Abstract: This paper analyzes the fault characteristics by using the difference of current loop when different faults occur in the equivalent circuit composed of modular multilevel converter（MMC）converter and input series output parallel-dual active bridge（ISOPDAB）converter,and verifies it with theoretical data analysis. A protection method using the absolute value change characteristics of current fault components of double ended lines to judge the internal and external faults and fault pole selection is proposed. Finally,a three terminal flexible DC distribution network model with distributed energy is established on PSCAD/EMTDC to output fault data. The protection algorithm is implemented using Matlab to verify the feasibility of the proposed protection scheme.