Electric field distribution in natural-ester retrofilled transformers under AC stress

Abstract

Retrofilling a transformer with natural or synthetic esters leads to a reduction of its fire and environmental risk without making a large investment. Nowadays, most of the retrofilled transformers are medium power and voltage units, but their use in higher rated power units is rapidly expanding. Transformer retrofilling presents certain challenges due to the physico-chemical differences between mineral oil and esters. The insulation system of a transformer is designed to work with mineral oil, performing a double function as dielectric and coolant agent; when replacing mineral oil with an ester, whose relative permittivity and viscosity are different, the ability to perform these two functions must be reassessed.In this work, the electric field distribution of a retrofilled transformer has been analyzed using Finite Element Method. The study was conducted on a 280 MVA transformer which was originally designed to operate with its tank filled with mineral oil. The dielectric margins of the different parts of the insulation were calculated, and then compared with the dielectric margins of the transformer insulation before retrofilling. To improve the accuracy of the study, an experimental study was driven to obtain the permittivity values of mixtures of MO and NE in different proportions and the composition of the solid insulation in retrofilled transformers. The study also investigates the influence of the quality of the retrofilling process on the electric field distribution. The analysis shows that some parts of the insulation, such as the oil channel, operate with lower stress when the insulating liquid is changed to an ester, but other regions, such as the solid insulation in the most stressed areas of the insulation, reduce their dielectric margins significantly.