Crossing Thyristor Branches-Based Hybrid Modular Multilevel Converters for DC Line Faults

The modular multilevel converter (MMC) is attractive for high-voltage direct current (HVdc) applications. The dc line short-circuit fault is one of the key challenges for the HVdc system. In this article, a crossing thyristor branches (CTB)-based hybrid MMC (HMMC) with current interruption capability is proposed to protect the HVdc system under dc line short-circuit faults. In the CTB-HMMC, the thyristor branches are proposed to crossing-connect different arm inductors and each arm consists of unipolar full-bridge (UFB) submodules (SMs) and half-bridge SMs, where the CTB effectively reduces the UFB-SMs number in the arm of the HMMC with current interruption capability. The current interruption operations, including dc-side and ac-side current interruption, are also proposed for the CTB-HMMC in case of dc line short-circuit faults, where the proposed CTB-HMMC can interrupt the fault current with the advantages of fewer UFB-SMs in the arm, short current interruption time, low semiconductor cost, and low power losses. The simulation studies are conducted and a down-scale prototype is built, and the study results verify the effectiveness of the proposed CTB-HMMC and operation principles.