DC fault protection of diode rectifier unit based HVDC system connecting offshore wind farms

DC fault ride-through operations of the offshore wind farm connecting with diode rectifier unit (DRU) based HVDC link are presented in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during DC faults and to provide fault current. This contributes the control of the offshore AC voltage, which does not drop to zero but is remained relatively high to facilitate fast system recovery after clearance of a temporary DC fault. The WT converters operate on current limiting mode during DC faults and automatically restore normal operation after fault clearance. The full-bridge based modular multilevel converter (MMC) is adopted as the onshore station and its DC fault current control ability is explored to effectively suppress the fault current from the onshore station around zero, which reduces semiconductor losses and potential overcurrent risk of the MMC station. Simulation results confirm the robustness of the system to DC faults.