Logic of conventional backup power switchover unit（BPSU） is merely adapted to the inner bridge connection or sectionalized single-bus configuration. When the operation mode changes in double-bus scheme， the logic has obvious defect， causing the locking pattern and abnormally complex functions of relay protection. According to the changes in BPSU function and actual operation mode of double-bus scheme， the special problem that logic of conventional BPSU cannot adapt to double-bus scheme is analyzed. The solution is put forward. Independent from changes of operation mode， locking relations between differential protection of bus bars and conventional BPSU can be simplified， reducing the risk of relay protection operating frequently and ensuring the accuracy of locking logic of BPSU.