Transformer Oil (or Insulating Oil)


Transformer oil or insulating oil is generally a highly refined mineral oil that is stable at high temperature and has excellent electrical insulating properties. It is used in oil-filled power transformers, some types of high voltage capacitors, high voltage switches and circuit breakers.

Transformer oil and its functions

Transformer/insulating oil are stable at high temperatures. Other than that, the following are functions of insulating oil:

  • Heat transfer

With an increase of voltage, the temperature of the transformer’s windings also increases, the oil is used to dissipate the heat. Under normal operation conditions, heat is generated from the load on the transformer as well as the ambient temperature. The oil dissipates the heat from the core and coils.

  • Insulation

It acts as an insulator between the windings. It increases the resistance between them and avoids short circuits. Both the paper and oil inside a transformer have dielectric strength. Combined, the paper and oil yield a 23% increase in dielectric strength. The increase in dielectric strength makes this combination popular.

  • For security

It operates on the level of oil inside the transformer. The temperature display of the oil describes the inner condition of transformer and makes any short circuit easy to spot. In this way, severe losses and damages in the transformer can be prevented.

Types of Transformer oil

  • Paraffin based – derived from special crudes that contain substantial amount of n-paraffin. This type of insulating oil is less oxidised than naphtha based.
  • Naphtha based – derived from special crudes that contain low amount of n-paraffin.

Physical properties of Transformer oil

  • Moisture content – The amount of free and dissolved water present in the oil is measured in ppm. The presence of water is detrimental as it adversely affects the electrical characteristics of the oil and accelerates the deterioration of the insulating paper. For good transformer perfromance the Oil Moisture Content must be kept low.
  • Flash point – It is the the lowest temperature at which a liquid gives off vapor to form an ignitable mixture in air near the surface of the liquid. A minimum flash point is specified in order to prevent the risk of an accidental ignition. For good Transformer Oil Flash Point should be high, .e.g., 140 degrees Centigrade.
  • Viscosity – It measures the oil resistance to the flow. The oil must be mobile to transfer the heat in a better manner from the core to the transformer radiators. So Viscosity of the transformer oil should be low.
  • Pour point – The temperature at which transformer oil will just flow under the prescribed conditions is known as the pour point. Pour Point should be low for good transformer oil.

Chemical properties of transformer oil

  • Neutralization value – It is the measure to determine the free organic and inorganic acids present in the oil expressed in terms of milligrams of KOH. Good transformer oil exhibit low Total acidity.
  • Sulphur content – Crude petroleum contains sulphur compounds. Most of it is removed at the time of refining process. Presence of corrosive sulphur results in pitting and black deposit.

Electrical properties of transformer oil

  • Dielectric strength – The dielectric strength is determined taking note of at what voltage, sparks between two electrodes immersed in oil and a specific gap between them. Low value of dielectric strength shows presence of moisture and maybe other conducting substances in the oil.
  • Specific oil resistance – It is measure of DC resistance between two opposite sides of one cm3 block of oil. Its unit is ohm-cm at a particular temperature. With an increase in temperature, the resistivity of oil decreases rapidly. The insulating oil’s resistivity must be high at room temperature, and it should have good value at high temperatures.

Transformer oil filtration

Almost 75 percent of the transformer failures happen due to contaminated and deteriorated oil. It is important to filter the transformer oil. Transformers require filtered and dried oil. During its usage, the insulating oil absorbs moisture and gets polluted by sopping fibers, dirty particles, aging products, and soot. The efficiency of the oil as an insulating material is highly reduced as the moisture level increases. Hence, transformer oil filtering is an important process which eliminates solid particles, dissolved gasses, and dissolved water. The electrical properties of the oil can be enhanced by filtering, dehydration, and degassing. Oil Filtering keeps the transformer in good condition and increases its life.

Transformer oil analysis

Fluid testing for oil-filled transformers ensures the oil perfroms as it should. Insights on oil health, contamination, and lubricant life help to drive informed decisions that maximize transformer life and support safety measures—saving money and preventing accidents.


New trends in transformer oil

In one of research paper authored by B Koti Reddy, vegetable oils have proved to be a good alternatives to transformer oil. Though vegetable oils present some limitations such as an inferior oxidative resistance, a poorer low temperature properties, a higher viscosity, and higher production cost. They can be used in their raw form in low voltage transformer but have to be refined and purified to to be used in high voltage transformers. In terms of economic costs and environmental considerations, Soya bean oil, Coconut oil and palm kernel oil are the most viable alternatives to transformer oils.


  1. Transformer oil testing. 
  2. A guide to transformer oil analysis
  3. Properties of transformer oil that affect efficiency
  4. Latest Trends In Use Of Transformer Oils by B. Koti Reddy
  5. Transformer oil: Testing, Types, and Properties
  6. 4 Things You Need to Know About Transformer Oil
  7. Why Transformer Oil Filtration Is Required
  8. Everything to know about Transformer Oil Filtration
  9. Transformer oil


Harshvardhan Singh works as a Senior Service Engineer at a mining firm in India. He is currently working into oil analysis field. Has worked in the filed of tribology and lubrication and loves to write about the same.

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