Commutation Failure and Mitigation Strategy with Lightning Shielding on AC Line near the Converter Stations of UHVDC Line

Compared with the typical Ultra HVDC (UHVDC) system, the inverters of the UHVDC system with a hierarchical connection mode (UHVDC-HCM), connect to 500 kV and 1000 kV voltage levels respectively under HCM. The thyristor is voltage-sensitive load. If the 500 kV or 1000 kV line near the converter station (CS) suffers lightning shielding (LS) failure, it will be vulnerable to cause the UHVDC-HCM system to commutation failure. In this paper, the model of the UHVDC-HCM system is developed in PSCAD/EMTDC and the principle of commutation failure with LS failure is analyzed. Then, a control strategy to suppress commutation failure is proposed. Simulation results demonstrate that the proposed control strategy effectively suppresses the commutation failure of UHVDC-HCM with LS and improves the recovery performance of post-fault.