Double-Thyristor based Protection for Valve-Side Single-Phase-to-Ground Faults in HB-MMC based Bipolar HVDC Systems

The valve-side single-phase-to-ground (SPG) faults in the half-bridge modular multilevel converter (HB-MMC)-based bipolar high-voltage direct-current (HVdc) systems induce some special fault consequences: overvoltage on the submodule (SM) capacitors in converter upper arms and nonzero-crossing currents in the grid-side ac circuit breaker (ACCB). In this letter, a protection strategy based on converter-embedded double-thyristors is proposed to address these issues. The double-thyristors are installed in parallel with each SM in the lower arms of the converter. By triggering the double-thyristors, the SPG fault is converted into a three-phase short-circuit that ensures current zero-crossings in the grid-side ACCB. Moreover, as the voltages of converter ac buses are clamped by the ground through the triggered double-thyristors, the upper arm overvoltage is mitigated. The effectiveness of the proposed protection strategy has been verified in a bipolar HB-MMC HVdc link built in PSCAD/EMTDC.