Current Ratio Based Breakage Protection for Flexible DC Distribution Systems

Unlike the short circuit faults, the breakage fault does not offer distinctive characteristics compared with normal operation transients in DC distribution system and the fault information is difficult to be analyzed, which bring great challenges to breakage protection. In this case, this paper introduces a fault characteristic analysis method for breakage faults and proposes a novel breakage protection scheme based on ratio of positive and negative fault currents for DC distribution network. By establishing the fault circuit equations, the breakage fault features are analyzed for protection. Meanwhile, the ratio of current summation over their subtraction is used to further amplify fault characteristics, which overcomes the reliability problem of conventional breakage protection. A six-terminal flexible DC distribution network is built in PSCAD/EMTDC to verify the proposed protection method. Simulation results show that the protection is robust to the measurement noise and can identify an internal breakage fault and realize the broken pole selection rapidly.