Which liquid is used in a transformer

What is the best transformer coolant?

Reasons of warming

One of the main sources of losses and causes of temperature rise in different parts of a transformer is the magnetic circuit and windings. What are the real reasons for the transformer heating up?

Siemens Energy has built a power transformer with an insulating liquid based on vegetable oil for the German energy supply company EnBW. EnBW is using the transformer with the alternative insulating liquid at the Teinach substation near Bad Teinach-Zavelstein in the Black Forest in order to examine and document its operating behavior there under real operating conditions. The transformer with an output of 40 MVA (107/21 kV) was manufactured in the Dresden transformer factory.

Core losses, copper losses in windings (I2R loss), leakage losses in windings and leakage losses due to leakage / high are responsible for the heat generation in the transformer - that is the answer.

Every transformer uses coolant to prevent overheating. I will try to name only the most important ones with the following description.

mineral oil

Mineral oil, which surrounds a transformer-core-coil arrangement, improves the dielectric strength of the winding and prevents oxidation of the core.

Dielectric improvement occurs because oil has a greater electrical resistance than air and because the dielectric constant of oil (2.2) is closer to the insulation. As a result, when oil replaces air in a dielectric system, insulation stress is reduced. While in contact with the conductors, oil also absorbs heat and conducts the heat to the tank surface by self-convection.

Such a transformer dipped in oil may have smaller electrical clearances and smaller conductors for the same voltage and kVA values.

Mineral oils specifically used for power generation distribution applications were in commercial production as early as 1899. Later, halogenated dielectric fluids - mainly Askarel fluids, which are known for their excellent fire protection properties - became the preferred choice for indoor transformers.


From about 1932 onwards, a class of liquids called askarels, or polychlorinated biphenyls (PCBs), began to be used as a substitute for mineral oil, where flammability was a major problem.

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

Although these coolants have been considered non-flammable as used in electrical equipment, they could decompose if exposed to arcing or fire to form hydrochloric acid and toxic furans and dioxins. The compounds have also been undesirable because of their persistence in the environment and their ability to aggregate in higher animals, including humans.

Tests by the US The Environmental Protection Agency has shown that PCBs can cause cancer and other health effects in animals. Human studies provide supportive evidence of possible carcinogenic and non-carcinogenic effects of PCBs (http://www.epa.gov). The use of askarels in new transformers was banned in 1977 (Claiborne, 1999).

Work Still Retires and Properly Dispose of transformers that contain Askarels or Askarel-contaminated mineral oil. Current ANSI / IEEE standards require transformer manufacturers to state on the nameplate that new devices have left the factory with less than 2 ppm PCB in the oil (IEEE, 2000).

High temperature hydrocarbons

Among the coolants that formerly took the place ofAskarel's in distribution transformers are high temperature hydrocarbons (HTHC), also called high molecular weight hydrocarbons. These refrigerants are classified as "less flammable" by the National Electric Code if they have a fire point above 300 ° C.

The disadvantages of HTHCs include increased cost and decreased cooling capacity due to the higher viscosity that accompanies the higher molecular weight.


Another coolant that hits them National Electric Code (NEC) Requirements for a less flammable liquid is a silicone, chemically known as polydimethylsiloxane. Silicones are only used occasionally because they have biological persistence when spilled and are more expensive than mineral oils or HTHCs.

Halogenated liquids

Mixtures of tetrachloroethane and mineral oil have been tried as an oil substitute for a few years. These and other chlorine-based compounds are no longer used because they are non-biodegradable, create toxic by-products, and have potential effects on the earth's ozone layer.


Synthetic esters are used in Europe, where high temperature capability and biodegradability are most important and their high cost can be justified for example in train transformers.

Transformer manufacturers in the US are now investigating the use of natural esters made from vegetable seed oils. It is possible that agricultural esters offer the best combination of high temperature 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 are currently considered the most popular choice in applications that require less flammable liquids. To a much lesser extent, synthetic ester-based fluids and synthetic hydrocarbons are also used. Dielectric synthetic ester fluids have suitable dielectric properties and biodegrade much faster than mineral oil and hydrocarbon fluids. Because of their high cost compared to other, less flammable liquids, synthetic liquids are generally limited to use in traction and mobile transformers and other specialty applications.

Having a biodegradable fluid is of significant savings potential for utility companies as it should simplify cleanup and remediation plans and procedures. However, the real savings come when a transformer starts to leak or when an overflow occurs. This is especially true for utility companies in environmentally sensitive areas who fear that marine life will be endangered by leaks or leaks from transformers near the water.

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