A Protection Method for DC Faults Using a Half-Bridge MMC with Consolidated Arm Inductors in HVDC Transmission Systems for Offshore Wind Farms

This paper proposes a new protection method for offshore terminals in HVDC transmission systems connecting offshore wind farms to onshore ac grids, specifically focusing on dc faults. The conventional single-arm short protection method allows a half-bridge modular multilevel converter (HB-MMC) to suppress the fault current to zero without external devices. However, this method is effective only for pole-to-ground faults in symmetrical monopole configurations and cannot handle pole-to-pole faults in asymmetrical monopole or bipolar configurations. The proposed method employs an HB-MMC with arm inductors consolidated into a single arm. By performing the single-arm short protection, this converter can suppress fault currents to zero for both pole-to-pole and pole-to-ground faults. Therefore, it is applicable to asymmetrical monopole and bipolar configurations. The validity of the proposed method is verified by simulations.