Transformer Oil Testing-Types, Methods Properties

Transformer oil testing is a crucial part of transformer maintenance, as the oil serves as electrical insulation between the winding and tank and acts as a cooling agent.

Over time, the oil can become contaminated, which can degrade its insulating properties and reduce its ability to dissipate heat.

Therefore, periodic transformer oil testing is essential to ensure that it operates efficiently and safely. different methods are used to test transformer oil properties.

In this article, we will discuss the types of transformer oil tests that are commonly performed on transformer oil.

Electrical Tests

Dielectric Breakdown VoltageASTM D877
Dissipation FactorASTM D924
ASTM stands for the American Society for Testing and Materials

Chemical Tests

Dissolve gas analysis DGA test ASTM D2945
Neutralization Number (Acidity Test)ASTM D664 AND ASTM D974
Transformer oil Water Content TestASTM D1533
Furan Test of transformer oilASTM D5837

Physical Tests

Flash and Fire Point TestASTM D92
Interfacial TensionASTM D971 ASTM D2285
Pour PointASTM D97
Relative DensityASTM D1298
Viscosity ASTM D88 ASTM D445

Electrical Tests of Transformer Oil

Dielectric Breakdown Voltage (BDV) Test

The BDV test measures the voltage at which the oil breaks down and conducts electricity.

This test determines the electrical strength of the oil and is a measure of the insulation properties.

A low BDV value indicates that the oil may not provide adequate insulation, which can lead to a potential fault.

Dissipation Factor ASTM D924

The dissipation factor measures the amount of power that is lost when transformer oil is exposed to an AC supply. This lost energy is transformed into heat within the oil. A low dissipation factor indicates that there is minimal power loss when an AC voltage is applied.

Chemical Tests of Transformer Oil

DGA Test of Transformer Oil

DGA means dissolved Gas Analysis is a critical test in the diagnosis of potential transformer faults.

When a fault occurs inside the transformer tank, it releases gases that dissolve in the transformer oil.

DGA measures the concentration of these gases, which provides an indication of the type of fault that is occurring.

By analyzing the gas composition, we can determine if the fault is thermal, electrical, or mechanical, and take appropriate corrective action

Neutralization Number Acidity Test

The acidity test measures the level of acid in the oil. As the oil degrades, it can become acidic, which can corrode the transformer’s internal components.

The acidity test is a measure of the oil’s ability to resist corrosion

Transformer oil Water Content Test: Karl Fischer Method

Transformer oilThe transformer oil may have water content in a different form, free water can be identified by visual inspection of the oil. the oil will become cloudy or separate water droplets can be seen at the bottom surface.

Water that is free can be separated from a substance through the filtration method. However, dissolved water cannot be detected visually and its presence may impact the dielectric breakdown strength of transformer oil. To measure the amount of water in the oil, a water content test is necessary.

Water can degrade the oil’s insulating properties, and can also cause corrosion of the transformer’s internal components.

The water content test is a measure of the oil’s ability to resist water penetration.

Furan Analysis Test

Furan analysis is done on transformer oil to determine the degradation of cellulosic material like paper insulation or pressboard.

Cellulosic compounds that are oil soluble are migrated into insulating oil. presence of a high concentration of furanic compound indicates cellulose degradation due to aging or an incipient fault inside the tank.

This measurement is done using high performance liquid chromatography or HPLC.

Physical Properties of Transformer Oil

Analine Test

The aniline point of transformer oil indicates the solvency of oil for materials that are in contact with insulating oil.The ASTM code of aniline test is ASTMD611.


Transformer oil should have a light color, changes in the shade of transformer oil indicate degradation of oil due to moisture and contamination.

Flash and Fire Point

The Flash Point of oil is the temperature at which oil must be heated to give enough vapor to form a flammable mixture with air.

The fire point of the oil is the temperature at which the oil ignites and maintains fire for 5 Sec. flash point test of transformer oil is essential to detect combustible contaminants present in the oil. The flash point of an oil is generally more than 1400 C.

Interfacial Tension IFT Test

Interfacial Tension is a measurement of the forces of attraction between water and oil. It is measurement unit is mN/m.

The IFT test measures the force required to separate the oil and water interface. this test is a measure of the oil’s ability to resist the penetration of water, which can degrade the oil’s insulating properties.

The IFT value decreases as the oil becomes more contaminated with water.

Pour Point

The pour point is the minimum temperature at which oil starts flowing. Paraffin base oil has a higher pour point than naphtha base oil.

Relative Density

The ratio of the weight of equal volumes of two materials is called relative density. in the case of transformer oil, it is water and oil. this generally measures at 150 C


The viscosity test measures the oil’s resistance to flow.

The viscosity of the oil can affect the transformer’s cooling properties.

As the oil degrades, it can become more viscous, which can reduce its ability to dissipate heat.


transformer oil testing is essential to ensure the safe and efficient operation of the transformer. The tests described above provide critical information about the transformer oil’s condition and its ability to provide adequate insulation and cooling. Regular oil testing can help prevent transformer failures and extend the transformer’s life.


Leave a Reply

Your email address will not be published. Required fields are marked *