Abstract: Based on the structure and characteristics of park DC micro-grid system, the fault characteristics of different grounding modes and the requirements for fault detection devices are analyzed in this paper, and the response curve of voltage and current is deduced. Based on the topological structure of power electronic transformer, it is proved that the current has no oscillation problem under the condition of under damping. And then, the short-circuit currents before and after installing protection allocation scheme are compared, and the constraint conditions for selection of protection allocation scheme are given with consideration of such factors as equipment safety and protection coordination in case of faults. Finally, a park DC microgrid model is constructed based on the PSCAD/EMTDC platform, and a simulation is made on the proposed scheme. The simulation results indicate that:the short-circuit current decreases significantly after installing protection allocation scheme, while the current response time increases; and the parameter value obtained according to the reference condition for protection device selection satisfies the time required for fault current detection and protection judgment.

Key words: DC microgrid, interpolar fault, voltage and current response curve, DC transformer, protection configuration