For Flexible Medium-voltage DC distribution grid, this paper proposes a DC line protection method. Frist, characteristic differences of transient current variation at both ends of the line are analyzed, afterwards a line protection is proposed based on the abrupt change of transient current. Through comparing the polarity of transient current, the method can discriminate the internal fault from the external fault. A MMC-based multi-terminal DC distribution networks is constructed, and extensive simulations are conducted to validate the effectiveness of the proposed protection scheme. Simulation results show that the proposed protection method can detect the fault section rapidly and accurately.

Multi-terminal DC distribution networks,MMC,Wavelet transform,Fault identification,Line protection