Abstract: The fault characteristics of multi-terminal flexible DC transmission system under various fault scenarios are studied.According to theoretical analysis,when an internal fault occurs,the fault current direction on both ends of the fault line is the same; however,the direction of the fault current on both ends of the healthy line is opposite. Then a fault line selection method based on current slope is proposed. By comparing the slope of fault current at both ends of the line,the fault line can be effectively identified,which can be used in the flexible DC transmission systems as a backup protection. Finally,the simulation model of Zhangbei four-terminal flexible DC grid is established by using PSCAD/EMTDC software,and the simulation is carried out to verify the effectiveness of the proposed protection principle.