Inner-bridge configuration is a kind of main electrical connection widely used in 110 kV substations. This paper describes the protection action process during a circuit breaker fault on the low voltage side of the main transformer in a 110 kV substation with inner-bridge configuration. The long-term overheat made the button contact of the low voltage side circuit breaker of the transformer lose its elasticity. The contact fall off caused an interphase short circuit. Since the high voltage side back-up protection of the transformer failed to operate

the line protection and recloser of the superior 220 kV substation acted. And automatic switchover acted irrationally while the backup line put in. The circuit breaker was damaged by the rush more than once until the substation lost power. The fault is to a certain extent typical of a power system with problems in the design of its protection configuration. An overall analysis is made by the authors on the protection action process of the fault alongside suggestions and countermeasures for optimizing main protection configuration and setting

as well as operation and maintenance of inner-bridge configuration.