Comparative Assessment of Twenty-Years Old Transformers Filled with Mineral Oil and Synthetic Ester Liquid

Synthetic esters have been used in transformers since the 1970s. They are known for being fire-safe, environmentally friendly dielectric liquids and can extend the transformer’s life. In addition, they have high oxidation stability and a low pour point, and the maintenance requirements are like mineral oil hence, simplifying their adoption. Although the experience has grown in the use of synthetic ester liquid, there is still a need to understand the performance of the liquid in in-service transformers. This paper features data from 10 $11 \mathrm{kV} / 0.415 \mathrm{kV}$ transformers (5 units with mineral oil and 5 units with synthetic ester) in-service for over 20 years sampled for liquid analysis, dissolved gas analysis (DGA), and the degree of polymerization (DP) estimation through furan analysis. This paper also features information on the reclamation of synthetic esters, which has never been commercially achieved in the past. Synthetic esters are sustainably manufactured; however, by being able to be reclaimed, the sustainability is further enhanced by minimizing the carbon footprint that would otherwise be generated in alternative uses.