Investigating Performance of Mineral Insulating Liquid and Less Flammable Ester Liquid Filled Distribution Transformers in Overload Conditions

Southern California Edison historically used mineral insulating liquid in distribution transformers but since 2018 have switched over to less flammable ester liquids as ester liquids are known to be fire safe, environmentally friendly and can extend the lifetime of the insulation system. In service, these transformers are operated under both typical loading patterns and emergency overloading conditions. To do a comparison in performance between mineral insulating liquid filled units to esters, 4 transformers (3 PH 75kVA, 12kV 95kV BIL) filled with mineral insulating liquid, synthetic ester, canola natural ester and soybean natural ester were subjected to approximately 180% load for a total period of 1 year (units being de-energized at intervals to perform testing). During that time frame, samples to analyze liquid quality as well as dissolved gases were collected. At the end of the project, tear down analysis was performed to evaluate the thermal aging on the solid insulation in mineral insulating liquid versus ester liquids. This paper summarizes the findings of the project including an in-depth analysis of the aging rate of cellulose as well as condition of the liquid in the transformers with the different dielectric liquids.