In No.1 unit of Tongchuan power plant, the bus coupler breaker tripping accident occurs which is caused by memory over-current protection malfunction of generator after main transformer differential protection acts in B phase ground fault of HV side of main transformer. The principles of memory over-current protection of generator are analyzed in detail. In addition, the treatment of accidents is put forward including increase of generator interconnection and cancellation of bus coupler breaker tripping. Combining with the development of relay protection for large units, the backup protection of generator is discussed, for simplifying which the corresponding suggestions are provided.