Two-Level Suppressing Controller of Offshore DC Overvoltage Using Zero-Sequence Voltage and GFL-GFM Switching

A valve-side single-phase-to-ground (VSSPG) fault in the converter station of the modular multilevel converter based high voltage direct current (MMC-HVDC) integrated offshore wind farms causes DC overvoltage. In severe cases, it leads to MMC blocking and off-grid of the offshore wind farms. To suppress DC overvoltage, this article first analyzes the VSSPG fault characteristics of the onshore and offshore converter stations. Further, a two-level suppressing controller of offshore DC overvoltage is proposed. Level I suppresses DC overvoltage peaks using fault-generated zero-sequence voltage components. Level II utilizes the grid-following (GFL) control and grid-forming (GFM) control to switch between each other to achieve complete DC overvoltage suppression. For permanent VSSPG faults, Level I can be used to mitigate the system impact from overvoltage before MMC lockout. For transient faults, when SPG faults occur at onshore converter stations, the use of Level I and Level II in concert with each other can achieve transient fault ride-through and reduce the probability of MMC-HVDC system blocking. Finally, the MMC-HVDC integrated offshore wind farms model is built in the PSCAD/EMTDC platform. The simulation results verify the correctness and effectiveness of the proposed two-level controller.