The characteristics of line breakage fault in DC distribution system are not distinctive compared with normal operation. The existing line breakage protections which rely on the single-terminal electrical quantity is difficult to identify the faulty line accurately and reliably. This paper proposes a novel protection method for line breakage fault based on ratios between sum and difference of single-ended positive and negative currents. By analyzing the characteristics of line breakage fault in open-loop and closed-loop operation modes of a four-terminal DC distribution system

the differences of line currents between fault pole and non-fault pole can be obtained when line breakage fault occurs. The differences between fault pole and non-fault pole are amplified by the ratios between sum and difference of the line currents to identify the fault pole reliably. The proposed protection principle is simple

straightforward and can avoid the setting calculation problem. In addition

fault identification and pole selection can be achieved quickly based on local measurement information. Finally

the reliability of the proposed method is verified by simulation results.