Abstract: After the 220 kV main transformer breaks down, the low-voltage fast-break overcurrent protection and differential protection action, the main transformer body pressure mutates, light gas alarms, and heavy gas trips.A large amount of characteristic gas is collected in the gas relay collecting box.Oil chromatographic analysis shows that H₂,C₂H₂ and total hydrocarbon in the main oil sample exceeds the standard, and the gas content in the lower oil sample is higher than that in the middle.Through the comprehensive diagnosis of winding deformation test, ratio test, winding DC resistance test and insulation resistance test, it is preliminatively determined that the low-voltage side short-circuit fault occurs.Under the strong electric power impact generated by the short-circuit current, the low-voltage A and B phase windings appear deformation phenomenon and arc discharge occurs when the insulation paper is broken down.This causes short circuit and broken strands between turns and cakes, and extrudes the balance winding inward to the core lap.Combined with the disintegration of the main transformer plant, the failure causes are analyzed, the correctness of the protection action and test results are verified, and the preventive measures are put forward.It provides a reference for solving the same type of transformer faults in the future.