What Is the Role of Oil in Oil-Immersed Transformers?

Why Oil-Immersed Transformers Use Insulating Oil

An oil-immersed transformer is a power transformer in which the core and windings are immersed in insulating oil inside a sealed tank. The oil performs two primary functions: electrical insulation and heat dissipation. Both functions together determine the rating, the reliability and the service life of the transformer, and both must be preserved through correct oil selection, sealing and maintenance.

Function One: Electrical Insulation

The first role of transformer oil is insulation. The oil fills the space between windings and between windings and the tank, providing a liquid dielectric that increases the dielectric strength of the insulation system. Impregnating the paper and pressboard insulation with oil raises its breakdown voltage and also prevents moisture from entering the insulation structure, because water is the most common cause of reduced dielectric strength and accelerated ageing. The dielectric properties of the oil are tested against the requirements of IEC 60296 and GB/T 7595, including breakdown voltage, water content and dissipation factor.

Function Two: Heat Dissipation

The second function is cooling. Transformer oil has a high specific heat capacity and acts as a coolant that transports heat away from the core and windings. As the transformer operates, heat raises the temperature of the oil near the core and windings; the heated oil becomes less dense and rises by natural convection, while cooler oil descends. This circulation carries heat to the tank wall and to radiators, where it is released to the surrounding air. In larger units, pumps and fans accelerate the circulation, which is reflected in the cooling class such as ONAN, ONAF, OFAF or OFWF.

Other Roles of Transformer Oil

Protection of solid insulation: oil keeps paper and pressboard dry and slows their thermal ageing.

Diagnostic medium: dissolved gas analysis of oil samples reveals developing faults such as overheating and partial discharge.

Arc suppression: in some designs, oil helps extinguish arcing and limits fault damage inside the tank.

Oil Types and Maintenance Essentials

Most power transformers use inhibited mineral oil, while natural ester and synthetic ester liquids are selected for their higher fire point and better biodegradability in special installations. Routine maintenance includes periodic oil sampling and testing in accordance with IEC 60599 and GB/T 7597, monitoring water content, acidity, breakdown voltage and dissolved gases. The conservator, breather and oil level indicators must be kept in good condition so that the oil remains sealed and dry over the transformer lifetime.

FAQ

1. Why is oil better than air as an insulating medium?

Oil has a much higher dielectric strength than air and also conducts heat away efficiently. Immersing the insulation system in oil both increases the withstand voltage and cools the active parts.

2. What happens if the oil level drops?

A low oil level exposes part of the winding to air, reducing dielectric strength and allowing moisture ingress. The transformer should be taken out of service and refilled with dry, tested oil.

3. How does dissolved gas analysis help maintenance?

Different fault types produce characteristic gases. Analyzing the gas composition and ratios lets engineers detect overheating, partial discharge or arcing before a catastrophic failure occurs.

4. What is the difference between ONAN and ONAF cooling?

ONAN relies on natural convection of oil and natural air flow over the tank. ONAF adds fans that force air over the radiators, increasing the heat transfer and allowing a higher rating.

5. Which standards govern transformer oil quality?

Unused oil is specified in IEC 60296, in-service oil quality follows GB/T 7595, sampling follows GB/T 7597, and dissolved-gas interpretation follows IEC 60599.