Analysis of the equivalent fault characteristics of DC system and its impact on distance protection based on power-frequency variation

Here, the response characteristics of high-voltage DC (HVDC) control system are studied in the case that fault occurs in the sending-end AC system. Then the characteristics of DC system which is equivalent to power-frequency variation impedance in the fault super-imposed network are analysed by mathematical derivation and verified by simulations on PSCAD/EMTDC. The result shows that the equivalent power-frequency variation impedance of HVDC system is affected by the rated capacity and the parameters of HVDC control system. The power-frequency variation impedance becomes capacitive under certain fault conditions, which reduces the zone of distance protection based on power-frequency variation and even leads to the operation failure.