The recent voltage-source converter based HVDC (VSC-HVDC) transmission systems apply a symmetrical monopole configuration with a modular multilevel converter (MMC) consisting of half-bridge submodules. Pole-to-ground faults in the symmetrical monopole system cause the voltage increase in the non-faulted pole. The overvoltage significantly increases the required insulation level of the transmission lines and converters. This paper proposes an overvoltage-less protection method for the pole-to-ground faults in the symmetrical monopole HVDC systems based on the half-bridge MMC. The proposed method combines the following two approaches. The MMC operating as a rectifier utilizes a short-mode protection instead of the conventional blocking protection. The MMC operating as an inverter is equipped with blocking diodes in its dc terminal. As a result, both the rectifier and the inverter immediately decrease the dc fault current, and avoid the overvoltage in the non-faulted pole. The simulation results verify that the proposed method can suppress the overvoltage of the non-faulted pole to 110% of the nominal voltage, whereas the conventional method reaches 210%.
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