Asymmetric Crossing Thyristor-Based MMCs for Valve-Side Single-Phase-to-Ground Faults in Bipolar MTdc Systems

The single-phase-to-ground (SPG) fault at the valve side of the converter transformer is one of the key challenges for modular multilevel converter (MMC)-based bipolar multiterminal direct current (MTdc) systems. This fault may cause severe submodule (SM) capacitor overvoltage, prevent the operation of ac circuit breaker (acCB) from quickly isolating the fault, and affect the normal operation of healthy MMCs. In this article, an asymmetric crossing thyristor (ACT)-based MMC is proposed, which can handle the valve-side SPG faults in bipolar MTdc systems with the advantages of low SM capacitor overvoltage, low semiconductor cost, and no arm overcurrent. In addition, the proposed method has little impact on the healthy MMCs. Simulations using PSCAD and experimental studies with a down-scale prototype are conducted to verify the effectiveness of the proposed ACT-MMC and its protection operation.