The article deduces analytic expression of voltage travelling wave for hybrid cascaded DC system and puts forward 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. 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 the 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 the external fault. Simulation verifies that the method can reliably discriminate the internal and external faults even with high fault resistances and has high robust, ss confronting noise interference.

A new line protection for HVDC systems using the ratio of currents of a DC filter and smoothing reactor is presented. Characteristic analysis on a DC filter shows that it behaves like an inductor at a high frequency band whose inductance is far less than the sum of smoothing reactor inductance plus converter equivalent inductance. Based on this characteristic, fault analysis shows that during high-frequency DC line faults (on either side), the DC filter current amplitude significantly exceeds that of the smoothing reactor, with a current ratio much greater than 1. Whereas for external faults, near faulty end, DC filter current amplitude is less than the one of a smoothing reactor and current ratio at this end is smaller than 1, while the current ratio at the distance faulty end is still far greater than 1. The effectiveness of the protection is verified by a large number of simulation experiments. It does not need synchronization of signals at both ends. In addition, it has high sensitivity for high-resistance faults, and is insensitive to the length of DC transmission line and power flow change.