The service of mineral insulating oils for power transformer insulation and cooling aspects cannot be disavowed. However, the continued use of mineral oils is questionable due to environmental unfriendliness and the divestment from fossil fuels. This has provoked the quest for green alternative insulating liquids for high-voltage insulation. Natural esters are among the remaining alternatives that are renewable and environmentally friendly. Regardless of their environmental and technical merits, natural esters have some limitations that are slowing down their total acceptance by transformer owners and utilities. Critical limitations and concerns include esters’ pour point, viscosity, oxidative stability, and ionization resistance. In this work, the state of the art of “natural esters for transformers” is explored with the aim of potential improvements. The sections of the article are geared towards technical viewpoints on improving the overall workability and serviceability of natural esters in high-voltage applications. A comprehensive review of the existing literature is achieved, based on performance improvements of the natural ester using “additives” and “chemical modification”. The authors hope that this report may be helpful to transformer owners as well as influence the progression of natural esters for power transformer applications.

Insulation cellulose paper is a basic measure for a power transformer’s remaining useful life, and its advantageous low cost, electrical, and mechanical properties have made it an extensive insulation system when impregnated in a dielectric liquid. Cellulose paper deteriorates as a result of ageing due to some chemical reactions like pyrolysis (heat), hydrolysis (moisture), and oxidation (oxygen) that affects its degree of polymerization. The condition analysis of cellulose paper has been a major concern since the collection of paper samples from an operational power transformer is almost impossible. However, some chemicals generated during cellulose paper deterioration, which were dissolved in dielectric liquid, have been used alternatively for this purpose as they show a direct correlation with the paper’s degree of polymerization. Furthermore, online and non-destructive measurement of the degree of polymerization by optical sensors has been proposed recently but is yet to be available in the market and is yet generally acceptable. In mitigating the magnitude of paper deterioration, some ageing assessments have been proposed. Furthermore, researchers have successfully enhanced the insulating performance of oil-impregnated insulation paper by the addition of various types of nanoparticles. This study reviews the ageing assessment of oil-paper composite insulation and the effect of nanoparticles on tensile strength and electrical properties of oil-impregnated paper insulation. It includes not only significant tutorial elements but also some analyses, which open the door for further research on the topic.

The reliability of the electrical grid is vital to economic prosperity and quality of life. Power transformers, key components of transmission and distribution systems, represent major capital investments. Traditionally, these machines have relied on petroleum-based mineral oil as an insulating liquid. However, with a global shift toward sustainability, renewable insulating materials like natural esters are gaining attention due to their environmental and fire safety benefits. These biodegradable liquids are poised to replace hydrocarbon-based oils in transformers, aligning with Sustainable Development Goals 7 and 13 by promoting clean energy and climate action. Despite their advantages, natural esters face challenges in high-voltage applications, particularly due to oxidation stability issues linked to their fatty acid composition. Various antioxidants have been explored to address this, with synthetic antioxidants proving more effective than natural ones, especially under high-temperature conditions. Their superior thermal stability ensures that natural esters retain their cooling and dielectric properties, essential for transformer performance. Furthermore, integrating machine learning and artificial intelligence in antioxidant development and monitoring presents a transformative opportunity. This review provides insights into the role of antioxidants in natural ester-filled power equipment, supporting their broader adoption and contributing to a more sustainable energy future.

The ageing of cellulose paper-based transformer insulation is a critical factor influencing the reliability and lifespan of power transformers, as insulating paper is not easily replaced or repaired. Therefore, this review explores the degradation mechanisms of insulating paper, emphasising the roles of dissolved gases, chemical markers, and macromolecular characterisation in assessing paper deterioration. Likewise, the impact of moisture and thermal stress on the breakdown of cellulose fibres are discussed, especially acid hydrolysis, which serves as the main degradation mechanism in cellulose insulating paper. Advanced diagnostic techniques for insulation condition monitoring, such as molecular simulations, glass transition temperature analysis, and DP estimation models, are highlighted. Furthermore, special attention is given to natural esters as alternative insulating liquids, demonstrating their ability to slow cellulose ageing through moisture absorption, hydrogen bonding stabilisation, and transesterification reactions. This paper also evaluates key chemical markers, including 2FAL and methanol, for estimating paper degradation. A comprehensive understanding of these mechanisms and diagnostic approaches can enhance predictive maintenance strategies and improve transformer longevity.