A Study on HVDC Overcurrent During Commutation Failure

Commutation failure is a common fault for HVDC transmission systems. The DC overcurrent induced during commutation failure not only endangers equipment safety but also threatens the system stability. In this paper, a reduced-order system model that can accurately describe the overcurrent dynamics during commutation failure in AC-DC system is first established using a singular perturbation method. Then a regular perturbation method is suggested to obtain the time-domain analytical solution for overcurrent estimation based on the reduced-order weak nonlinear system. A parametric study is further provided with phase portrait, showing how the equivalent reactance of AC system and relevant parameters of HVDC itself can affect the overcurrent caused by commutation failure, and how voltage stability is affected. The proposed overcurrent estimation method and qualitative analysis results are verified using several AC-DC test systems including an actual 177-machine provincial power system in China.