Power Quality Improvement by Mitigation of Current Harmonics using D

2017 Third International Conference on Science Technology Engineering & Management (ICONSTEM)

Power Quality Improvement by Mitigation of Current Harmonics using D - STATCOM H. Prasad

T. D. Sudhakar

Assistant Professor, Department of Electrical and Electronics Engineering St. Joseph’s college of engineering Chennai – 119, India [email protected]

Associate Professor, Department of Electrical and Electronics Engineering St. Joseph’s college of engineering Chennai – 119, India [email protected]

Abstract – Power Quality disturbances are a characteristic of typical distribution system. There are various issues like harmonics, voltage sag/swell, interruption etc which cause a serious impact on the quality of power supplied. In order to counter these power quality issues, Power Quality improvement devices are used. D – STATCOM or Distribution STATCOM is one such device. D – STATCOM finds widespread applications in Power systems. This paper describes the mathematical modeling of a three phase AC distribution network incorporated with D – STATCOM implemented to mitigate current harmonics caused due to non –linear loads in the system.

are discussed in [15] [8] [16] [13]. Bhim Singh, Jayaprakash et. al presented a comprehensive discussion about various topologies of D – STATCOM their features, enhancement and possible real time applications. This was found to be very useful for researchers [2]. Another discussion of the same regard was carried out by Pradeep Kumar, Niranjan Kumar et. al [7]. Mathematical modeling of D – STATCOM will help to predetermine its performance with respect to a particular system. It is a generalized objective approach which can be applied during simulation of a system. Various mathematical models of systems and their associated STATCOMs have been suggested so far. Vasudeo B. Virulkar and Mohan V. Aware modeled a D – STATCOM with BESS for a system affected by flickers [10]. Modeling of a typical 14 bus system with D – STATCOM incorporated for power quality improvement was carried out by P.Venkata Kishore and Dr. S. Rama Reddy [12]. The design of a D – STATCOM model using cross relation function approach was discussed by Bhim Singh et. al [17]. A D – STATCOM modeled using direct control strategy was discussed byAn Luo, Huagen Xiao et. al and which was found to have good dynamic response and adaptive ability [1]. Application of neural networks and other intelligent systems in real time power systems have always been a fascination for researchers globally. In this regard application of a neural network based learning vector quantization approach for control methodology of a DSTATCOM was discussed by Sabya Raj, Bhim Singh et.al [3]. The same duo also provided a detailed analysis of control of D – STATCOM based on leaky LMS algorithm [4].Control algorithm based on enhanced PLL or Phase Locked Loop was discussed with regards to application of D – STATCOM in maintaining power factor at the common point of coupling[5] [19]. Further Bhim Singh and Sabya Raj went on to discuss a neural net based back propagation learning algorithm for control of D – STATCOM. Under this method a prototype of D- STATCOM was realized using a Digital Signal Processor [6]. In this paper a three phase system feeding a rectifier as well as an unbalanced RL load is taken for analysis and is modeled along with D –STATCOM for Power Quality improvement by mitigation of various order harmonics injected into the system by non – linear loads.

Keywords – Point of Common Coupling (PCC), D – STATCOM, DVR, Rectifier, Voltage Source Inverter, Synchronous Reference frame, Fundamental Extraction, THD – Total Harmonic Distortion.

I. D – STATCOM – AN OVERVIEW D – STATCOM is a very recent development in the field of Power Quality [8].Distribution STATCOM orD – STATCOM is a shunt -Connected device incorporated at the Point of Common coupling or PCC inorder to improve power quality of the system taken into consideration [8] [9]. This performs more or less the operation analogous to that of an SVC connected to the bus. It can be used to provide shunt compensation in a network while a similar topology is also used analogous to a TCSC for providing series line compensation and is known as DVR or Dynamic Voltage Restorer [9]. Both D – STATCOM and DVR are similar in topology but only differ in the way they are connected to the system and thus the type of compensation they offer [8] [9].D – STATCOM topology consists of a Voltage Source Inverter with a capacitor on the DC side as the energy storage element. It works on the principle of generation of compensating current based on the reference signal obtained from extraction technique. There are various time as well as frequency domain reference extraction techniques in practice today [11]. This reference extracted from the raw waveform is processed through a current control algorithm such as the hysteresis band control [18] [20]. This will modulate the switching pattern of the VSI / D – STATCOM to generate the compensating voltage or current. Its various applications include reactive power compensation, voltage sag/swell mitigation, flicker mitigation, harmonic reduction etc… that

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2017 Third International Conference on Science Technology Engineering & Management (ICONSTEM) II. MODELING OF THE SYSTEM WITH D - STATCOM Consider a three phase AC system feeding an unbalanced R- L load and a DC load through a 6 –pulse rectifier setup. The system is subjected to power quality disturbances due to the unbalanced nature of loading and the rectifier acting as a non – linear load when seen from the AC side. The schematic of this system taken for analysis is given below in figure 1.

Figure 1. Three Phase System Schematic without D – STATCOM. The single line diagram of the above system with D STATCOM is given in figure 2 as follows.

and are the source resistance and Where reactancerespectively. Due to these there will be a drop in the voltage available at the PCC due to which the voltage available to the loads will drop to V. The expression for Vis given by V = System Voltage – Drop across source impedance The drop across the source impedance is given by two terms one a resistive drop and other and inductive drop given by equations 4 and 5 respectively = * (4) = * (d / dt)(5) and are the values of Where is the source current and source resistance and inductance respectively. Thus the equation of phase voltages will become. Va = – ( * )– (d / dt)(6) Vb = – * )– (d / dt)(7) – * ) – (d / dt) (8) Vc = B. Modeling of R-L Load: In order to raise the reactive power demanded by the load, the load designed will be a star connected unbalanced load. RL load is not a purely resistive load and hence the reactive power demand comes into play. This reactive power demand causes disturbances in the voltage profile of the system and hence there arises a necessity of D – STATCOM to be incorporated in the system to provide compensation so as to maintain a constant voltage profile at the buses. The schematic diagram of the unbalanced load is given in figure 3.

Figure 2. Single Line Diagram of the system with D – STATCOM incorporated at the PCC

The rectifier and the RL loadare modeled separately since the current drawn by them respectively differs and they sum up to the total current drawn from the source. Mathematical modeling of various components of the system is described in the sections to follow. A. Modeling of Source : The source is a three phase AC voltage source with magnitude ‘Vs’ with a frequency of 50 HZ. The individual phase vectors are given as (1) (2)

Figure 3. Three Phase Unbalanced RL load. Consider the load conditions as shown in the figure 2 above. Take the phase impedances (resistances and inductances) be given by suffixes a, b and c for the respective phases. Let the respective phase currents of phases a, b and c be Ia Ib and Ic. Now the differential equations respective to each of these phase currents can be stated as (9)

(10) (11)

The source impedance is given by

=

+j

C.

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Modeling of Rectifier

2017 Third International Conference on Science Technology Engineering & Management (ICONSTEM) is designed so as to account for the losses due to voltage source inverter switching operation. It can also absorb real power from the system to charge itself without causing a significant disturbance to the network’s equilibrium. The general schematic of a typical D – STATCOM is given in figure 4.

A Rectifier is the device which converts AC voltage or current to DC. Here a six pulse diode rectifier is taken for analysis which is the simplest bridge rectifier setup available. It converts three phase AC to DC. Since its operation involves power electronic switching, it acts as a non – linear load when seen from the AC side. This non – linear loading injects harmonics into the system which causes distortion of the supply current waveform. This has to be nitigated and here D – STATCOM comes in handy. The DC side of the rectifier is connected with an RL load with resistance Rd and inductance Ld. The current on the AC side of rectifier is of importance to us since it is a part of the total AC load current drawn. Let the DC current drawn by the DC side of the rectifier be Id. The diode rectifier consists of six switches. Each phase controls the switching of two switches. The switches are operated by a switching function given as follows.

Figure 4. Schematic of a D – STATCOM

From the figure CD denotes the energy storage element or capacitance of the energy storage capacitor while RD represents the losses associated with the energy storage DC bus part. Vf is the output voltage of the VSI and If is the filter current drawn by the series RL branch Xf of the filter. The main function of D – STATCOM is to inject compensating current at the point of common coupling or PCC to improve power quality. One of the important concepts in the operating principle of a D – STATCOM is its reference current generation technique which is discussed as follows. E. Reference Current Generation (Control) This is the process of extracting the fundamental component from a given waveform which has multiple frequency components or harmonics. There are various time domain as well as frequency domain techniques available for this. Of which the Synchronous reference frame theory [4] is widely used which is also discussed in this work. This method involves mathematical transformations namely Clark’s and dq0 transformations to extract the fundamental component alone from the given source waveform.The schematic of this method is given in figure 5.

Si = 1 when diode is ON and Si = 0 when diode is OFF. Thus from the above equations, the current drawn by the three phases on AC side of the rectifier in terms of its DC side current is given by the following equations. Ira= (12) Irb = (13) Irc = (14) Hence the total AC current drawn will be the sum of rectifier currents and RL load currents which is in general given by (15) I t = I r+ I Where I r = I r a + I r b + I r c and I = Ia + Ib + Ic. This current It will contain the harmonics caused due to non linear nature of loads discussed. The harmonic or non fundamental part of this current is to be compensated using the D – STATCOM. D. Modeling of D - STATCOM A D – STATCOM modeled with a voltage source inverter along with a charging capacitor on the DC side. The capacitor

Figure 5. Synchronous Reference Frame Method of Reference current Extraction

STEP 1

(16)

The various steps involved in reference current extraction are as follows.

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Convert the abc machine variables to α – β frame using Clark’s transformation. Using this equation 16

2017 Third International Conference on Science Technology Engineering & Management (ICONSTEM) STEP 2 Convert Clarke’s variables to dq0 variables of rotating reference frame with frequency ‘ ’ in rad/s. This frequency component can beF. obtained from a phase locked loop using voltage input as reference signal. Using this equation 17shown below the transformation is done. (17) STEP 3

Using a low pass filter or suitable averaging technique, extract the fundamental component. STEP 4 Using inverse transformation, convert this reference signal obtained in previous step, from dq0 to Alpha-Beta variables. STEP 5 Using inverse transformation, convert Clarke’s variables to abc machine variables. STEP 6 This obtained waveform contains fundamental separated from supply waveform. This so obtained current is known as reference current I ref. This fundamental component is then subtracted from the entire load current drawn which will give the harmonic component. It is given by the expression in equation 18 I comp = Il I ref(18) ThisI ref after being regulated by hysteresis current control will act as reference for the switching operation of inverter. The amplitude of the output voltage of the voltage source inverter which is the principal component of the D – STATCOM will be controlled by the energy storage capacitor CD which is connected to its DC side. The switching function is generated in the following pattern. Let the modulating control current value be Icomp. This current will alter the switching pattern of the VSI based D – STATCOM using the following algorithm. IF Input >= I comp allow the input to pass through the load. ELSE Output is Zero. The output of VSI will be a voltage source and is converted to a current source using a series RL branch whose parameters are given by R f and L f respectively. This current is given by the expression (19) Where V f is the D – STATCOM output with resistive drop considered and is the compensating current derived from it.The voltage of the DC bus side of D – STATCOM is moderated by a PI regulator. Thedifferential equation for final compensating current derived from the D – STATCOM which is to be fed back on source side can be depicted as follows. (20) (21)

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(22) F. Modeling of DC side capacitor of D – STATCOM The dc side capacitor voltage has to be contained within limits so as to maintain consistency in filter operation despite load variations. The output voltage of D – STATCOM’s VSI is combined with filter current which is operated using dot product operator to get the DC power from which the current order can be derived. From this the power loss can be calculated which can be expressed in terms of a term proportional to the capacitor value CD. This can be expressed as an equation 16 as shown below. (23) Where i dc is the current attributed to the losses incurred by the D – STATCOM. Regulation of DC bus voltage on the DC side of D – STATCOM is essential for its proper operation for which proper choosing of CD value becomes a prerequisite. The loss components are also considered before modeling of DC capacitor and are included in DC side as RD as shown in figure 4. III. SIMULATION AND RESULTS The system with D – STATCOM was modeled for a system with loads,using simulink/MATLAB. The load specifications are given in table 1 shown as follows. TABLE 1 SIMULATION PARAMETERS PARAMETERS

VALUES

SOURCE VOLTAGE 415 Volts, 50 HZ SOURCE IMPEDANCE Rs = 1.57 ohms, Ls = 40 mH UNBALANCED RL LOAD (R in ohms and L in milli Henry) PHASE A Ra = 50, La = 300 PHASE B Rb = 700, Lb = 500 PHASE C Rc = 250, Lc = 800 DC LOAD FED BY RECTIFIER (R in ohms and L in milli Henry) DC LOAD R = 200, L = 500 e-3 D – STATCOM FILTER SPECIFICATIONS (R in ohms and L in milli Henry, C in micro Farads, Voltage in Volts) FILTER RLC VALUES Rf = 0.1, Lf = 25, Cf = 1 DC BUS REFERENCE 400 Volts VOLTAGE DC BUS LOSS COMPONENT Rd = 60000, Cd = 5000

The simulation results are tabulated in table 2. Initially during simulation, the D – STATCOM filter is not switched to provide compensation. It is switched to inject compensation at the PCC after a time period of 0.2 seconds. This is done in order to compare and analyze the harmonic distortion before and after the incorporation of D - STATCOM and improvement of power quality using the same.

2017 Third International Conference on Science Technology Engineering & Management (ICONSTEM) TABLE 2 RESULTS OF SIMULATION OF THE SYSTEM SIGNALS SOURCE CURRENT IN PHASE A SOURCE CURRENT IN PHASE B SOURCE CURRENT IN PHASE C

THD BEFORE FILTERING (%)

5TH ORDER DISTORTION (%)


THD AFTER FILTERING (%)



16.34

12.34

6.53

0.82

0.09

0.12

23.78

13.09

7.63

0.83

0.13

0.13

22.42

11.60

6.46

0.80

0.09

0.16

FFT analysis is one of the important tools to find the percentage distortion caused in an ideal waveform due to disturbances. The results of FFT analysis carried out on source current waveform is given as follows.

Figure 8 FFT analysis window of current in Phase B (Without D-STATCOM)

Figures 9 and 11 show that the distortion caused due tounbalanced nature of loading can be mitigated using the D – STATCOM which proves to be effective from the results obtained. Figure 6 FFT analysis window of current in Phase A (Without D-STATCOM)

Figures 6 and 7 show that after the implementation of D – STATCOM, the THD is reduced to a great level which is very much desirable. The THD reduces from 16.34 % to 0.82 % which is a very good result with regards to the IEEE – 519 standard recommendations by which THD of a waveform should not be greater than 5 %.

Figure 9 FFT analysis window of current in Phase B (With D-STATCOM)

Figure 7 FFT analysis window of current in Phase A (WithD-STATCOM)

Currents in Phase B and C appear to be even more distorted and here too the implementation has been really effective in reducing the distortion caused due to harmonics. (Figures 9 and 11).

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Figure 10 FFT analysis window of current in Phase C (Without DSTATCOM)

2017 Third International Conference on Science Technology Engineering & Management (ICONSTEM) [2]. Bhim Singh,

Figure 11 FFT analysis window of current in Phase C (WithD-STATCOM)

From figure 12 one can have an idea of how the quality of waveform improves after implementation of D –STATCOM. The D – STATCOM is switched at t =0.2 seconds . In the waveform shown it can be seen that after 0.2 seconds the distorted nature of waveform is reduced due to the compensation provided. Thus the concept of Active harmonic filtering can be implemented using this device also.

Figure 12 Source current waveform

IV. CONCLUSION Due to their generalized operation and flexibility in control D – STATCOM are an obvious choice for power quality improvement. Conventionally D – STATCOMs are used for voltage sag/swell mitigation, flicker mitigation etc… but they also prove effective in treatment of various order harmonics and inter-harmonics associated with the system. But various design considerations to be taken into account while modeling a D – STATCOM and operational constraints like switching and control make the design and operation of a D – STATCOM a complex task. But nevertheless it is very much achievable. D – STATCOM provides a robust and effective methodology of providing shunt compensation in a line. REFERENCES [1]. An Luo, Huagen Xiao, Fujun Ma, Zhikang Shuai, Yichao Wang, “Distribution static compensator based on an improved direct power control strategy” IET Power Electron., 2014, Vol. 7, Iss. 4, pp. 957–964.

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P. Jayaprakash et. al, “Comprehensive Study of DSTATCOM Configurations” IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 10, NO. 2, MAY 2014. [3]. Sabha Raj Arya, Bhim Singh , “Implementation of distribution static compensator for power quality enhancement using learning vector quantisation” IET Gener. Transm. Distrib., 2013, Vol. 7, Iss. 11, pp. 1244–1252. [4]. Sabha Raj Arya, Bhim Singh , “Performance of DSTATCOM Using Leaky LMS Control Algorithm” IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS, VOL. 1, NO. 2, JUNE 2013. [5]. Bhim Singh and Sabha Raj Arya, “Implementation of Single-Phase Enhanced Phase-Locked Loop-Based Control Algorithm for ThreePhase DSTATCOM” IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 28, NO. 3, JULY 2013. [6]. Bhim Singh and Sabha Raj Arya, “Back-Propagation Control Algorithm for Power Quality Improvement Using DSTATCOM” IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 3, MARCH 2014. [7]. Pradeep Kumar, Niranjan Kumar, A.K.Akella, “Modeling and Simulation of Different System Topologies for DSTATCOM” 2013 AASRI Conference on Parallel and Distributed Computing and Systems. Pg 249 – 261. Published in Science Direct/ Elseveir. [8]. S.V Ravi Kumar and S. Siva Nagaraju, “Simulation of D-Statcom and DVR in power systems”, ARPN Journal of Engineering and Applied Sciences, VOL. 2, NO. 3, JUNE 2007. [9]. C. Benachaiba and B. Ferdi, “Power Quality Improvement Using DVR”, American Journal of Applied Sciences 6 (3): 396-400, 2009. [10]. Vasudeo B. Virulkar and Mohan V. Aware, “Modeling and Control of DSTATCOMwith BESS for Mitigation of Flicker”, Asian Power Electronics Journal, Vol. 4 No.1 April 2010. [11]. K-L. Areerak and K-N. Areerak, “The Comparison Study of Harmonic Detection Methods for Shunt Active Power Filters”, World Academy of Science, Engineering and Technology 46 2010 pg 243-248. [12]. P.Venkata Kishore and Dr. S. Rama Reddy, “Modeling and simulation of fourteen bus system employing D-STATCOM for power quality improvement”, ACEEE Int. J. on Control System and Instrumentation, Vol. 02, No. 03, October 2011. [13]. Veeraiah Kumbha and N. Sumathi, “Power quality improvement of Distribution lines using DSTATCOM under various loading conditions”, International Journal of Modern Engineering Research (IJMER), Vol. 2, Issue. 5, Sep.-Oct. 2012 pp-3451-3457. [14]. Sai Kiran Kumar.Sivakoti, Y.Naveen Kumar, D.Archana, “Power Quality Improvement In Distribution System Using D-Statcom,in Transmission Lines”, International Journal of Engineering Research and Applications (IJERA) , Vol. 1,Issue 3, pp.748-752 [15]. Alpesh Mahyavanshi, M. A. Mulla and R. Chudamani, “Reactive Power Compensation by Controlling the DSTATCOM”, International Journal of Emerging Technology and Advanced Engineering ISSN 2250-2459, Volume 2, Issue 11, November 2012. [16]. Bhattacharya Sourabh, “Applications of DSTATCOM Using MATLAB/Simulation in Power System”, Research Journal of Recent Sciences ISSN 2277 - 2502 Vol. 1(ISC-2011), 430-433 (2012). [17]. Bhim Singh, Sabha Raj Arya “Design and control of a DSTATCOM for power quality improvement using cross correlation function approach”, International Journal of Engineering, Science and TechnologyVol. 4, No. 1, 2012, pp. 74-86. [18]. Ambarnath Banerji, Sujit K. Biswas , Bhim Singh, “DSTATCOM Control Algorithms: A Review”, International Journal of Power Electronics and Drive System (IJPEDS) Vol.2, No.3, September 2012, pp. 285~296 ISSN: 2088-8694. [19]. Tejwani, V.S. ; Kapadiya, H.B. et. Al “Power quality improvement in power distribution system using D-STATCOM” NuiCone Nirma University IEEE International Conference on Engineering 2013. [20]. Lakshman Naik Popavath , K. Palanisamy, D. P. Kothari, “Research and Topology of Shunt Active Filters for Quality of Power”

2017 Third International Conference on Science Technology Engineering & Management (ICONSTEM) Springer Information Systems Design and Applications Feb 2016

AUTHORS H Prasad received his Bachelors degree in the faculty of Electrical and Electronics engineering from Anna University Chennai and his Masters degree in the faculty of Power systems engineering from Anna University, Chennai. He is currently working as Assistant Professor in the Department of Electrical Engineering at St. Joseph’s College of Engineering, Chennai. His areas of interest include Power Quality, Harmonic filters and application to renewable energy systems. T. D. Sudhakar received the Degree in Electrical & Electronics Engineering (B. E.) from Madras University, Chennai, India, in 2001, the M.E. degree and the Ph.D. degree from Anna university, India in 2004 and 2012 respectively. He is currently an Associate Professor in the Electrical Engineering Department at St. Joseph’s College of Engineering, Chennai, India. He has more than 35 Journal and Conference at International and National level publications. His technical interests are in the field of power systems & power quality and their performance evaluation through simulations.

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