Fault-isolation method for smart DC distribution networks with multiple types of converters

Complete fault-isolation scheme for flexible DC distribution networks containing multiple types of converters is rare. In this work, in light of the fact that the half-bridge modular multilevel converter (MMC) is incapable of blocking a DC fault current, an external thyristor-based MMC (ET-MMC) is proposed to replace the traditional double-thyristor arrangement. It significantly reduces the number of thyristors required while blocking AC feeding, and a power-dissipation module is added at the DC link of the MMC to accelerate the dissipation of the remaining energy in the inductor. Second, energy-consuming branches are added to the voltage-source converters (VSCs) and DC transformers (DCTs) in the network to implement fault self-clearance. Afterwards, an integrated fault-isolation scheme is designed for flexible DC distribution networks with multiple types of converters. Finally, a six-terminal DC distribution ring network is established via PSCAD/EMTDC. The simulation results show that the proposed scheme can complete the isolation process within 25 ms. It is also economical and practical.