In an oil-immersed transformer, the liquid insulates and cools. The choice between mineral oil and ester comes down to fire safety, the environment and cost. Here is how to settle it.
In an oil-immersed transformer, the windings sit in a liquid with two jobs: electrically insulating the active parts, and carrying the heat out to the radiators. The nature of that fluid directly shapes safety, maintenance and how the unit behaves.
Mineral oil is the most widespread fluid: proven performance, wide availability and the lowest cost. Its limits are a relatively low fire point (class "O") and the fact that it is not biodegradable, which raises questions indoors, on sensitive sites or near a watercourse.
In return, esters, whether natural (plant-based, FR3 type) or synthetic (MIDEL type), cost more than mineral oil and are more viscous, which the design accounts for on the cooling side.
Rather mineral oil: standard outdoor installation, tight budget, no particular fire or environmental stake.
Rather ester: transformer indoors, a data center or a building open to the public, proximity to water or a protected area, reinforced fire-safety requirements. In many cases, ester is the alternative to a dry-type unit when you want to keep the advantages of oil-immersed.
For indoor fire safety, the other route is the cast resin dry-type, with no liquid at all. The choice between "ester-filled oil-immersed" and "dry-type" follows the power, the environment and the budget; the trade-off is laid out in the data center guide, and we help you settle it for your case.
Classifications and orders of magnitude are indicative; the final choice depends on your installation and the regulations applicable to the site.
Its fire point is far higher than mineral oil's (K-class fluids), which sharply reduces the risk of ignition. That is one of the main reasons to choose it indoors or on sensitive sites.
Because it costs more. For a standard outdoor installation with no particular fire or environmental stake, mineral oil remains the most economical and perfectly suitable choice.
They are brands of ester fluids: FR3 typically designates a natural (plant-based) ester, MIDEL a family of natural and synthetic esters. Both offer a high fire point and good biodegradability; the exact choice follows the project's requirements.
Both limit the fire risk. Dry-type removes any liquid; ester keeps the advantages of an oil-immersed unit (high power, cooling) with a high fire point. The choice follows the power, the layout and the budget.