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Home / Technical Articles / What is the best transformer coolant?

Reasons of heating

One of the main sources of losses and reasons for temperature rise in various parts of a transformer are the magnetic circuit and windings. So what are the actually reasons of heating the transformer?

What is the best transformer coolant?
Siemens Energy has built a power transformer with insulating liquid based on plant oil for German power supply company EnBW. EnBW is deploying the transformer with the alternative insulating liquid at the Teinach substation near Bad Teinach-Zavelstein in the Black Forest in order to investigate and document its operating behavior there under actual service conditions. The transformer which has a power rating of 40 MVA (107/21 kV) was manufactured in the Dresden transformer factory.

Responsible for heat generation within the transformer are core loss, copper loss in windings (I2R loss), stray loss in windings and stray loss due to leakage/high – that’s the answer.

To avoid overheating, every transformer is using some coolant. I’ll try to name only the main ones with following description.

Mineral Oil

Mineral oil surrounding a transformer core-coil assembly enhances the dielectric strength of the winding and prevents oxidation of the core.

Dielectric improvement occurs because oil has a greater electrical withstand than air and because the dielectric constant of oil (2.2) is closer to that of the insulation. As a result, the stress on the insulation is lessened when oil replaces air in a dielectric system. Oil also picks up heat while it is in contact with the conductors and carries the heat out to the tank surface by selfconvection.

Thus a transformer immersed in oil can have smaller electrical clearances and smaller conductors for the same voltage and kVA ratings.

Mineral oils used specifically for power distribution applications were in commercial production early as 1899. Later, halogenated dielectric fluids-principally askarel fluids noted for their excellent fire safety properties-became the fluid of choice for indoor transformers.


Beginning about 1932, a class of liquids called askarels or polychlorinated biphenyls (PCB) was used as a substitute for mineral oil where flammability was a major concern.

Askarel-filled transformers could be placed inside or next to a building where only dry types were used previously.

Although these coolants were considered nonflammable, as used in electrical equipment they could decompose when exposed to electric arcs or fires to form hydrochloric acid and toxic furans and dioxins. The compounds were further undesirable because of their persistence in the environment and their ability to accumulate in higher animals, including humans.

Testing by the U.S. Environmental Protection Agency has shown that PCBs can cause cancer in animals and cause other noncancer health effects. Studies in humans provide supportive evidence for potential carcinogenic and noncarcinogenic effects of PCBs ( The use of askarels in new transformers was outlawed in 1977 (Claiborne, 1999).

Work still continues to retire and properly dispose of transformers containing askarels or askarel-contaminated mineral oil. Current ANSI/IEEE standards require transformer manufacturers to state on the nameplate that new equipment left the factory with less than 2 ppm PCBs in the oil (IEEE, 2000).

High-Temperature Hydrocarbons

Among the coolants used to take the place of askarels in distribution transformers are high-temperature hydrocarbons (HTHC), also called high-molecular-weight hydrocarbons. These coolants are classified by the National Electric Code as “less flammable” if they have a fire point above 300˚C.

The disadvantages of HTHCs include increased cost and a diminished cooling capacity from the higher viscosity that accompanies the higher molecular weight.


Another coolant that meets the National Electric Code (NEC) requirements for a less-flammable liquid is a silicone, chemically known as polydimethylsiloxane. Silicones are only occasionally used because they exhibit biological persistence if spilled and are more expensive than mineral oil or HTHCs.

Halogenated Fluids

Mixtures of tetrachloroethane and mineral oil were tried as an oil substitute for a few years. This and other chlorine-based compounds are no longer used because of a lack of biodegradability, the tendency to produce toxic by-products, and possible effects on the Earth’s ozone layer.


Synthetic esters are being used in Europe, where high-temperature capability and biodegradability are most important and their high cost can be justified, for example, in traction (railroad) transformers.

Transformer manufacturers in the U.S. are now investigating the use of natural esters obtained from vegetable seed oils. It is possible that agricultural esters will provide the best combination of hightemperature properties, stability, biodegradability, and cost as an alternative to mineral oil in distribution transformers (Oommen and Claiborne, 1996).

Silicone oils and high-molecular weight hydrocarbons currently rank as the most popular choices in applications requiring less flammable fluid. To a much lesser extent, synthetic ester-based fluids and synthetic hydrocarbons are also used. Synthetic ester dielectric fluids have suitable dieletric properties and biodegrade much quicker than mineral oil and hydrocarbon fluids. Due to their high cost compared to other less flammable fluids, synthetic fluids are generally limited to use in traction and mobile transformers, and other specialty applications.

A biodegradable fluid represents significant potential savings for utilities because it should simplify cleanup and remediation plans and procedures. However, the real savings are realized when a transformer starts to leak or when there is a spill. This is particularly true for utilities in environmentally sensitive areas that have to worry about threats to marine life from spills or leaks form transformers located near the water.

Resource: Electric power transformer engineering by Dudley L. Galloway and Dan Mulkey

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Jeremy Capouya
    May 08, 2016


    Could one use a SN150 base oil as a transformer oil ?

    Thanks you for your confirmation.


  2. Victor A. Rincon
    Jun 20, 2014

    Hello Edvard,

    Do you know if byproducts from linear alkylbenzene production have been considered as transformer coolant? (Byproducts usually are a mix of DPA (diphenylalkane) and DAB (dialkylbenzene).

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