1) Short-circuit damage accident. Transformers suffer from various short-circuit accidents during operation, such as single-phase-to-ground, two-phase, or two-phase-to-ground, and three-phase short circuits, among which the short-circuit at the outlet is the most serious. During the operation of the transformer, it is mainly to check whether the auxiliary equipment of the transformer is in good condition and whether the temperature of the transformer is normal. Taking infrared imager measurement can effectively find some faults such as overheating and lack of oil inside the transformer.

2) Insulation accident. Transformer insulation damage accidents accounted for about 70% to 80% of the total accidents. Some of them were damaged by mechanical force or overheating, and some were not up to the required insulation strength when leaving the factory or the insulation was damaged so that the strength could not meet the requirements of endurance. The reasons for the analysis are as follows:

①The transformer is damp due to the water entering. The main problems are that the end of the casing is not tightly sealed, water is leaking, the water cooler is leaking, the explosion-proof cylinder is exposed, and there is water in the oil storage tank. The presence of moisture will reduce the breakdown strength of the transformer insulating oil and cause insulation accidents. The parts that cause more insulation accidents are windings, leads and enclosures. The development of creepage leads to short circuits between turns and layers.

② Foreign matter remaining in the transformer. The residual metal conductor foreign matter and impurities in the transformer body will cause insulation breakdown and damage due to partial discharge or wear of the insulation when overvoltage occurs or under normal working voltage.

③Lightning strikes. The lightning protection level of the lightning protection on the middle and low voltage sides of the transformer is too low and the transformer insulation structure is weak, resulting in a grounding short-circuit accident of the transformer during a lightning strike.

3) Overheating failure. There are overheating caused by abnormal currents, such as overheating caused by circulating currents and eddy currents, overheating caused by increased resistance of conductive loops, and overheating caused by hindered heat dissipation.