Grid Code Testing of Wind Turbine by VSC-based Test Equipment

Other conference contribution, 2017

To fulfill the EU target of large amount of wind power in the electric power system, several technical barriers have to be overcome. The main challenge for the wind energy development is to make the wind turbines efficient in respect of costs and safe operation. An important design criterion is the fulfillment of the Grid Codes given by the transmission system operators. The Grid Codes state how wind turbines/farms must behave when connected to the grid in normal and abnormal conditions. In this regard, wind turbine manufactures must show how well their wind turbines can withstand an abnormal condition. This is done by simulations and some simplified tests. Tests of the Grid Code compliance for voltage dips in the grid are today carried out by use of coils and contactors mounted in a container that is connected between the wind turbine and the grid. This equipment has limited testing capability while been able to test only a part of the technical requirements given in the Grid Codes. For example, wind turbine operation with the grid frequency of 49 Hz cannot be tested, but still the requirement exists in today´s Grid Codes. Another solution to realize the grid code tests is to use fully-rated Voltage Source Converter (VSC) in back-to-back configuration. By controlling the turbine-side output of the converter system, a wider spectrum of grid faults can be emulated. Thanks to the full controllability of the applied voltage in terms of magnitude, phase and frequency, the use of VSC-based testing equipment, provides more flexibility as compared with the standard impedance-based test equipment. In addition, the AC grid is decoupled from the tested object when performing the test; meaning that the strength of the grid is not a major limitation. The test results are from the test of the 4 MW wind turbine and the 8 MW test equipment, located in Gothenburg, Sweden. In the tests the wind turbine operates at rated power when the voltage dip is applied. The turbine copes very well with the dip while a minor influence on the current and in the reactive and active power is observed.