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Grid Connection and Integration of Wind Turbines Bernhard ARNDT University of Applied Science Würzburg-Schweinfurt, Germany [email protected] Abstract—The use of wind turbines sharply increased in the last decade. This poses problems for the MV/HV-grid. System perturbations are in the focus to maintain grid stability. In brief, sources of voltage fluctuations are shown and the current concepts of wind turbines are compared concerning this aspect. The recently modified standards in Germany are presented and a sample of network calculations for the integration of a wind park is given. I. INTRODUCTION In recent years the energy produced by wind turbines increased sharply and in Germany we expect in 2020 a 20-25% share of wind of the total electricity produced. This poses new challenges for planning the grid: in Germany most inland wind turbines are located in very small wind farms of two to five turbines of 1.5 to 2 MW each. This gives a total of 3 to 10 MW at the connection point, which is typically not in the vicinity of HVsubstations, but mostly at the end of low-power MV-lines. This has an impact on mains stability. This is different to big wind farms we have offshore and inshore with a larger number of wind turbines. Also in other European countries the harvesting of wind is concentrated in bigger farms, fig. 1 shows the situation near Vejer in southwest Spain. II. To maintain grid stability and keep system perturbations low, careful planning is necessary So, what are the perturbations: - harmonics voltage fluctuations flicker unbalanced loads frequency changes voltage dips short breaks The contributors are: static converters and inverters all other power electronics nonlinear loads like transformers, fluorescent tubes, arc-furnaces appliances with dynamic behaviour like automatic welding machines Fig. 1: Wind turbines near Vejer, Spain When tracing the sources of system perturbations caused by wind turbines at the connection point you will always end up with one input parameter: the wind and its fluctuating power delivered, as the power produced is proportional to the cube of the wind speed, which is definitely not constant. But also the construction of the wind turbine affects the wind flow: from the tower itself and when a blade passes the tower the wind flow is disturbed and also the power generated varies as the blades turn and pass different heights. Both contribute to voltage fluctuations and flicker. But also the technical concept of the generating unit affects the grid. Turbines with synchronous generators are connected today via static converters (direct connection isn’t state of the art anymore). In normal operation the converters produce harmonics, which have to be limited where necessary. Doubleinverter-fed asynchronous generators use a converter with lower power, so there are less harmonics produced. Comparing both concepts, the synchronous generator has a better efficiency and reactive power can be controlled by the field excitation, but it has a higher harmonics level due to the converter normally used. The asynchronous generator is cheap and robust, but requires power factor correction and has higher flicker and a slightly lower efficiency. Power frequency (mean value of fundamental measured over 10 s) Voltage magnitude variations Rapid voltage changes Supply voltage dips Harmonics ±1% (49.5 - 50.5 Hz) for 99.5% of week -6%/+4% (47- 52 Hz) for 100% of week ±10% for 95% of week, mean 10 minutes rms values 4% normal 6% infrequently Plt ≤ 1 for 95% of week Majority: duration <1s, depth <60%. Locally limited dips caused by load switching on: 10 - 15% See fig. 3 Fig. 2: for the permissible perturbations, a quick look at the relevant standard EN 50160 (values for MV networks) : nominal active power of the generator : nominal apparent power of the generator In case of a single CP in a system this condition is always complied to, if ≥ 50. If the grid’s source impedance is highly inductive, this calculation should be done using the complex source impedance. But his is still an approximation. phase shift between current and voltage of the generating unit in degrees at the maximum apparent power . : phase angle of the mains source impedance. If you search for the maximum voltage lift at a given power level, you can change the formula into You can write above formulae also as and Fig. 3: Harmonic levels EN 50160 Furthermore in Germany the transmission system operators have additional requirements laid down in the “Technische Regeln zur Beurteilung von Netzrückwirkungen (2. Edition 2007)” (Technical regulations for the evaluation of system perturbations). III. PERMISSIBLE SYSTEM PERTURBATIONS 1. Permissible voltage increase at the connection point: Δv ≤ 2%. With only one connection point, this can be easily estimated with the short-circuit ratio: : system short circuit power at the connection point (CP) : sum of the maximum apparent power of all generating sources at this CP. SAmax of a wind turbine can be calculated from the maximum apparent power of a single generator, where p1min is the relative maximum of the active power in one minute respectively. From the formula, you can easily see, that the voltage dip at the connection point can be negative. With an appropriate R/X and a sufficiently small ( for inductive reactive power) the voltage at the connection point decreases even without connecting a generating unit. This mode of operation leads to increasing losses and a reduced power transfer capability. If you have a weak connecting point or if your generated power is too high and the next strong point is too far, you may also go for this mode of operation. In intermeshed networks and/or operating your grid with multiple distributed generating units you have to check the voltage increase doing a complex loadflow analysis. You have to ensure, that the condition for is fulfilled at the most adverse connection point. The shown formulae present practical approximations. It is assumed that the angle between the voltage at the busses of the substations and the voltages at the connection points is negligible. Also the backlash of the voltage changes on voltage and current at the connection point are not cared for. The voltage changes calculated from above formulae are always slightly higher than the exact values. From many case studies you can assume that the limits of relevant standards (especially EN 50 160) for the voltage fluctuations are met both in MV and LV networks, if the voltage increase in the MV network is limited to the above mentioned 2%. In special cases of the network, the transmission system operator may require a lower voltage increase than the 2% mentioned.[1] IV. SWITCHING-INDUCED VOLTAGE CHANGES The voltage changes from switching generating units on and off the network must also be limited to 2%. This may not happen more frequent than every 1.5 minutes at maximum generator apparent power. If it is lower than half this power a time lag of minimum 12 seconds is permissible. For very infrequent operating cycles, i.e. once per day, the grid operator may permit a higher voltage change. You can estimate the voltage change by network dependent current factor : nominal apparent power of a single generating unit : network short circuit power The factor not only evaluates the amount, but also the time characteristics of the current during the switching operation. It is determined in the test report according FGW TR 3 [2] in relation of the network source impedance angle for every single generating unit and has to be provided by the turbine manufacturer. Concurrent switching operations are to be avoided. These technical regulations also govern flicker, harmonics, ripple control and protection schemes. The first edition was introduced in 1998, when the total installed wind power was 2,900 MW. Ten years later we had 23,903 MW, an increase by a factor of eight. Due to this massive increase in 2008 these regulations were tightened. The current values are in brief: Voltage fluctuations Flicker long term or (flicker factor and long term flicker amount) Harmonics the 5th order was cut by half to 0.058 for 10 kV and 0.029 for 20 kV networks and for the other harmonic levels see fig. 3. Ripple control, a maximum lowering of the audio level by 5% is now permissible, as 10 to 20% previously. V. SECURITY OF ENERGY SUPPLIES Wind turbines share 7% of the electricity produced in Germany today and the forecast is for 2020 around 20% (30% all renewables). The problem is, that the power delivered is not controllable but arbitrarily, just as the wind blows or the sun shines. But also the sudden switch-off of a large amount of generating units due to a short circuit in the higher network (as it has happened in 1998) causes frequency fluctuations. In the 2009 edition of the Renewable Energy Law (EEG 2009) in Germany some changes were introduced to help the transmission system operators: Equipment with a feeding power of more than 100 kW must have the option to reduce the active power in the case of network overload by remote control and the capability of remote reading the actual values of the power delivered. VI. GERMAN ORDER ON SUPPLY OF SYSTEM SERVICES (SYSTEMDIENSTLEISTUNGSVERORDNUNG SDLWINDV) With the enormous increase of wind power, these systems need to be more “grid-friendly”. In general, this covers the behaviour in case of failure, provisioning of reactive power, frequency control and voltage control. As this is involved with financial efforts, a bonus for every grid-friendly delivered kWh was introduced. The intention was mainly to upgrade also existing turbines over 100 kW. This bonus is currently at 0.5 cent/kWh. For pitch-controlled turbines, this can easily be achieved whereas for stall-controlled turbines it is only possible to switch them off. Either ripple-control or telephone-networks (fixed or mobile) are used to control the turbines. VII. Practical example: Wind farm Buchbrunn near Würzburg: 4 turbines with dual-speed asynchronous generators rated at 1.5 MW each and one turbine at 2.0 MW with a double-fed asynchronous generator. In this wind farm only the 2 MW turbine is fully controllable whereas the 1.5 MW turbines can only be switched off. The upgrade for enhanced system services consisted of changing the control-parameters for the 2.0 MW turbines and the installation of enhanced capacitor banks for the additional provision of reactive power. The remote-control was implemented through a fixed telephone line. Fig. 4 Wind farm Buchbrunn with 5 turbines VIII. CONCLUSION The use of wind power sharply increased in the last decade by the Renewable Energy Law (EEG) in Germany and other countries. Power system stability is now also in the focus for wind farms, as the share of wind power of the total electricity generation will increase to more than 25% in the next decade. We need to face further tightening of standards to maintain power system quality. In the future not only the amount produced but also the quality supplied will be in the focus. Wind turbines are becoming “conventional power generation” and need to be considered as real power plants with full controllability. REFERENCES [1] [2] [3] Eigenerzeugungsanlagen am Mittelspannungsnetz; Richtlinie für den Anschluss und Parallelbetrieb von Eigenerzeugungsamlagen am Mittelspannungsnetz. VDEW, 2. Ausgabe 1998 Technische Richtlinien für Erzeugungsanlagen, Forschungsgemeinschaft Wind (FGW), 2010. BDEW (Bundesverband der Energieund Wasserwirtschaft e.V., German Federal Association for Energy and Water) www.bdew.de