Partial discharge is primarily the discharge that occurs within the insulation of transformers, reactors, and other high-voltage electrical equipment under the influence of high voltage. This discharge is localized within the insulation and does not immediately result in complete insulation breakdown or flashover, hence termed partial discharge. The magnitude of partial discharge is extremely weak and imperceptible to human senses such as sight or hearing; only highly sensitive partial discharge measuring instruments can detect it.

The internal insulation of transformers, especially as voltage levels increase, is subjected to prolonged exposure to the operating voltage, leading to high electric field intensity within the insulation. In areas of weak insulation, partial discharge is prone to occur due to excessive concentration of electric field or high electric field intensity.

Causes of partial discharge include localized electric field concentration, the presence of bubbles or impurities in solid dielectric materials, moisture, gas, or suspended particles in oil, and severe electric field distortion at interfaces between different dielectric materials. Traces of partial discharge on solid insulation often manifest as small spots or dendritic burn marks, while in oil, small decomposition bubbles may appear.

Although partial discharge events are brief and low in energy, they pose significant hazards. Their prolonged existence can cause considerable damage to insulation materials. This damage occurs due to the direct impact on neighboring insulation materials by discharge particles, and the chemical effects of heat, ozone, nitrogen oxides, and other reactive gases produced by discharge, leading to corrosion, aging, and increased electrical conductivity of local insulation, ultimately resulting in thermal breakdown. The aging and deterioration of internal insulation in operational transformers often originate from partial discharge events.