It is known that the pole-to-ground fault in the symmetrical monopole HVDC transmission system causes overvoltage in the non-faulted pole. The overvoltage increases the required insulation level of the converters and transmission lines. To suppress the overvoltage, this paper proposes a protection method for symmetrical monopole HVDC systems. The proposed method introduces novel protection modes “lower arm short protection” and “upper arm short protection” to the modular multilevel converter (MMC) being composed of half-bridge (HB) submodules. In addition, blocking diodes are inserted to the DC terminal of the inverter. As a result, the method enables to clear the DC fault currents and to avoid the voltage increase in the non-faulted pole. Experimental results obtained by a 5.5-kW HB-MMC HVDC system demonstrate that the proposed protection method enables to suppress the voltage increase to 106 % of the rated voltage, while the conventional protection method caused overvoltage of 200 %. The proposed method is also effective to the multi-terminal HVDC systems. Even if a fault occurs in one of the transmission sections, the fault does not cause the overvoltage in the other non-faulted section. Therefore, the non-faulted section can continue the power transmission both during and after the DC fault.
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