By hvhipot 
Partial discharge is mainly the discharge that occurs in the internal insulation of transformers, transformers, and other high-voltage electrical equipment under the action of high voltage. This type of discharge only exists in localized areas of insulation and does not immediately form a complete insulation breakdown or flashover, hence it is called partial discharge. The amount of partial discharge is very weak and cannot be detected by human intuition, such as observation or hearing. Only highly sensitive partial discharge measuring instruments can detect it.
The internal insulation of transformers is subjected to the working voltage for a long time during operation, especially with the increase of voltage level. The electric field strength that the insulation can withstand is very high, and partial discharge is easily generated at weak insulation points. The reason for 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 high, such as solid media with bubbles and impurities that have not been completely removed; Oil contains water, gas, and suspended particles; There is severe electric field distortion at the interface in different combinations of media. The traces of partial discharge often leave only a small spot or tree shaped burn mark on solid insulation. In oil, some decomposed small bubbles appear.
Although the partial discharge time is short and the energy is small, it has great harm. Its long-term existence will have a significant destructive effect on insulation materials. Firstly, it will directly bombard adjacent insulation materials with discharge particles, causing damage to local insulation. Secondly, the chemical reaction of active gases such as heat, ozone, and nitrogen oxides generated by discharge will corrode and age local insulation, increase electrical conductivity, and ultimately lead to thermal breakdown. The aging and damage of the internal insulation of transformers in operation often begin with partial discharge.
The detection methods for partial discharge in transformers generally include:
(1) Electrical measurement method. Use an oscilloscope or radio interference meter to search for the characteristic waveform of discharge or the degree of radio interference.
(2) Ultrasonic measurement method. Detect the sound waves generated during discharge, convert them into electrical signals, record them on magnetic tape for analysis, and use the time difference between the transmission of electrical and acoustic signals to determine the distance from the detection point to the discharge point.
(3) Chemical measurement method. Detect the content and changes of various dissolved gases in oil. This testing method can detect changes in the composition, proportion, and quantity of oil, thereby determining the presence of partial discharge (or local overheating).
In addition, in recent years, online partial discharge detectors have been developed, which can automatically detect partial discharge during transformer operation.
To prevent the occurrence of partial discharge, the manufacturing unit should carry out reasonable structural design of the transformer; Carefully construct, improve material purity, and strictly handle the quality of each link. The operating unit should strengthen the maintenance and monitoring of transformers to effectively prevent the occurrence of partial discharge in transformers.