WIND ENERGY INTEGRATION IMPACT ON POWER QUALITY IN ESTONIA Hannes Agabus 1, Ivo Palu 2 1 Estonian National Gird (Estonian TSO), grid analyst, doctoral student, M.Sc, Address: Kadaka tee 42, Tallinn 12915, Estonia, Phone: +372 7151243, Fax: +372 7151200 e-mail: hannes@4energia.ee 2 Department of Electrical Power Engineering of Tallinn University of Technology, teaching assistant, doctoral student, M.Sc. Address: Ehitajate tee 5, Tallinn 19086, Estonia Phone: +372 6203769, Fax: +372 6203751 e-mail: ivo.palu@ttu.ee Summary In this paper, affect of the wind turbines to power quality in weak grid is described to analyze wind turbines cooperation with weak gird. During the implementation of this study, number of power quality measurement series was carried through in Läätsa 35/10 kv substation on Saaremaa island. On the first time the wind park consisted three 500 kw synchronous wind turbines connected to the Läätsa substation. Second measurements were made, when there was already six 500 kw wind turbines with total capacity up to 3 MW. During the measurements approximately 310 hours of analysed material was collected, involving full and part-time operation time and total outage of all wind turbines. Possible impact of wind turbines to power quality in 10 kv and 35 kv power grid was controlled and analysed and the results compared to standards EVS-EN 501600 Voltage characteristics of electricity supplied by public distribution systems and Eesti Energia s company standard EE 10421629 ST 7:2001. On the basis of measured results it can be concluded that at times there are moderate deviations from average voltage quality including brief emersions from the limits set in the standard at investigated substation. The analysis shows that limiting values of rapid voltage changes given in standard was frequently exceeded in comparison to contractual supply voltage. Permitted maximum value in an hour was exceeded several times. Also, large values of flicker, voltage and current harmonics factor have been noted during measured period. Keywords: weak grid; expansion of wind turbine generators, power quality, integration of fluctuating electricity generation,
Introduction Wind is the renewable energy sources (RES) with the highest potential in Estonia. Estonia has long coastline and many islands, where are good wind conditions. Nevertheless the wind energy projects were infeasible during a long period due to low electricity price in existing power system and Estonia s very limited possibilities to subsidize windgenerated electricity. In addition to that, multiple technical constraints exist as well. During last 10 years the electricity tariffs have raised several times and they continue to increase, the purchasing obligation and feed-in tariffs for electricity from RES have written into legislation and construction of wind generators have improved substantially. This has caused recently new peak of interest in wind energy investments and even in local manufacturing of windmills to reduce the price of equipment. This interest is supported also by national target to increase the share of renewable energy in the electricity production up to 5,1% from gross inland consumption in 2010. This target was set to Estonia by European Union during the accession negotiations. If the renewables share was only 0,1% in 2001 then in near future, the actual share of electricity produced from RES will be approx. 10% from gross inland consumption in 2010. Saaremaa island has large potential for wind energy use, but it cannot be considered as the solution for covering the local electricity balance due to various limitations. Currently there is installed capacity approx. 56 MW in Estonia. In the near future, this amount of wind generators should be at least double. The largest wind park is situated on eastern part of Estonia (Viru-Nigula wind park) and has amount of capacity 24 MW. 1. Renewable energy support mechanism The new Market Act was ratified by the Parliament in March 2007. In the new Market Act, there are provided measures (purchase obligation) supporting renewable energy. A network operator is obliged to buy electricity with a fixed feed-in tariff (was 0.81 EEK/kwh; presently 1.15 EEK/kWh = 0.074 EUR/kWh) produced from renewables within the network, where it operates. The wind boom posed by new Market Act have resulted in grid connection applications for more than 500 MW of wind capacity. This sum forms ca 20% of total net capacity of existing power plants. It immediately raised questions: how many wind parks can the present system integrate and what will be the costs of this expansion and what benefits can both sides be achieved. With no doubt, 500 MW integration requires corresponding heavy investments into electrical networks to provide transmission capacity and power plants for control services and required generation reserves. 2. Power system of Estonia There are currently 260 substations in 110-220-330 kv level in Estonia (8 of them are 330 kv substations). Estonian 110-330 kv grid condition is sufficient. Estonian has very good interconnection with Russia and sufficient interconnection with Latvia. Figure 1. Estonian power system
Net available capacity of Estonian power plants is ca 2300 MW. The annual electricity consumption is ca 6 TWh (excluding losses) and exports to Latvia and Russia are ca 2 TWh in total. Domestic peak load in wintertime reaches 1600 MW and low load in summer decreases to 400 MW. Since the end of 2006, Estonia finally made interconnection available with Finland via HVDC submarine cable Estlink (350 MW) witch made energy market participants possible to enter into the Nordic energy market. 2.1 Electrical network of Saaremaa Peak load in Saaremaa during wintertime reaches 40 MW and low load in summer decreases to 9 MW. The island is connected to the mainland through six 35 kv marine cables. The weakness of the network is it s poorly developed 35 kv western part, which does not enable to connect larger consumers or wind parks into the network [1]. Current network satisfies current loads and has enough resource left for 15 years. However, this network cannot guarantee needs of big consumers or wind energy projects. The main bottleneck is the connection to the mainland through Lihula 110kV substation and the double circuit main 110 kv feed line of Saaremaa on the dam of Small Strait [2]. 3. Power quality measurement in weak grid During the implementation of this study, number of measurement series was carried through in Läätsa 35/10 kv substation. Läätsa 35/10 kv substation is situated in the southwest corner of Saaremaa and substation connection to main 110 kv substation is made only trough one 35 kv overhead line which limits the WTGs installation in the region. During the first measurements in 2004 and 2005 the wind park consisted three 500 kw wind turbines. In year 2007 the wind park contains 6 turbines with total capacity of 3 MW. The investigated wind park is situating in Saaremaa island in Sõrve peninsula and connected to Läätsa 35/10kV substation on 10kV level. During the first measurements approximately 212 hours of analysed material was collected, involving full and part-time operation time and total outage of all wind turbines. Measurements made in the beginning of year 2007 added extra 98 hours of measured results. 3.1 Valid requirements for WTG-s There are several technical requirements what wind parks must follow when connecting itself into the power networks in Estonia. These are described in: National Grid Code (technical requirements for electrical installation due to security of supply etc.) Eesti Energia AS Company standard EE 10421629 ST 7:2001 Technical requirements for connecting wind turbine installations to the power network (specifications for WTG-s power quality; stability specifications etc.) [3]. When wind park is connected with transmission system, then there are two special standard documents for fulfillment: Specifications for wind parks control and automation and Required approval tests for wind parks [4]. The first document describes all needed control functions and description of transmitted information for control centre. Second documents points out needed test runs for WG final approval by the system operator (Fault ride trough capability etc.). During the measurements in substation possible impact of wind turbines to power quality in 10 kv power grid was controlled and analysed and the results were compared to standards EN 501600 Voltage characteristics of electricity supplied by public distribution systems [5] and Eesti Energia s company standard EE 10421629 ST 7:2001. 3.2 Power quality Wind turbines are installed in locations with sufficient wind energy. In many cases these locations are not in densely populated nor in industrial areas, which could guarantee an existence of stronger power grid. Currently in Estonia the existing wind turbines are installed to peripheral areas in terms of power grid. The installment of wind turbines in these areas affects normal operation of the existing grid (one-way power transmission) and can cause deterioration or improvement of power quality. Power quality that is frequency, voltage and supply interruptions. Wind turbines in common strong power system have a direct impact only on voltage: voltage change, flicker, harmonics and transients. These are values, which also characterise the so-called weak grid. Due to unique character of every grid and different types of wind turbines it is difficult to evaluate the impact. Load fluctuations cause the steady-state operation voltage fluctuations, especially in weak grids with long transmission lines and relatively low voltage. Connecting wind turbines to this grid increases those fluctuations because of short and long-term changes in output power.
3.3 Weak grid The grid could be characterized as weak or strong. If the grid is weak, the input of active and reactive power might change the voltage in connection point or in any point close to it. One thing that also describes weak grid is low shortcircuit power. In case of strong grid, there are fewer problems in connecting the wind turbines. Distribution grid in Estonian rural areas is usually weaker than in the cities and industrial areas. This is usually connected with the impact on voltage quality that the power produced by wind turbines might have and (although more rarely) the thermal restrictions of grid transmission capacity. These restrictions depend on the characteristics of the grid and the wind energy installation. Areas in Estonia with most wind resources are located far from main supply sources and have only 10-35kV grids with low transmission capacity. When connecting wind turbines to the existing distribution grids it should be kept in mind that these grids are initially planned for unidirectional transmission of electric energy. Until presently the opposite direction of wind turbines capacity and their special characteristics has not been taken into account in the grid development [6]. The most important factors in installing wind turbines and connecting the turbines to the existing grid are sufficient transmission capacity of the grid and the issues of power quality. If the grid is not substantially expanded, both problems can be considered technical restrictions that have to be fulfilled and used for calculating the permitted capacities of wind turbines connected to the grid depending on the planned connection point. There are three determining factors when connecting wind turbines to the weak grid: impact of wind capacity on the steady-state operation voltage; voltage fluctuations; possible distortions of voltage waveform. 3.4 Performed measurements Voltage values have been measured from the substation s 10kV bars on the secondary side of voltage transformer. In 2007 power quality was also measured on 35 kv side, but due to long data saving interval no extreme distortions were noted. The current has been measured from the secondary side of current transformers in the wind park s feeder. The following dimensions were measured and recorded: power coefficient, current, voltage, voltage and current harmonics until 50th rank, flicker emission P lt and P st, reactive, active and apparent power generated by wind turbine (Q, P and S), total harmonic distortion factor THD U and THD I, frequency and asymmetry. Measurements results have been presented in 0,2 second, 1 minute and 10 minute average values as described in standard. Due to limited scope of this article only two diagrams characterizing the power quality of wind turbines have been presented for the period of 15 19 January 2007, where noticeable flicker phenomena and extreme harmonic distortion caused by wind turbines can be noticed. On the basis of results it can be concluded that at times there are moderate deviations from average voltage quality including brief emersions from the limits set in the standard. Unfortunately there is no information about the voltage quality prior to the installation of wind turbines and therefore these measurements do not give cause to blame only the wind turbines for violating the voltage quality, although the incidents have occurred apparently are associated with them. Reactive power consumption or excessive production was not observed. Power coefficient during the full operation of wind turbines was close to one. The analysis shows that limiting values of fast voltage changes given in standard EN 501600 was frequently exceeded in comparison to nominal supply voltage. Permitted maximum value in an hour was exceeded several times. Also, large values of flicker (figure 2) and total voltage and current harmonic distortion (figure 3) have been noted during short periods. 9,00 8,00 Flicker Pst Plt 7,00 6,00 5,00 4,00 3,00 2,00 1,00 0,00 15.01 16.01 16.01 17.01 17.01 18.01 18.01 19.01 19.01 Figure 2. Measured short time flicker P st and long time flicker P lt values in Läätsa substation on 15.01-19.01.2007
40,00 70,00 35,00 60,00 30,00 50,00 THD U [%] 25,00 20,00 15,00 40,00 30,00 THD I [%] 10,00 20,00 5,00 10,00 0,00 0,00 19:00:00 19:15:06 19:30:06 19:45:06 20:00:07 20:15:06 20:30:05 Figure 3. Measured THD U and THD I values on 18. January 2007 in Läätsa substation 3.5 Measurements results Based on the measurements made in Läätsa 35/10 kv substation, the wind park has not caused considerable power quality modifications according to EN 501600 standard. As there was no direct contact with wind park owners, thus source of any abnormal event caused by planned work or regulation in wind turbines was unnoticed. Graphics presented in this paper are valid only for this measured grid with above mentioned wind turbines and are not expandable as typical characteristics. As described in this paper, the integration of wind turbines into weak distribution grid requires precise evaluation of grid conditions with reviewing the wind turbine characteristics. As the standard EVS-EN 501600 standardises voltage quality in consumer connection point and does not regulate disturbances caused by single devices, the validity of standard EE 10421629 ST 7:2001 has also to be considered when connecting the wind turbines. 4. Required grid development for extra WTGs installations in Saaremaa The fist main thing to do for opening more extra WTGs capacities in Saaremaa, is to replace existing six 35 kv cables with at least one 110 kv cable. The 110 kv cable is planned to build in 2014-2015. Also in mainland, very important Lihula 110 kv substation must be renovated from 110 to 330 kv to provide more grid capacity and better grid quality. If we are looking trade possibilities for wind energy, then second DC link with Finland and new DC link from Baltic to Sweden must be built (Estonia-Sweden; Latvia-Sweden or Lithuania-Sweden etc.). Likewise, to strengthen Baltic States internal grid, new 330 kv overhead line between Estonia and Latvia must be constructed. Currently Tallinn-Sindi-Riga 330 kv overhead line study, with the co-finance by the TEN-E, is in progress for selection of accurate right-of way. Conclusions Based on the measurements made in 35/10 kv substation, the wind park has caused power quality modifications according to EVS-EN 501600 standard. Integration of wind turbines into weak distribution grid requires precise evaluation of grid conditions with reviewing the wind turbine characteristics. If installed WTG is technically adequate to fulfill the electrical grid requirement basis, then success of integration should be guaranteed.
Special thanks Authors thanks Estonian Science Foundation (Grant 5885) for the financial support of this study. References 1. Landsber M, Agabus H., Liik O. Possibilities to develop the use of renewable energy and co-generation in Saaremaa. 2nd International Symposium Topical Problems of Education in the Field of Electrical and Power Engineering ; 2005. 2. Ojangu J., Kilk K., Kivirand J., Landsberg M., Kirpu K.. Keila, Western-Estonia and islands region 20 110 kv network development plan. Tallinn: Elpec AS; 2003 (in Estonian). 3. Company standard EE 10421629 ST 7:2001 (2001) Technical Requirements for Connecting Wind Turbine Installations to the Power Network Eesti Energia s official edition. Tallinn: 18. (in Estonian) 4. http://www.pohivork.ee/ [WWW] 01.05.07 5. EVS-EN 501600 (2000) Voltage characteristics of electricity supplied by public distribution systems. Estonian centre for standardisation. Tallinn: 22. 6. Liik, O., Oidram, R., Keel, M., Ojangu, J., Landsberg, M., Dorovatovski, N. (2005) Co-operation of Estonia s oil shale-based power system with wind turbines. Oil Shale 22(2) Special. Tallinn: 127 142.