Analysis of the Impregnation Process of Cellulosic Materials by Ester-Based Insulating Liquids

Abstract

Ester insulating liquids have higher viscosities than mineral oils. This complicates the impregnation of cellulosic insulation and has slowed down the adoption of these dielectrics in high voltage power transformers. This paper studies the dependence of the impregnation characteristics of a mineral oil, a natural ester and a synthetic ester on temperature. For this purpose, impregnation experiments were carried out and the results were compared with an analytical model. The main factors governing the impregnation mechanism related to dielectric liquids are surface tension and viscosity. Results show that increasing the temperature effectively reduces the viscosities of esters, allowing the performance of the esters to approach that of mineral oil. In addition, an exponential model is proposed to predict the impregnation rates associated to a given temperature.