Dielectric Ester Transformer Fluids: An Alternative to Mineral Oil

Published under Technical Articles

The liquid used in transformers serves two main purposes. First is to act as an insulation between the energized internals of the transformer and the tank, as well as the separation of conductors. The second is to transfer heat from the core of the transformer to external radiators and/or the tank depending on the design. In each of these cases, the liquid that is used must have specific dielectric and thermal properties to be able to accomplish these tasks.

There have been many dielectric fluids used throughout the last half century. Some have been made obsolete because of new alternatives, and others restricted due to health risks. The two most commonly used dielectric fluids in modern transformers and electrical equipment are mineral oil and natural ester-based coolants. Mineral oil comes standard in most transformers. It is an inexpensive solution and has great dielectric and thermal properties. Although mineral oil meets the requirements of most applications, it has not stopped engineers from developing new alternatives.

Dielectric esters have become increasingly popular due to several special properties. We will discuss these in later articles. Mineral oil is a distillate of the petroleum refining process. Dielectric fluids are derived from seed oils. Currently, the most common dielectric ester coolant on the market is FR3® produced by Cargill; This soy-based coolant is marketed mainly for its small environmental footprint. It and other ester fluids inherently come from a more natural and renewable source than mineral oil, making them more prominent in environmentally sensitive areas.

As previously mentioned, the main requirements for transformer fluids are to have acceptable dielectric and thermal properties. The dielectric constant of FR3 is 3.1. That is only slightly higher than mineral oil’s rating of 2.2. These are both on the low side when compared to other insulators. As for the dielectric strength, FR3 and mineral oil have ratings of 45 kV and 35 kV respectively. FR3 will tend to resist dielectric breakdown at higher voltages than mineral oil. Both would be considered to have acceptable dielectric properties for applications in transformers. The thermal properties of FR3 are very close to mineral oil. It has a slightly lower thermal expansion and has a specific heat of 90% of mineral oil. Dielectric ester fluids pass these two main tests to be used as a transformer fluid.

Along with their basic electrical and thermal properties, dielectric ester transformer fluids have other benefits that make them a great alternative to mineral oil. If you are interested in learning more or want to find a transformer that contains these fluids, don’t hesitate to contact us. Keep us in mind and we can help you source your next transformer.


Quinn Alleman

Utility Transformer Brokers


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