Abstract: Based on the abnormal voltage of the positive bus in a 220 kV substation inspection and repair work, this paper analyzes the causes of the accident from various aspects of on-site construction, professional management, and comprehensive maintenance and overall management. The direct cause of the accident was the poor contact of the internal AC voltage neutral line N of the newly replaced 220 kV bus voltage parallel device during the inspection, modification, and construction, which in turn caused the three-phase imbalance of the secondary voltage of the 220 kV positive bus voltage transformer. After the maintenance personnel rewired, the line resumed normal operation. At the same time, by organizing the person responsible for the incident, professional management and other relevant personnel to hold an accident special analysis meeting, carefully analyze the cause of the incident, educate and deal with the relevant responsible persons, and propose the next improvement measures to avoid the recurrence of similar incidents.