First, let's understand what partial discharge is.
According to the definition of IEC 60270, partial discharge (PD) is a partial discharge, which only partially bridges the insulation between conductors and may or may not occur near conductors.
The main reason for partial discharge is that when the dielectric is not uniform, the electric field intensity in each area of the insulator is not uniform. In some areas, the electric field strength reaches the breakdown field and discharges, while other areas remain insulated.
Partial discharge can be divided into four types:
1. Internal discharge: occurs in the gap or cavity in the solid or liquid dielectric;
2. Surface discharge: occurs at the junction of different insulating materials;
3. Corona discharge: occurs in gaseous dielectric with uneven field;
4. Tree discharge: the continuous impact of discharge in solid dielectric forms the discharge channel.
Main phenomena of partial discharge:
1. Electric pulse current
2. Dielectric loss
3. Radiation (light)
4. Sound (noise)
5. Increase gas pressure
6. Chemical reaction
Partial discharge hazard
Partial discharge will corrode the insulation structure. Its damage mechanism to insulation includes the following aspects:
1 Charged particles (electrons, ions, etc.) impact the insulation and destroy its molecular structure, such as fiber fragmentation, thus damaging the insulation.
2 Due to the influence of charged ions, the insulation will have local temperature rise, which is easy to cause overheating of the insulation, and carbonization will occur in serious cases;
3 The ozone (O3) and nitrogen oxides (NO, NO2) produced by partial discharge will corrode the insulation, and nitric acid will be produced in case of moisture, which will aggravate the corrosion of the insulation.
4 During partial discharge, due to the electrolysis of the electrode and Schottky radiation, the oil will decompose the oil, plus some impurities in the oil. Therefore, it is easy to make the oil sludge formed by polymerization accumulate on the paper layer (mostly in the insulating layer or other insulating oil wedge). The formation of oil sludge will increase the dielectric damage angle tg of insulation δ ， Reduce the heat dissipation capacity and even cause thermal breakdown. possibility. The continuous development of partial discharge will gradually increase the damage of insulation deterioration, eventually shorten the normal life of insulation, shorten the insulation strength in a short time, and may even damage the entire insulation