Abstract: The symmetrical and bipolar flexible high voltage direct current(HVDC) transmission system has higher transmission capacity and higher reliability than the monopole system. Therefore, it is gradually being applied to practical engineering. In view of AC/DC outlet ground faults of modular multilevel converter in bipolar systems, the transient characteristics of voltage and current after converter fault blocking were studied, and the mathematical analytical formula of the fault component was derived. The research results show that a single phase-to-ground fault in the AC outlet lead to the up-arm of blocked converter on the non-fault phase over-voltage and the lower-arm over-current on the non-fault phase. And the DC bias on the AC-side current causes the fault phase short-circuit current to have no zero-crossing. When the DC outlet occurs a pole-to-ground fault, the short-circuit current of the bridge arm of the blocked converter is mainly composed of the steady-state current injected by the AC system and the circulating current between the upper and lower arm. A phase-selective trip protection strategy is proposed for this special fault characteristic of single-phase ground fault at AC outlet. The problem that the AC circuit breaker cannot be normally opened when the zero-crossing point of the fault current does not exist is solved.A simulation model of the zhangbei flexible DC grid was built. The simulation results show that the analysis of the fault characteristics in the converter outlet is accurate and the proposed strategy is effective and feasible.