What is Transformer Oil : Types & Its Testing

The oil used in the transformer is a key element to check the physical condition of the unit. In the last five decades in India, there is a drastic change in production technology. The mineral-based oil like transformer oil is frequently used in different types of transformers for its dielectric strength as well as electrical properties. The oil used in the transformer works like a cooling agent & an insulator. This article discusses an overview of what is transformer oil, its functions, different types, properties, testing, and factors to test.

What is Transformer Oil?

Definition: The transformer oil can be defined as a special kind of oil that has outstanding electrical insulating properties. It is also known as insulating oil. At high temperature, this is stable and it is used in electrical power transformers to prevent arcing, dissolve the heat of the transformer. So this kind of oil acts as a coolant protects the windings and core of the transformer because these two are submerged within the oil.

Transformer Oil
Transformer Oil

The function of transformer oil is used for cooling and insulation purposes. We know that there are different kinds of materials available which have dissimilar dielectric strength. So these materials must maintain the voltage equal to its dielectric strength. For instance, if the voltage of the material is enhanced over the specific dielectric strength of the material, then the current flow will be there throughout it.

This oil absorbs the moisture within nature very quickly. The oil dielectric strength will get deteriorated by the absorption of the dampness. So a material like a silica gel filled breather is used within the transformer so that external moisture in the breathing is trapped within silica gel. The main functions of transformer oil are coolant and insulator


The primary function of this is as a coolant. The designing of transformer coils can be done with copper that carries high current so that these coils will become hot. It is an excellent heat conductor, so by using this, the temperature of the copper coils can be reduced. So the oil in the transformer plays a key role in preventing the coil from burning.


It works as an insulator. It includes high dielectric strength to resist with a high voltage. This is the reason it is used as an insulator within the transformer.


Different Types

There are two types of transformer oil that include the following.

  • Naphthenic Oil
  • Paraffinic Oil

Naphthenic Oil

  • The mineral insulating oil is derived from particular crudes which include extremely low n-paraffin known as wax.
  • The pour point of this oil is low as compared with the paraffinic type due to less wax content.
  • The boiling point of this oil is approximately 425 °C.
  • As compared with other oil, this is more readily corroded.
  • The products of oxidation are soluble within the oil.
  • The corrosion of paraffin-based crudes generates an unsolvable sludge to increase the viscosity. So it will reduce the capacity of heat transfer, service life, and overheating.
  • These oils include aromatic compounds at relatively fewer temperatures like -40°C.

Paraffinic Oil

  • Mineral insulating oil derived from special crudes contains a substantial amount of n-paraffin i.e. wax.
  • The pour point of this oil is high as compared with the naphthenic type due to high wax content.
  • The boiling point of this kind of oil about 530 °C.
  • Oxidation of this oil is less.
  • Oxidation products are insoluble within the oil.
  • Even though the naphthenic type is more readily corroded as compared with paraffinic however the oxidation products are soluble within the oil that results in a decrease of problem.

Why Transformer Oil Testing is Important?

Its testing is very important because of the following reasons.

Transformer Oil Filtering
Transformer Oil Filtering
  • It determines important electrical properties
  • Recognizes if particular oil is appropriate for future use
  • Notice whether filtration/ regeneration is required
  • It decreases oil costs
  • Component life can be enhanced
  • Untimely failures can be prevented
  • Exploit safety


Transformer oil properties mainly include the following.

  • Power loss is less due to excellent dielectric properties
  • Better insulation values will be there between windings due to high resistivity.
  • High performance
  • Evaporation losses will be reduced due to thermal stability and high flash point.
  • Outstanding aging characteristics even in severe conditions
  • Temperature range is wider
  • The electrical properties of transformer oil mainly include dielectric strength, specific resistance, and dissipation factor of dielectric for tan delta.
  • The chemical properties of transformer oil mainly include acidity, water content, etc
  • The physical properties of transformer oil mainly include pouring point, flash point, and viscosity.


Testing of transformer oil is mandatory in yearly once for its maintenance. Early testing will set up a bottom line for contrast & yearly testing will design any changes inside the transformer. The transformer quality can be performed through the following tests.

  • Dielectric Strength
  • Dissolved Gas Analysis
  • Moisture
  • Sludge Test
  • Acidity
  • Viscosity
  • Pour Point
  • Interfacial Tension
  • Flash Point
  • Oxidation Stability Test
  • Resistivity

The faults which occur while testing include the following.

  • Corona Discharge Fault: When low energy discharges then it will generate hydrogen, methane minor quantities of ethane & ethylene.
  • Arcing Acetylene Fault: In this fault, huge amounts of acetylene/ hydrogen/small amounts of methane & methylene can be generated.
  • Overheated Cellulose Fault: Whenever the cellulose is excited, then it will generate carbon monoxide
  • Overheated Oil Fault: Overheating oil generates ethylene & methane

Factors to Test

The following factors need to test while doing a test

  • The power factor of liquid
  • Acid number
  • Visual examination
  • The breakdown voltage of the dielectric
  • Corrosive sulfur
  • Specific resistance
  • Interfacial tension

The above tests will determine the condition of the oil in the transformer.

Thus, this is all about an overview of transformer oil which includes its functions, why it is important, different types, properties, testing, and factors to test. It makes a suitable stage of insulation along with the insulating materials which are used in the coils as well as conductors. It also works as a coolant for removing the heat from the windings & core. Here is a question for you, what are the types of transformer oil