## 1 Introduction  
The operational reliability of a transformer depends on effective electrical insulation and cooling, as direct coil-to-coil contact can lead to sparking, resulting in thermal and electrical breakdown. According to the literature, dielectric insulation failures account for approximately 75% of high-voltage transformer failures [1]. These unforeseen and premature failures, primarily caused by insulation breakdown, impose significant financial burdens due to costly repairs or replacements, revenue losses, and compromised power delivery efficiency [2]. The insulation system in power transformers consists of passive components, which are broadly classified into solid, liquid, and gaseous insulators [3]. The selection of these materials depends on their specific dielectric properties and the intended application within the transformer. Solid insulators include ceramics, epoxy resin, pressboard, Kraft paper, and thermally upgraded Kraft paper. Liquid insulators comprise mineral oil, synthetic esters, silicone oils, and vegetable oils. Gaseous insulators include compressed air and SF\({}_{6}\) gas. Power transformer insulation majorly depends on two fundamental insulating materials, which are solid insulators like Kraft papers and liquid insulators like mineral oils. The primary source of the Kraft paper insulators is majorly from wood pulps, which contain 40-55% cellulose, 15-35% lignin, and 25-40% hemicellulose [4]. This Kraft paper demonstrates exceptional electrical insulation properties, offering mechanical support and insulation between various conductive parts to prevent electrical discharges. Kraft paper also exhibits good thermal stability, which is vital for maintaining performance at high temperatures [5]. Transformer oils, typically composed of mineral oil or synthetic ester, serve multiple functions as a liquid dielectric, providing electrical insulation between energized components to prevent arcing and short circuits [3]. It facilitates cooling by transferring heat away from the windings and core, ensuring optimal operating temperatures. Additionally, the oil serves as a barrier against moisture and air, protecting the insulating materials from degradation and extending the transformer's lifespan [6]. Mineral oil insulating liquid, derived from refined crude oil hydrocarbons, has been utilized in the electrical industry for over 130 years. It possesses excellent dielectric and cooling properties, along with remarkable oxidation stability, allowing it to function effectively in both hermetically sealed and free-breathing transformers. Its low pour point is an added advantage, enabling optimal performance in subpolar regions. However, mineral oil has a low ignition point, poor heat resistance, and is non-renewable, posing environmental risks in the event of spills [7]. These drawbacks conflict with the United Nations Sustainable Development Goals (SDGs) 7 and 13, which promote affordable and clean energy as well as climate action. As a result, researchers have shifted their focus to green alternatives, such as vegetable-based insulating liquids [8; 9; 10; 11; 12; 13]. Vegetable-based insulating liquids, commonly known as natural esters, emerged in the late 1990s as an alternative to conventional insulating liquids. Their application has seen significant success, with commercially available examples including FR3, derived from soybean oil, and Midel 1204, based on rapeseed oil [7; 14; 15]. Natural esters exhibit excellent insulating properties while having minimal environmental impact. They offer strong fire resistance, high breakdown voltages, and improved electric field distribution [16]. Additionally, their high moisture tolerance helps delay the aging of cellulose paper insulation, enhancing the overall longevity of transformer systems [17]. Despite the remarkable properties of natural esters, their widespread adoption remains a challenge due to several factors.