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Transformer Oil: Stick With Mineral Or Switch to Synthetics?

Transformer Oil: Stick With Mineral Or Switch to Synthetics?

Mineral oil has long been the go-to choice for electric transformer oil since the 19th century when renowned engineer Elihu Thomson patented its use in transformers to disperse heat away from the machine’s core, prolonging its service life. While mineral oil still works well to this day, new alternative fluids are increasingly popular as they offer improved advantages in prolonging transformer life, cost reductions, and lower environmental impact.

Below, we compare these two options and see which is better for your equipment.

The rise of synthetic and natural transformer oil alternatives

Over the years, transformer technology has seen little changes, and the shift towards newer fluids is among the more notable improvements in recent times, hence why there is growing attention as to how these new ester fluids perform in real-life operations.

There are a couple of driving factors behind the increasing popularity of synthetic and natural esters today, such as Cargill’s FR3 dielectric fluid. The first is their high flash point and fire point, which are often double what mineral oil-filled transformers can withstand and are thus excellent fluids for outdoor and indoor transformer installations.

The second reason is that these oils are inherently biodegradable. Biodegradability is a must-have feature for fluids used in sensitive areas close to water, like dams and other installations, since it eliminates the need to invest in containment structures to hold 110% of nameplate capacity. Furthermore, any failures or leaks from transformer units will have a significantly lower environmental impact than traditional transformer fluids.

Key advantages of natural esters

Improved longevity

Reliability is paramount to every organisation that relies on electrical equipment for its operations. Thankfully, synthetic and natural esters are proven to be incredibly stable fluids with higher cooling capacities than hydrocarbon dielectric fluids and high-molecular-weight silicone fluids. Moreover, transformers with thermally upgraded insulation paper using FR3 last approximately eight times longer than their mineral-filled counterparts. That said, maximising the service life of natural and synthetic ester fluid transformers requires that the transformer units used must maintain low oxygen and moisture levels, similar to mineral oil-filled transformers.

Better moisture retention

Water is the greatest threat to a transformer’s solid insulation. Since synthetic and natural esters can keep less moisture in the paper insulation but hold more of it in the fluid, they work well to keep the transformer’s most critical part dry.

Cargill’s FR3 holds around 1000 parts per million (ppm) at room temperature, while most mineral oils average at around 50 ppm. In transformers with high moisture content, using esters will generate long-chain fatty acids that react and form new ester links over the solid insulation, serving as a protective layer that safeguards the component from future decay.

Is it possible to retrofill mineral oil filled transformers to take natural ester fluid?

Based on extensive lab and field testing, transformers that use mineral oil and even R-temp or high-temperature hydrocarbon fluids can be retrofitted with esters. Doing so brings many benefits, including decelerating the transformer’s ageing process, improving short-term overloading capabilities, and lowering environmental risk.

However, should you wish to top off your transformer with a different fluid, it is vital to ensure compatibility between the fluids first. This step is important since not all transformer fluids are compatible. A good example is silicone, which is not miscible with other fluids. When silicone is used to top off a mineral oil-filled transformer, the resulting fluids will not mix well and may look similar to a lava lamp. When in doubt, always seek a professional for help. Just as you would only trust certified professionals to conduct electric motor rewinds and other services on your critical equipment, it only makes sense to get an expert’s opinion when retrofilling your transformers.


The shift towards new fluids is one of the biggest changes in the transformer industry in recent years, and many organisations are extremely interested in learning more about these synthetic and natural esters and how they can use them in their operations.

As the industry continues to evolve, it is crucial for organisations to stay informed about these changes and determine transformer capability in relation to these innovative fluids. But for now, mineral oil will most likely retain its popularity since it is an established fluid more readily available from many manufacturers. Nonetheless, though the change to the new fluid alternatives may take some time, the market for it is coming on strong. Many organisations that do not have a transformer that uses them may begin to adopt them sooner rather than later.