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Flexible direct current (DC) distribution systems have emerged as the development trend for future distribution grids. However, these systems are vulnerable to DC faults, rapid fault identification and faulted line selection method are required to enhance the security of the entire system. A novel transient highfrequency impedance comparison based DC protection for flexible DC distribution systems is proposed in this paper. The controlindependent high frequency impedance model of power converter is also investigated. Based on this model, the proposed method identified the faulted lines by comparing high frequency impedance measurement differences. For DC bus with multiple branches, this technique minimizes the threshold calculation job, which is usually difficult to process for the transient value based protections. Strict synchronization of data is also not required for this method. The simulation model of four-terminal flexible DC distribution networks is built in PSCAD/EMTDC to verify the effectiveness of the proposed protection model. Simulation results prove that the protection is robust to fault transition resistances and the measurement noise.

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