Best ways to test transformer oil

Testing of oil plays a vital role in keeping transformers running efficiently and effectively. Here’s a comprehensive analysis on the different ways to test transformer oil.

Picture Courtesy: SRL OIL & GREASE

Oil is an integral part of transformers and plays a critical role in keeping a transformer running efficiently. Transformer oil is also known as mineral insulating oil and it serves as liquid insulation in electrical equipment and also dissipates heat of such equipment. All winding and core are fully immersed in the oil inside the equipment. The transformer oil also prevents direct contact of atmospherics oxygen to avoid any type of oxidation. It can be said that the transformer oil is life line of equipment.

Analysis of transformer oil enables operators to determine whether it is working at its best and detect possible problems before they result in costly downtime, repairs and loss of production. According to SK Jain, Director, Powerlink Oil Refinery Ltd, “It is very essential to monitor the quality of transformer oil and needs to be tested regularly as per standard laid down under various test methods. These all tests are conducted under various standards for each parameter.”

Divyansh Kohli, Executive Director, NDL Power Ltd, believes: “Transformer oil analysis is the most effective method to detect incipient faults and monitoring a transformer’s condition and its most important credit is that it is non-destructive analysis. Moreover, the analysis is conducted in live transformer condition.”

Alok Agarwal, Chairman, IEEMA, Distribution Transformers Division opines that several factors need to be considered while selecting the best suitable way for transformer oil testing. “Budget, volume of samples to be tested and the need for real-time continuous monitoring are the most critical factors,” he explained while accepting that cost is generally the first consideration.

Most effective method to determine transformer condition

Oil is the one element that surrounds the transformer body and is in contact with all the elements. Transformer unlike any other asset is a static equipment and therefore has a very minuscule operator feedback. “Transformer oil analysis is by far the most effective method to determine transformer condition and to predict and prevent failure with highest success rate,” Kohli observes.

Oil in a transformer works for two essential function: Dielectric property and cooling property. IS 335 defines and explains about it. Both the properties are equally important and essential for a transformer to live and work healthy. The oil property is generally tested at laboratories available across India. It is done on sampling basis and does not take much time. “It is advisable to take sample oil from transformers as a part of routine maintenance and keep a chart for noting values for monitoring and analysis. The breakdown voltage (BDV) gradually goes down with time when transformer is in use. Once in a year oil should be filtered to arrest dissolved moisture,” suggests Sanjib Mitra, Country Head and Sr VP – Transformer Division, Electrotherm (India) Ltd. He adds, “Dissolved gas analysis (DGA) is used for oil property checks at site to avoid poor oil performance on a longer period.”
However, Kohli of NDL Power Ltd suggests that a few points the consumers should understand before conducting the oil analysis:

The testing parameters and its significance of analysis: Most engineers only conduct OST which only observe the oil condition and some add DGA. However, there are several other parameters in transformer oil analysis that surpass the abilities of electrical diagnostics only after understanding its significance shall one add the parameters.
• It is utmost important to entrust a laboratory for oil testing. Not only shall the laboratory be NABL or ILAC accredited, but it should be specific to transformer diagnostics. There are several generic laboratories in India that also document transformer oil analysis. But apart from the quality or understating the critical elements of analysis, they fail in diagnosing a problem or incipient failure by interpretation of the data.
• The reliability managers should first develop and then strictly follow the schedule of testing oil to ensure accurate condition monitoring.
Sampling of oil: It is very important to have a true sample to get accurate results. ASTM or IEC guided syringe sampling from transformer oil analysis is recommended.

Transformer oil testing: A proven capital loss prevention technique

Transformer oil testing is a proven capital loss prevention technique which should be a part of any condition-based predictive maintenance program, advocates Anil Kadam, GM – Business Development, Solution Architect, Schneider Electric. He said, “This early warning system can allow maintenance management to identify maintenance priorities, plan work assignment schedules, arrange for outside service, and order necessary parts and materials. The whole process can turn predictive.”

He adds, “The transformer’s fluid not only serves as a heat transfer medium, it also is part of the transformer’s insulation system. It is therefore prudent to periodically perform tests on the oil to determine whether it is capable of fulfilling its role as an insulant.”
Some of the most common tests for transformer oil are: Dissolved gas in oil analysis, screen tests, water content, metals-in-oil, and polychlorinated biphenyl. The various test methods are explained here:

Test methods

American Society of Testing and materials (ASTM), Indian Standard (IS), British Standard (BS) and International Electro technical Committee (IEC) are widely accepted standards worldwide. In India, major test methods are adopted as per IS-335 and IS-1866 for acceptance of new oil as per agreement with seller and purchaser and maintenance and supervision of electrical equipment in service. Sampling of liquid oil for carrying out the various test is also very important and Bureau of Indian Standard (BIS) recommends and publishes standard for sampling (IS-6855). The sampling of transformer oil for testing from container or equipment should be as per this standard to get proper and correct results, recommends Jain.

Testing a new transformer oil

For new transformer oil, certain tests are to be carried out such as appearance, flash point, pour point, viscosity, breakdown voltage, acidity, neutralisation value, power factor and water contents etc. which is called routine tests. Other tests are long duration test such as corrosive sulphur, oxidation and ageing tests which are helpful to determine the oil life in equipment. As per Jain, all these tests should be kept within the acceptable limits as agreed upon under various standards.

Tests on transformer oil in service

In service transformers, oils are subjected to normal deterioration due to the equipment conditions. The oil darkens in colour and its acidity begins to increase. All these changes may have effect on both solid and liquid insulating material. There are different best ways for carry out the tests on transformer oil in service. “Field test-cum-onsite testing are usually limited to visual inspection such as colour, appearance, breakdown voltage and neutralisation value. The complete examination is only possible to fix the appropriate action to ensure the reliable operation of equipment,” informs Jain. He further points out, “The laboratory tests merely seek to establish the quality of transformer oil for long life of equipment and tests include are breakdown voltage, flash point, power factor, resistivity, water contents and presence of sediments and sludge.”


As testing of oil has become a critical component in order to achieve optimum efficiency and reliability of any transformer, there should be a system in place to ensure that the testing is timely carried out and records are maintained. Also, as no “one-size-fits-all”, the end-user must select the most suitable testing methods considering affordability, cascading impacts of downtime in case of failure, and volume of samples to be tested.

If you want to share thoughts or feedback then please leave a comment below.


  1. Dear sir,
    After collecting oil samples from transformer for testing. How many days we have to kept for controlled environment condition without testing. Plz answer us.

    From Suresh. S. P

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