MMC-MTDC System Based on Improved Voltage Control Analysis of DC Fault Characteristics

DC fault protection is one of the key technologies to achieve the reliability of a multi-terminal flexible DC transmission (VSC-MTDC) system. For half-bridge MMC, in the event of a unipolar ground fault on the DC side, the converter valve will withstand overvoltage. Under short-circuit faults, the system will generate a very large short-circuit fault current. This paper proposes an improved control strategy for the voltage outer loop, which reduces the over-voltage under unipolar ground fault and the short-circuit current under inter-pole short-circuit fault. Finally, a half-bridge MMC-MTDC system model is built in PSCAD/EMTDC. The traditional control strategy and the improved control strategy are compared under two DC fault conditions. The comparison simulation results show the effectiveness of the proposed control strategy.