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 oil’s primary functions are to insulate and cool a transformer.

Transformer oil is also used to preserve the transformer’s core and windings – as these are fully immersed inside the oil. Another important property of the insulating oil is its ability to prevent oxidation of the cellulose-made paper insulation. The transformer oil acts as a barrier between the atmospheric oxygen and the cellulose – avoiding direct contact and hence minimizing oxidation. The level of transformer oil is typically measured using a MOG (Magnetic Oil level Gage).
It must therefore have high dielectric strength, thermal conductivity, and chemical stability, and must keep these properties when held at high temperatures for extended periods.[1] Typical specifications are: flash point 140 °C or greater, pour point −30 °C or lower, dielectric breakdown voltage 28 kV (RMS) or greater.

To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Power transformers with capacities of thousands of kVA may also have cooling fans, oil pumps, and even oil-to-water heat exchanges.

What are the characteristics of insulating oil?

High Flash Point: To minimize risk of the formation of a flammable mixture of oil vapor and air at high operating temperatures.
Low Pour Point: To reduce risk of inefficient cooling or of slow O.C.B. tripping due to high oil viscosity.

Low Viscosity: To ensure good impregnation of cellulose insulation and free flowing circulation and heat transfer.

Low Density: To avoid ice crystals floating in oil as dissolved water precipitates and freezes.

Note: The above physical properties will be unlikely to alter significantly in service. Except by contamination by other materials such as diesel, paraffin, lubricating oil or other liquid insulants.

Good Chemical Stability: To resist oxidation in “hostile” environment i.e. high temperature, oxygen, copper, iron, water and other oxidation products.

Note: Oxidation is the natural process of aging of a transformer oil. It will not only produce sludge, which may reduce the cooling efficiency of the transformer but can lead to tank corrosion by volatile acids. Even low levels of oxidation of the transformer’s oil will reduce the life of its cellulose insulation. The only treatment is oil-change and reclamation of old oil or in-situ regeneration.

Low Dissolved Water Content: To preserve electrical properties, especially where other contaminants may be present, and to avoid the precipitation of free water at extremely low temperatures.

Note: It is important to understand the dynamic relationship between oil and cellulose water content and temperature. The electrical, chemical and physical integrity of cellulose will be adversely affected by high water content. Prolonged or repeated on-site processing via a high vacuum processing plant will dry the oil and hence the paper.

Good Appearance: A “safety net” to exclude any visible contaminants e.g. dirt, dust, sediment or water, that may either be missed by the objective tests, or which would degrade any test equipment into which they were deposited.
Note: Visible contaminants can usually be removed by filtering.

High Breakdown Voltage: To ensure the oil’s ability to withstand a sustained high A.C. voltage without breakdown.
Note: Filtering/Dehydrating/Degassing (i.e. treatment via a high vacuum processing plant) will remove the contaminants, which cause a reduction in breakdown voltage.

Low Dielectric Dissipation Factor (DDF): To minimize dielectric loss or leakage current. Sometimes referred to as “power factor”.

Types of Transformer Oil

There are two types of Transformer Oil, Naphthenic and Paraffinic based Transformer Oils. A comparison between the Naphthenic and Paraffinic Transformer Oil is listed below.

Sr. No. Naphthenic Oil Paraffinic Oil
 1) Mineral insulating oil derived from special crudes containing very low n-paraffin i.e. wax. Mineral insulating oil derived from special crudes containing substantial amount of n-paraffin i.e. wax.
 2) The pour point of Naphthenic Oil is less than the Paraffinic Oil as the wax content is low. The pour point of Paraffinic Oil is more than the Naphthenic Oil as the wax content is high.
 3) Boiling point of Naphthenic Oil is around 425 °C. Boiling point of Paraffinic Oil is around 530 °C.
 4) Naphthenic Oils are more readily oxidized than Paraffinic Oils. Oxidation of Paraffinic Oil is less.
 5) Oxidation products of Naphthenic Oils are soluble in the oil. Oxidation products of Paraffinic Oils are insoluble in the oil.


The oxidation of paraffin-based crudes produces an insoluble sludge, which increases the viscosity. It results in reduced heat transfer capabilities, overheating and reduced service life. Although naphthenic oils are more readily oxidized than paraffinic but the oxidation products are soluble in the oil results in a reduction of problem.
 7) Naphthenic oils contain aromatic compounds which remain fluid at comparatively low temperatures such as -40°C.

Moisture content in Transformer Oil is highly undesirable as it affects the dielectric properties of oil. The water content in oil also affects the paper insulation of the core and winding of Transformer.
Paper is highly hygroscopic in nature. Paper absorbs maximum amount of water from oil which affects paper insulation property as well as reduced its life. But in loaded transformer, oil becomes hot, hence the solubility of water in oil increases as a result the paper releases water and increases the water content in Transformer Oil.

Testing transformer oil

At site, two types of tests are conducted. One is BDV Test i.e. Break Down Voltage Test and moisture content test. Moisture content in oil is measured as ppm (parts per million). Normally the BDV value for Transformer oil shall be more than 70 kV/cm and moisture content shall be less than 10 ppm.