Natural ester insulating liquids for power transformers in subpolar regions: Low-temperature challenges and mitigation strategies

In the context of the modern industrial era, sustainability and the adoption of environmentally friendly materials have become central to reducing pollution and supporting a cleaner energy transition, in line with Sustainable Development Goal 7. Since the early 1900s, mineral insulating oils have been widely adopted in the electrical industry due to their efficient insulation and cooling attributes. However, growing concerns regarding land, water, and air pollution associated with hydrocarbon-based materials have intensified the search for alternative insulating liquids derived from renewable sources. Natural esters insulating liquids, primarily produced from vegetable seed oils, have emerged as a promising alternative owing to their biodegradability, low ecotoxicity, and superior fire safety characteristics. These properties make them particularly attractive for applications such as transformer installations in environmentally sensitive and densely populated areas. In addition, natural esters exhibit high moisture solubility, which can mitigate moisture-related insulation degradation, as well as high dielectric strength that enhances overall insulation performance. Despite these advantages, the fatty acid of natural esters leads to crystallization and significant viscosity increases at low temperatures, resulting in reduced fluidity and potential operational challenges in cold and subpolar regions. This paper presents a comprehensive review of the low-temperature behavior of natural ester insulating liquids, focusing on cold-flow limitations, crystallization mechanisms, and their impact on transformer operation. Mitigation strategies aimed at improving low-temperature performance, including fluid formulation, blending approaches, and operational measures, are critically discussed. Furthermore, the dielectric behavior of natural esters during cold start-up and under low-temperature operating conditions is discussed.