Introduction  The paper aims to improve the reliability of the multi-terminal circular flexible DC distribution network, the characteristics of the fault in the different grounding mode of the AC side and its influence on the protection configuration are studied. The fault characteristics of multi-terminal circular flexible DC distribution network under different AC grounding modes were studied, and the impact on protection configuration had been given.

  Method  First of all, the equivalent AC/DC fault path of two-level and modular multi-level converter equipment was determined, and the influence of different grounding modes were analyzed from the aspects of fault overvoltage, overcurrent level and fault recovery. Then from the aspect of protection principle configuration, setting value setting, export mode and cooperation principle, the typical protection configuration scheme of multi-terminal circular flexible DC distribution network was given. Finally, the electromagnetic transient model of the three-terminal flexible DC distribution network was established under the PSCAD /EMTDC simulation software.

  Result  The simulation results show that when the AC side adopts a high resistance grounding mode, a reasonable protection configuration can improve the stability of the system when the AC asymmetrical fault occurs, and achieve zero interruption time fault ride through when the DC asymmetrical fault occurs.

  Conclusion  The research provides some reference for the design of grounding mode and the research on protection configuration of flexible multi-terminal DC distribution network.