Grid emulator requirements for a multi-megawatt wind turbine test-bench

Full-scale nacelle test-benches accelerate the development process of new multi-megawatt wind turbines. The device-under-test (DUT) is connected to an artificial grid that is emulated by a power-electronic converter system. Since the desired sinusoidal waveform of the voltage can only be approximated, filters have to be applied to suppress voltage harmonics sufficiently. But still, the output voltage is not purely sinusoidal due to a remaining share of harmonics. For the verification of the total harmonic distortion (THD) caused by the DUT, the influence of the grid emulator on the measurements of voltage and current has to be known. Otherwise the source of the harmonics cannot be identified clearly and the DUT might be characterized incorrectly. Within this work the requirements for grid emulators to allow the sufficiently high measurement of the THD caused by the DUT are derived analytically and validated with simulation models. Furthermore, the dynamic of voltage dips that have to be emulated to test the low-voltage ride-through (LVRT) capability of the DUT has to fulfill the established standards (IEC 61400-21) for emulating a faulty grid realistically. The consequently derived requirements for the power electronic converters are presented within this paper.