What is the Best Transformer Coolant

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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.

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?
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

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
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 (http://www.epa.gov). 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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