TRANSFORMER OIL AND IT’S INSULATING PROPERTIES

What Is Transformer Oil?

Transformer oil actually is Insulating oil that has excellent stable electrical insulation properties at long-time high temperatures. Due to the mass use of this insulating oil in an electrical oil-filled transformer, commonly it is called transformer oil. Not only in transformers, but it also uses in high-voltage capacitors, oil circuit reclosers, high-voltage switches & circuit breakers, florescent lamp ballast, etc.

Transformer Oil or Insulating Oil also known as mineral insulating that is normally obtained by fractional distillation and subsequent treatment of crude petroleum.

The chemical name of oil used in transformer

Transformer oil is called mineral oil which is free from FATTY acids. And the Chemical name of Transformer oil is Hydrotreated Light Nepthanic Distillate.

Polychlorinated biphenyls (PCBs) were formerly used as transformer oil since they have high dielectric strength and are not flammable. Unfortunately, they are also toxic, bioaccumulative, not at all biodegradable, and difficult to dispose of safely. When burned, they form even more toxic products, such as chlorinated dioxins and chlorinated dibenzofurans. Wikipedia

Properties of Transformer Oil

Before using any substances in electrical equipment, their physical, chemical, and electrical properties should be known to get maximum efficiency using safe conditions. The main properties of Transformer oil are as below-

It creates an acceptable level of insulation in combination with the insulating materials used in the conductors and coils; 

It also acts as a coolant to extract heat from the core and the wingdings. 

A good transformer oil must have the following Physical, chemical, and electrical properties. Physical constants of Transformer oil:

Property Recommended value

Permittivity 2.2
Thermal conductivity 0.12 W/m deg C
Specific Heat 2.06 kJ/kg deg C
coefficient of Expansion 0.00078/ deg C
Mean density factor 0.00065/ deg C
The above parameters are based on IS 335(Indian standard)

To know more about insulating you may read the complete separate article named A Guide to Use Electrical Insulation Materials and Insulating Oil. High dielectric strength Polychlorinated biphenyls (PCBs) formerly used as transformer oil, It is toxic, bioaccumulative, not flammable, and biodegradable also difficult to dispose of safely.

What is BDV Test in Transformer Oil?

BDV in terms of transformer oil is the abbreviation of Break Down Voltage. BDV of Transformer Oil or Insulating Oil means the dielectric strength of the oil. Break-down voltage actually is measured by observing the voltage at sparking straits between two electrodes separated by a specific gap that emerged in the testing insulating oil.

During a long period of time operation of a transformer, its insulation oils will become degraded which results in it becoming physically dirty and chemically losing the function of the oil in the transformer as insulation and cooling, in that case, the transformer will face some serious problems.

Periodically BDV Testing is the main way to monitor and check the oil’s quality. When the testing result shows a very low value, the oil has to be replaced or purified.


The BDV value tested at a gap of 2.5 mm between two terminals of 50 kV and above is considered good for operation, while 60 kV and above means the oil is in excellent condition.

What Is the DGA Test in Transformer Oil?

DGA is short for Dissolved Gas Analysis in Transformer Oil. DGA of Insulating Oil is very useful to analysis to transformer health. There are two main causes of gas formation within an operating transformer- I) electrical disturbances and ii) thermal decomposition. Actually, the breakdown of electrical insulating materials and related components inside a transformer generates gases within the transformer. The major gas produced is Hydrogen. The minor gas produced is Methane.

DGA usually consists of sampling the oil and sending the sample to a laboratory for analysis. Mobile DGA units can be transported and used on-site as well; some units can be directly connected to a transformer.

International standard IEC 60599 is followed to DGA- Dissolved Gas Analysis for transformer oil testing.

What Would be the Transformer Oil Testing Frequency?

Testing standards and procedures are defined by various factors.

• Overheated Oil: Methane and Ethylene
• Arcing: Acetylene
• Corona discharge: Hydrogen
• Overheated Cellulose: Carbon Monoxide

The frequency of Transformer oil testing is normally twice a year for general and physical tests. Dissolved gas analysis once a year, and Furan testing every 2 years for transformers in operation at least.

The transformer oil lifetime maybe 20-25 years and even 30 years for well-purified oil. But, incorrect use may reduce the service life of transformer oil.