What Is Transformer Oil?

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

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

Properties of Transformer Oil

Before use any substances in electrical equipment, it’s physical, chemical and electrical properties should be known to get maximum efficiency using at safe condition. Main properties of Transformer oil is as below-
  1. It creates an acceptable level of insulation in combination with the insulating materials used in the conductors and coils;
  2. 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, bio accumulative, not flammable and at all 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 oil. Break down voltage actually  is measured by observing the voltage at sparking straits between two electrodes separated by specific gap which emerged in the testing insulating oil.


During long period of time operation of a transformer, it’s insulation oils will become degradation which results it becomes physically dirty and chemically in loosing function of the oil in transformer as insulation and cooling, in that case, 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 gap 2.5 mm between two terminals of 50 kV and above is considered good for operation,  while 60kV and above means the oil is in excellent condition.

What Is DGA test in Transformer Oil?

DGA is the shorten of 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.

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