Partial discharge is mainly the discharge of internal insulation of dry transformers, transformers and other high-voltage electrical equipment under the action of high voltage. This discharge only exists in the local position of the insulation, and does not immediately form a breakdown or flashover of the entire insulation, so it is called partial discharge. The local discharge is very weak, and it is imperceptible by people's intuition, such as the eye and ear, and only a highly sensitive partial discharge measuring instrument can detect it.

The internal insulation of the dry transformer is under the action of the working voltage for a long time in operation, especially with the improvement of the voltage level, the electric field strength of the insulation is very high, and it is easy to generate partial discharge in the weak insulation. The cause of partial discharge is that the electric field is too concentrated in a certain point, or the electric field strength is too large at a certain point, such as the solid medium has bubbles, and the impurities are not removed. The oil contains water, gas and suspended particles; In different media combinations, there is serious electric field distortion at the interface. Partial discharge marks often leave only a small spot on the solid insulation, or a tree-shaped burn mark. In oil, small bubbles of decomposition appear.

Although the partial discharge time is short, the energy is also very small, but it has great harm, its long-term existence on the insulation material will have a greater destructive effect, one is to make the insulation material near the partial discharge, by the direct bombardment of the discharge particles caused by the local insulation damage, the second is the chemical action of heat, ozone, nitrogen oxide and other active gases generated by the discharge. The local insulation is corroded and aged, the conductivity increases, and eventually leads to thermal breakdown. The aging and destruction of the internal insulation of dry transformers in operation mostly starts from partial discharge.

Transformer partial discharge detection methods are generally:
1, electrical measurement method. Using oscilloscope or radio interferometer, find the characteristic waveform of discharge or the degree of radio interference.
2, ultrasonic measurement method. The sound wave appearing in the discharge is detected, and the sound wave is transformed into an electrical signal, recorded on the tape for analysis, and the distance from the detection point to the discharge point can be obtained by using the difference of the transmission time of the electrical signal and the sound signal.
3, chemical measurement method. The content of various dissolved gases in the oil was detected and the law of change was increased and decreased. This test method can detect changes in the composition, proportion, and quantity of the oil to determine whether there is partial discharge (or local overheating).
In addition, in recent years, partial discharge online detector has been developed, which can automatically detect partial discharge in transformer operation.

In order to prevent the occurrence of partial discharge, the manufacturer should carry out reasonable structural design of the dry-type transformer. Careful construction, improve the purity of materials, strict treatment of the quality of each link. The operation unit should strengthen the maintenance and monitoring of dry type transformer to effectively prevent the occurrence of partial discharge of dry type transformer.