The internal insulation of the transformer is under the effect of working voltage for a long time during operation, especially with the increase of the voltage level, the electric field intensity of the insulation is very high, and it is easy to produce partial discharge at the weak insulation. The reason for the partial discharge is: electric field Too much concentration at a certain point, or the electric field intensity at a certain point is too strong, such as solid media with bubbles and impurities that have not been removed; the oil contains water, gas, and suspended particles; in different media combinations, there is serious electric field distortion at the interface . The traces of partial discharge often leave only a small spot on the solid insulation, or a dendritic burn mark. In the oil, some decomposed small bubbles 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 destructive effect on the insulating material. First, the insulating material adjacent to the partial discharge will be directly bombarded by the discharge particles. The damage to the local insulation is caused by the chemical action of heat, ozone, nitrogen oxide and other active gases generated by the discharge, which causes the local insulation to be corroded and aged, and the electrical conductivity increases, which ultimately leads to thermal breakdown. The aging and destruction of internal insulation of transformers in operation mostly start from partial discharge.

Transformer partial discharge detection methods generally include:

1. Electrical measurement method. Use an oscilloscope or radio interference meter to find the characteristic waveform of the discharge or the degree of radio interference.

2, ultrasonic measurement method. The sound waves appearing in the discharge are detected, and the sound waves are converted into electric signals, which are recorded on a tape for analysis. The distance between the detection point and the discharge point can be obtained by using the difference in the transmission time of the electric signal and the acoustic signal.

3. Chemical measurement method. Detect the content of various dissolved gases in the oil and the law of increase or decrease. This test method can find changes in the composition, proportion and quantity in the oil to determine whether there is partial discharge (or local overheating).

In addition, in recent years, an online partial discharge detector has been developed to automatically detect partial discharge during transformer operation.

In order to prevent the occurrence of partial discharge, the manufacturer should carry out a reasonable structural design of the transformer; carefully construct, improve the purity of the material, and strictly handle the quality of each link. The operating unit shall strengthen the maintenance and monitoring of the transformer to effectively prevent the occurrence of partial discharge of the transformer.