Grounded Thyristor Loop-Based Hybrid MMCs for Valve-Side Single-Phase-to-Ground Faults in Bipolar HVdc Systems

The valve-side single-phase-to-ground (SPG) fault is one of the key challenges for the MMC-based bipolar HVdc system. In this article, a grounded thyristor loop (GTL)-based hybrid MMC (HMMC) is proposed to protect the bipolar HVdc system under valve-side SPG faults. In the GTL-HMMC, the thyristor branches are proposed to connect the different arm inductors and the ground. The operation for the GTL-HMMC in case of valve-side SPG faults is also proposed, where the GTL-HMMC can handle the faults with the advantages of short-fault current interruption time, low capacitor overvoltage, low arm overcurrent, low semiconductor cost, and low power loss. Electromagnetic transient (EMT) simulation studies using Power system computer aided design / electromagnetic transients including DC (PSCAD/EMTDC) tool and experimental studies with a laboratory scaled hardware prototype are conducted to confirm the effectiveness of the proposed technique.