Transformer Oil


Transformer Oil

The operation of the transformer produces heat. Transformer oils perform basic functions in an oil‐immersed transformer. Oil provides insulation, cooling of winding and helps to quenching arcs . The mineral oil, with the main role of insulating and cooling in power transformers. Transformer oil is also known as insulating oil. 

This transformer oil is use to, helps to preserve the core and winding as there are fully immersed oil and other important purpose of this oil is , prevent direct contact of atmospheric oxygen with paper insulation of winding.

Types of Transformer Oil

Generally two types of transformer Oil used in transformer,

  1. Paraffin based transformer oil
  2. Naphtha based transformer oil

paraffin oil is less oxidized than Naphtha oil, though oxidation product for instance sludge in the naphtha oil is extremely soluble compared to paraffin oil. Hence the sludge content in naphtha based oil is never precipitated at the transformer bottom. Therefore it allows oil circulation, hence the transformer cooling system running normally.

For paraffin oil whose oxidation is lower compared to that of Naphtha oil is sludge, it is precipitated at bottom of the tank therefore obstruct the transformer cooling system.

Good transformer oil acts as a liquid insulation in an electrical power transformer and dissipates heat from the transformer , because it helps preserve the core and winding of the transformer for they are dipped it.  paper insulation made for the primary and secondary winding, the transformer oil prevents direct contact of atmospheric oxygen. paraffin based oil is high pour point due to the wax content, but this does not effect its use due to warm climate condition of India.

Properties of insulating oil.

Electrical Parameter of insulating oil

Dielectric strength of insulating oil

  • minimum breakdown voltage of insulating oil of transformer is 30KV. it is measure by testing of transformer oil.
  • dielectric strength of insulating oil is also known as breakdown voltage of transformer(insulating) oil.
  • break down voltage is measured by observing at which voltage sparking occur between two electrode

Specific resistance of insulating oil

  • resistivity of insulating oil should be measure at 27º C as well as 90º C.
  • standard specific resistance of insulating oil at 27°C it is 1500 × 1012 ohm/cm and  90°C is 35 × 1012 ohm/cm.

Chemical characteristic of insulating oil

Acidity of insulating oil

it is harmful property. if oil become acidic , water content in oil become more soluble to the oil.

Water content in insulating oil

  • Moisture is highly undesirable. it is affect the dielectric strength of insulating oil.
  • The water content in oil is measured as pm(parts per million unit). Water content in oil is allowed up to 50 ppm as recommended by IS–335(1993).
  • The water content in oil also affects the paper insulation of the core and winding of transformer. Paper is highly absorbing air from nature.
  • Paper absorbs maximum amount of water from oil which affects paper insulation property as well as reduced its life.

Physical characteristic of insulating oil

flash point of insulating oil

  • Flash point of transformer oil is the temperature at which oil gives enough vapors to produce a flammable mixture with air.
  • flash point of transformer oil is more than 140°(>10°).


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