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This study makes a detailed analysis about the influence of AC system fault moments on commutation process of inverter valves and commutation failures. Taking into account the time-varying characteristic of the commutation voltage effective value after a three-phase fault of the AC system, a new method to calculate critical voltage drop is proposed. Also, the relationship between different fault moments and the critical voltage drop is deduced theoretically. In addition, considering the regulatory effect of the control strategy, detailed analysis is elaborated mathematically about the influence of different fault moments on the commutation process of valves connected in a 12-pulse pair at the inverter station. According to this, the variation curve of the critical voltage drop along with different fault moments is developed. All the results are verified based on the simulation model established in power systems computer aided design (PSCAD)/electromagnetic transients including DC systems (EMTDC).