Proceedings of the 9th WSEAS International Conference on POWER SYSTEMS
Aspects Regarding Integration of Wind Power Plants into the Power System NICOLETA ARGHIRA1, DOINA ILISIU2, IOANA FAGARASAN3, SERGIU STELIAN ILIESCU4, VIVIANA ANDREEA ASAN5 1,3,4,5
Department of Automatic Control and Industrial Informatics University “POLITEHNICA” of Bucharest Splaiul Independentei 313, 060042 Bucharest ROMANIA 2 Transelectrica SA Str. Armand Calinescu, nr. 2- 4, sector 2, Bucharest ROMANIA {ioana, iliescu}@shiva.pub.ro;
[email protected] [email protected]
Abstract: Considering the need for greenhouse gas emissions reduction, the power system is facing the additional variability and uncertainty introduced by the generation of renewable energy. Wind farms are important power generation units that produce “clean” energy and induce disturbances to the power system according to the fluctuating nature of the wind that is very difficult to forecast. The power system has to be flexible in order to integrate large amounts of wind power. In this paper several problems are presented, but also advantages of connecting the wind generation unit to the system are shown. Keywords: wind farm, wind power integration, frequency control, power quality, load flow but that are not commonly used in traditional power systems. Aspects concerning the integration of wind power are related to the new perspectives in operating the electrical power system, problems at the connection point of the wind power plants (WPP), the transformation of the grid infrastructure and the influence of wind power on system adequacy.
1 Introduction The purpose nowadays for wind power is to integrate large amounts of wind energy into the grid in many countries. This raises important technical issues and many aspects have to be considered. Although wind power plants can be analyzed separately from the power system, most of the time, the generation units are part of the grid, so they must be studied in this combination. The size and the flexibility of the power system determine the system’s capability of accommodating an important amount of wind power. The variable character of the wind energy resource has a great importance in the context of variable power system, rather than in the context of an individual wind farm or turbine. Even though the wind is not available 100% of the time at one particular site, there is little overall impact if we consider that the areas with high penetration levels of wind power are part of very large national or even multinational strongly interconnected power systems. An important issue concerning the challenge that integration of wind power into power system design and operation is related to the fluctuating nature of the wind, that is very difficult to forecast. Also it is related to the comparatively new generator types (e.g. double fed induction generator) that are used in wind turbines
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2 The WPP Conditions for Integration into the Power System The basic challenge regarding the network integration of wind power consists of the following two aspects [1]: − Keeping an acceptable voltage level for all consumers as well as for wind farm operators of the power system: consumers should be able to continue to use the same appliances that they are used to, and wind turbines are usually designed to operate within a specific voltage range; − Maintaining the power balance of the system: wind power production and other generation units continuously should meet consumers needs. 127
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Proceedings of the 9th WSEAS International Conference on POWER SYSTEMS
The first aspect mentioned above is presented in figure1:
the wind owner loses possible financial income; These requirements concern the reliability of the power system at the connection point of the wind farm. If the reliability requirements are high, the cost will increase. Regarding the balancing effect over the power system, wind power production brings new challenges for the system operators, as shown in [4]. Unlike conventional power production, the output of wind generators is variable and for long time in advance unpredictable. These two aspects require new solutions to enable system operations, especially when wind power penetration level rise. The two major risks that currently are experienced by the transport system operators (TSOs) are: o Large power flows that lead to congestion in the grid and especially in the grids of other control areas o Reduction of available system reserves
Consumers
Consumers' voltage level
Availability for selling electricity at any time
Power System
Availability for buying the wind farm production at any time
Wind generation unit voltage level
The wind farm power production in the connection point is generally limited to a maximum value to avoid overloading of the power grid. In case of low wind velocity (under 4 m/s), the farm can not operate, so system reserves have to be used. Because of the variability of wind production, most countries require reserves of at least 50% from the installed capacity.
Wind Generation Unit
Figure 1. Consumers’ and wind generation unit requirements for the power system
These two risks have a high probability of occurrence as well as a big impact on system performance when wind power penetration levels rise. In several countries priority dispatch of wind power production is legally imposed. It removes the responsibility for balancing from wind power suppliers and gives it to the TSOs in these countries. Under conditions of high penetration rates of wind energy, imperfect control and imperfect forecasting lead to an increasing balancing need in the control areas. The impact of wind affects the volatility of market prices. In some control areas it even may happen that the wind power production exceeds the total load in a control area.
From consumers point of view, there are certain conditions that has to be accomplished which affects also the integration of the wind generation units into power systems: • The voltage level at the connection point has to stay within an acceptable range, as most consumer appliances require specific voltage range for reliable operation; • The power should be available at exactly the time the consumers need it in order to use their various appliances; • The consumed power should be at a reasonable cost. The first two requirements also concern the reliability of the power supply. Greater reliability leads to higher costs and in this way a conflict arises between the first two requirements and the demand for reasonable costs. Wind farm owners or operators also have certain demands on the existing power system in order to be able to sell the wind power production: • Wind farms require a certain level voltage at the connection point as wind turbines are usually designed to operate within a specific voltage range; • Wind farm owners want to be able to sell their power production to the grid when wind power production is possible, otherwise, the production has to be spilled, which means that ISSN: 1790-5117
3 Effects of wind power integration in operating the power system There are several problems that have to be solved when integrating wind power into the grid. Aspects such as reactive power compensation, reducing harmonics, frequency control, fault behavior must be considered. The impacts in the system can be considered both locally and system wide, as exposed in figure 2.
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A long term prediction for wind is not possible to obtain. The system operator collaborates with the meteorological institute for wind forecast, but good predictions can be made only for 4-8 hours, 1 day the most. Taking into consideration this aspect, on a generation level, system balancing requirements and costs are higher. The system also has to supply the operating power reserve to cover the ancillary services for wind generation unit functionality. On the transmission and distribution level, the problems that may be caused to the system are related to possible congestions in the grid. Changes in the magnitude and direction of cross border power flows can appear. Most of wind turbines are connected at the distribution level, which brings advantages such as: − The power may be consumed inside the distribution grid, which reduces the costs; − Remote users can be supported by wind power; − Wind power may keep parts of the system operational in case of transmission failures which otherwise would cause black-outs. Another impact on the operation of the power system and over the quality of the power is the accumulation of wind power into the power system [6]. Two of the most important aspects on system operation are: • The number of wind turbines in a wind farm; • The distribution of wind farms over large regions. Due to the movement of the air masses and the roughness of the ground, the wind speed necessary for wind turbines may be turbulent. A larger number of wind turbines in a wind farm reduce the turbulence effect over the entire farm as not all the turbines are affected. The repartition of wind farms over larger regions leads to the smoothing effect as changing weather do not affect all turbines at once. This effect is conditioned by the local weather prognosis and by the total size of the distribution region.
System wide scale influences:
Local scale influences: Active power control
Grid voltage
Power flow
Grid voltage level
Power quality
Distribution losses reduction
High costs in infrastructure for remote wind sites
Figure 2. Local and system wide wind power influence On a local scale, wind power plants can affect the grid voltage, and the requirements to consider are: power quality and voltage control at or near wind farm sites. Wind power can provide active power control. WPP are able to reduce transmission and distribution losses when the power is used for local power generation (the transport distance is reduced). On the system-wide scale there are major effects to consider. Wind power plants affect the voltage level and active and reactive power flows in the networks. As an advantage, it can participate in frequency control in the system. Therefore, wind power may require additional upgrades in transmission and distribution grid infrastructure. In order to connect remote wind sites such as offshore to the load centers, new lines have to be erected, which brings additional costs for system operators. One solution to reduce this problem is cross-border power flows which help in reducing the costs of geographically distributed farms. System operation is influenced by wind power production in different points: reserve capacities and balance management, short-term forecasting of wind power, cross border flow management. Figure 3 shows the effects of WPP over the power system. Generation level - higher system balancing costs due to random WPP fluctuations - the system has to provide the power reserve for the wind generation units
4 Grid codes requirements for wind power connection
Transmission and distribution level - possible congestions in the grid - changes in the magnitude and direction of cross-border power flows
The nowadays necessity to produce more and more wind energy has revealed the need to create or improve the transmission and distribution grid codes. These have demands related to power quality as reducing harmonics, reactive power supply, voltage and frequency control, behavior during fault conditions. From another point of view, the present grid infrastructure has the capacity to operate wind power to the system’s limits. For the grid to accept wind power
Figure 3. Effects of WPP on power system
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ISBN: 978-960-474-112-0
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in best conditions, it is necessary to take into account aspects such as: • It is necessary to connect the wind generation zones with the consumption centers; • When the energy consumption level and wind power output are different one from another (for example weak wind and high load), it is possible to appear congestions in the grid; • The appearance of geographic sectors with high wind power concentration. This paragraph presents some aspects regarding frequency control, as several grid codes require the participation of wind generation units in primary and secondary control, [7,8]. In a power system, frequency is influenced by the active power, so frequency control is responsible of the balance between power consumption and production. Primary control allows a balance to be re-established (in 1-30s) at a frequency different from the nominal one, in response to a sudden imbalance between power supply and demand. Secondary control restores the system frequency at its setpoint value in a time span of 10-15 minutes. Figure 4 depicts the activation of reserves as a function of time, for the situation when a large power plant is disconnected from the system. The primary reserve is activated automatically within 30 seconds after the incident happened. The secondary reserve is activated in 5-10 minutes after the frequency drop. This replaces the primary reserve and operates until tertiary reserve takes action.
The requirements for frequency control in different countries are presented in Figure 5. Areas marked in blue depict frequency ranges where wind turbines should be able to operate continuously at full power output. Red frequency ranges require the limited operation of wind turbines, where they should stay connected in order to contribute to frequency restoration. The time mentioned shows the period that the WPP should stay connected.
Figure 5. Requirements for frequency range in which power production units should stay on-line [2] The power system capability of matching the evolution in the electricity demand is expressed in the term ‘system adequacy’. The system adequacy can be considered from two points of view: − The capacity of the production units in the power system to cover the demand (load). − The ability of the transmission system to transport the power flows between the generator units and the consumers. In order to maintain the grid adequacy at the integration of wind power into the power system, grid codes demand that wind power plants should participate in voltage control in the network as well as continuously produce energy in the event of grid faults. Due to the high levels of penetration and the generated power flows of wind power into the interconnected power systems, the evaluation of the impact of wind power should be made on cross border interconnected network level instead of the national one.
Frequency
50 Hz Frequency
Time Load decrease ggdfjhjg
Power
Kinetic energy
Load Secondary reserve
Primary reserve hgjh
Long-term reserve
Seconds
Minutes
Time Hours
Figure 4. Frequency and power reserves activation in case of disconnection of a large power plant
5 Conclusions The efficient integration of wind power plants into the electrical interconnected power system is necessary in order to achieve greater use of renewable energies. On medium term, wind energy has the greatest potential for increasing the proportion of electricity consumption covered by renewable energies. The overall purpose of the power system operation, independent of wind power generation levels, is to supply an acceptable voltage to consumers and continuously to balance production and consumption.
In normal operation, the output active power of a wind farm may vary up to 15% of installed capacity in range of 15 minutes. Thus, wind production can take part in primary and secondary control. The wind farm should produce more in order to provide secondary control for frequency below the nominal value. In case of overfrequency in the power system, some wind turbines may be shut down.
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ISBN: 978-960-474-112-0
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The power system should have an acceptable reliability level, independent of wind power penetration. All grid codes requirements should aim to better adapt the grid requirements of wind turbine, as well as introducing extended and more specific control and protection rules. To maintain the power system’s adequacy, it is important to prevent the loss of large amount of wind power generation following grid faults. Measures like fault ride through behavior, wind turbines voltage control and additional system automation based on frequency characteristic have to be taken into consideration.
References [1] T. Ackermann, Wind Power in Power Systems, John Wiley & Sons Ltd, Stockholm, 2005. [2] EWEA, Large scale integration of wind energy in the European power supply: analysis, issues and recommendations, 2005 [3] Holttinen, Hannele, Lemström, Bettina, Design and operation of power systems with large amounts of wind power, ISBN 978-951-38-6633-4, 2007 [4] European Transmission System Operators, “European Wind Integration Study (EWIS) Towards a Successful Integration of Wind Power into European Electricity Grids”, Final-Report, 2007 [5] J. C. Smith, M. R. Milligan, E. A. DeMeo, B. Parsons, Utility Wind Integration and Operating Impact State of the Art, ISSN: 0885-8950, IEEE Transactions on Power Systems, Atlanta, 2005 [6] Doina Ilişiu, Ioana Făgărăşan, S.St. Iliescu,Nicoleta Arghira, Viviana Asan, Integration Issues for Wind Generation Units, Int. Conf. ETECA, Târgovişte, 2009 [7] Ioana Făgărăşan, S. St. Iliescu, C. Soare, Doina Ilişiu,F. Biliboacă, Nicoleta Arghira, Power Generation Unit Models for Load Frequency Control Simulator, Proceedings of the 17th Int. Conf. Control Systems And Computer Science, Bucureşti, 2009 [8] Ioana Făgărăşan, S. St. Iliescu, C. Soare, Doina Ilişiu, F. Biliboacă, Process Modelling for LoadFrequency Control in Power Systems, Proceedings of the Int. Conf. PowerTech, Bucharest, 2009
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