Novel direct voltage control by wind turbines

Wind turbine controllers are generally designed for grid connected operation. Under certain grid conditions such as islanding of offshore wind farms following HVDC converter blocking, voltage and frequency of the island remains uncontrolled. Measurements in real wind farms show that the voltage in this situation can rise to unacceptably high values. In this paper the performance of the well-known current control approach which controls the terminal voltage via the injected currents will be compared with a new control approach which controls the terminal voltage directly. The results show that the existing approach has deficits with respect to its performance in islanding conditions because the integrators of the PI controller reach their limits and thus force overvoltages. In contrast the proposed direct voltage control is able to keep the terminal voltage in the transient period of islanding within an acceptable tolerance band and thus avoids damages to the remaining components in the network.