Abstract: This article uses a composite sequence network analysis method based on positive sequence, negative sequence, and zero sequence components to analyze single-phase disconnection faults in feeder lines. The theoretical analysis results show that the voltage of the disconnection phase is closely related to whether the neutral points of the main transformer on both sides of the disconnection are grounded. If the neutral points of the main transformer on the feeder line are not grounded, it will lead to an increase in the zero sequence voltage of the busbar opening, which in turn will cause the zero sequence overvoltage action of the main transformer protection; This article analyzes the protection action behavior based on the electrical characteristics during faults and the actual fault waveform. It points out the shortcomings of relay protection logic in substations in dealing with single-phase disconnection faults, and proposes improvement plans to improve the reliability of power supply