Earlier works have reported some critical commutating voltage or commutation failure immunity index calculation methods for commutation failure (CF) mitigation capability evaluation of line-commutated-converter-based high-voltage direct-current. However

accuracy of earlier methods is limited since they only consider commutating voltage drop and DC current rise. Hence

a commutation failure fault level calculation method under three-phase-to-ground faults considering multi-factor influence is proposed in this paper. First

transient time-domain response of DC current after fault is gained and DC current harmonics are calculated by Fast Fourier Transform. Then

AC current harmonics are estimated based on the coupling relationship of DC current and AC current of the inverter. Furthermore

supply voltage-time area and zero-crossing considering the harmonic voltage are estimated. Then

dynamic interaction between AC and DC system caused by constant extinction angle controller is considered by the power flow equation. Finally

discrete commutation processes are constructed based on commutation voltage-time area rule to determine occurrence of CF under different fault levels. Calculation results of the tradition method and the proposed method are compared with simulation results based on PSCAD/EMTDC.