Characteristics of a good transformer oil

Transformer oil:

  • Transformer oil (also known as insulating oil) is a special type of oil which has excellent electrical insulating properties and is stable at high temperatures.
  • Transformer oil is used in oil-filled electrical power transformers to insulate, stop arcing and corona discharge, and to dissipate the heat of the transformer (i.e. act as a coolant).
  • Transformer Oils are mineral electrical insulating oils processed from fractional distillation and treatment of crude petroleum.


There are two main types of transformer oil used in transformers:

  • Paraffin based transformer oil
  • Naphtha based transformer oil


Naphtha oil is more easily oxidized than paraffin oil. But the product of oxidation – i.e. sludge – in the naphtha oil is more soluble than the sludge from the paraffin oil. Thus sludge of naphtha-based oil is not precipitated in the bottom of the transformer. Hence it does not obstruct convection circulation of the oil, which means it does not disturb the transformer cooling system.

Although Paraffin oil has a lower oxidation rate than Naphtha oil, the oxidation product (sludge) is insoluble and precipitated at the bottom of the tank. This sludge acts as an obstruction to the transformer cooling system.

Another problem with paraffin-based oil that the dissolved waxes inside of it can lead to a high pour point. Although this is not an issue in warmer climate conditions (such as India).

Despite the disadvantages mentioned above, paraffin-based oil is still commonly used in many countries (such as India) due to its high availability.

Properties of transformer oil:

Electrical properties:

  • Very high dielectric strength. The minimum breakdown voltage of transformer oil or dielectric strength of transformer oil at which this oil can safely be used in the transformer, is considered as 30 kV.
  • High specific resistance or resistivity. The minimum standard specific resistance of transformer oil at 90°C is 35 × 1012 ohm-cm and at 27°C it is 1500 × 1012 ohm-cm.
  • Low dielectric dissipation factor.


Chemical properties:

  • Moisture or water content in transformer oil is highly undesirable as it affects the dielectric properties of the oil adversely. The water content in an oil is allowed up to 50 ppm as recommended by IS–335(1993).
  • Acidic transformer oil is a harmful property. If oil becomes acidic, the water content in the oil becomes more soluble in the oil. The acidity of oil deteriorates the insulation property of paper insulation of winding. Acidity accelerates the oxidation process in the oil.


Physical Properties:

  • Good Transformer oil generally exhibits high interfacial tension. Inter facial tension between the water and oil interface is the way to measure the attractive molecular force between water and oil in Dyne/cm or milli-Newton/meter.
  • It is desirable to have a very high flash point of transformer oil. The Flash-point of transformer oil is the temperature at which oil gives enough vapors to produce a flammable mixture with air. In general, it is more than 140°C.
  • Lower pour point. If the oil temperature falls below the pour point, the transformer oil stops convection flowing and obstruct cooling in a transformer.
  • Transformer oil has a low viscosity and low volatility with good gas absorbing properties, while It resists oxidation, slugging, and emulsification with water.
  • Viscosity is the resistance of flow, in normal condition. Resistance to flow of transformer oil means obstruction of convection circulation of oil inside the transformer. Good oil should have a low viscosity so that it offers less resistance to the conventional flow of oil thereby not affecting the cooling of a transformer

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