Natural ester fluids beat mineral oil for use in renewable energy applications

In the history of fluid-immersed transformers, mineral oil has long been used due to its wide availability, closely linked to the extensive development of the petroleum industry. Today, alternative natural ester fluids like FR3^TM fluid – manufactured by Cargill – are well known and largely chosen for use in the renewables sector. Compared with mineral oil, FR3™ fluid offers higher thermal class capability, better preservation of the insulation system, and high overload tolerance.

The advantages of FR3^TM fluid potentially allow higher permissible temperature rises in both oil and windings. When combined with thermally upgraded (TU) paper, it is possible to achieve a thermal class of 140 C in accordance with both IEC and IEEE product standards. This requirement can be effectively met by using FR3 ^TM fluid, while a mineral oil alternative would be either unfeasible or not cost-effective.

For example, if a customer is evaluating achievable power density using mineral oil or natural ester, they may set a goal to remain within the following dimensions: L = 1950 mm , W = 1300 mm , H = 1950 mm. Using mineral oil, the maximum feasible power rating is 3100 kVA. By switching to natural ester, it is possible to achieve 4000 kVA within the same dimensions. This means nearly 30% higher power density. The reduction in dimensions can substantially reduce CAPEX – including lower installation, civil works, and fire protection costs – while also potentially reducing OPEX through lower maintenance requirements.

The lower weight also simplifies shipping and handling operations, reducing delivery complexity, transportation effort, and associated logistics costs.

The same approach also allows the feasibility of much larger transformers with improved environmental performance. While a sealed-type integrally filled mineral oil-immersed unit can typically reach a rated power of 6-8 MVA (depending on the design characteristics), an ester-filled unit can reach up to 12 MVA or more. FR3™ fluid is 100% biodegradable in as little as 10 days according to OPPTS 835.3100. Additionally, it is readily biodegradable in accordance with OECD 301B. In contrast, mineral oil is not biodegradable and typically requires oil-containment systems to manage potential leaks or spills.

For Westrafo, which is strongly active in the renewable energy sector, FR3 ^TM fluid is the most widely used insulating fluid, with consumption four times greater than mineral oil. Westrafo’s primary customers are involved in photovoltaic (PV) plants, battery energy storage systems (BESS), and wind power installations. In many cases, these customers request transformers with a reduced footprint in order to increase power density and reduce the total CAPEX of the installation. Westrafo says FR3™ fluid’s advantages clearly demonstrate its superiority over conventional mineral oil solutions.