The shift towards using natural or synthetic esters to rejuvenate older transformers has sparked a remarkable trend in the energy sector over the past decade. Aimed at slashing the risks of fire and environmental damage while bolstering the transformer’s ability to handle increased loads, this approach has gathered momentum. These esters, distinguishable by their biodegradability and significantly higher fire points compared to conventional mineral oils, promise a safer and more sustainable alternative. Beyond safety, they offer a boon for the longevity of transformers by decelerating the aging process of the solid insulation within. This is achieved through enhanced moisture migration from the solid insulation to the liquid and a transformative chemical process known as transesterification, which occurs in the presence of esters. As a result, transformers can endure higher operational demands without shortening their expected service life, a leap forward in enhancing power system reliability and sustainability.

Leading an innovative study, Andrés Montero, Belén García, and Juan Carlos Burgos from Charles III University of Madrid explore the electric field distribution in transformers retrofilled with natural esters under alternating current (AC) stress. Their findings, published in the International Journal of Electrical Power and Energy Systems, underscore a pivotal shift from traditional mineral oil to natural esters. This transformation aims at mitigating fire hazards and minimizing environmental impact without requiring significant financial outlays.

Central to their investigation is the nuanced impact of retrofilling—replacing mineral oil with natural esters—on the electric field distribution within transformers, a crucial factor for ensuring operational safety and efficiency. The team employed the Finite Element Method, a sophisticated modeling technique, to scrutinize the electric field distribution within a high-capacity transformer initially designed for mineral oil. This approach allowed the researchers to provide detailed insights into how the change in insulation material affects the transformer’s internal electric fields.

Montero underscores the essence of their methodology, explaining, “We modeled a high-capacity transformer designed for significant electrical loads using advanced simulation software. Our goal was to understand how replacing mineral oil with natural esters would impact the transformer’s electric field distribution.” This approach illuminates the potential benefits and challenges of using natural esters, showcasing the team’s commitment to enhancing transformer safety.

The findings of Montero and his colleagues reveal a complex picture of electric field distribution, highlighting areas where insulation stress decreased, thereby potentially improving safety. However, certain regions, particularly those involving solid insulation in high-stress zones, experienced a decrease in dielectric margins. Montero notes, “When a transformer is retrofilled, weaknesses may appear in the solid insulation of the surface of the conductor in the areas of highest electric field of the transformer,” emphasizing the intricate balance between material properties and transformer performance. The implications of this research stretch beyond immediate safety improvements. Montero adds, “Retrofilling is not inherently dangerous for the transformer dielectric performance, although there is a significant change in the electric field distribution that should be taken into account.” This nuanced understanding is crucial for the development of safer, more environmentally friendly power systems.


Andrés Montero, Belén García, Juan Carlos Burgos, “Electric field distribution in natural-ester retrofilled transformers under AC stress,” International Journal of Electrical Power and Energy Systems, 2024.



Andrés Montero (BSc’17, M’20, PhD’23) was born in Cuenca in 1995. In 2017, he obtained a B.Sc. Degree in Industrial Engineering from Universidad Politécnica de Valencia, Spain, and in 2020 an M.Sc. Degree in Industrial Engineering from Universidad Politécnica de Valencia and M.Sc. Degree in Electrical Engineering from Politecnico di Milano, Italy. Since 2020 he has been a Ph.D. student in the Electrical Engineering Department of Universidad Carlos III de Madrid. His main topic of interest is the study of the retrofilling process of transformers with biodegradable fluids.