Influence of transition resistance and fault moment on DC system commutation process caused by two-phase fault of AC system

Commutation failure caused by the fault of AC system at inverter side seriously affects the safety and stability of high-voltage direct current (HVDC) system. Although two-phase fault is a typical asymmetrical fault, the studies on commutation failure caused by two-phase fault are few compared with the studies on commutation failure caused by three-phase fault or single-phase fault. In this study, the features of two-phase fault occurring on commutation bus is analysed theoretically. The analysis shows that the transition resistance will affect the amplitude and phase of the commutation voltage at Y-Y and Y-D1 transformer valve sides. Furthermore, the change regularity of the commutation voltage along with the change of transition resistance is given. Taking interphase short circuit as an example, the influence of different fault moments on commutation process is studied. For the first commutation process before the fault, the possible cases of commutation failure are given for fault moments in one cycle. For the first commutation process after the fault, a calculation method of critical voltage drop is given. Simulation results verify the correctness of the analysis.