Distributed Voltage and Frequency Control of Offshore Wind Farms Connected With a Diode-Based HVdc Link

This paper introduces a new technique for the distributed voltage and frequency control of the local ac-grid in offshore wind farms based on synchronous generators. The proposed control technique allows the connection of the offshore wind farm using a diode based HVdc rectifier. The use of microgrid control techniques allowed the system comprising the wind farm and the diode HVdc rectifier to be operated in current or voltage control mode. Fault response to on-shore voltage sags of up to 80% has been shown to be comparable to that of thyristor rectifiers. The proposed control technique has been shown to be robust against load changes in islanded operation, capacitor bank switching, diode-rectifier ac-breaker tripping and wind turbine power limitation due to slow wind speeds. PSCAD simulations are used to prove the technical feasibility of the proposed control techniques both in steady state and during transients.