Abstract: This article analyzes the cause of a tripping fault triggered by a defect in the high-voltage bushing of a 110 kV transformer. Through diagnostic electrical tests such as insulation resistance measurement, DC resistance measurement, and oil chromatography analysis on the transformer, combined with the results of the factory disassembly inspection, it was confirmed that the cause of the failure was a defect in the end of the oil-paper capacitor core of the C-phase bushing on the high-voltage side of the transformer. The insulation strength of the bushing capacitive screen cannot withstand the applied voltage, leading to discharge between the bushing and the transformer′s upper iron yoke. This causes the lower porcelain sleeve of the bushing to crack, resulting in further discharge between the bottom of the bushing and the transformer′s iron yoke.