Managing harmonics and resonances in HVDC connected 66 kV offshore windfarms

Many HVDC connected windfarms with 66 kV AC collector systems are under development and in the offshore AC collector systems, there is a significant risk of lightly damped resonances. Managing the resonances and resulting harmonic stresses and possible instabilities is of paramount importance to avoid problems during commissioning and subsequent operation. One practical problem, however, is that, because of the long lead-time of platform construction, the HVDC system design typically needs to be frozen before the windfarm developer even selects a turbine supplier - mitigation strategies must therefore be chosen based on limited information. This paper analyses harmonics in a typical 66 kV windfarm using several turbine models and mitigation strategies to help developers decide on a mitigation strategy early in the development phase. The paper concludes that the HVDC always contributes negligible harmonics to the system, but that the total distortion is likely to be unacceptable unless extra mitigation measures are implemented, e.g., 66 kV high-pass filters at the HVDC platform or within the wind turbine system.