Study on the Contamination Rate of Mineral Oil in Natural-Ester Retrofilled Transformers

The replacement of mineral oil (MO) by ester (retrofilling) is a practice used in power transformers to reduce fire and environmental risk. Although measures are taken to remove traces of oil on the paper, a certain amount of MO remains, which migrates over time. This article presents a study to evaluate the migration of MO from paper to ester using techniques such as the iodine value or the measurement of permittivity and dielectric losses.