Optimized circulating current suppression scheme for permanent DC line grounding fault

For the problem that the abnormal circulation of bipolar double twelve-pulse ultra high voltage direct current (UHVDC) line permanent grounding fault can not be effectively isolated, an optimized scheme for suppressing the abnormal fault circulation is proposed. Firstly, the influencing factors of abnormal circulating current value for DC line permanent grounding fault are analyzed. Secondly, combined with the existing removal strategies of DC line fault in China, the causes of abnormal circulation are analyzed, which are caused by restarting the high-end valve group with communication condition, unsuccessful restart with communication condition or without communication condition after DC line grounding fault happens. Then, a circulating current suppression scheme for DC line permanent ground fault based on pole isolation and blocking after shifting phase 90° strategy is proposed accordingly. Finally, the simulation model is established through real time digital system (RTDS) to verify the effectiveness of the improved scheme. The results show that the scheme proposed in this paper can effectively solve the abnormal circulation problem of DC line permanent grounding fault under different working conditions.