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Partial discharge is an electrical discharge in which the insulation between conductors is only partially bridged. This type of discharge can occur near or away from the conductor.

For the air gap insulation defects that occur in the production process of power transformers, when the electric field strength is high, the air gap insulation is first broken down and discharged. If this happens repeatedly, the breakdown and extinction will occur again and again. If the surrounding insulation is still good, that is, the partial discharge channel has not been formed. In other words, the phenomenon that a part of the insulating medium is electrically broken down is called partial discharge.

Insulation gaps in the transformer manufacturing process, untreated air bubbles in the insulating oil, and the tip of the equipment are all parts that are prone to partial discharge. The discharge along the surface of the dielectric, the discharge of the laminated material inside the transformer, the creepage of the inner tree branches, etc., all belong to the category of partial discharge of the transformer. Partial discharge is divided into surface partial discharge and internal partial discharge, which respectively refer to the discharge phenomenon that occurs at different locations on the surface or inside of the transformer insulator.

In the study of partial discharge in electrical equipment, discharge corona is also an important discharge phenomenon. It mainly occurs around the conductor surrounded by gas, and occurs in uneven field strength, causing the surrounding gas to ionize. As shown in Figure 1-a, the tip discharge of the insulating medium. The discharge pulse of the corona is near 90° of the negative half cycle of the applied voltage, and the pulses are of equal amplitude and interval, as shown in Figure 1-b. With the increase of the voltage, the size of the discharge remains unchanged and the number of times increases. With the gradual increase of the voltage, a small amount of large-amplitude discharge appears in the positive half cycle

(1) There are several forms of partial discharge in power transformers: glow discharge, which is a non-pulse type discharge; spark discharge, which is a pulse type discharge; sub-glow discharge, which has discrete pulses with relatively small amplitudes, and belongs to the previous two types Intermediate transition of discharge;

(2) The internal discharge of the winding mainly has the following forms: breakdown of the oil-paper barrier insulation in the middle of the winding oil passage; breakdown of the oil passage at the end of the winding; close to the insulated wire and electrical paper (lead insulation, lap insulation) , Interphase insulation) the breakdown of the oil gap; the breakdown of the longitudinal insulating oil channel between the coils; the discharge of branches around the insulating cardboard; the discharge of other solid insulation; the discharge of other metal foreign bodies infiltrated in the insulation.