1) the insulation of the current transformer is very thick, some insulation wrapping is loose, there are creases between the insulation layers, in addition, the vacuum treatment is bad, the impregnation is not complete and causes the air-containing cavity, thus easy to cause the partial discharge fault. 

2) the size and arrangement of the capacitor screen do not meet the design requirements, or even discharge the capacitor screen, the capacitor plate is not smooth, even dislocation or fracture, so that its voltage sharing characteristics are destroyed. Therefore, when the electric field intensity along the surface of local solid insulation reaches a certain value, partial discharge will be caused. The direct consequence of the partial discharge is that the insulating oil is cracked and a large amount of x-wax is produced between the insulating layers, and the dielectric loss is increased. This discharge is a cumulative effect, let it develop, gas in oil analysis will likely appear arc discharge characteristics.