Experience In Retrofilling of Mineral Insulating Liquid Filled Transformers With Ester Liquids

Ester liquids have been used in transformer applications for over four decades and are known to be fire safe, environmentally friendly and they can improve the life of insulation systems when compared to mineral insulating liquid (mineral oil). These dielectric liquids are increasingly being used to retrofill (removal and replacement of one insulating liquid with another) transformers filled with mineral insulating liquid to improve the fire safety of the transformer and equipment around it, extend the life of the transformer and make the transformers environmentally friendly. This paper will discuss the differences in the dielectric liquids and the properties that affect the transformer design and the effects on the performance of the transformer after a retrofill. Details will be provided on how to determine if a transformer is a candidate for retrofill and the retrofilling process. A relevant industry case study will be examined regarding transformers that have been retrofilled and the in-service condition of the transformers.