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 paper, 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 PSCAD/EMTDC tool and experimental studies with a laboratory scaled hardware prototype are conducted to confirm the effectiveness of the proposed technique.