Thyristor Branch Loop-Based Hybrid Modular Multilevel Converters for DC Short-Circuit Faults

DC-side short-circuit fault is one of the key challenges for the modular multilevel converter (MMC) based high-voltage direct current systems. In this article, a thyristor branch loop (TBL) based hybrid MMC (HMMC), where the thyristor branches are adopted to connect the upper arm inductors in three phases, is proposed to handle the dc faults. The protection operation for the TBL-HMMC in case of dc faults is also proposed, where the TBL-HMMC can handle the faults with the advantages of short dc current interruption time, short ac current interruption time, and low ac-side overcurrent. In addition, the proposed TBL-HMMC requires much fewer unipolar full-bridge submodules than the conventional HMMC, which results in low semiconductor cost and low power loss. Electromagnetic transient simulation studies using power systems computer aided design (PSCAD)/electromagnetic transients including dc (EMTDC) tool and experimental studies with a laboratory scaled hardware prototype are conducted to confirm the effectiveness of proposed TBL-HMMC and its operation under dc faults.