Investigation of the impregnation of cellulosic insulations by ester fluids

This paper confronts the practical problems associated with ester fluids, due to their higher viscosities, for the impregnation of cellulosic insulation systems used in power transformers. The impregnation mechanism by an insulating fluid was studied using an analytical model and then laboratory tests were carried out to investigate the impregnation process of cellulosic materials in the forms of paper, pressboard and laminated pressboard blocks by various insulating fluids. The impregnation of both papers and pressboard barriers brings no practical problems due to their thinness; however the impregnation procedure for thick laminated pressboard blocks by esters needs to be established. It was shown that by increasing the impregnation temperature to 60degC, the viscosities and the capillary action of esters can achieve a similar impregnation to that of mineral oil at room temperature and consequently a full impregnation can be achieved for the ester filled power transformers in the same length of time.