Abstract: As the characteristics of commutation failure of high-voltage direct current ( HVDC) systems vary with not only fault close angle but also fault positions,the existing assessment methods that only consider the influence of voltage dip of converter bus are not accurate enough.In order to solve this problem,firstly,the space-time dis- creteness of commutation failure is investigated.Secondly,the influence of fault close angle on the process of com- mutation failure is analyzed,and the most unfavorable angle is pointed out.Then,two main causes for the impact of fault position on commutation failure,namely the electrical distance and the phase difference between fault bus and DC inverter bus,are discovered.Moreover,in light of the analytical results of space-time discreteness under the condition of commutation failure,a criterion of extinction angle,which takes both voltage amplitude and phase distortion into consideration,is proposed.Thus,a method to assess commutation failure is presented.Finally,a comparison of the proposed method is made with PSCAD/EMTDC and BPA on a three-node AC/DC system in Chi- na Southern Power Grid.The results show that the proposed method is effective and practicable.

Key words: high-voltage direct current, commutation, space-time discreteness, voltage amplitude, phase change, extinction angle, assessment method