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 afault, the MMC at the fault side may experience different conditions of blocking failure. The blocking failures may occur on: the whole converter; ` one converter arm; ′one sub-module (SM)/several SMs of one converter arm; ?other conditions. The phenomenon of the multi-terminalHVDC (MTDC) system following the fault is analyzedunder the first three conditions with real-time simulations using the real-time digital simulator (RTDS). Based on theimpact of different conditions on the MTDC system, the necessity of utilizing special control and protection is dis-cussed. A special control and protection strategy is proposed for emergency conditions, and its effectiveness is verified by real-time simulation results.