Control and protection strategy for MMC MTDC system under converter-side AC fault during converter blocking failure

This paper investigates a control and protection strategy for a four-terminal modular multilevel converter (MMC) based high-voltage direct current (HVDC) system under a converter-side AC fault. Based on the system operating condition, a control and protection strategy against the fault with normal blocking of the converter is proposed. In practical, applications encountering such a fault, the MMC at the fault side may experience different conditions of blocking failure. The blocking failures may occur on: ① the whole converter; ② one converter arm; ③ one sub-module (SM)/several SMs of one converter arm; ④ other conditions. The phenomenon of the multi-terminal HVDC (MTDC) system following the fault is analyzed under the first three conditions with real-time simulations using the real-time digital simulator (RTDS). Based on the impact of different conditions on the MTDC system, the necessity of utilizing special control and protection is discussed. A special control and protection strategy is proposed for emergency conditions, and its effectiveness is verified by real-time simulation results.