Fault voltage amplitude coefficient based protection for DC lines in hybrid multi-terminal DC system

The article deduces analytic expression of voltage travelling wave for hybrid cascaded DC system, and puts forward a protection based on amplitude coefficients of line-mode fault-component voltage (LFCV). By analyzing time-domain expressions of fault voltage travelling waves, it is derived that under internal faults, for the head of line, the ratio of two amplitude coefficients of the LFCV is 1: 2, while for the end of line, the ratio of two amplitude coefficients of the LFCV is 1: 0. And then it is further derived that under internal faults, for either side of line, the sum of two amplitude coefficients of the LFCV is far less than 0. However, under external faults, for the near faulty terminal of line, the ratio of two amplitude coefficients of the LFCV is 1: −1, thus, the sum of two amplitude coefficients is 0. Therefore, the amplitude coefficients of LFCV can be utilized to distinguish the internal fault from external fault. Simulation verifies that the method can reliably discriminate the internal and external faults even with high fault resistances, and has a high robustness confronting the noise interference.