Abstract: In allusion to the abnormal phenomenon that commutation failure occurred in the fault recovery process of UHVDC transmission system cannot be explained by commutation voltage reduction or DC current rising, this paper proposes a method for commutation failure analysis based on harmonic decomposition and commutation voltage-time area, with considering the influence of commutation voltage waveform distortion. The method can qualitatively and quantitatively analyze the influence of harmonic voltage on commutation failure through the nature and size of every maximum possible harmonic commutation voltage-time area of distorted commutation voltage. Noting the uncertainty of harmonic phase, we use the harmonic risk coefficientas the maximum risk index to evaluate harmonic to promote commutation failure. Then we adopt the proposed methodto analyze the abnormal commutation failure occurred in the fault recovery process of a UHVDC system.The results show the cause of commutation voltage waveform distortion, reveal the stimulative or inhibited influence degree and risk of the main harmonic on commutation failure, and confirm the effectiveness of the method.

Key words: UHVDC transmission system, abnormal commutation failure, DC-bias, harmonic decomposition, commutation voltage-time area, harmonic risk coefficient