Partial discharge is mainly the discharge of the internal insulation of transformers, transformers and other high-voltage electrical equipment under the action of high voltage. This kind of discharge only exists in the local part of the insulation and will not immediately form the entire insulation breakdown or flashover, so it is called a partial discharge. The amount of partial discharge is very weak, and it is imperceptible by human intuition, such as seeing and hearing. Only a very sensitive partial discharge measuring instrument can detect it.

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 in the weak part of the 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 are serious electric field distortions 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, there are some small bubbles that decompose.

Although the partial discharge time is short and the energy is small, it is very harmful. Its long-term existence will cause greater damage to 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. In the transformer in operation, the aging and damage of the internal insulation mostly start from partial discharge.

The detection methods for partial discharge of transformers 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. Detect the sound waves appearing in the discharge, convert the sound waves into electrical signals, record them on a tape for analysis, and use the difference in the transmission time of the electrical signals and the acoustic signals to obtain the distance from the detection point to the discharge point.

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).