Abstract:Power system has a large spatial range and all links are closely connected. Extreme environments such as lightning strikes, earthquakes, and extreme cold are likely to have direct and indirect impacts on power equipment, and further cause drastic changes in electrical quantity of the system. Studying fault protection of MMC sub-modules in extreme environments is of great significance to improve the safe and stable operation level of power system. The fault characteristics of MMC sub-modules in extreme environments are analyzed, and a fault diagnosis method based on sub-module capacitance-voltage comparison is used to propose a bypass fault protection strategy with high reliability for MMC sub-modules in extreme environments where thyristors are installed between the output ports of sub-modules. The redundant energy supply circuit is configured to ensure the controllable state of sub-modules, and the overvoltage breakdown of thyristor is used as a backup passive protection. Combined with the multi-level protection of submodules and the input of hot and cold backup redundant submodules, it can realize the highly reliable bypass protection for faulty sub-modules in extreme environments. Taking the ±800 kV UHV HVDC flexible transmission system for example, the proposed strategy has good feasibility and can improve the reliability and quickness of sub-module fault protection in extreme environments. In order to meet the long-term bypass flow requirements, forced cooling must be carried out through the radiator after the breakdown of the installed thyristor.