The transient process caused by commutation failure in an AC/DC interconnected system is not the same as that of the purely AC system

which affects the performance of distance protection that uses the fundamental frequency. The transient characteristic of equivalent power-frequency current on the inverter side is analyzed first by introducing the concept of quadrant. Then

the effect of commutation failure on distance protection is studied in detail by considering the connection mode of relay. The analysis results show that the fault position and fault resistance both significantly influence the performance of distance protection. Furthermore

novel countermeasures based on the zero-sequence directional component and the successive trip used for measuring the change of reactance are put forward. Simulation results based on PSCAD/EMTDC have proved the conclusion correct. This work is supported by National Basic Research Program of China (973 Program) (No. 2012CB215206).