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Transformer Oil and Its Properties.

Updated on November 3, 2020

Transformer oil is one of the very important components of oil-immersed transformers. The life expectancy and satisfactory operation of transformers mainly depend on the oil in which it is immersed.

Transformer oil performs two very important functions.

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 windings.

Transformer oil

The oil used in transformers is hydrocarbon mineral oil. It is mainly composed of four generic classes of organic compounds, aromatics, paraffins, napthenes and olefines. Transformer oil gives a better insulation when aromatics, paraffins, napthenes and olefines are present at the right proportion. For better insulation Transformer oil is desired to have more of saturated paraffins and less of aromatics, napthenes and olefines. For more stability, more aromatics and napthenes are necessary. To get both insulating property and stability at the same time, there must be an optimum mix of four organic components. This can be obtained by careful refining of crude oil. Vegetable and animal oils form fatty acids and attack the fibrous insulating materials and hence cannot be used in transformers.

A good transformer oil must have the following Physical, chemical and electrical properties.

Physical properties

Oil density

The maximum value of density of transformer oil at 29.5 oC must not be more than 0.89 gm/cc.


It is the measure of resistance to the flow of a fluid with the effect of external forces. This property of the fluids is inversely proportional to temperature. It increases with decrease in temperature. But in case of transformer oil it is necessary that viscosity remains low at low temperature. The maximum permissible level of viscosity at 27 oC is 27 cSt.

Flash point.

It is the temperature at which oil gets vaporized and when this vapor mixes with air, forms an ignitable mixture and may cause momentary flash. To prevent the risk of fire the transformer oil must have a high flash point. The flash point must be greater than 140oC.

Moisture constant

It is the amount of dissolved present in oil expressed in ppm/kg. the insulating property of the transformer oil will be degraded by the presence of moisture. The maximum allowable moisture constant is 50 ppm.

Pour point

It is the temperature at which oil just commences its flow under prescribed conditions. The specified pour point is -6oC.

Chemical Properties

Neutralization value

It is the measure of acidity of oil. It is the measure of organic and inorganic acids present in the oil. It is expressed in terms of milligrams of base required to neutralize the total force acid present in one gram of oil. The recommended maximum value of neutralization is 0.03 mg KOH/gm. Higher the neutralization value, higher the acidity leading to higher sludge formation.

Corrosive sulphur

The presence of chemicals in the transformer oil will lead to the formation of black deposits over the copper parts of the transformer. Hence the heat dissipation is highly affected. Since transformer oil is derived from petroleum it definitely contains traces of sulphur. The presence corrosive of sulphur in the transformer oil is not recommended.

Oxidation stability

The neutralization value of transformer oil increases on getting oxidised. The neutralization value must not go beyond 0.4 mg KOH/gm after oxidation and total sludge after oxidation must not be more than 0.1% of weight of oil used.

Sediment and perceptible sludge

Sludge is poor conductor of heat. Sludge deposited over the transformer parts leads to poor heat dissipation. It blocks the flow of oil in ducts and impairs cooling. Therefore once sediments or perceptible sludge are detected oil is considered as not usable.

BDV testing instrument

Transformer oil is placed between a sphere gap and the voltage across the electrodes is raised gradually till Break down of oil occurs. Break down is characterized by formation of arc between the electrodes.
Transformer oil is placed between a sphere gap and the voltage across the electrodes is raised gradually till Break down of oil occurs. Break down is characterized by formation of arc between the electrodes.

Electrical properties

Breakdown voltage (BDV)

It is the voltage is the voltage at which transformer oil losses its dielectric property and starts conducting. It represents the electrical property of transformer oil. The presence of moisture, sludge contaminating agents and sediments decreases the dielectric property of oil. For a new sample of oil the breakdown voltage of transformer oil is 30kV and for a sample after filtration must have BDV of 60kV.


It is one of the most sensitive properties of transformer oil. The resistivity of the oil decreases with increase in temperature. The transformer oil must have a minimum resistivity of 30 x 1012 ohm-cm at 90oC and 1500 x 1012 ohm-cm at 27oC.

Dielectric dissipation factor (DDF)

It is numerically equal to the sine of loss angle. A High value of DDF refers to poor quality of oil. The maximum recommended value of DDF factor of oil at 90oC is 0.002.

Physical constants of Transformer oil

Recommended value
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

The transformer is affected by its operating conditions such as operating temperature, atmospheric conditions, electric strength, moisture content and other contaminations and sludge formation. The presence of moisture or suspended particles in transformer oil affects its dielectric property. Hence transformer oil it should be tested periodically. If the oil is containing moisture or suspended particles it should be filtered or replaced by fresh oil.

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