Product Code Database
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 wet transformers,Frank D. Petruzella, Industrial Electronics, p. 51, Glencoe/McGraw-Hill, 1996 . some types of high-voltage Capacitor, Fluorescence lamp ballasts, and some types of high-voltage switches and circuit breakers. It functions to insulate, suppress corona discharge and arcing, and serves as a coolant.
Most often, transformer oil is based on mineral oil, but alternative formulations - with different engineering or environmental properties - are growing in popularity.
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 discharge formation and subsequent electrical breakdown under load.
Oil filled transformers with a conservator oil reservoir may have a gas detector relay like a 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.
Pentaerythritol tetra fatty acid synthetic and natural Ester 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 are over . They are biodegradable, but are more expensive than mineral oil. Natural esters have lower oxidation stability in the 120C oxygen saturated test of approximately 48-hours compared to 500-hours for Mineral oils, and are therefore used in closed transformers.
Hermetic seals are important for larger transformers due to thermal expansion and contraction. Mid-size and large power transformers will typically have a conservator and employ a rubber bag with the use of natural ester to reduce oxygen ingress and prevent the natural ester from experiencing a faster oxidation than utilities are accustomed to with mineral oils. Silicone oil or fluorocarbon-based oils, which are even less flammable, are also used, but they are more expensive than esters.
There are over 3 million transformers in service with vegetable-based formulations, using soy or rapeseed based formulations in up to 500 kV transformers so far. However, coconut oil-based formulations are unsuitable for use in cold climates or for voltages over 230 kV. Researchers are also investigating for transformer use; these would be used as additives to improve the stability and thermal and electrical properties of the oil. (2025). 9781424494682, Conference on Electrical Insulation and Dielectric Phenomena.. ISBN 9781424494682
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 US, production of PCBs was banned in 1979 under the Toxic Substances Control Act. In many countries significant programs are in place to reclaim and safely destroy PCB contaminated equipment. 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.
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.California Code of Regulations, Title 22, section 66261
The details of conducting these tests are available in standards released by International Electrotechnical Commission, ASTM International, International standard, British Standards, 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:
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:
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