Overvoltage phenomena in offshore wind farms following blocking of the HVDC converter

This paper presents measurement and simulation results in offshore wind farms following blocking of the HVDC converter used for connecting the wind farm to the main onshore grid. It is shown that the voltage will rise by approximately 30% within 50-100 ms in the ensuing island operation. The process is superimposed by transients caused by short but repetitive periods of saturation of the transformer resulting in excitation of the resonance frequency between transformer and cable capacitance predominantly in the zero sequence. It is demonstrated that in EMT type simulation the phenomena can be reproduced provided that the wind turbine control and the saturation of the transformer are properly modelled. A simulation tool has been used to demonstrate the effect of the level of wind farm loading at the instant of islanding on the observed phenomena.