New Control of Wind Turbines Ensuring Stable and Secure Operation Following Islanding of Wind Farms

This paper first presents the phenomena of the offshore wind farm (OWF) experience following the blocking of the high voltage direct current converter used for connecting the wind farm (WF) to the main onshore grid. Measurements in real large WFs show 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 transformers. Based on the analysis of common wind turbine (WT) control concepts, it has been found that the reason for the voltage rise is the integral characteristic of the classical current controller. Generally, it seems that the current injection control, as it is used in almost all WT controls, is not suitable in grids dominated by converter in-feed like in OWF. The authors are suggesting a new WT control concept that is characterized by voltage injection instead of the usage of an explicit current controller. Simulation results show that the voltage rise following islanding can be avoided, whereas the fault ride-through response to grid short circuits remains almost the same as before.