Abstract: In order to improve the success rate of power transmission and the accuracy of fault troubleshooting after the main transformer overhaul and technical modification, this paper introduces the ratio differential protection action during the impact closing of the main transformer overhaul with a practical case of unsuccessful impact closing. A new voltage method is used to analyze the fault troubleshooting and accurate fault location, and the fault causes are verified. At the same time, the suggestions to prevent accidents are put forward for the problems of wiring and short circuit between turns in the balance winding.