The transformer, mutual inductor and some other high voltage electrical equipment in the action of high pressure to make discharge in interior insulation. This kind of discharge exists in a part of transformer which do not form the whole penetration breakdown or flashover immediately. The magnitude of partial discharge is weak. Only highly sensitive electrical measuring instruments can detect it.
The internal insulation of the transformer is under the action of the working voltage for a long time in operation, especially with the increase of the voltage level, the electric field strength of the insulation is very high, and the partial discharge is easily generated in the weak place of the insulation. The reason for the partial discharge is that the electric field is too concentrated at a certain point, or the electric field strength at a certain point is too large. In different media combinations, there is severe electric field distortion at the interface. Traces of partial discharge often leave only a small spot on the solid insulation. In the oil, some small bubbles that decompose appear.
Although the partial discharge time is short and the energy is small, it is very harmful. Its long-term existence will have a greater damage to the insulating material. First, the insulating material adjacent to the partial discharge is directly bombarded by the discharge mass to cause local insulation damage. Second, the chemical action of the heat generated by the discharge and the reactive gas causes the local insulation to be corroded and aged, and the conductance is increased, eventually leading to thermal breakdown. The aging and damage of the back insulation of the transformer in operation is mostly caused by partial discharge.