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.