Analysis and calculation method of critical voltage of DC commutation process considering AC system single-phase fault moments

This paper takes a close research on the impact of voltage reduction in a single phase as well as its occurring moments on commutation failures (CFs). When there is a sudden voltage reduction in a single phase, the change rule of post-fault magnitudes and phases of commutation voltages is researched theoretically. On the basis of the change characteristic, valves that are most probable to suffer from CFs are carefully elaborated. Moreover, a method for calculating critical voltage of a single phase considering different fault moments is developed. As long as the phase voltage is lower than the critical voltage, it may result in a CF. In addition, the change regularity of critical voltage along with different fault moments and phases is given. All the analysis and theoretical calculation results are validated by simulation in PSCAD/EMTDC.