Partial discharge refers to the phenomenon of discharge that occurs within the insulation of electrical equipment such as transformers, reactors, and other high-voltage devices under the influence of high voltage. This discharge occurs only at localized points within the insulation and does not immediately lead to complete insulation breakdown or flashover, hence termed partial discharge. It is characterized by its minute magnitude, undetectable by human senses such as sight or hearing, and can only be detected by compassionate partial discharge measuring instruments.

The internal insulation of transformers is subjected to prolonged exposure to operational voltages, especially with increasing voltage levels, leading to high electric field intensities within the insulation. At points of weak insulation, partial discharge is prone to occur due to excessive concentration of electric field or high electric field intensity. Common causes of partial discharge include the presence of air bubbles or impurities in solid dielectrics, moisture, gas, or suspended particles in oil, and severe electric field distortion at interfaces between different dielectric materials. The traces of partial discharge on solid insulation often manifest as small spots or dendritic burn marks, while in oil, they may appear as small decomposed bubbles.

Despite their short duration and low energy, partial discharges pose significant hazards. Their prolonged presence can lead to substantial damage to insulation materials. Firstly, adjacent insulation materials near the site of partial discharge may suffer direct damage from discharge impacts. Secondly, the chemical effects of heat, ozone, nitrogen oxides, and other reactive gases produced by discharge can corrode and age the local insulation, increasing electrical conductivity and ultimately leading to thermal breakdown. The aging and deterioration of internal insulation in operational transformers often originate from partial discharge events.