Abstract: According to the analysis of the short-circuit accident at the exit of No. 2 main transformer in Sishui substation, it is very effective to determine the fault of the main transformer winding based on the combination of the main transformer winding frequency response method and the chromatographic analysis. At the same time, the lines issued by the cables are elaborated in order to avoid step-to-step tripping accidents. The conventional cable distribution cabinet should be changed to a quick-disconnecting fuse cabinet or a set of drop-type switches should be added; Ability test report.