Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona discharge and arcing, and to serve as a coolant.

Transformer oil is most often based on mineral oil, but alternative formulations with different engineering or environmental properties are growing in popularity.

Function and properties

Oil filled transformer with air convection cooled heat exchangers in the front and at the side
Oil filled transformer with air convection cooled heat exchangers in the front and at the side

Transformer oil's primary functions are to insulate and cool a transformer. It must therefore have high dielectric strength, thermal conductivity, and chemical stability, and must keep these properties when held at high temperatures for extended periods.[1] Typical specifications are: flash point 140 °C or greater, pour point −40 °C or lower, dielectric breakdown voltage 28 kV (RMS) or greater.[2]

To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Power transformers with capacities of thousands of kVA may also have cooling fans, oil pumps, and even oil-to-water heat exchangers.[3]

Power transformers undergo prolonged drying processes, using electrical self-heating, the application of a vacuum, or both to ensure that the transformer is completely free of water vapor before the insulating oil is introduced. This helps prevent corona formation and subsequent electrical breakdown under load.[citation needed]

Oil filled transformers with a conservator (oil reservoir) may have a gas detector relay (Buchholz relay). These safety devices detect the buildup of gas inside the transformer due to corona discharge, overheating, or an internal electric arc. On a slow accumulation of gas, or rapid pressure rise, these devices can trip a protective circuit breaker to remove power from the transformer. Transformers without conservators are usually equipped with sudden pressure relays, which perform a similar function as the Buchholz relay.[citation needed]

Mineral oil alternatives

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Mineral oils are still widely used in the industry. Mineral oil is generally effective as a transformer oil, but it has some disadvantages, one of which is its relatively low flashpoint versus some alternatives. If a transformer leaks mineral oil, it can potentially start a fire. Fire codes often require that transformers inside buildings use a less flammable liquid, or use dry-type transformers with no liquids at all. Mineral oil is also an environmental contaminant, and its insulating properties are rapidly degraded by even small amounts of water. Transformers are well equipped to keep water outside the oil for this reason.

Pentaerythritol tetra fatty acid natural and synthetic esters have emerged as an increasingly common mineral oil alternative, especially in high-fire-risk applications such as indoors due to their high fire point, which can be over 300 °C.[4] They are readily biodegradable. Pentaerythritol tetra fatty acid natural and synthetic esters are more expensive than mineral oil. Transformers require special design change to operate with Pentaerythritol tetra fatty acid natural and synthetic esters. Natural esters have very poor oxidation stability (typically only 48 hours in the same test vs 500h for Mineral oils and they produce acids) as a consequence, natural esters are only really a viable solution in hermetically sealed transformers in a distribution context. As transformers get larger than around 1 MVA and above 33kV it becomes more challenging to achieve a hermetically sealed design (due to thermal expansion and contraction). Mid-size and large power transformers will typically have a conservator and even if a rubber bag is employed the use of natural ester should be carefully considered because if there is oxygen ingress the natural ester will experience much faster oxidation than utilities are accustomed to with mineral oils.

Silicone or fluorocarbon-based oils, which are even less flammable, are also used, but they are more expensive than esters, and less biodegradable.

A 380 kV transformer with vegetable oil [5]
A 380 kV transformer with vegetable oil [5]

Researchers are experimenting with vegetable-based formulations, using coconut oil for instance. As yet these are unsuitable for use in cold climates or for voltages over 230 kV.[6]

Researchers are also investigating nanofluids for transformer use; these would be used as additives to improve the stability and thermal and electrical properties of the oil.[7]

In 2019 a new bio-based transformer liquid was introduced, with lower viscosity.[citation needed] This makes it a better coolant, which could reduce the transformer’s winding hot spot temperature and improve over-loading capability.

Polychlorinated biphenyls (PCBs)

Polychlorinated biphenyls are a man-made substance first synthesized over a century ago and found to have desirable properties that led to their widespread use.[8] 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.[citation needed]

Beginning in the 1970s, production and new uses of PCBs were banned in many countries, due to concerns about the accumulation of PCBs and toxicity of their byproducts. For instance, in the USA, production of PCBs was banned in 1979 under the Toxic Substances Control Act.[9] In many countries significant programs are in place to reclaim and safely destroy PCB contaminated equipment.[citation needed]

One method that can be used to reclaim PCB contaminated transformer oil is the application of a PCB removal system, also called a PCB dechlorination system. PCB removal systems use an alkali dispersion to strip the chlorine atoms from the other molecules in a chemical reaction. This forms PCB-free transformer oil and a PCB-free sludge. The two can then be separated via a centrifuge. The sludge can be disposed as regular non-PCB industrial waste. The treated transformer oil is fully restored, meeting the required standards, without any detectable PCB content. It can, thus, be used as the insulating fluid in transformers again.[10]

PCBs and mineral oil are miscible in all proportions, and sometimes the same equipment (drums, pumps, hoses, and so on) was used for either type of liquid, so PCB contamination of transformer oil continues to be a concern. For instance, under present regulations, concentrations of PCBs exceeding 5 parts per million can cause an oil to be classified as hazardous waste in California.[11]

Testing and oil quality

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Transformer oils are subject to electrical and mechanical stresses while a transformer is in operation. In addition there is contamination caused by chemical interactions with windings and other solid insulation, catalyzed by high operating temperature. The original chemical properties of transformer oil change gradually, rendering it ineffective for its intended purpose after many years.[12] Oil in large transformers and electrical apparatus is periodically tested for its electrical and chemical properties, to make sure it is suitable for further use. Sometimes oil condition can be improved by filtration and treatment. Tests can be divided into:

  1. Dissolved gas analysis
  2. Furan analysis
  3. PCB analysis
  4. General electrical & physical tests:
    • Color & Appearance
    • Breakdown Voltage [13]
    • Water Content
    • Acidity (Neutralization Value)
    • Dielectric Dissipation Factor
    • Resistivity
    • Sediments & Sludge
    • Flash Point
    • Pour Point
    • Density
    • Kinematic Viscosity

The details of conducting these tests are available in standards released by IEC, ASTM, IS, BS, and testing can be done by any of the methods. The Furan and DGA tests are specifically not for determining the quality of transformer oil, but for determining any abnormalities in the internal windings of the transformer or the paper insulation of the transformer, which cannot be otherwise detected without a complete overhaul of the transformer. Suggested intervals for these test are:

On-site testing

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Main article: Transformer oil testing

Some transformer oil tests can be carried out in the field, using portable test apparatus. Other tests, such as dissolved gas, normally require a sample to be sent to a laboratory. Electronic on-line dissolved gas detectors can be connected to important or distressed transformers to continually monitor gas generation trends.

To determine the insulating property of the dielectric oil, an oil sample is taken from the device under test, and its breakdown voltage is measured on-site according to the following test sequence:

See also


  1. ^ Gill, Paul (2009). Electrical power equipment maintenance and testing (2nd ed.). Boca Raton: CRC Press. p. 193. ISBN 978-1-57444-656-2.
  2. ^ Hirschler, Marcelo M. (2000). Electrical insulating materials : international issues (Online ed.). West Conshohocken, Pa.: ASTM. pp. 82–95. ISBN 978-0-8031-2613-8.
  3. ^ Kenneth R. Edwards, Transformers, American Technical Publishers Ltd., 1996 ISBN 0-8269-1603-1 pp.138-14
  4. ^ "Fluids Comparison". Midel.
  5. ^ "Siemens has produced the world's first large-scale transformer that uses vegetable oil".
  6. ^ "Coconut Oil As An Alternative To Transformer Oil" (PDF). ERU Symposium. November 2001.
  7. ^ "Properties Of Transformer Oil | HERING VPT". Retrieved 2020-12-09.
  8. ^ "PCB-RS Series – PCB Removal System | HERING VPT: The standard in oil purification and transformer drying technology". Retrieved 2020-05-20.
  9. ^ Blackmore, Carolyn. "Classification and Handling of PCB Waste" (PDF). Lawrence Berkeley National Laboratory. Retrieved 20 October 2017.
  10. ^ "PCB Dechlorination System". Hering-VPT GmbH. Retrieved 20 October 2017.
  11. ^ California Code of Regulations, Title 22, section 66261
  12. ^ "Transformer Oil Detoriation - Why is transformer oil purification essential?".
  13. ^ "What is BDV Test of Transformer Oil?".
  • Less and nonflammable liquid-insulated transformers, approval standard class Number 3990, Factory Mutual Research Corporation, 1997.
  • McShane C.P. (2001) Relative properties of the new combustion-resistant vegetable oil-based dielectric coolants for distribution and power transformers. IEEE Trans. on Industry Applications, Vol.37, No.4, July/August 2001, pp. 1132–1139, No. 0093-9994/01, 2001 IEEE.
  • "The Environmental technology verification program", U.S. Environmental Protection Agency, Washington, DC, VS-R-02-02, June 2002. [1]
  • IEEE Guide for loading mineral-oil-immersed transformers, IEEE Standard C57.91-1995, 1996.