

G 'c

j

2 WA2j G '2cWAj WR2j G '2cWRj , W WR2j 2 Aj

(16)

where WAj ,WR j - active and reactive energy at the beginning initially j-part, defined on evidences of counters.

i 1 n

n

i 1

i 1

i 1

¦ d i ¦ d 'j

n m § NB ¦ G ci d i ¦ G 'c j d 'j n ¨ i 1 i 1 ¦ ¨ G ci d i d i n n i 1¨ ¦ d i ¦ d 'j ¨ i 1 i 1 ©

In expressions (15) are used of systematic inaccuracies of the determination of losses of the electric power on parts of main scheme under the low quality of the electric power, are defined on results of measurements as:

m

NB ¦ G ci d i ¦ G 'c j d 'j

j

* (T ) WEPQ j

(15.1)

n m · ª º NB ¦ G ci d i ¦ G 'c j d 'j » ¸ m « i 1 i 1 ¸ ¦ «G 'c j d 'j d 'j » n n ¸¸ i 1 « » d i ¦ d 'j ¦ «¬ »¼ i 1 i 1 ¹

(15.2)

CONCLUSIONS

1. The method, allowing to define a value EPC, stipulated by the low quality of the electric power, in the scheme of electrical network of the power system of any difficulty and which non-discount (rediscount) by the measurement complex of the electric power, is offered. 2. The mathematical model of measurement complex of the electric power of input or output feeder of the substation under the low quality of the electric power, is received.

They are herewith WAj , WR j defined apart for each

REFERENCES

area of pathways from the beginning (main part) by the end of. Systematic inaccuracy of active and reactive energy with provision for low EPQ on each part of main scheme inhere on expressions (8), (9), i.e. as in the preceding event. The corrected values of the electric power on main scheme W * (T ) and losses in her under the normal

1. Myasoedov Y.V.: Increasing of accuracy of active systems of the technical account of the electric power in systems of electrical supply/ Paper on reception of scientific degree of the candidate of technical sciences. Mariupol (Ukraine), 1995.. 2. Savina N.V.:. Modern aspects of the problem of the determination of losses of the electric power in distributing networks of power systems. The Second conferences of Russia with the international participation " Energy: control, quality and efficiency of using the power resources - Blagoveschensk, 2000

quality of the electric power 'W * (T ) are defined on totality factors, which are affecting on inaccuracy. The value of EPC, which stipulated by the low quality of the electric power and vague on measurement complex (non-discount or rediscount), is inheres as a difference: * ª¬G WEPQ (T ) º¼

* ª¬WEPQ (T ) º¼ ª¬W * (T ) º¼ .

(17)

Similarly, for losses of the electric power * ª¬G'WEPQ (T ) º¼

456

* ª¬ 'WEPQ (T ) º¼ ª¬ 'W * (T ) º¼ .

(18) Electrical Power Quality and Utilisation

Yury V. Myasoedov was born in 1962 in t.Kazimagomed, Azerbaijan. He received the candidate of science degree at 1996 years in Priazovsky state technical university, Mariupol, Ukraine. The rank of docent of the chair of power engineering was conferred him in 1998. At present – assistant of dean of Power engineering of the Amur State University. His areas of interests include the quality of the electric power in power systems and design of means of communication in power systems. Mailing address: Yury V. Myasoedov Amur State University, Faculty of Power engineering 21 Ignatevskoe shosse, 675027 Blagoveschensk Amur region RUSSIA phone: (4162) 35-61-29 e-mail: [email protected] [email protected]

Nataly V. Savina was born in 1956 in t.Kazimagomed, Azerbaijan. She received the candidate of science degree at 1983 years in Dnepropetrovsk mining institute, Ukraine. In 1990 her was conferred rank of docent of the chair of electrical supply of industrial enterprises. At present – chief of the chair of Power engineering of the Amur State University. Her areas of interests include the quality of the electric power in power systems. Mailing address: Nataly V. Savina Amur State University Chair of Power engineering 21 Ignatevskoe shosse, 675027 Blagoveschensk Amur region RUSSIA phone: (4162) 35-05-56 e-mail: [email protected] [email protected]


457

458




DYNAMIC CHARACTERISTICS OF THE UNIT A IN HPP VRUTOK IN ISLANDED OPERATION

Vangel FUSTIK, Atanas ILIEV Faculty of Electrical Engineering, Skopje, Skopje (Macedonia)

Harald WEBER, Fred PRILLWITZ University of Rostock, Rostock (Germany)

Abstract-This paper illustrates a brief overview of the specific experiments performed on one unit in HPP Vrutok. According to the main tasks of the joint project DYSIMAC (Dynamic Simulation of the Macedonian Power Plants in a New Technological and Market Environment), the authors investigate dynamic characteristics of the hydro units in power system. Since the power plants shall operate in a new technological and market environment additional technical requirements have to be met by rehabilitated equipment and the unit itself. The results and graphical presentation of one typical experiment – islanded operation of the hydro unit is presented in original form.

the most important experiment [1,2] that was realized is operation of the unit on islanded load. In this paper, a method and procedure for realization, as well as, obtaining results in graphical form from this experiment on the hydro unit are described.

1. INTRODUCTION Modern control theory is now well developed to provide a genuine design technique for developing practical control systems. However, to apply such theory on a particular technical system in power plant, data acquisition of dynamic parameters should be provided. Moreover, the advent of fast and powerful computers, as well as, reliable software packages reduce the computational burden for process analysis and increase possibilities for creation of a modern unit and plant control and monitoring. The rehabilitation of 6 hydropower plants in Macedonia is underway. The project includes rehabilitation of the main power components, as well as, the control system, protection and auxiliaries. In order to check dynamic characteristics of the units in HPP Vrutok, after installing a new equipment for turbine and voltage control, set of experiments were performed on the Unit A. One of

2. GENERAL DESCRIPTION OF THE HPP VRUTOK The HPP Vrutok is located in the north-western part of the Republic of Macedonia, 7 km on the west of Gostivar city and 64 km south-west of Skopje. It is the largest hydropower plant in Republic of Macedonia and with the Mavrovo accumulation lake has an irreplaceable role in the regulation of the load-frequency control and electricity consumption daily diagram. The Vrutok plant with the entire hydro and electromechanical equipment is arranged in an underground building. It is a derivational, storage type of hydro plant. Main data of HPP Vrutok are, as follow: Number of units………… 4 Rated power ……………. 42 MVA Rated voltage …………… 12 kV Rated current …………… 2020 A Rated speed ……………. 500 RPM Reated flow ….…………… 4 x 8 m3/s Net head ..………………… 525 m Average yearly production …. 350 GWh The following plant equipment is already being rehabilitated:

Turbine regulator for all units, Voltage regulator for all units,


459

Control of the turbine inlet valve and brakes, Synchronization system for each unit.

This equipment is state of the art with up-to-date technology and it is already built-up and prepared for modern control systems.

3. THE OBJECTIVES OF RESEARCH ACTIVITY IN HPP Following the framework of the main objectives and research program defined under the DYSIMAC project scheme, the project team has been performed practical experiments on one unit in hydro power plant Vrutok.

Such experiments are inevitable task in creating a consistent basement for development of mathematical model, which could be used for computer analysis and support in modern control tasks. Each experiment on a real system, as it is hydro unit with a high rated power, is followed by risk and sometimes non-justified additional costs. Considering that the required control tasks lead to producing electrical energy in non-optimal points and limits, all procedure for experiments from the beginning to the end, should be prepared with a great care and responsibility. Furthermore, such experiments are necessary for establishing simulation procedures, training sessions and testing the mathematical model on a real system in various modes of operation. In that sense data acquisition shall be as reliable as possible and as complete, as the analysis needs. As a first significant step forward in the research was performing experiments on a real system to be modeled. Those experiments enable determination of dy-

namic characteristics of the unit and identification of the main parameters of the installed control system.

4. AVAILABLE HARDWARE AND SOFTWARE Data acquisition (DAQ) of the reliable measured values depends on a computer-based system compose for this purpose (Fig.1) and the quality of the following equipment: 1. Engineering station (Laptop computer or PC with peripherals and accessories), 2. Transducer, 3. Signal conditioning, 4. DAQ hardware with complete cabling and connecting terminals, 5. LabVIEW Software support. The obtained real-time data have been up-dated with the programs developed in LabVIEW 5.0 [3] and MatLab 5.3 [4,5] environment. LabVIEW (Laboratory Virtual Instrument Engineering Workbench) is a rapid developing environment based on a graphical programming language G. LabVIEW is fully integrated for communication with the following hardware GPIB, VXI, PXI, RS232, RS-485 and plug-in cards for data acquisition. Also, LabVIEW software has already developed libraries for using actual software standards like ActiveX and TCP/Networking. Thus, LabVIEW enables development of prototypes, design, testing and implementation of the measuring instrumentation, improving the efficiency of the research team activities. signals.

Fig.1. A typical computer based system for data acquisition

460


With the best features of the LabView Software, special, original Virtual Instruments (VI) were developed by the project team, in order to have reliable and continuous acquisition of the processed For complete realization of the experiments the following hardware was used: Laptop computer with PCMCIA card, Pentium 500 MHz, 384 RAM, 5 GB Hard disk, 13,3" Color Display, Brother HL 6L Laser printer, 2xSCXI 1120 8 channel isolation amplifier, SCXI 1327 Direct Mount Terminal Block, SCXI 1000 - 4 Slot chassis, AC. All used equipment and software were property of the University of Rostock. The research team was able to follow on-line 13 measured dynamic parameters. Obtained graphical presentations of the dynamic characteristics have been printed out directly and the results have been analysed instantly by program developed in MatLab 5.3. Furthermore, an applicable database was created in order to be used for identification and testing of the simulation model of the HPP Vrutok.

5. THE PARTICULARITIES OF ISLANDED OPERATION OF HPP VRUTOK HPP Vrutok has a very important role in Macedonian power system restoration and stability. There are a few very important issues that should be analyzed such as reliability onto tripping onto auxiliary supplies, criteria for automatic disconnection from the network, transient stability of the unit during short-circuits, steady-state stability etc. Since deregulation of Macedonian Power System is not established, such network and system operational rules [6], [7], yet are not applicable. However, rehabilitation of hydro plants in Macedonian Power System is underway and it is very important to answer the question: “Are already rehabilitated systems capable to meet the requirements of new technological and new foreseen market environment ?” The research team has focused its interest on capability of isolated operation of Unit A in HPP Vrutok as a very important issue in restoration of supply. In that sense according to the practice in Germany [6], two important conditions should be considered:

The Control system of the unit must be designed and adjusted such that synchronization to a isolated partial load is controlled as reliably as synchronization to the generating unit

auxiliary supply. Isolated operation of this kind must be sustainable for a few hours. When operate on a partial load the generating unit must be capable to compensate for impulsive load connections of up to 10% of the nominal capacity

6. A LIST OF ON-LINE MEASURED PARAMETERS All preparations have been made successfully and the experiments were performed in September 2000. The experiments were organized and performed by joint research expert team with approval of the ESM (Electric Company of Macedonia) management. The measurements have included on-line monitoring of 13 dynamic parameters of the unit A, relevant for the foreseen research activity: 1. Generator active power (MW), 2. Generator reactive power (MVAr). 3. Rotating speed (RPM), 4. Frequency (Hz), 5. Excitation voltage (V), 6. Excitation current (A), 7. Voltage on generator’s terminals (kV), 8. Hydraulic pressure of the water (Pa), 9. Position of the deflector (%), 10. 11, 12 and 13. Position of the Pelton turbine needles (%). After performing a precise identification of the corresponding process parameters, the range of each measured value have been adjusted. All identified input signals have been transformed into voltage signals in the range from 0-5 V. High precise resistors have enabled the transformation of the current signals into voltage signals, while hardware programmable voltage amplifier has additionally adjustded the value of the voltage signal. 7. EXPERIMENTAL RESULTS With the presented hardware and software a number of experiments have been performed. All planed experiments have been performed in close cooperation with all relevant subjects (HPP operational staff, National Dispatch Center, Local consumers etc.). The results and the recorded dynamic characteristics of one unit were prepared for further investigations planed with DYSIMAC joint team activities [8]. Since the database for dynamic characteristics of the unit are very important for evaluation of the overall plant performances, the results of typical


461

experiments were collected in digital form. Furthermore, in this paper, for one representative experimental case some measured parameters are presented in graphical form. The results are used for evaluation of analysed unit capabilities. Parallel operation of the unit and switching-off to a local consumption, as one of the most important cases for analysis and testing the quality of installed control system, was one of the most important tasks in our research. The fact that a local consumption with loading of approximately value of 700 kW (Å2% of the unit rated active power) was

real consumption in the plant surroundings made our concept feasible. The diagrams (shown on Fig.2–9) present recorded real measured dynamic parameters from experiment: Parallel operation of the unit with power system – with loading approximately 4.5 MW and than switch-off (reclose) to the islanded operation with loading of local consumption up to 700 kW.

Data: 06.09.00_21-55-45.dat Active power 4

3.5

Fig.2. Dynamic characteristic of the active power in islanded operation

[MW]

3

2.5

2

1.5

1

0.5

75

80

85

90

95

100

t [s]

Data: 06.09.00_21-55-45.dat Reactive power 5.5 5 4.5 4

[MVar]

3.5

Fig.3. Dynamic characteristic of the reactive power in islanded operation

3 2.5 2 1.5 1 0.5 75

80

85

90

95

t [s]

462


Data: 06.09.00_21-55-45.dat Frequency 50.25 50.2 50.15

Fig.4. Dynamic characteristic of the frequency in islanded operation

[Hz]

50.1 50.05 50 49.95 49.9

75

80

85

90

95

100

t [s]

Data: 06.09.00_21-55-45.dat Speed 508 507.5 507 506.5

Fig.5. Dynamic characteristic of the speed of rotation of the unit in islanded operation

[rpm]

506 505.5 505 504.5 504

80

85

90

95

100

t [s]

Data: 06.09.00_21-55-45.dat Excitation current

360

350

[A]

340

Fig.6. Dynamic characteristic of the excitation current in islanded operation

330

320

310 300

75

80

85

90

95

100

t [s]


463

Data: 06.09.00_21-55-45.dat Excitation voltage 70 65 60 55

Fig.7. Dynamic characteristic of the excitation voltage in islanded operation

[V]

50 45 40 35 30 25 20 75

80

85

90

95

t [s]

Data: 06.09.00_21-55-45.dat Needle 1

11.5 11 10.5

[%]

10

Fig.8. Dynamic characteristic of the needle 1 of the Pelton turbine in islanded operation

9.5 9 8.5 8 7.5

75

80

85

90 t [s]

95

100

105

Data: 06.09.00_21-55-45.dat Deflector

56

54

Fig.9. Dynamic characteristic of the position of the deflector of the Pelton turbine in islanded operation

[%]

52

50

48

46

44

75

80

85

90

95

100

105

110

t [s]

464


The analysis of the experimental results justified all expectations of the experts from HPP Vrutok. For the research team it was useful case study for further research activities towards the identification on developed simulation models and understanding the dynamic behavior of the entire unit. The research team concludes that those experiments have proven the unit capability in islanded mode of operation to meet all the requirements for the quality of produced electricity for local consumers.

8. CONCLUSION The paper presents a brief overview on typical experiments in HPP Vrutok, performed by the DYSIMAC research project team. The obtained results are a step forward for creating a reliable database of the hydro unit dynamics. Created database and graphical presentations of the dynamic characteristics have completed project resources for further application of the control theory and control paradigms on a real hydro system. Nevertheless, such experiments and practical research of dynamic operation of hydro power plant are inevitable tasks in everyday operational practice. Especially, such tasks have to be solved acquiring all constraints influencing the hydro power plant in a new technological and market environment. Since the quality of produced electric power and availability of supply is the most important issue in such environment, the unit itself should meet all requairements of black-start capabilities and islanded operation on consuption with a very low loading. REFERENCES [1] H. Weber, D. Zimmermann: Investigation of the Dynamic Behavior of a High Pressure HPP in the Swiss Alps during Transition from Inter-Connected to Isolated Operation, 12-th Power System Computation Conference, 1920 August 1996, Dresden. [2] P.Kundur: Power System Stability and Control, McGraw Hill, Inc, New-York, USA, 1994. [3] National Instruments: LabView 5.0 User's Manual, 1999. [4] MATLAB User's Guide, Version 2, The Math Works, Inc, 1998.

[5] MATLAB, The Language of Technical Computing, The Math Works, 1997. [6] Network and System Rules of the German Transmission System Operators, DVG, May 2000. [7] The GridCode, DVG, Heidelberg 1998. [8] V. Fustik, H. Weber, A. Iliev, F. Prillwitz, I. Kukovski, E. Bekiri: Computer Analysis of Dynamic Characteristics of the Unit A in HPP Vrutok Using LabView 5.0 Software, Project DYSIMAC Report 4/2000.

Prof. Vangel Fustik was born in 1956 in Skopje, Republic of Macedonia. He received M.Sc. and Ph.D. degrees from Faculty of Electrical Engineering – Skopje in 1987 and 1990 respectively. His areas of interest include: Automation of Power Plants and Project Management of Power Plants e-mail: [email protected]

Prof. Harald Weber obtained his Ph.D. degree from University of Stuttgart in 1990. He has worked in EGL Elektrizitats Gesellschaft Laufenburg AG and currently he is professor at the University of Rostock, Department of Electrical and Electronic Engineering. He is also IFAC Chairman of TC on "Power Plants and Power Systems". e-mail: [email protected]

Assist. Atanas Iliev was born in 1963 in Strumica, Republic of Macedonia. He received B.Sc. and M.Sc. degree from Faculty of Electrical Engineering – Skopje in 1987 and 1993 respectively. His area of interest includes: mathematical modeling of power system, automatic control of power plants and application of optimization techniques to power system operation problems. e-mail:[email protected]


465

466





QUALITY PARAMETERS OF ELECTRIC POWER AND THEIR INFLUENCE ON THE WORK OF ELECTRIC RECEIVERS L.V. TOROPCHINA S.V. TOROPCHINA The Amur State University Blagovestchensk (Russia)

Abstract - The presented article describes the experiments categories and electroenergy quality influence on their safety and technical characteristics. In this connection the solitary criteria are described depending on elementary energy parameters as well as the methods of every education calculation. The characteristics of given functions are examined. Electric loads are defined according to the types of the consumers and can be divided into the following groups: - industrial enterprises; - municipal consumers; - electrified transport; - other consumers. Industrial load may vary depending on the profile of the enterprise. As a rule large industrial enterprises are supplied by separate networks having their own specific characters. The group of municipal consumers includes lighting receivers, consumer appliances, small motor and other load of dwelling and public buildings, consumer services centers and etc. The load of electrified transport includes the load of tractive rectifying substations. Pumping water and sever-pipe plants are included to the group of other consumers. One of the most important and urgent problems is the raising of quality of electric power in electricity supply networks. The quality of electric power is characterized by the reliability of the electricity supply system and the quality of electric power on terminals of electric receivers and is estimated by a number of indices. According to the GOST 13103-87, permissible values of fluctuations and deviations of tension and frequency, asymmetry of tension of industrial frequency and asinusoidal of tension function of the form of a curve of tension while being supplied are

normalized by three conducting networks; the value of removal of a neutral is normalized while being supplied by four conducting networks. The values of fluctuations and deviations of tension and frequency as well as asinusoidal functions of the form a curve are normalized in the network of onephase current. Normalized values of the quality indices must not exceed 0.95 (integral probability). Permissible deviations of frequency by its changes are restricted to the value r 0.1 Hertz. Fluctuation of frequency, i.e. its quick change, is limited by the value 0,2 Hertz of super permission value of deviation of frequency. Permissible value of deviations of tension from nominal on terminals of electric engines and appliances under the conditions of normal starting and control must be within the limits of 5 y 10%; for the lamps of lighting of the working spaces and for protections these limits are reduced to 2,5 y 5%. On terminal of other electric receivers deviations of tension are permitted within the limits of r 5%. Additional reduction of tension under afterbreakdown conditions is permitted by no more than 5%. The change of tension, speed of which is no less than 1% of nominal tension per second, are referred to the category of fluctuations. Permissible fluctuations of tension vt for the lamps and radio sets are expressed in % of super permissible deviations of tension and are calculated according to the formula

't 10 where 't – the average interval between deviations, calculated within an hour. For equal variable tension vt = 1,5% and the frequency of reiterations are not limited. Fluctuations of tension on other receivers are not limited. vt


1

467

Asymmetry of tension is characterized by percentage of tension of inverse sequence to nominal: ' K asym

U2 *100 Un

(1)

' Asymmetry is permitted by K asym < 2% on the out-

puts of any three phase symmetrical electric receiver. On terminals of asynchronous engines asymmetry may be larger. Asinusoidal function of tension is characterized by the coefficient of asinusoidal function f

K a sin

¦U X X 2

Un

2

*100%

sumed active capacity and the loss connected with the change of frequency of rotation; E - the cost of 1 KVt-h of electricity; Y – the additional expenses caused by the change of the service life of insulation; K – the specific cost of reactive capacity of a source. As the deviation of tension influence the value of losses in electric engines, the dependence of thermal wear of insulation on the deviations of tension and load of electric engine take place as well. By the positive deviations the service life of insulation TS in comparison with TN nominal tension and load change inversely proportionally to the square of the coefficient of load, i.e. TN TS m2

(2)

U X - the current value of X harmonica. Permissi-

By m < 1 thermal wear is reduced. By the negative deviations the service life on insulation is reduced and is defined by the equation

ble value K a sin = 5% of any electric receiver.

TS Deviations of tension a) The asynchronous and the synchronous electric engines. By deviation of tension on the outputs of the asynchronous electric engine, the frequency of turning of a rotor changes, as well as the value of the active losses and the consumed reactive capacity, that leads to the changes of economic indices, characterizing the works of the electric engine. The changes of the active losses in the asynchronous engines by deviations of tension within the limits 5 y 10% Un are slight (no more than 0,03 Pn), however they turn out to be at that very character that the losses in supplying networks are. In practice the changes of tensions by 1% for the engines of the series A of capacity 20 y 100 Volt by permissible limits of deviations of capacity leads to the changes of consumed creative capacity by 3%. The increase of consumed creative capacity with the growth of tension is explained by the increase of the expenses of reactive energy on magnetization of steel of the machine. For the asynchronous engines the given expenses change by a quantity in comparison with conditions of normal tension. 'La

KG ('Q H ) E >G ('PH ) G ( P'I )@ 'I Y ,

(3)

where G ('QH ) and G ('PH ) - the increase of the value of consumed reactive capacity and active losses in comparison with these quantities by normal tension; G ('PI ) and 'I - the increase of con-

468

TN [47('U ) 7,55'U 1]m 2 2

Within the normalized limits TN | TS. If the productivity doesn’t depend on the level of tension, it is recommended to support nominal tension on the buses by full load of appliances and by load 50 y 75% - slightly reduced. These are the recommendations for the equipment connected with the networks having the batteries of diametral compensation. On pumping stations, equipped with the asynchronous electric engines, changes of tension are reflected on the productivity of an appliance. In this case this advisable to support nominal tension by low and average loads and heightened tension the maximum by nominal load. The deviation of tension by the work of synchronous electric engine with invariable current of arousing gives rise to the corresponding changes of the margin of static stability and leads to the change of reactive capacity, defined by thermal load of the electric engine. The losses of active capacity 'P are increased with the growth of tension in the networks and with the loading of the engine according to reactive capacity Q Q 2 ) D2 ( ) , 'P D1 ( QN QN where Q and QN – generated reactive capacity and its normal value. Coefficients D1 and D2 are defined by the parameters of a definite machine.


Fluctuations of tension Fluctuations of tension in the networks arise mainly by the work of sharply variable loads (the value converters, steel-smelting furnaces, welding units). Fluctuations of tension adversely affect visual perception of objects, machine parts and finally – the labour productivity and the sight of workers. Fluctuations of tension adversely affect the work of a large number of consumers. Short-term fluctuations of tension by the application of arc welding practically do not influence the quality of a welded joint that is explained by inertness of the processes in metal. Fluctuations and deviations of tension in the networks of the supplying machines of the contact welding greatly affect the quality of dot welding. According to the results of the research, for different types of the welded units the permissible fluctuations of tension are within the limits of 3y5%. Asymmetry of tension By the asymmetry of tension in three conducting networks the loss arises conditioned by the reduction of the service life of the equipment and lowering of economic indices of its work. In the asynchronous electric engines the reduction of the useful rotating moment MY by the certain sliding S is proportionally to the square of the coefficient of asymmetry: 2

MY

Z S 2 x 1a x H HC , 2 2S Z 2a

where Z1a and Z2a – full resistances of direct and inverse sequances of the electric engines. There may be admitted Z2a/Z1a = 0,15 and S = 2 0,04y0,05, with a | H HC . In practice by HHC | 0,05y0,06, lovering of MY is turned out to be very small. The losses conditional by asymmetry: 'PHC

2 2,41'PC K st2 H HC ,

where PC – nominal losses in copper of a stator of the electric engine; Kst – multiplness of starting current. By the nominal rotating moment and the coefficient of asymmetry of tension, equal 4%, the service life of insulation is reduced approximately twice.

In tension of one of the phases considerably exceeds the nominal, the service life of insulation will be larger. Besides in the synchronous machines, dangerous vibrations are observed as a result of the sign variable rotating moments and the tangencial powers pulsating with double frequency of the network. Asymmetry of tensions does not affect the work of HVL (High-voltage lines) and CLET, at the same time, the heating of transformers and, consequently, reduction of their service life can be essential. The calculations show that by SN of a transformer and by the coefficient of asymmetry of currents H *(1) 0,1 , the service life of insulation is reduced by 16%. The formula of total annual loss conditioned by asymmetry by the stable condition of work of the network, is: E - the cost of 1 KVt/h of electric power, rouble/KVt-h; T – the quantity of hours of work of electric equipment a year/an hour; (S ) 2 ) - the losses of active capacity in S 'PHC f (H HC element of electrical supply system, KVt; 2 'U (ps* ) f (H HC , H HC ) - reactive increase of deductions for renovation from capital expenses of KSS type of equipment; YTHC - the technological loss conditioned by asymmetry of tension, rouble/year; ( 0) 2 YHC f 3 (H HC , H HC ) - the loss conditioned by lowering the light stream of lamps, installed in the phases with lowered tension, rouble/year; ( 'Q ) 3 YHC f H (H HC , H HC ) - the loss, conditioned by underproduction reactive capacity of the batteries of condensers. By H = 0,05y0,06 'Q = 0,01y0,04 QN Asinusoidal function of tensions Higher harmonicas of tension and current adversely affect the electrical equipment, systems of automatic devices, relay protections and etc. In this connection the additional losses in the electrical machines, transformers and the networks are arisen, the compensation of reactive capacity with the help of the batteries of the condensers is made difficult, the service life of the electrical machines and appliancesis reduced, the breakdown rate in the cable networks is increased. The distortion of the form of a curve of tension affect the value of the coefficient of capacity and the rotating moment of the electric engines as well.


469

By the asinusoidal function of tension the accelerating aging of insulation is observed as a result of irreversible physical and chemical processes taking place under the influence of the fields of higher harmonicas, hightened heating of current conducting part. The value of the economic loss, conditioned by harmonicas by the asinusoidal function of tension: (c)

(a)

/[3,65K au.e. U *t a 3,65K stc.eu. U * p m KT (k ) 0,062u U(T* )W T 0 , 7 u W K cq U* k 2 q 1 s 1 x K* S

Yt

¦

¦

EK r S k T x 10 3 ] 0,087 K b u WUV

n

¦ (X 1)uX X

2 *

2

1,6 K b u U(V* )

n

¦U X X

*

Yc ,

2

where Ka.e., Ks.e., KT, Kc, Kb – the cost of the correspondingly asynchronous, synchronous electric engines, the transformers, the cables, the batteries of the condensers; u U( a* ) , u U( c* ) , u U(T* ) , u U( k* ) , u U(V* ) - the specific normative deductions for renovation of corresponding types of equipment; W a ,W s ,W T ,W c ,W V the temperatures of overheating of insulation of the mentioned types of the electric equipment by sinusoidal tension; W a W s 40q, W T 25q, W k 35q by the laying by air and 45q - by the laying in the earth, W V 40q; U X* - the relative (in the parts of tension of the first harmonica) values of X harmonica of tension; KT – the ratio of active and reactive resistances of the network by industrial frequency; x*S - the relative reactive resistance of a trans-

p n - the probability of transition of the phase fault to two phase in the place of damage (average p n = 0,8); c 0 - the cost of repair of damage.

Toropchina Ludmila V. Was born in 1965 in Blagoveshchensk, Amur region, Russia. She is a pro-rector in the Amur State University. Her qualification is characteristics eleboration. Mail address: Ludmila V. Topopchina Amur State University Ignatyevskoe Shosse, 21, 675027, Blagoveshchensk Amur region Russia Phone: (8-416-2)-35-06-01 Fax: (8-416-2)-35-03-77 e-mail: [email protected] Toropchina Svetlana V. Was born in 1973 in Blagoveshchensk, Amur region, Russia. She obtained the doctor’s degree in 1998. Mail address: Svetlana V. Topopchina Amur State University Ignatyevskoe Shosse, 21, 675027, Blagoveshchensk Amur region Russia Phone: (8-416-2)-35-05-58 Fax: (8-416-2)-35-03-77 e-mail: [email protected]

former and load. For the transforming substantions x* = 0,35 +uk, where uk – the relative value of the short curcuit of a transformer; E - the cost of 1 KVt-h of electric power; T – the quantity of hours of the work of electric equipment; Sk – capacity the short curcuit on the buses of the substantions, MVt; Yc – the expected loss conditioned by the damaged cable lines; Yc = 0,61 p c p n c 0 , where p c - the quantity of homogeneous earth faults, referred the capacity of current, equal 100 A (average p c =13);

470




THE INFLUENCE OF SUPPLY VOLTAGE QUALITY TO THE ACTUATORS BEHAVIOR Adrian BARABOI Maricel ADAM Catalin PANCU “Gh. Asachi” Technical Universitu of Iasi Iasi (Romania)

Abstract - Starting from the electromechanical complex model of the behavior in dynamic state for an DC actuator, it realize a program of numerical simulation in EMT Program and it analyze the deviation’s influence from the parameters of the supply voltage quality to the electromechanical characteristics. It shows the calculus results.

In Fig.1 is shown schematically the realization of polar pieces in the zone of working air gap, for basic constructive variants of an DC actuator electromagnetic system: with translation balanced armature (Fig.1a), respectively rotation (Fig.1b). G1 m

1. INTRODUCTION Actuators, having sophisticated structures or more simples other represent, in behavior, the premises for some processes of different nature and high complexity, which can be advantageous analyzed through numerical simulation with computer. The modeling and numerical simulation of phenomena and complex systems represents modern procedures of scientific investigation, having the capacity to transform a much more important part of scientific research and design, from the real space of laboratory tests, into virtual space, located in computers memory [1], [2], [6]. The modeling and numerical simulation of behavior allows to evaluate electromechanical systems’ conduct in the application for which it has been conceived, designed and realized The behavior’s numerical simulation leaves from the electric modeling of all non-electric phenomena associated, followed by the numerical simulation of the model’s behavior which, usually, is an electric network. The solution’s advantage consists in the possibility of using, in simulations, unaffected by state, some specialized software medium, for example EMTP. 2. THE DYNAMICAL STATE MODEL

N

U

Fa Fr G x

a D D1 N

U

x J Ma Mr

b Fig. 1 Basic DC actuator components: a-translating armature; b-rotating mobile armature.


471

After the way in which the balanced armature has a translation or rotation movement, the remove on transient state duration corresponds with one of the equations [4]:

½ d2x dx at e t x Fa ( x ) Fr ( x ), ° 2 dt dt ° 2 d x dx °° J 2 ab e b x M a ( x ) M r ( x ), ¾ dt dt ° ° dx x ( 0) x 0 , 0 ° dt 0 °¿ m

(1)

U T

) G2 ,M a P 0SG

2FG

i

R

FG being the active force reported to a working air gap, r-the arm of this force; )G-the magnetic flux from the air gap; SG-the polar surface, and P0=4S10-7 H/m the magnetic permeability of air.

Fr, Mr Fr2, Mr2

L0

U

Rp

i0 Fig. 3 The actuator electric equivalent circuit 3. THE NUMERICAL SIMULATION IN ACTING DYNAMIC STATE The mathematical model of actuator acting is considered as a differential equations system, where one describes the shift of balanced armature, but the others to the electric circuit behavior.

Rs

is

Ls

ucs

us

Fr1, Mr1

Cs

Fig. 4 Equivalent simulating circuit

O D2, G2

D1, G1

D, G

Fig. 2 Mechanic characteristic In Fig.2 is presented the mechanical characteristic for an actuator. It can observe that the resistant force, respectively resistant moment decrease linear in fuction of the linear air gap, respectively angle air gap. For the electric circuit is adopted a complex equivalent for an iron-core coil, [7], represented in Fig.3. The equations of this circuit have the form:

472

Ld ip

(2)

rFG ,

(3)

where: TF is the magnetomotive force corresponding to Foucault currents, N-the number of coil’s turns, and respectively K-a constant coefficient.

where: at, ab and et, eb are amortization and elastic constants, corresponding to the actuators from Fig.1a, Fig.1b. The active force, Fa, respectively active moment, Ma, are presented in relations: Fa

di d) G ½ , N dt dt °° ¾ d) G ° Ni TF ,TF K , dt °¿ Ri L V

Thus, for Fig.1a, this model is formed by the equations (11), (2), (3), in the case of Fig.1b, the equation (11) being replaced with (12). The numerical simulation is realized in EMTP medium software (ATP 1999 version), composed to solve the problems of the transient state in a complex electric circuit. The constructive variant analyzed is in Fig.1b, for the behavior with rotation balanced armature. For the calculus, the parameters are referenced, using the relations: y

)G ,z )r

x , xr

(4)


y, z being the useful magnetic flux, respectively the shift of balanced armature, expressed in relative units, through a difference in the reference values )*, x*. The electric model attached to the equation (12) is represented through the circuit shown in Fig.3, the parameters similitude, considered in conditions (4), being expressed under the form:

r ) r y M r D1 , 2P 0SG

½ ° ° ° 3 L s >mH@ 10 Jx r , R s >:@ a b x r ,¾ ° 10 6 ° , z Cs u cs . Cs >PF@ ° eb x r ¿ u s >V @

2

(5)

having the configuration of Fig.4 and the parameters calculable with similitude relations (5), (7). 4. THE INFLUENCE OF SUPPLY VOLTAGE CHARACTERISTICS TO THE ACTUATORS DYNAMICAL BEHAVIOR

By using the simulation program in EMTP, one can study the dynamic acting state of a DC electromechanical actuator, for different characteristics of the supply voltage. In Fig. 5 is given the electrical scheme of the supply, which was simulated in EMTP.

L

The magnetization characteristic, referenced to a homogeneous part of the iron-core, is approximate to the polynomial function:

Hj

a1 jB j a 3 jB3j

LV K) r , N

b3 ) r y b5 ) r y . 5

Fig. 5 The DC actuator rectifier supply What interests us is the dynamic behavior when the actuator is acted upon, for a supply voltage obtained through the bialternating rectification and different values of the C capacity of the filtering condenser. In Fig. 6, are presented the calculus results of the passing of current in the coil, i* and of the displacement of the mobile armature, z, both expressed in relative units. 1.0

1000 750

i* 0.8

250

(7)

0.6

C [PF]

z

0.2

0.0 0.00 0.05 ACT1000.pl4: t: Z t: IR

(8)

The conclusion attained is that, in the mentioned conditions, the acting transient state of the actuator can be numerically simulated in EMTP medium software through two circuits of second order, each

250

1000 750

0.4

where b1, c1, b3, b5 are constants having values depending on constructive parameters of core an coil, but n(y) is a function of the form: 3

R3

(6)

½ ° ° § RK · ° R s >:@ ¨ b1L V N ¸) r , ° © N ¹ ° ° 106 Cs >PF@ , ¾ b1R) r ° ° u cs ,u s >V @ U c1R) r x r yz ° y b1R) r ° ° d d Rn ( y) c1LV) r x r yz L V >n y @,° dt dt ¿

n ( y)

C

R1 e(t) ~

a 5 jB5j ,

Hj, Bj being the intensity of magnetic field, respectively the inductance, but akj being coefficients with constant values. In the mentioned conditions, the conclusion reached is that the electromagnetic subsystem is also simulated, also, through an electric circuit of second order (Fig.4), with the following similitude relations between the parameters: Ls >mH@ 103

R2

RB

ACT500.pl4: t: Z

t: IR

ACT250.pl4: t: Z

t: IR

t [s] 0.10

0.15

0.20

0.25

0.30

Fig. 6 Filtering capacity influence on the dynamic mechanic and electric actuator characteristics The simulation is made for values of 250, 750 and 1000 PF, given to the filtering capacity C and for values of 63 V for an average value of the DC supply voltage. The results obtained show a significant alteration of dynamic parameters of the actuator. The acting time of the actuator has


473

increasing values; from 0,060-0,070 s, obtained for C=750-1000 PF, to 0,170 s, if C=250 PF.

of the tension influences characteristics of the actuator.

the

dynamic

1.2

0.6

i*

1.0

69

M*a

63

0.5

0.8

0.4

0.6

0.3

0.4

0.2

0.2

0.1

0.0 0.04 0.06 DC57.pl4: t: IR t: Z

0.0 0.00 0.05 ACT1000.pl4: t: MA t: MR ACT750.pl4: t: MA

t: MR

ACT250.pl4: t: MA

t: MR

t [s]

M*r 0.10

0.15

0.20

0.25

0.30

1.0

0.8

i* 0.6

0.4

0.2

z t [s] t: Z

ACTBA.pl4: t: IR

t: Z

0.15

0.20

0.25

0.30

0.35

0.40

Fig. 8 Dynamical (mechanic and electric) actuator characteristics at single and double alternating rectified supply voltage In Fig. 8 are shown characteristics of the dynamic state (the current absorbed by the coil, i* and displacement of the mobile armature, z) calculated for the supply of the actuator with non-filtered voltage, obtained by double- alternating rectification, respectively a mono-alternating one. To obtain the functioning of the actuator within acceptable limits (acting time of 0,230-0,280 s), the supply voltage must be increased to 75-125 V maximal value. In the case of a supply voltage storage battery or by a rectifier with powerful filters, only the level 474

z 0.08

t [s] 0.10

0.12

0.14

0.16

0.18

t: IR

t: IR

t: Z

Fig. 9 Dynamic mechanic and electric characteristics at some values of DC voltage supply

Similar conclusions result if the mechanic characteristics of the dynamic state are viewed. These, having the active and resistant moments expressed in relative units, are presented in Fig. 7 for the same values of the supply voltage and of the filtering capacity. The excessive increase of the active moment’ s values after action is limited by the behavior of the DC actuators in schemes with economizing resistance.

0.10

69 63 57 U [V]

DC69.pl4: t: Z DC63.pl4:

Fig.7 Dynamical states of the active and resistant moments

0.0 0.00 0.05 ACTMA.pl4: t: IR

57

In Fig. 9 is given the graphical representation of the calculus results obtained in this case. The supply voltage takes values close to the nominal one (63 V). The speed of the actuator increases, reaching acting times of only 0,050-0,075 s. 5. CONCLUSIONS It is presented a possibility of numerical simulations for an electromechanical DC system, consisting in the electric modeling of non-electric (mechanics) processes corresponding to their behavior, followed by numerical simulations of transient state for models (non-linear RLC circuits) in EMTP medium software (ATP 1999 version). The proceeding is presented through the analysis of a transient state for a DC actuator electromagnetic system and curves whit the calculus results obtained are shown. What is underlined is the influence of the supply voltage characteristics, being considered as a rectifier with an RC filter, respectively an storage battery, on the electromechanical response of the dynamic state of the actuator. We present the calculus results and curves obtained by numerical simulation, meant to illustrate the analyzed influences. 6. REFERENCES 1. Adam M., Baraboi A., Ciutea I., Popa Coculeana, Leonte P., Simulation and Monitoring of the Circuit Breakers Mechanical System. Proceedings of ELECTRIMACS, Saint-Nazaire, France, 3, 1183 (1996). 2. Adam M., Baraboi A., Leonte P., Modelling of the thermal stress and the monitoring of circuit breakers. The 5th International Conference on Optimization of Electric and Electronic Equipments OPTIM'96, BraÆov, Romania, III, Electrical Power Quality and Utilisation

3.

4. 5.

6.

641 (1996).Baraboi A., Leonte P., Adam M., Solicitrile echipamentelor electrice. Ed. BIT IaÆi, 1997. Baraboi A., Hnatiuc E., Leonte P., Le régime transitoire d'attraction des électro-aimants à courant continu. Bul. Inst. Polit. Iai, XXV(XXXIX), 3-4, s. III, 47 (1980). Baraboi A., Hnatiuc E., Electromagnet de curent continuu. Brevet 68336 (1977). Baraboi A., Adam M., Leonte P., Baraboi T. A., Simulation des contraintes thermiques de l'appareillage électrique. The International Conference on Applied and Theoretical Craiova, Electrotechnics "ICATE'96", Romania, vol. II, 11 (1996). Savin G., Rosman H., Circuite electrice neliniare i parametrice. Ed. TehnicÇ, BucureÆti, 1973.

Prof. Adrian Baraboi was born in 1948 in Iasi, Romania. He received the M.Sc. degree in electrical engineering from “Gh. Asachi” Technical University of Iasi and his Ph.D. degree also from Technical University of Iasi, in 1980. Presently, he is Professor at Electrical Power Department of the Faculty of Electrical Engineering of the Technical University of Iasi. His areas of interest include electrical apparatus, EMC, power electronics and non-linear analysis.

areas of interest include electrical and electronical apparatus monitoring and diagnostic, modeling and simulation problems. Mailing address: Maricel Adam “Gh. Asachi” Technical University of Iasi Faculty of Electrical Engineering D. Mangeron 51-53, Iasi-6600 ROMANIA e-mail: [email protected]

Assist. Catalin Pancu was born in 1973 in Iasi, Romania. He received the M.Sc. degree in electrical engineering from “Gh. Asachi” Technical University of Iasi. Presently, he is Assistant at Electrical Power Department of the Faculty of Electrical Engineering of the Technical University of Iasi. His areas of interest include electrical apparatus, EMC, the study of the electrical apparatus using the artificial inteligence. .

Mailing address: Catalin Pancu “Gh. Asachi” Technical University of Iasi Faculty of Electrical Engineering D. Mangeron 51-53, Iasi-6600 ROMANIA e-mail: [email protected]

Mailing address: Adrian Baraboi “Gh. Asachi” Technical University of Iasi Faculty of Electrical Engineering D. Mangeron 51-53, Iasi-6600 ROMANIA e-mail: [email protected]

Prof. Maricel Adam was born in 1960 in Iasi, Romania. He received the M.Sc. degree in electrical engineering from “Gh. Asachi” Technical University of Iasi in 1985 and his Ph.D. degree also from Technical University of Iasi, in 1996. Presently, he is Associate Professor at Electrical Power Department of the Faculty of Electrical Engineering of the Technical University of Iasi. His


475

476


Section 7 Reliability and Continuity of Supply

7.1. PASKA J., MOMOT A., BARGIEL J., GOC W.: Application of TRELSS and Implementation of Value Based Transmission Reliability Approach at Polish Power Grid Company (Poland)..............487 7.2. BARGIEL J., GOC W., PASKA J., SOWA P., SZEWC B., TEICHMAN B.: Reliability in Contracts for Electric Energy Supply and Settlements (Poland) .........................................................495 7.3. RUSEK S.: The Relation Between Classical and Global Indices of Reliability (Czech Republic) .....505 7.4. HRADÍLEK Z.: Reliability and Continuity of Towns Electic Power Supply (Czech Republic) .........509 7.5. MIENSKI R., PAWELEK R., PAWLIK M., WASIAK I.: Supply Reliability Improvement by Means of Unconventional Energy Sources (Poland) ...........................................................................513


485

486





APPLICATION OF TRELSS AND IMPLEMENTATION OF VALUE BASED TRANSMISSION RELIABILITY APPROACH AT POLISH POWER GRID COMPANY Józef PASKA Andrzej MOMOT Warsaw University of Technology, Institute of Electric Power Engineering Abstract - Even, or may be particularly, in the time of disintegration, deregulation and competition in a power system, its reliability is one of the most important criteria which must be taken into consideration during planning and operation phases of its life. This paper sums the results of Electric Power Research Institute (EPRI) and Polish Power Grid Company’s (PPGC) project entitled “Application of TRELSS and Implementation of Value-Based Transmission Reliability Approach at Polish Power Grid Company”. The following issues are presented in this paper: background, target and scope of the project “TRELSS for PPGC”, short description of TRELSS (Transmission Reliability Evaluation of Large Scale Systems) computer program, model of Polish power transmission system, reliability and system load data, results of calculations, valuing the reliability, and areas of TRELSS application at PPGC.

1. BACKGROUND, TARGET, AND SCOPE OF EPRI-PPGC PROJECT Currently introduced common rules of the European electricity market predict a rise of importance of development planning and optimal performance of high voltage transmission and distribution networks - mainly from the reliability point of view. The new Energy Law in Poland also requires enlargement of responsibility of electric power utilities for the reliability of electricity generation, transmission and supply to the customers. Technological progress in measurement, telecommunication and computerization creates a need for new reliability approach in planning and Section 7. Reliability and Continuity of Supply

Joachim BARGIEL Wiesaw GOC Silesian Technical University, Institute of Electric Power Eng. and Systems Control utilization of transmission and distribution networks. The USA and Western Europe countries also change the philosophy of reliability issue [1-2, 5]. Among others, the following questions await answers: x Whether present-day approach of “internal pressure” is sufficient to maintain the reliability level on competitive electricity market? x Who should establish reliability standards and criteria and what role should play the state and state-owned companies in this issue? x Are technological progress and strengthening of electric power system really necessary to secure sufficient reliability level? x Who and how pays for it? Do we know real cost and worth of transmission and distribution reliability? x Whether and to what extend the customers are inclined to pay for reliability? Similar and new questions and problems are facing Polish electric energy sector in its deregulation era. In traditional approach, transmission system planning and operation were based on specified deterministic criteria, for example “n-1”. Nowadays in transmission system analyses more and more common are probabilistic criteria. The EPRI has elaborated a whole VBTRA (Value-Based Transmission Reliability Approach) strategy, oriented on supplying to electric utilities the methods and tools for quantitative assessment and analysis of transmission resources [9]. The core idea of VBTRA is treatment of transmission reliability as a resource, valued in analogous way as, for instance, DSM resources or classical supply resources. The key component of VBTRA is TRELSS program, because it provides the important quantitative measure for transmission subsystem assessment - a very broad set of 487

computed reliability indices both for the whole system or its part. The preliminary assessment of TRELSS usefulness in Polish conditions has justified an interest in this EPRI product. It has also confirmed a need to start the activities oriented on application and implementation of this tool for transmission reliability evaluation and methodology of its valuing in the Polish practice of transmission system planning and utilization. The objective of the project 1 was detailed familiarization with and implementation at PPGC of the computer program for reliability assessment of transmission systems - TRELSS and to create the fundamentals for application of the VBTRA. To secure this objective was vital: x to create for TRELSS needs a transmission and generation outage database; x to identify and specify expected future changes in TRELSS program, taking into considerations PPGC’s needs, approach and practice; x to prepare or adapt existing methodology of outage cost assessment and evaluate economic characteristics of losses caused by unreliability for various transmission planning and reliability analysis levels; x to run Polish transmission system reliability calculations for chosen years 1998 and 2010 data, using TRELSS program; x to prepare Polish version of user’s reference manual of TRELSS program and conduct hands-on training in TRELSS and interpretation and utilization of its results for Polish specialists. 2. SHORT DESCRIPTION OF TRELSS The TRELSS model and program, elaborated in EPRI (USA), is more and more widely applied in North America and worldwide. In the approach used in TRELSS contingency enumeration, the choice and analysis of contingencies for defined failure criteria in the system and calculation of reliability indices are performed. For reliability assessment effective ranking of contingencies, based on component overloads and analysis of voltage conditions for many load levels is used. 1

Under EPRI contract WO7057-05: Principal Investigator - Józef Paska, EPRI Project Manager Nicholas Abi-Samra, PPGC Project Coordinator Janusz Putorak, Project Team: Janusz Bartczak, Andrzej Kos, Andrzej Momot, El¶bieta Nowakowska, Józef Paska from Warsaw University of Technology and Joachim Bargiel, Wiesaw Goc, Pawe Sowa, Bogusaw Teichman from Silesian Technical University.

488

Load flow is computed either by DC method or by de-coupled AC method. For the optimization of remedial actions to reduce effect of system failures (re-dispatch, shunt switching, transformer tap adjustment, phase shifter adjustment, and 3 classes of load curtailment), linear programming is used [8]. It is also possible to model the operation of protection and control devices (PCG - Protection and Control Group) and simulate situations, when a failure of a transmission system component may cause the outage of a group of components including the faulted component through the action of relays and circuit breakers. Common mode outages are treated in TRELSS like disconnection of single components but the program interprets them in a special way. Modeling of scheduled maintenance of branches is done at the following assumption: If an outage of a defined combination of branches results in a system failure, none of these branches will be outaged for scheduled maintenance when one or more of the branches has been already disconnected. The capabilities and limitation of TRELSS are listed in Table 1. TABLE 1. TRELSS limitations and capabilities Model Size Up to 4500 buses, 9900 lines, 1000 generators, and 1200 transformers. Components Standard system components. DC lines are not currently modeled. Base Cases Maximum of ten; may all be usersupplied or TRELSS can create lower load level base cases from a user-supplied peak case. Study Modes System Problem Approach (system (system problem analysis) – reliability emphasis on system overloads and analysis voltage problems. Calculates approach) frequency, duration and magnitude of problem type indices. System Capability Approach (based on the system possibilities) - emphasis on loss of load due to remedial actions. Calculates frequency and duration of loss of load type indices. Generation Three types are available - unit Re-dispatch margin, participation factor and dispatch as specified in the Load Flow file. PCG Analysis Automatic identification of PCGs using network trace, computes reliability impact of PCG outages due to temporary and permanent faults. Electrical Power Quality and Utilisation

Contingency Depth

Max. Cont. run: Contingency Ranking Contingency Enumeration Commonmode Failures "Must run" contingencies Power Flow Solution Remedial Actions

System Failure Modes

Bus Specifications Islanding Bus Load Specifications Indices Calculated

Up to six simultaneous outages due to repair; two circuits and four generator units outaged by automatic selection. Any combination, up to six, by manual selection. Up to 60,000. Severity of contingencies estimated using performance index methodology. "Wind-chime" methodology utilized with user specified success cutoff. User-defined common mode failures accommodated. User-defined contingencies can be analyzed. Both AC and DC solutions are available. Both AC and DC models; utilizes OPF style linear programming based solution to modify system controls to alleviate system problems such as overloads and voltage violations. Control actions include generator MW dispatch, generator voltage control, MVAR dispatch, phase shifter adjustment, transformer tap changing, shunt reactive switching and load curtailment. Circuit loading, Divergent load flow, High/low bus voltage, System separation, Bus voltage deviation, Load curtailment. Independent voltage tolerances for each bus. Will attempt to keep islands viable through generation re-dispatch. Firm, critical and interruptible load (specified by bus). System Indices: Probability of loss of load, Frequency (occ./year), Duration (hours/year), Duration (hours/occ.), EUE (MW-Hr./year), EUE (MW-Hr./occ.), EUD (MW/year), EUD (MW/occ.) Customer Indices: Outages (customer-occ./year), Outages (customers/occ.), Outages (occ./customer), Duration (customer-hours/year), Duration (hours/occ.), Duration (hours/customer), Service


availability (p.u.) Normalized Indices: Energy curtailment (MWHr/MWHr-year), Power interruption (MW/MW-year) 3. POLISH TRANSMISSION SYSTEM MODEL IN THE YEARS 1998 AND 2010 At the end of the year 1998 the composition of Polish power transmission system contained: x 16 system power stations, being independent economic entities; x Transmission network of 750 kV (750 kV OHL from Ukraine is currently not in operation), 400 and 220 kV, operated by PPGC; x Distribution network of 110 kV and below, operated by 33 distribution utilities (only part of 110 kV network plays transmission role). Polish transmission system was connected with UCTE and CENTREL at two points with Germany, at three points with the Czech Republic and at one point with Slovakia. For dispatching purposes transmission system was divided into five areas: PPGC – Center, PPGC – East, PPGC – South, PPGC – West, PPGC – North. Program of the transmission network modernization up to the year 2010 takes into account mainly: x Adaptation of the national transmission network to the UCTE requirements; x Adaptation of some of the existing objects to the increased requirements resulted from system development and environment protection; x Limitation of the operating costs. Two general representations of the transmission system were taken into consideration: x With detailed model of the 400 kV and 220 kV network, but with equivalents of the 110 kV network (which gives for year 1998 - 140 buses and 300 branches: 245 lines and 55 transformers, and 195 generating units). x With detailed model of the 400 kV, 220 kV and 110 kV network (which gives for year 1998 2284 buses and 2825 branches: 2638 lines and 187 transformers, and 195 units). 4. RELIABILITY AND SYSTEM LOAD DATA At least the following basic data files should be prepared to run successfully reliability calculations in TRELSS:

489

a) Load flow data file – this file contains bus data, branch data and interchange data needed for base-case load flow calculation. b) Generator unit data file – this file contains the data of all the individual generating units (50, 120, 200, 360 and 500 MW) in the system. c) Circuit outage data file – this file contains reliability data for all existing branches (transmission lines 110 kV, 220 kV and 400 kV, transformers of 220/110, 400/110 and 400/220 kV) in the system. It may also contain data for protection group analysis if such is to be done. d) Generator units’ outage data file – this file contains reliability data of all the individual generating units in the system (i.e. FOR probability of forced outage and its duration). e) Bus characteristics data file – this file contains data characterizing each individual bus in the system (voltages, type of load, number of customers). f) Chronological hourly load curve data file – this file contains numbers representing the load in the system given every hour from 1st January to 31st December of a given year. One of the project tasks concerned the preparation of the statistical reliability database of individual HV power system components. The task additionally required preparation of the reliability database for individual groups of protection and weather data. The generator reliability data is attainable on the basis of long-term analysis of reliability indices of national generating units made by Energy Market Agency. Values of FOR indices were taken from the national statistics but the average duration of the outages required carry out some calculation. TRELSS distinguishes so-called “branches” in reliability analyses. Branches define transmission lines and transformers. Six reliability indices are required for each branch: D - annual frequency of outages (outages/year), t - average outage duration (h/outage), dpo - number of outages during adverse weather (outages/year), tpo - average outage duration during adverse weather (h), v maintenance outage probability (-), tv - average duration of maintenance outage (h/maintenance). Majority of branch reliability data is currently available on the basis of statistics and failure reports [10] of HV transmission lines and transformers, carried out in National Power Dispatching Center and local departments of PPGC. Continuous restoration and updating of the records about branch failures are necessary. An example of branch reliability data is given in Table 2. An example of 400, 220 and 110 kV branch reliability data tpo v tv D t dpo

U002 H001 H002 DBN -A3 401 DUN -A1 N001 N003 GRU -A3 408

D 0.290 0.760 0.680 0.124

t 15.0 15.0 15.0 12.0

dpo 0.087 0.230 0.200 0.037

tpo 18.0 18.0 18.0 14.5

v 0.0086 0.0123 0.0116 0.0114

tv 9.5 13.5 13.0 12.5

0.700 15.0 0.210 18.0 0.0118 13.0 0.124 12.0 0.037 14.5 0.0114 12.5 0.940 15.0 0.280 18.0 0.0137 15.0 0.800 15.0 0.240 18.0 0.0126 14.0 0.124 12.0 0.037 14.5 0.0114 12.5 0.730 15.0 0.220 18.0 0.0120 13.0

Receiving of a correlation between weather conditions and branch failures is currently possible only on the basis of own investigations. Such investigations were done and sufficient weather data were prepared. In TRELSS there are two ways of introducing variation of load during the calendar year: either to input so called chronological hourly load curve data file, or to provide probabilities of load occurrence at certain levels of demand. In the calculations that were done the first possibility was used, because the chronological hourly load curve data file, for instance for the year 1998, was the “truest” file of all ones used in calculations – the data was simply measured in working system. Hourly variation of load in the Polish power system is presented in Fig. 1. 1,1 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 1

14 27

40

53 66 79 92 105 118 131 144 157 170 183 196 209 222 235 248 261 274 287 300 313 326 339 352 365

min

max

average

Fig. 1. Hourly load variation in 1998. Its transformation into load levels used in TRELSS for load scaling during calculations is shown in Table 3. TABLE 3. Load levels for the year 1998 Winter Autumn Spring Summer Load 12 - 02 09 – 11 03 - 05 06 - 08 2 level 2 - 6 7-1 2 - 6 7-1 2 - 6 7-1 2 - 6 7-1 100%

TABLE 2.

490

2

In TRELSS Sunday has the number 1, so days 2-6 are weekdays, and 7-1 weekends.


Winter Autumn Spring Summer Load 12 - 02 09 – 11 03 - 05 06 - 08 2 level 2 - 6 7-1 2 - 6 7-1 2 - 6 7-1 2 - 6 7-1 93% 87% 80% 72% 69% 64% 60% 58% 51% 4. MAIN RESULTS OF CALCULATIONS Firstly the year 1998 was chosen as the study year. Two general representations of the Polish transmission system were taken into consideration: (1) with equivalents of the 110 kV network and (2) with the whole system together with 110 kV network. In the first situation the study area was defined as the entire system. Contingencies were selected from that area, the study area was monitored for system problems, and then reliability indices were calculated for this area. The solved peak load level base case was input and then TRELSS created additional base cases by load scaling (ten or eight load levels were analyzed). It was stated in all the calculations that contingencies consisting of up to four generating units and zero branches do not cause any limitation of supply. Available capacity of the Polish system exceeds maximum demand by almost 10000 MW and the sum of nominal active power generated by the four largest units in the system is only 1720 MW (2*500 MW + 2 360 MW). In second case the system had 2284 buses, 2825 branches (2638 lines and 187 transformers) and 195 generating units. Because circuit breakers exist on both sides of every branch protection group analysis could not be run, as their number exceeded possibilities of TRELSS. At first calculations were conducted for 100% base case level with the study area defined as the entire system. Contingency depth up to 4 generators and 0 branches had no influence on reliability indices (all the indices were equal to zero). In the next step data files were modified and prepared for ten load levels. The program created lower base cases by load scaling. In this case contingency depth up to 4 generators and 0 branches had also no influence on reliability indices. Problems started when the program Section 7. Reliability and Continuity of Supply

calculated contingencies with branches. The deepest contingency, which could be calculated at probability cutoff level equal to zero (probability that such contingency would exist), was 0 generators and 1 circuit. Deeper contingency levels were not calculated for the whole system because they exceeded possibilities of the program (even at probability cutoff level equal to 0.00001). The worst reliability indices of electricity supply have load buses in the north of Poland (the 5th zone) and in the north - east of Poland (the 1st zone). TABLE 4. Comparison of system reliability indices for the years 1998 and 2010 System reliability 1998 2010 (1998/ indices 2010) Probability of load 0.0162 0.0027 6.0 loss Frequency of load 19.74 4.88 4.0 loss (occ./year) Duration of load 141.78 23.92 5.9 loss (hrs/year) Duration of load 7.18 4.90 1.5 loss (hrs/occ.) Expected unserved 19760.74 1832.88 10.8 energy (MWh/year) Expected unserved 914.44 344.93 2.6 energy (MWh/occ.) Expected unserved 2948.06 426.89 6.9 demand (MW/year) Expected unserved 136.42 80.34 1.7 demand (MW/occ.) Energy curtailment 0.000189 0.000014 13.5 (MWh/annual MWh) Power interruption 0.168189 0.019216 8.75 (MW/Peak MW) Contigencies 18902 5573 3.4 causing load loss The next test of TRELSS program were the calculations carried out for Polish power system for the year 2010. The calculations were made with equivalents of 110 kV network. During the calculations all network investment predicted for realization, as well as loads forecasted for the year 2010 in individual network nodes and quantity of generation in power plant nodes were taken into account. Moreover the prognosis of 400 kV and 220 kV network component reliability expected for analyzed year, was taken into consideration. A comparison of results of calculations for Polish transmission system structures in the year 1998 and planned for the year 2010, is shown in Table 4. These results were obtained for

491

transmission system model with equivalents of the 110 kV network, and for calculation case 2G2LP(E-5): contingency depth – 2 generators and 2 branches, with PCG analysis, cutoff probability equal to 0.00001. A significant improvement of the system reliability indices for the year 2010 has been stated. It concerns especially expected unserved demand (MW/year) and expected unserved energy (MWh/year). A relatively lower improvement has been stated for the expected unserved demand (MW/occ) and load loss duration (hrs/occ). This general improvement of all indices can be substantiated by prospective network investments, particularly in the northern part of the PPGC system. Limitations in energy supply occurred in 1998 in 47 nodes and in 2010 in 32 nodes. The greatest values of unserved energy occurred for one-busbar one-way-feed nodes: x Year 1998: NAR, ELK, OST, OLM, LSY, OLS, ATA, WLA; x Year 2010: LSY, BYD, ATA, GOR, NAR. A significant improvement for nodes: ELK and NAR (a change to a two-way feed) but worsening for nodes LSY, BYD, GOR, LSN (an increase of demand at still the same two-way feed) has been observed. 5. RELIABILITY VALUING The goal of all the actions on the field of reliability is: Maintaining of existing system reliability level. Identification of investment projects which have the most cost effective contribution to maintain system reliability. Development and determination of quantified reliability measures for electric system planning. Assurance that future system parameters will meet predicted requirements of reliability. Valuation of reliability in terms of interruption service cost. The evaluation of economic losses caused by electric power system unreliability is especially needed for analysis of alternatives of system development planning. Usefulness of an investment, which improves system reliability, may be estimated on the basis of relationship between costs and benefits. The tool for these development planning is cost benefit analysis of power system reliability, known as value-based reliability approach (VBRA). The VBRA idea and its application to transmission system VBTRA are based on this method [3, 9]. The major components in VBRA are (Fig. 2): 492

Identification of alternatives. Assessment of capital costs and operating costs (connected with activities, which improve system reliability). Computation of reliability indices of planned power systems. Assessment of supply interruption costs. Ranking of alternatives, by the total costs of solution. Evaluate Capital Cost

Identify Alternatives

Evaluate Operating Cost

Total Cost

Compute

Total Outage Costs

Reliability Indices System, Operating, and Outage Data

Customer Outage Costs

Rank Alternatives

Utility Outage Costs

From customer surveys

Fig. 2. General idea of Value-Based Reliability Analysis. In order to estimate economic consequences of outages a survey was carried out [7]. The questionnaires taken from EPRI reports [3-4], after its adaptation to Polish conditions were used in this survey. Consumers were divided into two groups: (1) residential sector, (2) industrial and commercial sector. For each scenario of interruption the customer was asked a question what would happen in his house (for housing sector) or in his business (for industrial or commercial sector) as a result of interruption and what would be his reaction on this event. The customer was asked to determine the value of this hypothetical event in PLN 3 . The value in PLN may be determined in one of the three ways: direct costs, readiness for payment, or readiness for approval of payment. The results showed that service reliability is an important issue for customers. The residential customers as a rule do not apply any preventive measures, which could lower effects of outages. They predominantly estimate their one-hour losses within the range from 5 to 10 PLN. From this results that IEAR (Interrupted Energy Assessment Rate) values are within the range 1.5 ÷ 5 PLN/kWh. Wider investigations require participation (including also financing) of local power system utilities. They should be based on polling investigations using questionnaires, prepared in the project. These works should allow for determination of: 3

Polish currency unit (zloty).


cost sector functions of losses caused by supply interruptions (sector customer damage functions - SCDF and cumulative customer damage functions - CCDF); IEAR index value for load nodes of high voltage grid, for voltage levels, for areas of transmission network and for the whole country. 6. AREAS OF TRELSS APPLICATION The TRELSS program is one of the best tools for analysis of large scale electrical power systems reliability on, so-called, 2nd hierarchical level, i.e. comprising both generation and transmission. The main application area of TRELSS is cost/benefit analysis in system planning. The program can compute MW-hours not served and other customer service indices. Thus, having capital cost of the system improvement, evaluating operating cost and total outage cost, the total cost could be calculated. Fields of its application at PPGC are the following: 1. Reliability calculations. 2. Analysis and assessment of planned operational structures of the entire Polish transmission system or its chosen areas. 3. Effectiveness assessment of remedial actions undertaken during operational management of power system, and in consequence, optimization of these actions. 4. Investigations of the influence of network component parameter values on system reliability indices and indices for specific system load nodes. 5. Prognosis of reliability indexes values for specific settlement (clearing) nodes and application of these indices in new contracts. 6. Identification of system threats for realization of already concluded contracts.


7. Calculation of unserved energy costs in the whole system and in specific nodes. The project work, which summary is given in this paper, created a base for implementation and application of the TRELSS program and VBTRA at PPGC. It is basic but also initial stage of longer process. In the authors’ opinion for fully effective realization of the TRELSS program and VBTRA implementation and use some conditions are necessary to be fulfilled. These conditions are following: x Initialization of TRELSS program database (done by the authors of the report [6]). x Training of recommended staff (realized in January 2000). x Urgent starting of works for development of consistent, automated reliability database of network components and generating units, in the shape proposed in the report. x Continuous updating of TRELSS program database and reliability database. x Starting of works for valuation of electricity supply unreliability, comprising of polling investigations (in cooperation with distribution utilities). x Cyclical run of computations by the TRELSS program. x Active membership of PPGC in the works of TRELSS Users Group (TUG). x Use of the most actual version of the TRELSS program. 7. ACKNOWLEDGEMENTS The work presented in this paper was performed for the needs of Polish Power Grid Company, under a tailored collaboration project between the EPRI and the PPGC. Prof. Roman Janiczek, Director of Transmission Services Management, PPGC was the project initiator. He is greatly acknowledged for his encouragement and support. During the course of the project, General Reliability (GR) was consulted on running TRELSS studies. Dr Sudhir K. Agarwal from GR is acknowledged for his help in making TRELSS runs. The proposal of new system of reliability data collecting was consulted with Mr. G. Parcinski from Energy Market Agency (ARE S.A.). We thank for his valuable comments.

493

8. REFERENCES [1] Billinton R., Allan R.N.: Reliability Assessment of Large Electric Power Systems. Kluwer Academic Publishers, 1988. [2] Billinton R., Salvaderi L., McCalley J.D., Chao H., Seitz Th., Allan R.N., Odom J., Fallon C.: Reliability Issues in Today’s Electric Power Utility Environment. IEEE Transactions on Power Systems, Vol. 12, No. 4, 1997. [3] Cost-Benefit Analysis of Power System Reliability: Determination of Interruption Costs. TR-2878, Final Report, Palo Alto, 1990. [4] Outage Cost Estimation Guidebook. EPRI TR106082, 1995. [5] Paska J., Reliability of power supply under energy market conditions (in Polish), Rynek Energii, Nr 4, 1999. [6] Paska J. (Principal Investigator), Bargiel J., Bartczak J., Goc W., Kos A., Momot A., Nowakowska E., Sowa P., Teichman B.: Application of TRELSS and Implementation of Value-Based Transmission Reliability Approach at Polish Power Grid Company. EPRI TR-114816, March 2000. [7] Paska J., Abi-Samra N., Bargiel J., Putorak J.: Experience with Application of Reliability Analysis in Polish Transmission System Planning. North American Power Conference – NAPS2000, Waterloo, Canada, 2000. [8] Transmission Reliability Evaluation for Large Scale Systems (TRELSS). User’s Reference Manual for Version 4.0. EPRI Research Project 3833-1, September 1997. [9] Value-Based Transmission Resource Analysis. EPRI Research Project 2878-02, Final Report, April 1994. [10] Working instruction of regulation of a course of action in domain of failure recording which occur in energy industry (in Polish). Zjednoczenie Energetyki, Warszawa, 1986. Józef Paska (Ph.D., MEEng.) was born in Lubomin, Poland, on Sept. 29, 1950. He received his M.Sc. and Ph.D. from the Electrical Engineering Faculty of the Warsaw University of Technology in 1974 and 1982, all in Electrical Power Engineering. His main scientific interests: applications of probabilistic methods in power system planning, operation and maintenance; power system reliability; electricity economics and planning. In 1998-1999 he was the principal investigator of EPRI project „Application of TRELSS and implementation of valuebased reliability approach at Polish Power Grid Company”. Author of over 70 papers and 2 academic

494

textbooks on power system reliability, electricity generation, renewable energy sources. Member of the Polish Society of Theoretical and Applied Electrical Engineering and the Polish Nuclear Society. Mailing address: Józef Paska Warsaw University of Technology Institute of Electrical Power Engineering 75 Koszykowa Str., 00-662 Warsaw POLAND Phone: (+48/22)6218646 e-mail: [email protected] Andrzej Momot (MEEng.) graduated from the Electrical Engineering Faculty of the Warsaw University of Technology. Currently he is researcher in Institute of Electrical Power Engineering. His area of interest includes electric power system analysis. Mailing address: Andrzej Momot Warsaw University of Technology Institute of Electrical Power Engineering 75 Koszykowa Str., 00-662 Warsaw POLAND Phone: (+48/22)6227189 Joachim Bargiel (Ph.D., MEEng.) graduated from the Silesian Technical University (Poland). Assistant Professor at Silesian TU. Main scientific interests: planning and operation of transmission and distribution networks, reliability of electricity generation, transmission, distribution and supply of consumers. Author of above 50 publications. Mailing address: Joachim Bargiel Silesian Technical University Institute of Electric Power Eng. and Systems Control B. Krzywoustego Str., 244-100 Gliwice POLAND Phone: (+48/32)2372602 E-mail: [email protected]. Wiesaw Goc (Ph.D., MEEng.) graduated from the Silesian Technical University. Assistant professor at Silesian TU and ód¦ TU. Member of Commission of Power Engineering of the Polish Academy of Science. Main scientific interests: electric power networks’ reliability; reliability of supply of consumers in designing, planning and operation; settlements of electricity quality and reliability; rationalization of energy consumption; energy audit. Mailing address: Wiesaw Goc Silesian Technical University Institute of Electric Power Eng. and Systems Control B. Krzywoustego Str., 244-100 Gliwice POLAND





RELIABILITY IN CONTRACTS FOR ELECTRIC ENERGY SUPPLY AND SETTLEMENTS Joachim BARGIEL Wiesaw GOC Silesian Technical University of Gliwice Gliwice (Poland)

Józef PASKA Warsaw University of Technology, Warsaw (Poland)

Pawe SOWA Bogusaw SZEWC Bogusaw TEICHMAN Silesian Technical University of Gliwice, Gliwice (Poland)

Abstract - A trade of the specific good, which is an electric energy, needs a determination of its delivery reliability in points of settlements between the supplier and consumer. A proposal of determination of reliability in contracts for electric energy delivery, as well as rules of settlements is presented in the paper.

1. DESCRIPTION OF THE TOPIC Power purchase agreements are contracted between different subjects of power engineering, as well as between these subjects and consumers. Consumers impose determined requirements regarding electric energy parameters and its delivery – with elimination or at last strong curtailment of outages. Requirements of consumers made another subjects participating in energy market to keep high level of service rendering quality. Thus it occurs transfer of consumers’ requirements to the upper hierarchical levels of power system. The requirements must be taken into consideration in contracts between power firms acting on energy markets. The following enterprises belong to the above mentioned power firms: large power producers, Polish Power Grid Co. and many distribution companies. Like in another branches of trade, also here the following partition occurs: a commodity (i.e. electricity), its transport and deterioration of the commodity because of different reasons. These three segments are described below. Commodity: It originates in power stations and its quality is described by voltage parameters [1, 2] determined in regulations. A question appears: do really all power stations keep these parameters? The parameters are connected with the generated Section 7. Reliability and Continuity of Supply

energy, so they made a feature of the commodity. Among power stations it is to distinguish large producers, which play a leading role, and control over parameters (mostly over frequency), and another local producers, sometimes connected with the network by a converter. Transport: It means electric energy transmission and distribution by transmission and distribution networks of different voltages and with use of transformers. The networks supply customers (wholesale consumers and end users). Also points of settlements between power sellers and buyers exist there. Transport of energy to consumers depends on transmission and distribution system configuration and on random events among network elements. Power curtailments and outages of power supply can appear in unfavourable situations. Therefore supply reliability gives an evaluation of quality of the specific service, which is electric energy supply. Commodity deterioration: It means worsening of electric energy quality because of impact of consumers on the network. Consumers influence the network by: - cumulating of load in some period of a day (peak hours). It can lead in extreme cases to worsening of energy quality (decrease of voltage and/or frequency), and even to power curtailments when generation or transmission capabilities are inadequate, - generation of harmonics that make difficult operation of another consumers (failures of machines and devices), - power strokes and voltage fluctuations. It must be added, that equipment and devices of producers also influence networks and introduce harmonics, overvoltages, overcurrents and ano495

ther distortions. All these influences bring immediate results whose magnitude depends on state of the power system. It can infect systems of generation, transmission, distribution and supply. A service, which is electric energy delivery, was here separated from its parameters, i.e. supply reliability was separated from the commodity (= electric energy) quality. It is principally right, because different objects decide about advantages of each of them. One should remember that even interruptible supply but at low energy quality can be not very useful for a consumer. That is why the problem of the commodity deterioration is so important. Therefore, supply reliability can be defined as electric network ability to supply consumer with electric energy of determined admissible parameters accordingly to its requirements, i.e. intake of the proper power in the determined moment. Consumer, who knows features of electric energy and meaning of electric energy in its operation, tries to ensure delivery of the energy by closing the proper contract – or for energy sale, or contract for transmission. In both kinds of contracts [1] attention should be paid to the quality standards of the consumers servicing and responsibility of parties for not keeping those standards. Values of many of standard quantities cannot be taken freely, because they influence energy quality and supply reliability of many consumers in the same time. It makes some limitations and need of standardisation of those indices [2, 3, 4, 5]. Parameters of supply reliability can be taken more freely. In following sections of the paper problems of supply reliability contracts and settlements between the transmission firm PPGC and distribution companies have been considered. 2. PROPOSALS OF THE GUARANTEED RELIABILITY STANDARDS FOR DISTRIBUTION COMPANIES Differentiation of the EHV networks, particularly EHV/110 kV nodes (i.e. in Poland 400/110 kV and 220/110 kV) in operational areas of the particular distcoes causes diverse values of parameters characterising reliability/continuity electric energy delivery from those nodes to the 110 kV networks of distcoes. Parameters of power supply reliability from the PPGC networks to the networks of distcoes can be determined for: a) any node EHV/110 kV, here usually the 110 kV side of the EHV/110 kV transformer without consideration of possibility of reserve from another source, b) the distco as a whole in the same time, taking it as a consumption area supplied from many nodes EHV/110 kV [6]. 496

Each one of objects being a subject of considerations has different set of parameters, which characterises power supply reliability. For single point of settlements (case a) the set consists of the following indices: 1) number of the short outages (of duration comparable with the operating times of protection devices or shorter), Dkr, 2) number of another outages (so called long outages), Dr, 3) total time of outages in a year, Ta, 4) unreliability index, Q Ta T , where T is the annual operating time, usually equal 8760 h, 5) time of the longest single outage in a year, t, 6) unserved energy as result of outages in a year, And, 7) number of outages lasting longer than contracted border value of the single outage time, Dgr, 8) yearly time of outages lasting longer than a border value of the single outage time, Ta,gr, 9) border value of the single outage time, tgr, 10) distribution of number of outages as a function of their duration, without the short outages, d f (t ) , 11) number of curtailments in power supply, except the short outages, DoP, 12) annual time of curtailments in power supply, ToP, 13) maximal curtailment in power supply in a year, Pomax, 14) unserved energy as a result of curtailments in a year, AoP, 15) number of days with curtailments in a year, DDoP. Parties of the contract (buyer and seller) negotiate, which of the mentioned above indices will be put into the power purchase agreement. The reliability indices are determined on the base of statistical investigations and calculations carried out for points of settlements, usually for the 110 kV side of the EHV/110 kV transformer. Results of computations (carried out with use of the NIEZ program [7]) have been set in the table 1. Also results of statistical investigations, which enable recognition of the outages’ distribution for the points of settlements – see figs. 1 and 2 – can be used. More sophisticated calculations allow to determine distributions of outage times [8]. Results of calculations and investigations can make a proposal to negotiations and to choice of the proper set of indices taken in the contract as the guaranteed values [11]. According to the regulation [1], two obligatory reliability indices for the low voltage networks have been stated: the total time of outages in a year (Ta), and the time of the longest single outage (t) together with their guaranteed parameters.


TABLE 1. Results of calculations of supply reliability indices for the selected consuming nodes on the side of 400 kV, 220 kV and 110 kV D Q And And D Q Name [MWh] outages/a [×10-6] [MWh] outages/a [×10-6] A413 0.035 7.17 1.49 K223 0.055 108.06 10.02 A423 0.047 7.74 0.00 L213 0.382 1334.74 24.00 B413 0.042 7.34 0.00 L223 0.413 1527.87 27.00 B423 0.045 7.80 0.00 A113 0.081 193.95 40.35 C423 0.045 8.26 2.20 D113 0.100 209.49 24.06 C413 0.046 8.34 2.43 D123 0.100 209.49 24.06 D413 0.054 22.71 5.22 E113 0.113 197.84 27.89 E423 0.062 10.57 1.49 E123 0.108 197.34 27.82 E413 0.068 11.07 1.56 C113 0.128 130.46 10.74 F413 0.123 22.42 0.00 C123 0.152 134.86 11.10 B213 0.056 10.23 1.23 F113 0.125 386.36 0.00 B223 0.094 17.16 0.00 F123 0.125 386.38 0.00 C213 0.115 20.95 1.72 H113 0.082 206.64 31.71 C223 0.092 16.71 1.38 H123 0.083 204.55 31.39 H223 0.045 92.90 14.26 L113 0.420 1448.64 26.05 H213 0.047 90.81 13.94 L123 0.450 1641.77 29.52 K213 0.054 107.67 9.98 Zero values of unserved energy mean delivery of energy into the system. In the name of the node – the first number after the character means the voltage level: “4” is 400 kV, “2” is 220 kV, “1” is 110 kV. Name

N u m b e r o f o u ta g e s 80 70 60 50 40 30 20 10 0 0

50

100

15 0

20 0

250

300

35 0

40 0

45 0

50 0

550

600

650

700

75 0

80 0

70 0

75 0

800

O u ta g e tim e in h o u rs

Fig. 1. Distributions and diagrams of outages of 160 MVA transformers (after [11]) N u m b e r o f o uta g e s 50 45 40 35 30 25 20 15 10 5 0 0

50

100

15 0

200

25 0

30 0

350

40 0

45 0

50 0

55 0

60 0

65 0

O u ta g e tim e in h o u rs

Fig. 2. Distributions and diagrams of outages of 250 MVA transformers (after [11])


497

Results of reliability calculations shown in the table 1 allow computing the mean outage time (emergency outage) from the following relation:

ta

Ta , D

5) unserved energy expected annual value, MW·h, Auo, 6) unserved energy expected unit value, MW·h per one occurrence, Ajuo, 7) not covered demand expected value, MW·h in a year, Puo, 8) energy curtailment index as a quotient of unserved energy (Auo) to energy demand (A), expressed in MW·h/ MW·h, QAu, 9) outage rate as a quotient of not covered demand expected value (P.uo) to power peak demand (P.), expressed in MW/MW, Qpu, 10) number of states of outage which cause loss of load, in a year, Duo. Reliability indices for the power system have been obtained from statistic investigations or calculation programs. Results of calculations carried out with use of the TRELLS program [10] for several selected areas of the power system have been presented in the table 2. The calculations, together with results of statistic investigations, make a base for determination of standards of reliability parameters for energy consumption areas [11].

(1)

where: ta – mean outage time, h/outage, Ta – annual time of outages, D – expected yearly number of the long outages. Usually the 110 kV side of the transformer is taken as a point of settlements. Reliability indices for different kinds of nodes vary in a wide interval – see table 1. For energy consumption areas as a whole (case b) the following indices characterise power supply reliability: 1) loss of load frequency, measured in occurrences in a year, Cuo, 2) loss of load annual time, Tuo, 3) loss of load mean time, h/occurrence, tuo, 4) loss of load probability, Quo,

TABLE 2. Power system reliability indices, peak load and annual energy consumption in 5 energy consumption areas in Poland Index

Energy consumption area 3

1

2

4

5

0.032797

0.0081611

0.0496635

0.0587791

0.0293706

Loss of load frequency, occurrences/a

37.774

9.437

56.795

67.046

33.989

Loss of load duration, h/a

287.30

71.49

435.05

514.91

257.29

7.61

7.58

7.66

7.68

7.57

Unserved energy expected value, MW·h/a

3839.2

391.5

2904.4

4957.7

2698.6

Unserved energy expected value, MW·h/occurrence

101.6

41.5

51.1

73.9

79.4

Not covered demand expected value, MW·h in a year

511.7

51.3

383.7

650.7

363.4

Unserved energy expected unit value, MW·h per one occurrence

13.5

5.4

6.8

9.7

10.7

Energy curtailment index

0.00013612

0.00002232

0.00007223

0.00015327

0.00015761

Outage rate

0.100804864

0.017429

0.05683957

0.11980911

0.12639333

Number of states of outage causing loss of load, in a year

2204

839

6546

2423

1552

Peak load, MW

4736

2945

6751

5431

2875

28204.3

17540.0

40210.5

32345.9

17121.6

Loss of load probability

Loss of load duration, h/occurrence

Annual energy consumption, GW·h

498


Values of reliability indices obtained from calculation programs for particular energy consumption areas show influence of 400 kV and 220 kV networks’ differentiation on supply reliability for those areas. The determined indices can made a base to determination of the guaranteed parameters jointly for several points of settlements but now it is only an initial stage. 3. EVALUATION OF RISK WHEN CLOSING CONTRACTS

Both described above sets of parameters give an evaluation of electric energy supply reliability for a distribution company, for current and possible prospected requirements (see tables 1 and 2). The first set of parameters (case a) enables determination of supply reliability levels for particular nodes – points of settlements. The second set (case b)

allows carrying out an evaluation of supply reliability for whole energy consumption areas. Risk of exceeding of the outage admissible time for a single point of settlements can be determined on the base of: - computed indices D, Q (set in the table 1), - diagrams of outage times obtained from statistic analyses (figs. 1 and 2) or calculation programs. Exemplary statements concerning supply reliability for the above mentioned nodes have been presented in the table 3. In those statements was paid attention to the single outage time, which in many cases make a subject of the contract. Evaluation of risk when closing contract is very important. The risk depends on the outages frequency and outage times’ distribution. An attempt to estimation of the risk for selected contract’s paragraphs in several distribution companies is presented below.

TABLE 3. Statements in contracts for transmission service concerning emergency outages (examples for selected distribution companies) Emergency outages duration determined in the contract for Code of the transmission service distco or the annual – for each single outage of in cases not counted to power station, No. which close one service point each one service emergency outages point imposing replacement the contract or repair by a producer 1 2 3 4 5 220/110 kV „G” – no SD1 1 longer than 2 months 2

SD2

3

SD3

Another, additional statements

6

cannot exceed 2 weeks for any place of cannot exceed 2 TR 250 MV·A in sub- Admissible duration of delivery, for the weeks station 400/110 kV simultaneous emergency particular outages . „T” – no longer than 3 outages is 36 hours. cannot exceed 3 months; weeks from the ATR 160 MV·A in moment of substation 220/110 kV notification „K” – no longer than 2 months.

For the distribution company SD1 exceeding of the admissible time (equal 2 months, i.e. 1440 hours) of break of supply from the 220/110 kV autotransformer is very little. In the diagram (fig. 1) duration of the longest outage was equal 744 hours and the outage time of range of 1 months embraces about 94% of the all emergency outages. Therefore it can be drawn a conclusion that risk of exceeding of the contracted admissible value of emergency outage time is very little. For the company SD2 the admissible time of outage of supply from the 220/110 kV autotransformer was determined as two weeks, i.e. 336 hours. Section 7. Reliability and Continuity of Supply

Probability of exceeding of this value - determined from the diagram (fig. 1) - makes about p t ! t gr 0.17 . Outage frequency makes about d 0.55 outages/year. So the expected outages number of duration longer than 336 hours can be estimated as: d t ! t gr p t ! t gr d 0.094 outages/year ,

therefore - in practice - once in 10 years. For supply from the 400/110 kV transformer probability of exceeding of the admissible time – determined from the diagram (fig. 2) – makes 499

about p t ! t gr 0.21 . Outages’ frequency is ca. d 0.75 outages/year . There from expected number of outages lasting longer than 336 hours is: d t ! t gr pt ! t gr d 0.158 outages/year , therefore - in practice - once in 6 years. It can be drown a conclusion that risk of exceeding the contacted time (2 weeks) for both discussed supply configurations is similar, and its level can be admitted average. In the same way one can analyse the case of SD3. The settled in the table 3, col. 6 admissible duration of simultaneous emergency outages of several feeding lines (equal 36 hours) is advantageous also for the supplier. Let us consider it for a case of feeding from two transformers. Resultant indices D in the table 1 vary in such case from 0.081 to 0.152 outages/year – the taken in calculations average value is d 0.11 outages/year. It has been proved [8] that probability of exceeding of 36 hours – if such outage happens – is about p t ! t gr 0.4 .

There from expected number of outages lasting longer than 36 hours for the whole node is: d t ! t gr p t ! t gr d 0.044 outages/year,

so once in 22 years. Therefore risk is small. The put into contracts guaranteed reliability parameters are result of negotiations. It is distinctly seen matching those parameters to the worst operating conditions of the given node (i.e. the worst supply reliability). These parameters represent supplier’s (seller’s) reliability and give very low risk of exceeding. In the present conditions – determined by the power system abilities in relation to consumers’ (buyers’) demand, i.e. by reserves in power stations, and transmission capabilities of lines and transformers – the consumer accepts such statements in contracts. It can partially result from insufficient orientation of the consumer. Negotiations in this area will be in the future more difficult for the supplier. 4. SUMMARY

The Act “Energy Law” introduces fundamentally new rules into the Polish power industry and imposes on power firms (carrying on transmission and distribution) much increased responsibility for supply of consumers. The responsibility embraces also supply reliability on particular hierarchical stages of the power system, quality of delivered energy, and a level of service. Reliability of supply and electric energy quality are in the present time a subject of particular interest of the energy market participants (power traders, power producers, producers of equipment, consu-

500

mers and so on). In the same time – while changing power industry, managing structures, while introducing competition, costs’ reduction and new mechanisms of regulation of the power sector – it becomes more and more difficult ensuring the proper level of those parameters. Power firms, despite of their wishing, will (partially they do it already in the present time) square up with customers for quality of the rendered services, particularly for reliability of supply and for supplied energy quality. In order to establish parameters that determine supply reliability, results of reliability indices’ calculations as well as statistic data on unreliability of network elements are necessary. Energy supplier can use them as a base for determining of risk of not keeping parameters stated in the power purchase agreement. 5. REFERENCES

1. Rozporzdzenie Ministra Gospodarki z dnia 25.09.2000 w sprawie szczegó~owych warunków przy~czenia podmiotów do sieci elektroenergetycznych, obrotu energi elektryczn, wiadczenia us~ug przesy~owych, ruchu sieciowego i eksploatacji sieci oraz standardów jakociowych obs~ugi odbiorców [The regulation of Minister of Economy of 2509.2000 on detailed conditions of connecting of subjects to electric power networks, electric energy trade, wires services’ rendering, network operation, maintenance and quality standards of consumers’ servicing]. Dz. U. [Acts’ Diary], No 85, pos. 957. 2.Paska J., Bargiel J., Goc W., Sowa P.: Jako energii elektrycznej a niezawodno [Electric energy quality in relation to reliability]. Prace Nauk. IE Pol. Wrocawskiej, No. 91, seria Konferencje, No. 34, 2000 r. 3.Hanzelka Z., Kowalski Z.: Kompatybilno elektromagnetyczna i jako energii elektrycznej w dokumentach normalizacyjnych [Electromagnetic compatibility and electric energy quality in standardisation documents]. “JakoÉ i U¶ytkowanie Energii Elektrycznej”, No 1/1999. 4.PN-EN 50160 Parametry napičcia zasilajcego w publicznych sieciach rozdzielczych [The Polish Standard: Supply voltage parameters in public distribution networks]. 5.Polish Power Grid Co: Instrukcja ruchu i eksploatacji sieci przesy~owej [Directions of transmission network operation and maintenance]. Warsaw 1999. 6.Goc W.: Porównawcza ocena niezawodnoci zasilania krajowej sieci 110 kV w etapach jej rozwoju [Comparative assessment of reliability of supply of the Polish 110 kV network on its development stages]. Proc. of the Sc. Symp. “JaElectrical Power Quality and Utilisation

koÉ zasilania z ukadów sieciowych”, Gliwice 1986. 7.Bargiel J., Goc W., Teichman B.: Ocena niezawodnoci uk~adów sieci elektroenergetycznych dla potrzeb optymalizacji ich rozwoju [Network system reliability evaluation for its development optimisation]. Proc. of the 7th Int. Sc. Conf. APE’95, Gdask, 12-14.06.1995. 8.Bargiel J., Ciura S., Goc W., muda K.: Uwzglčdnienie niecig~oci zasilania w rozliczeniach hurtowych energii elektrycznej [Supply unreliability in wholesale settlements for electric energy]. Proc. of the 4th Int. Sc. Conf. APE’93, Gliwice 1993. 9.Bargiel J., Goc W., Szewc B, Paska J.: Computer tools for estimation of the transmission and distribution networks’ unreliability. Proc. of the 4th Int. Sc. Conf. “Efficiency and power quality of electrical supply of industrial enterprises”, Mariupol (Ukraine), 24-26.05.2000. 10.Multi-author work of Technical Universities from Warsaw and Gliwice: Metoda oceny niezawodnoci przesy~u [Transmission reliability evaluation method]. Bull. of the PPGC “Elektroenergetyka”, No 2/2000. 11.Multi-author work: Badanie uwarunkowa zarzdzania niezawodnoci dla optymalizacji pracy systemu elektroenergetycznego [Investigation of reliability management conditions for optimisation of the power system operation]. Project No 8T10 B00216, Project Manager Joachim Bargiel, Silesian Technical University of Gliwice, 2000.

Joachim Bargiel, PhD, MEEng. Graduated from the Silesian Technical University. Assistant professor at Silesian TU. Main scientific interests: planning and operation of transmission and distribution networks; reliability of electricity generation, transmission, distribution and consumers’ supply. Author of above 50 publications. Mailing address: Silesian Technical University of Gliwice Institute of Power Systems and Control ul. B. Krzywoustego 2 44-100 Gliwice, Poland E-mail: [email protected] Wiesaw Goc, PhD, MEEng. Graduated from the Silesian Technical University. Assistant professor at Silesian TU and TU of Lodz. Member of the Power Engineering Commission at the Polish Academy of Science. Main scientific interests: electric power networks’ reliability; consumers’ supply reliability in designing, planning and operation; electricity quality and reliability settlements; rationalisation of energy consumption; energy audit.


Mailing address: Silesian Technical University of Gliwice Institute of Power Systems and Control ul. B. Krzywoustego 2 44-100 Gliwice, Poland E-mail: [email protected] Józef Paska, PhD, MEEng. Graduated from the Warsaw University of Technology. Assistant professor at Warsaw UT. Main scientific interests: application of probabilistic methods in power system planning, operation and maintenance. 1998-99 the principal investigator of EPRI project “Application of TRELLS and implementation of value-based reliability approach at PPGC”. Author of over 50 papers and 2 academic textbooks on power system reliability, electricity generation, and renewable energy sources. Mailing address: Warsaw University of Technology Institute of Electrical Power Engineering ul. Koszykowa 75, Gmach Mechaniki 00-662 Warszawa E-mail: [email protected] Pawe Sowa, DSc, PhD, MEEng. Graduated from the Silesian Technical University. Professor at STU, head of section, professor at TU of Czčstochowa. Current scientific interests: system identification, power system transient analysis, power system control, modelling of transmission system. Member of the IEEE, Polish Electricians’ Assosciation, Polish Association of Theoretical and Applied Electrotechnics. Mailing address: Silesian Technical University of Gliwice Institute of Power Systems and Control ul. B. Krzywoustego 2 44-100 Gliwice, Poland E-mail: [email protected] Bogusaw Szewc, MEEng. Graduated from the Silesian Technical University. Research staff engineer in STU in organisation of researches. Main scientific interests: power system protection, operation of the neutral point, market economy in power engineering, reliability of consumers’ supply, organisation of researches. Author of above 30 publications. Mailing address: Silesian Technical University of Gliwice Institute of Power Systems and Control ul. B. Krzywoustego 2 44-100 Gliwice, Poland E-mail: [email protected] Bogusaw Teichman, EEng. Research engineer in STU. Area of activity: reliability of electric energy supply of large industrial consumers or subsystems. Application of computer techniques to power engineering. Author of above 10 publications. Mailing address: Silesian Technical University of Gliwice Institute of Power Systems and Control ul. B. Krzywoustego 2 44-100 Gliwice, Poland E-mail: [email protected]

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502





THE RELATION BETWEEN CLASSICAL AND GLOBAL INDICES OF RELIABILITY Stanislav RUSEK VŠB-Technical University of Ostrava Ostrava (Czech Republic)

Abstract - The contribution deals with analysis of classical and global indicators of electrical energy supply reliability. Analysis of indicators according to UNIPEDE is performed. A comparison of these classical and global reliability indicators is performed in the contribution. Calculation analysis is performed, i.e. how is it possible to calculate global indicators of reliability at common reliability calculation.

1. INTRODUCTION At present, the reliability of electrical energy supply is a matter more and more discussed. Reliability calculations in the area of electric networks are constantly more frequent nowadays. With the growing importance of reliability calculations, the quantification of reliability changes as well. In the past, classical reliability quantities were used. These days, socalled global indices of reliability are applied increasingly. These global indices can be understood more easily from the electrical energy customer’s point of view. The global indices of reliability are, altogether, easy-to-determine from the analysis of individual power outages. At present the collection of data on failures and outages from almost all the power distribution join-stock companies in the Czech Republic is in progress at the Department of Electric Power Engineering of VŠB - Technical University of Ostrava. In virtue of this database it will be possible not only to determine the global indices of reliability, but also the reliability of important elements of the electric power system, i.e. classical indices of reliability. Section 7. Reliability and Continuity of Supply

2. CLASSICAL INDICES OF RELIABILITY The quantification of reliability may be various according to input data and a methodology employed. The commonest expression of reliability is as follows:

failure rate O [year-1] mean failure duration W [h] probability of failure-free operation R [-] probability of failure Q [-] mean time between failures tS [h]

The failure rate is usually expressed in the number of failures per time unit (with equipment in electric power engineering usually per year). The mean failure duration is given in hours or days. The probability of failure-free operation as well as the probability of failure is given as a proportional number (decimal fraction) or is given in per cents. These values are related to the time, for which the probability is being determined. The mean time between failures is stated in days or years and is a ratio of the total time of operation to the total number of failures during this time. The mean time between failures is proportional to the inverse value of the rate of failures. These classical indices of reliability are used mainly in reliability calculations, when reliability indices of individual elements of the reliability diagram are known and the calculation of resultant reliability of the whole system is executed.

503

3. GLOBAL INDICES OF THE RELIABILITY OF ELECTRICAL ENERGY SUPPLY Methods of calculations of the reliability of electrical energy supply usually lead to the determination of the reliability of electrical energy supply in a certain point (or more points) of the electric network. To enable the quantification of the reliability of electrical energy supply to a given area, so-called global indices of the reliability of electrical energy supply must be applied. These global indices rest upon the recommendation of the international organisation UNIPEDE. In the Czech Republic, the professional group called in English “Reliability” at K (Czech Committee) of CIRED worked out a document named in English “A methodology for the determination of both the reliability of electrical energy supply and that of elements of the distribution systems”, in which I participated. In the framework of this document, global indices of the reliability of electrical energy supply are defined too. The global indices of supply reliability are as follows: outage frequency (number of outages/year/customer) the total duration of all outages (min/year/customer) the duration of one outage (minutes/outage) These indices recommended for this purpose by UNIPEDE characterise the mean average reliability of delivery and its effects from the power customer’s point of view. They will be exploited especially in relation to consultation firms, authorities and mutual comparisons between power distribution joint-stock companies (furthermore referred to as REAS). In relation to common power customers, important limits, however, exist within which these indices move in REAS (or in some of its part) and also the distribution of their frequencies. The subject of observation is, within the meaning of EN 50160, events of the duration more than 3 minutes (so-called long interruptions of delivery). Events of shorter duration belong to the area of electromagnetic compatibility (EMC) and their observation is not included in this directive. For the calculation of the global indices of the reliability of electrical energy supply, it is necessary to have the following data on each event (outage):

504

T0 T1 T2 T3 T4

TZ P1

P2

D1 D2 Z1 Z2

Date and time of the beginning of the event (failure). Date and time of the beginning of manipulation. Date and time of the end of manipulation for failure detection. Date and time of supply restoration in the section affected by the event. Date and time of the end of the event, i.e. time of restoration of equipment ability to fulfil its function. Date and time of earth fault. Performance at the time T0 in kVA. For the calculation of undelivered energy, P1 is taken as the performance undelivered (installed) in the time from T0 till T1. Performance at the time T2 in kVA. For the calculation of undelivered energy, P2 is taken as performance undelivered (installed) in the time from T2 till T3, in the time from T1 till T2 the mean value of P1 and P2 is considered. The number of distribution stations being without voltage at the time T0. The number of distribution stations being without voltage at the time T2. The number of customers being without voltage at the time T0. The number of customers being without voltage at the time T2.

There are three basic approaches to the determination of global indices of the reliability of supply from the distribution networks caused by random or planned interruptions of supply, namely: outage effects are related to the number of customers affected by the outage outage effects are related to the undelivered performance (installed or agreed performance) outage effects are related to the number of affected stations or transformers It is expected that for the purposes of inter-annual comparisons any selected approach can ensure sufficient accuracy. From the standpoint of long-term point of view, the observation of proposed indices (related to the customer) should gradually pass to the observation of the number of affected customers. The indices can be calculated for specific voltage levels according to one of ways given below. In the assessment, the method employed for the calculation must be stated. One event in the distribution system can lead to several outages that will affect some or all originally affected Electrical Power Quality and Utilisation

customers, in some cases, however, also other customers. All relevant outages and their effects on customers must be considered in the calculation of the indices.

The expression in the numerator determines the time limitation of customers and is divided into the following three parts: The first part is the time from failure occurrence to the beginning of manipulation. During this period, the most customers are being affected, i.e. Z1. The second part is the time of manipulation till the failure detection. Within this period, the number of affected customers is expected to decrease from the value Z1 to the value Z2. Relation (3.9) presupposes that the decrease in the number of affected customers is linear in time. The third part is the period of time from failure detection to the complete restoration of power supply. In this period, Z2 customers are being affected.

-

-

3.1 The variant of customer limitation This is the variant when the number of affected customers and the duration of the outage are recorded or can be estimated. For this variant, the following relations are valid:

-

The frequency of outages OG

¦n

OG

-

j

j

[outage . year-1]

NS

(3.1)

The total duration of all outages related to one customer WGV

¦ n

W GV

j

.tj

j

NS

[min . year-1]

(3.2)

The duration of one outage WG

WG

¦ n j

j

3.2

.tj

¦n

[min . outage-1] (3.3) j

j

nj . . . . the number of customers in the group of affected customers j [-] tj . . . . . the mean duration of outage for the customer of the group j [min] Ns . . . . the total number of customers supplied [-] The mean duration is given by the following relation: tj

If no change in the number of affected customers occurs in the course of failure detection, the duration of outage is equal to the time difference T3 – T0. The relation between the global reliability indices and the basic reliability quantities is clear from the following example.

Z1(T1 T0 ) (Z1 Z 2 ) (T2 T1 )/2 Z 2 (T3 T2 ) Z1

(3.4)

This variant is similar to the previous. However, instead of the number of customers affected during the outage, installed performance with the affected customers is considered. For this variant, the following relations are valid: The frequency of outages OG

OG

The mean duration of outage is related to all affected customers Z1, i.e. to customers that were affected at the beginning of the event (Z1 is the largest number of affected customers during the given event; any increase in the number of affected customers in the course of manipulation is not expected).


¦l

j

j

[outage . year-1]

LS

(3.5)

The total duration of all outages related to the unit of installed performance WGV

The philosophy of relation (3.4) is as follows: -

The variant of installed performance limitation

W GV

¦ l

j

.tj

j

LS

[min . year-1]

(3.6)


505

¦ l

WG

j

.tj

j

¦l

The duration tj is given by the following relation: [min . outage-1] (3.7)

tj

j

D 1 (T1 T0 ) (D 1 D 2 ) (T2 T1 )/2 D 2 (T3 T2 ) D1

j

(3.12)

lj . . . . . installed performance in the group of affected customers j [kVA] tj . . . . . the mean duration of outage for the customer of the group j [min] Ls . . . . the overall installed performance [kVA] The duration tj is given by the following relation: tj

P 1 (T1 T0 ) (P1 P2 ) (T2 T1 )/2 P2 (T3 T2 ) P1

(3.8) The philosophy of relation (3.8) is similar to that of relation (3.4). 3.3 The variant of the limitation of distribution transformer substations

The philosophy of relation (3.12) is similar to that of relation (3.4). In all three variants described above, individual global reliability indices are designated in the same manner. Always merely one variant of expression is used depending upon available data on individual outages. The order of importance of individual variants is in accord with the order of their description in this chapter. In case of analysing the area, where the number of customers per DTS is the same and where each customer has the same installed performance, all the described variants are equivalent. Individual global reliability indices will be the same regardless of the fact which variant is selected for the calculation. 4.

This variant is similar to the previous variants. However, instead of the number of affected customers or installed performance with affected customers, the number of affected HV/LV distribution transformer substations (DTS) is taken into account. For this variant, the following relations are valid: The frequency of outages OG

OG

¦s

j

j

[outage . year-1]

SS

(3.9)

The total duration of all outages related to one DTS WGV

¦ s

W GV

j

.tj

j

SS

[min . year-1]

On the basis of analysis of definitions of the global and classical reliability indices it is possible to make their comparing. As already stated, it is possible to use the global reliability indices that are related to the number of limited customers, or to the limited performance, or to the limited distribution transformer substations. For the purpose of comparison with the classical reliability quantities, the global indices will be taken that are related to the number of limited customers (chapter 3.1). From the comparison of the definitions, the following relations can be written: As for the frequency of outages, the following can be written: n

(3.10)

WG

j

j

i

[outage . year-1]

i 1

n

¦N

(4.1)

i

.tj

¦sj

¦O .N i

OG


¦ s

THE RELATION BETWEEN GLOBAL AND CLASSICAL INDICES OF RELIABILITY

i 1

[min . outage-1]

(3.11)

j

The total duration of all outages related to one customer WGV may also be expressed by the following relation: n

sj . . . . . DTS number in the group of affected customers j [-] tj . . . . . the mean duration of outage for the customer of the group j [min] Ss . . . . the total number of DTSs [-]

506

W GV

¦O

i

. t i . Ni

i 1

n

¦N

[min . year-1]

(4.2)

i

i 1


The last global reliability index WG can be expressed as follows: n

WG

¦O

i

. t i . Ni [min . outage-1] (4.3)

i 1

n

¦O

i

. Ni

i 1

Oi . . . . outage rate at the point ”i“ of the network [year-1] ti . . . . . the mean duration of outage at the point “i“ of the network [min] Ni . . . . the number of connected customers at the

Each of the lines V1 – V4 is set by the values of the outage rate O and the mean duration of outage W. (Lines in this scheme are considered including the power lines into substations.) At each consumption point (L1 – L4 bus bars) the N1 – N4 number of customers is connected. In contrast to the classical setting of the reliability calculation, the data on the number of connected customers is added here. The input data may be seen in the following table. TABLE 4.1

n

point “i“ of the network (

¦N

i

V1 V2 V3 V4

NS )

i 1

Relation 4.1, in principle, coincides with relation 3.1. The frequency of outages can be also expressed as a ratio of the number of customers affected by one outage per year to the total number of customers. Relation 4.2 is basically equal to relation 3.2. The value of this index can also be defined as a ratio of the number of customers affected by a minute’s outage per year to the total number of customers. Relation 4.3 is in the main equal to relation 3.3. The value of the index is also defined as a ratio of the number of customers affected by a minute’s outage per year to the number of customers affected by the outage. The definitions of reliability indices that are given in this chapter express the basic relations between the classical reliability quantities and the global reliability indices. These relations are clear most from the example that is presented in the following chapter. 4.1 An example of the calculation of reliability indices The calculation of reliability indices will be executed on the example of a radial distribution network. A simplified network diagram is illustrated in the following picture.

O -1 Year 0,20 0,35 0,10 0,15

Line

t h 1,00 0,30 0,20 0,05

Consumption point L1 L2 L3 L4

Customer number N 400 250 150 200

Because this is the classical case of a series reliability diagram (providing that lines are dimensioned to supply all customers), relations for the series connection can be used for resultant values of reliability quantities in individual consumption points. [2] The resultant values of reliability indices of individual consumption points are presented in the following table: Table 4.2 Consumption point L1 L2 L3 L4

O -1 year 0,20 0,55 0,65 0,80

t h 1,0000 0,5545 0,5000 0,4156

O.t -1 h . year 0,2000 0,3050 0,3250 0,3325

By means of the tables 4.1 and 4.2 the calculation of values of reliability indices can be executed by applying formulas 4.1 – 4.3: n

OG

¦ Oi . Ni i 1

n

¦ Ni i 1

0,2 . 400 0,55 . 250 0,65 .150 0,8 . 200 400 250 150 200 0,475 [outage . year-1] n

W GV

¦ Oi . ti . Ni i 1

n

¦ Ni i 1

Fig. 4.1 The network is fed from the source, with which absolute reliability can be presupposed. Section 7. Reliability and Continuity of Supply

0,2 . 400 0,305 . 250 0,325 . 150 0,3356 . 200 400 250 150 200 0,272 [h . year-1] 507

n

WG

¦ Oi . ti . Ni i 1

n

¦ Oi . Ni i 1

0,2 . 400 0,305 . 250 0,325 . 150 0,3356 . 200 0,2 . 400 0,55 . 250 0,65 .150 0,8 . 200 0,573

[h . outage-1] It is clear from this simple example how to quantify the global indices of reliability in common reliability calculations. As was already stated, for non-specialists the basic reliability indices have mostly no predicative ability. In the given example, the customerorientated global reliability indices (it is necessary to know the number of consumers in the given feed point) are quantified. Similarly, the global indices could be quantified that are related to the outage of installed performance, or to the number of affected DTSs. For the purpose of these calculations it would be necessary to know, in each feed point, the installed performance of consumption or the number of DTSs fed. 5. CONCLUSION

As it follows from the submitted contribution, it is possible to express reliability in various ways. For reliability calculations, classical reliability indices of elements are stated. When determining the reliability of electricity supply to customers, global reliability indices are used almost ever at present. At the observation of outages of electrical energy supply that has been introduced in the Czech Republic lately, it is possible to quantify merely the global reliability indices either. An effort is devoted to the gradual extension of outage databases so that the reliability of specific elements may be assessed as well. In this contribution a methodology for the calculation of global reliability indices from classical reliability indices is described briefly. This methodology is very important and is employed mainly in the determination of global reliability indices of wholesale customers connected newly, when no real data on outages is available. In this case classical reliability calcultions must be applied and resultant values must be converted into global indices.

508

5. REFERENCES 1. Rusek S., Procházka K.: Metodika urování spolehlivosti dodávky elektrické energie a prvk distribuních soustav (Methodology for the determination of reliability of electrical energy supply and elements of distribution systems), Collected papers from the conference K CIRED, pp. 4/16 – 4/20 Tábor 1999 2. Rusek S.: Spolehlivost elektrických sítí (Reliability of electric networks), Monograph. VŠB TU Ostrava 2001, ISBN 80-7078-847-X 3. Rusek S.: Globální ukazatelé spolehlivosti (Global reliability indices), Collected papers from the international scientific symposium “I. Mezinárodné vedecké symposium Elektroenergetika 2001“, pp. 99 - 103 Vysoké Tatry – Stará Lesná 2001, ISBN 80-88922-34B 4. Billinton R., Allan N.: Reliability Evaluation of Power Systems, Plenum Press 1987

Doc. Ing. Stanislav Rusek, CSc. was born in 1957 in HavíÊov, Czech Republic. He received his Ing. degree from VŠB-TU Ostrava and CSc. degree from VUT Brno. At present he is Associate Professor of Department of Electrical Power Engineering of VŠB-Technical University of Ostrava. His areas of interest include: Electric Power Engineering Theory Transmission and Distribution of Electrical Energy Control of Electrical Distribution Systems Reliability in Electric Power Engineering

Mailing address: Stanislav Rusek VŠB-Technical University of Ostrava Faculty of Electrical Engineering and Computer Science Department of Electrical Power Engineering 17. listopadu 15 708 33 Ostrava-Poruba Czech Republic phone: +42 069 699 43 77 fax: +42 069 691 95 97 e-mail: [email protected]





RELIABILITY AND CONTINUITY OF TOWNS ELECTRIC POWER SUPPLY Zden k HRADÍLEK VŠB Technical University of Ostrava Ostrava (Czech Republic)

Abstract - Differences of load among the towns supply areas can be seen in daily load diagram. The time course of electric power consumption influenced by several factors, such as week's day, period season and day's or night's hour. Very important factor of power fluctuation is outside temperature fluctuation. The time course may be approximately wellbalanced from the reason of economical using of electric power plants and of loading distribution network. By management of consumers, which use an electric space heating, can be affected loading of distribution transformers in the town. The opportunity of used combination of distribution transformers, which feed areas with different character of load, it is suitable to notice. Although the load of distribution transformers will not be well balanced, the load

of superior transformer 110 kV/22kV may be more well – balanced by suitable management of customs. 1. INTRODUCTION The areas, feeded by distribution transformers DT connecting in distribution network, can be divided to following categories: x x x x

Building with central long-distance heating (fig.1) Building with electric space heating (fig.2) Business consumption (such as small shops and manufactories in the centre of towns (fig.3) Air-conditioned banks and big offices (fig.4)

Fig. 1 Load- time characteristic of distribution transformer 22/0,4 kV DT- 149 Keplerova v Olomouci, which was measured on Sunday and Monday


509

Fig. 2 Load time characteristic of distribution transformer 22/0,4 kV DT-149 Keplerova in Olomouc, which was measured on Sunday and Monday

Fig. 3 Load time characteristic of distribution transformer 22/0,4 kV DT- 094 Dolní námstí in Olomouc, which was measured on Sunday and Monday. Consumers use the electric off-peak heating.

Fig.4 Load -time characteristic of distribution transformer 22/0,4 kV DT-082 Finanní úÊad in Olomouc, which was measured on Sunday and Monday.

510


Differences of load among the town supply areas can be seen in daily load diagram DLD. The time course of electric power consumption influenced by several factors, such as week's day, period season and day's or nights hour. Very important factor of power fluctuation is outside temperature fluctuation. The time course may be approximately well-balanced from the reason of economical using of electric power plants and of loading distribution network. By management of consumers, which use an electric space heating, can be affected loading of distribution transformers in the town DT. The other groups of customers influence DLD of distribution transformers without possibility of management.

2. OUTSIDE TEMPERATURE INFLUENCE TO LOAD OF DISTRIBUTION TRANSFORMERS Outside temperature load dependence is especially significant in areas with electric space heating. This influence is caused by increase of heat losses of heating buildings when the outside temperature decreases. The heat losses have to be covered. The basic heat losses Q0 are equal to sum of heat flows go through walls of the building both to the outside medium and to the other rooms. The magnitude of basic heat losses is:

¦k

j

S j (t i t e j ) [W] Active power P

The peaks of load of DT feeding the consumers with direct acting heating increases as the peaks of temperature decreases in the longer time and the other way round (see the figure 5).

3. BALACING DIAGRAM

OF

§ S· ¸ ' P ' P0 ' Pk ¨ © Sn ¹

Powers P,S [kW,kVA]

LOAD

2

[kW]

(2)

S …. load of transformer in kVA Sn …. rated power of transformer in kVA

(1)

Apparent power S

Outside temperature °C

160,0 150,0 140,0 130,0 120,0 110,0 100,0 10.2.1996

–

The DLD of some above mentioned categories are very unbalanced and minimum load of DT feeds this categories is only very little part of maximum load. Loses of transformers consist of two components. First is – no load loses ËP0, which donÌt depend on load of transformer and second is short-circuit loses ËPk, which depends on load of transformer. Total loses:

170,0

90,0 6.2.1996

DAILY

15.2.1996

20.2.1996

25.2.1996

1.3.1996

11,0 9,0 7,0 5,0 3,0 1,0 -1,0 -3,0 -5,0 -7,0 -9,0 -11,0 6.3.1996

Outside temperature [°C]

Q0

S j….area of cooled wall in m2 kj … overall coefficient of heat transfer in W.m-2. K-1 ti … calculating inside temperature in oC tej … temperature of surface of j- th wall in oC

Days of measured interval

Fig. 5 – Average load time characteristics of distribution transformer 22/0,4 kV DT-078 Selské námstí in Olomouc, which feeds area with direct acting heating. There is outside temperature shown on this graph too.


511

Similar relation applies to reactive input of transformer. From this results, that the loses of little loaded transformer will be create big part of transferred power. If the DLD is approximately well-balanced and its magnitude is nearby of nominal power of transformer, the transmission costs of demanded power will going down. Power distribution plant has supply electric energy to consumers immediately and with the demanded power quality. If we know restrict big peaks of transferred power, we will be able to take away reconstruction’s of transformers and construction of new power sources. The DLD may be influenced by switching clocks or ripple control receivers. However, those devices may be only used, in our case, in areas with electric space heating. With the help of those devices the direct acting heating may be switched off in period of peak load or the off-peak heating may be switched on in period of low load DT. Big customers, such as banks or big offices, use their own transformers for feeding. A quarter hoursÌ maximum is arranged with them. Change of magnitude of the quarter hoursÌ maximum is only one chance to influence a load during the working hours. IsnÌt possible to influence load of households, witch use central longdistance heating. 4. SUMMARY Directly manage is possible only in the case of distribution transformers feeds customers with direct acting heating. Motivation of the other customers is possible only by prices of electric energy at the frame of tariffs of electric energy. The opportunity of used combination of distribution transformers, which feed areas with different character of load, it is suitable to notice. Although the load of distribution transformers will not be well balanced, the load of superior transformer 110 kV/22kV may be more well – balanced by suitable management of customs. 5. REFERENCES 1. Hradílek, Z.: Elektrotepelná zaÊízení. Knižnice ELEKTRO sv. 35, Praha INEL 1997.

512

2. Pokluda, M.: Îízení odbru distribuních trafostanic s predikcí zatížení pÊi rzných venkovních teplotách. Doktorská disertaní práce, Ostrava 1999. 3. Hradílek, Z.: Theoretical aspects of alternative proposals of electric heating. Sborník vdeckých prací VŠB Technická univerzita Ostrava . 1,1999, roník V., lánek . 34 This work is founded by agency of GAR no 10299-1158. 4. Hradílek Z., Gurecký J., Rusek S.: Reliability of Disconnecting Switch for Automatic Control in Outdoor High-Voltage Networks. Proc. of VIIth Symposium PPEE 1997, UstroÏ, Poland, pp.391-393. 5. Hradílek Z., Gurecký J., Rusek S.: Optimisation in Design of Automatic Control of Disconnecting Switches in Outdoor Voltage Networks. Proc. of IVth Symposium "Metody matematyczne w elektroenergetyce 1998", Zakopane, Poland, pp.227-234, ISBN-83908240-5-2. 6. Hradílek Z., Gurecký J.: Application of Methods of Multicriterion Analysis in the Optimisation of Introducing the RemoteControlled Disconnecting Switches in Outdoor High-Voltage Networks. Proc. of VIIIth Symposium PPEE 1999, Wisla, Poland, pp.356-360, ISBN 83-907217-4-0. 7. Ruses S.: Simulaní metody výpotu spolehlivosti elektrických sítí. In: Proc. of scientific conf. "Elektroenergetika na poátku XXI. století, pp.103-108, Brno, Czech Republic, 1997. 8. Rusek S.: Výpoet spolehlivosti napájení rozvoden s respektováním soubhu vedení. In: Elektrotechnika v praxi, vol.8. no.1-2, pp.7072, Ostrava, Czech Republic, 1998, ISSN 0862-9730. Prof.Ing.Zden k Hradílek, DrSc. born 1940, works as a head of the Department of electrical power engineering VŠB-Technical University Ostrava since 1977. His scientific work is located on electrical distribution network, electrothermal engineering and research of the reliability of electric plants. He has been working in the special commitee of the U.I.E. international union since 1978. Mailing address: Zdenk Hradílek Department of electrical power engineering tÊ. 17.listopadu, 708 33 Ostrava-Poruba Czech Republic phone: 420 69 699 1235, fax: 420 69 6919597 e-mail: [email protected]





SUPPLY RELIABILITY IMPROVEMENT BY MEANS OF UNCONVENTIONAL ENERGY SOURCES Rozmysaw MIENSKI

Ryszard PAWELEK

Maciej PAWLIK

Irena WASIAK

Technical University of Lodz Lodz (Poland)

Abstract - An improvement of supply reliability can be obtained by means of unconventional energy sources. This may be synchronous machines driven by a gas engine or Diesel engine, which when supplying basic receivers can ensure continuity of a technological process during faults in the supplying network. For assessment of such systems operation the information about voltages, speeds and other transients of technological devices is necessary. The paper describes a computer simulation method which enables to determine these transients for the network supplying a sewage treatment industry. Results of calculations are presented. 1. INTRODUCTION In many technological processes, unconventional sources of energy may be used to improve reliability of supply to basic receivers of electric energy. Typical examples are biological sewage treatment plants where biogases generated in a technological process may be utilised to energie synchronous generators. The obtained efficiency of energy medium (biogas) is generally not sufficient to meet total power demand. However, use of generators operating synchronously with a supply network allows to decrease energy consumption (costs), and they may be a source of energy for basic devices ensuring the continuity of a technological process during supply interruptions. It is of great importance both for environmental protection and efficient operation of a treatment plant. Economic analysis for that kind of an undertaking and simulation investigation made in order to check for correctness of technical solutions are necessary in each case.


The paper presented deals with selected technical aspects related to the improvement of reliability of electrical supply to basic receivers in one of biological sewage treatment plants.

2. DESCRIPTION OF THE SUPPLY NETWORK OF THE TREATMENT PLANT The plant under investigation is insensitive to short power interruptions when supplied from a public distribution network (interruption duration to 30 min). However, the energy supplier does not ensure to recover power in a period less than 2 hours, so it should be checked if the treatment plant may be operated under fault conditions, i.e. when energy from a public distribution network is not delivered. Basic receivers ensuring continuity of a technological process in the treatment plant are as follows: x pumps in a main pumping station (12 motors with power ratings 17 kW) x aeration tanks x high-pressure pumps (8 motors with power ratings 110 kW). In order to ensure continuity of supply to a.m. basic receivers in the treatment plant, two K 2876 LN type heat and power units will be installed, each including the synchronous generator with power rating 223 kVA (173 kW) driven by the MAN K 2876 LN type gas engine rated of 180 kW. Each unit will be equipped with a control system providing both automatic synchronisation with a public distribution network and independent operation (at power rating reduced by ca 10 %).

513

Supply from public distribution network MAIN SUPPLY SUBSTATION 15/0,4 kV 4 transformers x 800 kVA

MAIN PUMPING STATION

HIGH-PRESSURE PUMPS

12 motors x 17 kW in two groups of 6 motors

8 motors x 110 kW in two groups of 4 motors

GENERATORS

DIESEL GENERATORS

2 units x 173 kW driven by 180 kW gas engines

2 units x 700 kW

AERATION TANKS load 2 x 230 kW

Fig.1. Simplified diagram of energy distribution in the treatment plant Those generators will be connected to the separate sections of a switchgear in a main pumping station. Generating capacity of generators does not meet total power demand of receivers installed in the treatment plant. Deficiency will be covered from a public distribution electrical network. In addition, installation of two generating sets driven by Diesel engines is planned as an energy source for a highpressure pumping station under fault conditions. In Fig.1, there is presented a simplified diagram of energy distribution in the treatment plant. Receivers that are insensitive to long power interruptions are omitted (only receivers whose operation is necessary are included). When energy from a public distribution network is not delivered, the industrial network shall provide a supply to basic receivers by means of either gas generators or Diesel generators. Two separate networks will be formed: x The first network supplied by gas generators operating either independly (two subnetworks) or in parallel (mutual synchronization of generators is required). From that network, the aeration tanks and drives for pumps in the main pumping station will be powered. For this purpose a network configuration shall be changed to supply the aeration tanks from the switchgear of the main pumping station. Receivers shall be controlled (by turning on the 514

proper number of pumps and aeration tanks) by personnel manually, depending on sewage flow, taking care that generators are not overloaded. x The second network supplied by Diesel generators. The separate drives for highpressure pumps (4 motors with ratings 110 kW) will be powered from that network. The drives can be controlled either manually or automatically, depending on sewage flow. Specific character of operation of pump drives powered from particular networks shall be checked, especially start conditions. 3. SIMULATION 3.1. Subject, purpose and program of investigation Subjects of investigation were two separate networks supplying electric motors of selected pumps in the sewage treatment plant under fault conditions, i.e. on a failure of supply from a public distribution network: x the network with a synchronous generator driven by a Diesel engine, supplying four asynchronous motors with power ratings 110 kW,


x the network with a synchronous generator driven by a gas engine, supplying six asynchronous motors with power ratings 17 kW. The purpose of investigation was the check by means of a computer simulator on possibility of operation of both separate networks. The program of investigation for each network included: x simulation of starting a synchronous generator driven by a gas engine and switching on a generator excitation, x simulation of switching on a motor that drives one pump at rated load, x simulation of switching on successive motors that drive pumps at rated load, x simulation of the steady state of a separate network with a running synchronous generator and all pumps driven at rated load by motors.

running) in a separate network supplied from 173 kW generator driven by a gas engine. Results of simulation in that case are presented in Fig. 2, 3 and 4, which are: x schematic diagram of a separate network under investigation with generator driven by gas engine and 17 kW motors for pumps (Fig. 2), x diagrams of slip s, electromagnetic torque m, mechanical torque mop, voltage on the generator stator terminals _u_ and its rms value Urms calculated for a predetermined time (Fig. 3), x diagrams of slip s, electromagnetic torque m, mechanical torque mop, stator current _i_ of the motor driving a pump and its rms value Irms calculated for a predetermined time (Fig. 4). It should be pointed that a slip for each machine is calculated in relation to the rated synchronous speed of a machine, while machine torques are calculated in relation to the rated torque of a machine.

3.2. Assumptions 4. SUMMARY Simulators for both separate networks were built by means of SYMEL program developed at the Institute of Electric Power Engineering, Technical University of Lodz. In the simulators, synchronous and asynchronous machines are represented with mathematical models that by means of differential equations enable to simulate transient states of electromagnetic and electromechanical machines. Due to lack of exact data, parameters of machines that operate in separate networks were assumed on the basis of data accessible in a catalogue for machines with similar rating, i.e. for SZJe64b and SZJe 94b squirrel-cage motors, and CD12-46-8 synchronous machine. It assumed that synchronous generators were equipped with excitation regulators, set value of, which is 380 V, and that motors driving generators were provided with simplified speed controllers. It was also assumed that drive motors for pumps normally run at rated load, a mechanical torque of pumps was dependent on a speed squared, and initial mechanical torque (for pump speed about zero) was 20 % of a rated value. It was assumed to disadvantage of calculation that the load of a pump driven by motor at start-up is the same as at rated duty. Cable parameters of the simulated networks are omitted as insignificant ones. 3.3. Examples of simulation results For presentation, a part of simulation investigation has been selected that is related to start-up of 17 kW pump No 7 (pumps from No 1 to No 6 are


In many technological processes, unconventional sources of energy may be used to improve reliability of supply to basic receivers of electric energy. Engineering and economic analysis is required in each case. To check for correctness of selection of device parameters, a computer simulation may be used in an engineering investigation. For this purpose, the SYMEL computer program developed at the Institute of Electrical Power Engineering, Technical University of Lodz is useful. It has been proved by the investigation, a part of which is presented in this paper. Results achieved during a simulation may be characterised as follows: x On the basis of slip diagrams for pump drive motors, it may be concluded that after any motor under investigation was switched on to a proper separate network, the motor completed correctly its start-up, though each start-up was simulated at rated load of a pump driven by that motor. x After a successive motor is switched on, a momentary reduction in motor supply voltage is noted, which is limited in a short time by means of a field regulator. x It should be pointed that after the start-up of a successive pump is completed, the voltage always returns to its rated value (see the calculated rms voltage Urms of a generator). x The speed of synchronous generators is reduced and so the frequency in separate networks decreases when successive pumps are turned on, but it is not reduced below 90 % of a rated synchronous speed. 515

Fig. 2. Schematic diagram of a separate network under investigation for simulation of the start-up of 17 kW pump No 7

516


|u|

s

mop

m

Fig. 3. Diagrams of slip s, electromagnetic torque m, mechanical torque mop and voltage _u_ on the generator stator terminals during the start-up of pump No 7

s

| i|

m

mop

Fig. 4. Diagrams of slip s, electromagnetic torque m, mechanical torque mop and stator current _i_ of a pump motor during the start-up of pump No 7.


517

Rozmysaw Mieski received M.Sc. and Ph.D. degrees from Technical University of Lodz. At present he is a senior lecturer at the Institute of Electrical Power Engineering of Technical University of Lodz. His area of interest is power quality and AC/DC power network simulator. e-mail: [email protected] Ryszard Paweek was born in 1952 in Chocz, Poland. He received M.Sc. and Ph.D. degrees from Technical University of Lodz. At present he is a senior lecturer at the Institute of Electrical Power Engineering of Technical University of Lodz. He is a secretary of the Editorial Board of Polish periodical “Electrical Power Quality and Utilisation”. His field of interest is power quality. e-mail: [email protected]

518

Maciej Pawlik was born in 1940 in Sarny, Poland. He received the M.Sc. degree in electrical engineering from Technical University of Lodz. He received his Ph.D. and D.Sc. degrees in power stations from Technical University of Lodz. He was made Professor in 1990. Since 1994 he is a foreign member of Ukrainian Academy of Sciences. Presently he is a Director of Institute of Electrical Power Engineering in Technical University of Lodz. His area of interests included optimization of auxiliary systems of power stations and cogeneration. E-mail: [email protected] Irena Wasiak graduated from the Technical University of Lodz, Poland. There she received the Ph.D. degree in electrical power engineering. Presently she is a senior lecturer at the Institute of Electrical Power Engineering, Technical University of Lodz. She is a secretary of the Program Board and a member of the Editorial Board of Polish periodical “Electrical Power Quality and Utilisation”. Her area of interest includes modelling and simulation of transients in power systems, and power supply quality. e-mail: [email protected]


INDEX OF AUTHORS 1.

Abreu J.P.G.

339, 401, 425

2.

Adam M.

471

3.

Al.-Khayat N.

433

4.

Arango H.

401

5.

Ballocchi G.

265

6.

Banko S.

153

7.

Baraboi A.

471

8.

Baranenko T.K.

253

9.

Bargiel J.

485, 493

10.

Barkan J.

241

11.

Beck H.P.

419

12.

Benysek G.

279

13.

Bernardes D.F.

339

14.

Bicova E.V.

327

15.

Biovská B.

195

16.

Bien A.

231, 237

17.

Bajszczak G.

273, 393

18.

Brasil D.O.C.

369

19.

Brenna M.

265

20.

Carolsfeld R.

225

21.

Dmitriev E.

159

22.

Dominigues E.G.

401

23.

Dzie¶a J.A.

285

24.

Dziuba R.

433

25.

Eguia P.

377

26.

Ertunc H.M.

113

27.

Faranda R.

265, 317

28.

Fernandez E.

377

29.

Fickert L.

139

30.

Fustik V.

459

31.

Gavlas J.

195

32.

Gellings C.W.

33.

Goc W.

485, 493

34.

Gomes R.J.R.

369

35.

Grabowski D.

353

36.

Grinkrug M.

411

Index of Authors

25

519

37.

Hanzelka Z.

231

38.

Hartman M.

231

39.

Hashad M.

231

40.

Howard M.W.

383

41.

Hradílek Z.

507

42.

Hrková J.

43.

Iliev A.

459

44.

Janson K.

445

45.

Järvik J.

333, 445

46.

Kempski A.

439

47.

Klimash V.

303

48.

Koczara W.

433

49.

Koponen P.

217

50.

Korczyski M.J.

245

51.

Kot E.

279

52.

Kovernikova L.I.

123

53.

Kowalski Z.

54.

Kozlov A.

179

55.

Kumierek Z.

245

56.

Kuznetsov V.

159, 327

57.

Kwasnicki W.T

58.

Kysnar F.

203

59.

Leonarski J.

433

60.

Leschenko S.

241

61.

Leva S.

145, 165

62.

Lezhnyuk P.

129

63.

Lo Schiavo L.

51

64.

Lobos T.

35

65.

Loginov V.

179

66.

Lopes G.

309

67.

Lukianenko Y.

129

68.

Malaman R.

51

69.

Mceachern A.

63

70.

Mcgranaghan M.

211

71.

Medeiros J.R.

369

72.

Meyer J.

73.

Mielczarski W.

363

74.

Mienski R.

103, 291, 511

75.

Mircea I.

347

520

85, 203

15

95

71


76.

Momot A.

77.

Moncrief W.A.

78.

Morando A.P.

145, 165

79.

Myasoedov Y.V.

451

80.

Nesterovich V.V.

119

81.

Olaru D.

133

82.

Ozdemir E.

113

83.

Ozdemir S.

113

84.

Pacholski K.

259

85.

Pancu C.

471

86.

Paska J.

485, 493

87.

Paulillo G.

401, 425

88.

Pawelek R.

103, 291, 511

89.

Pawlik M.

511

90.

Ploutenko A.D.

187

91.

Postolati V.M.

327

92.

Prillwitz F.

459

93.

Procházka K.

203

94.

Renner H.

139

95.

Rösner J.

419

96.

Rozkrut A.

237

97.

Rusek S.

501

98.

Rusinaru D.

347

99.

Rusiski J.

279

100.

Sabarno L.

153

101.

Saenz J.R.

377

102.

Sakkos T.

333

103.

Sakulin M.

139

104.

Samotyj M.J.

363

105.

Santarius P.

195

106.

Sarv V.

333

107.

Savina N.V.

173, 451

108.

Sayenko Y.L.

109.

Semczuk M.

225

110.

Sevastuk I.

153

111.

Smirnov S.S.

123

112.

Smoleski R.

439

113.

Sobierajski M.

95

114.

Sowa P.

Index of Authors

485 63

79, 119, 253

493 521

115.

Špaek Z.

116.

Strzelecki R.

439

117.

Szewc B.

493

118.

Szkutnik J.

407

119.

Teichman B.

493

120.

Tironi E.

309, 317

121.

Tkacheva Y.

411

122.

Toropchina L.V.

467

123.

Toropchina S.V.

467

124.

Torres E.

377

125.

Trach I.

153

126.

Trusca V.

133

127.

Tugay Y.

159

128.

Ubezio G.

309, 317

129.

Valadè I.

265, 309, 317

130.

Varetsky Y.

297

131.

Vasiljev A.

241

132.

Vehviläinen S.

217

133.

Vinnal T.

445

134.

Vydmysh V.

129

135.

Vyskoil V.

136.

Waclawiak M.

211

137.

Walczak J.

353

138.

Wasiak I.

103, 291, 511

139.

Wasiluk-Hassa M.

363

140.

Weber H.

459

141.

Widlok H.

477

142.

Wilkosz K.

95

143.

Winkler G.

71

144.

Yarnyh L.

129

145.

Zaninelli D.

145

146.

Zhezhelenko I.V.

522

85

85, 203

79, 119, 253
