DC line fault is one of the key problems that must be solved in the further DC grid. In order to solve this problem

a scheme of DC grid protection based on hybrid DC circuit breakers is proposed. The current differences between fault lines and non-fault lines are analyzed based on the most serious pole-to-pole fault in DC side. Considering the coordination of lines protection

a directional protection principle based on current increment is proposed

and the corresponding setting calculation method is given. Finally a four-terminal DC grid model is established by the PSCAD/EMTDC software to verify the effectiveness of the proposed protection scheme. The simulation results indicate that the proposed protection scheme identifies and isolates the fault based on single-terminal current measurement

meanwhile it has inverse-time action characteristics and is insensitive to transition resistance

thus this scheme is suitable for backup protection of DC grid.