Study on Protection Logic of Last Circuit Breaker in Ultra High Voltage Converter Station

The last circuit breaker protection of the inverter station of UHVDC transmission system is a very important protection, which is mainly to prevent the AC load of the inverter station from suddenly complete breaking during the operation of the DC system, resulting in the over-voltage of the AC side and other parts of the converter station, resulting in the insulation damage of the AC and DC equipment. When all AC lines are lost in the AC field, the protection of the last circuit breaker is started, the DC system is shut down, and all AC filters are cut off at the same time, which can be summarized as bipolar shutdown without AC lines. Using the logic of the last circuit breaker protection and selectively locking the valve for different operation modes can avoid unnecessary double pole locking, which plays an important role in the availability and stability of the UHV(Ultra High Voltage) transmission system. This paper focuses on the the application assumption of the last circuit breaker protection of a ± 800kV layered access converter station. In order to prevent the DC double pole blocking and the mis-operation of the last circuit breaker, it is suggested to further improve the protection scheme , which has certain reference significance for the design of UHVDC transmission project in the future.