What are the reasons for the occurrence of partial discharge in transformers?

Generally speaking, there are two main reasons for the generation of partial discharge. Firstly, it may result from an unreasonable structural design, leading to uneven distribution of the internal electric field in the insulation and the formation of concentrated local electric fields. In areas of concentrated electric fields, partial discharges may occur in oil gaps or localized solid insulation, or along the surface of solid insulation. Secondly, improper manufacturing and processing, such as metal components with sharp angles or burrs, impurities mixed into the insulation, and the presence of local defects. In these areas, the electric field undergoes distortion, causing an increase in field strength, thereby triggering partial discharge. Additionally, when the gas content in insulating oil is excessively high, creating voids, or when the water content is too high, leading to the generation of hydrogen and the formation of voids, these voids with low dielectric coefficients and high field strength may experience breakdown, resulting in partial discharge.

In transformers and reactors, partial discharge mainly occurs in areas with concentrated electric fields and weak insulation. In transformers, common occurrences include discharge in gaps between conductors and insulation, gaps in joints between insulation and insulation, oil gaps discharge at the junctions of leads and winding terminals with paper insulation, oil gaps discharge in insulation at winding ends, inter-turn insulation discharge, sliding discharge along the surfaces of paper and angle rings, insulation discharge around metal impurities mixed into various insulation components, and discharge in areas of high electric field strength due to ground potential. In high-voltage current transformers, there may be discharge between oil-paper insulation gaps retained between each equalizing screen, discharge around impurities in insulation, discharge in gaps or oil gaps at the ends of equalizing screens, and discharge in oil gaps at the points of rupture in equalizing metal screens. In high-voltage voltage transformers, there may be discharge in oil gaps at the ends of insulation, and discharge in gaps or oil gaps in inter-layer insulation. For resin-cast solid-insulated transformers, discharge typically occurs in areas with air bubbles in the solid insulation.

There are two types of destructive effects of partial discharge on insulation: Firstly, the direct impact of discharge particles on insulation causes gradual and expanding damage to local insulation, eventually leading to insulation breakdown. Secondly, the chemical effects of heat, ozone, nitrogen oxides, and other reactive gases generated by discharge corrode local insulation, increase electrical conductivity, and ultimately result in thermal breakdown. Generally, the damage or localized aging of electrical insulation often begins with partial discharge, and its harmful effects manifest in reducing insulation life and affecting safe operation. This is why there has been increasing attention to partial discharge in recent years.

In China's standards, specific measurement methods for partial discharge in transformers and reactors and allowable discharge levels under certain voltage conditions have been clearly defined. The current "Power Transformers" standard serves as the basis for conducting partial discharge tests.

LuShan, est. 1975, is a Chinese professional manufacturer specializing in power transformers and reactors for 49 years. Leading products are single-phase transformer, three-phase transformers, DC inductors, AC reactors, filtering reactor, expoxy resin high-voltage transformer and intermediate, high-frequency products. Our transformers and reactors are widely used in 10 application areas: rapid transit, construction machinery, renewable energy, intelligent manufacturing, medical equipment, coal mine explosion prevention , excitation system, vacuum sintering, central air conditioning.

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