A Review of Eco-Friendly Insulating Fluids in Transformers: Mineral Oil, Natural Esters, and Synthetic Esters

Insulating liquids are easy to overlook in transformer design, but they influence almost everything that matters in service: electrical reliability, heat removal, fire safety, maintenance practice, and insulation life. For many decades, mineral oil has remained the standard liquid in power and distribution transformers because it is familiar, relatively economical, and supported by a large amount of design and field experience. However, the expectations placed on transformers have changed. Today, engineers and utilities are asked to think not only about dielectric performance and cooling, but also about environmental risk, fire behavior, urban installation constraints, sustainability targets, and long-term asset management. This shift has pushed natural esters and synthetic esters from being secondary alternatives to being serious engineering options. This review paper examines eco-friendly insulating fluids used in transformers, with emphasis on natural and synthetic esters as alternatives to mineral oil. The discussion is organized around the questions that matter most in practice. What role does the liquid actually play inside the transformer? How do mineral oil, natural esters, and synthetic esters differ in dielectric behavior, thermal performance, viscosity, oxidation stability, moisture tolerance, fire safety, and environmental impact? How do these fluids influence paper ageing, maintenance practice, diagnostics, and retrofilling decisions? Rather than treating one liquid as universally superior, the paper shows that each fluid family offers a different balance of advantages and limitations. Mineral oil remains strong in cost and conventional cooling design. Natural esters stand out in biodegradability, fire safety, and moisture behavior. Synthetic esters offer high fire safety together with broader practical robustness in some applications. The paper concludes by summarizing current standards, highlighting the most important technical gaps, and identifying the directions that are likely to shape the next generation of safer and more sustainable transformer insulation systems.