Comprehensive Fault Current Limiting Control for Isolated Modular Multilevel DC Transformer

This paper proposes a comprehensive fault current limiting control (CFCLC) strategy for isolated modular multilevel DC transformer (IMMDC). The proposed CFCLC strategy includes a combination of hardware and software-based approaches to suppress the fault current. In hardware approach, a bidirectional switch and a limiting-current inductor are configurated in series with the DC-link capacitor and the high frequency circuit loop, respectively. In software approach, an active current limiting control strategy based on inner phase shift control at MMC side is proposed. With the proposed CFCLC, the low-voltage side current can be effectively suppressed in the event of a bipolar short-circuit fault at LVDC bus. In addition, the soft restart control based on the CFCLC strategy after the fault clearance can reduce the internal surge current caused by the transformer's restart, and the restart speed is fast. Finally, the effectiveness of the proposed strategy is verified by simulation.