A Comparative Study of the Dielectric Strength of Ester Impregnated Cellulose for Use in Large Power Transformers

It is essential to ensure that the dielectric strength of an oil-cellulose insulation structure is not reduced when using esters to replace mineral oil in power transformers. One advantage proposed to using esters is that their higher dielectric constants, when compared to mineral oil, provide a better match to the dielectric constant of the impregnated cellulose. This results in the electric stress being reduced in the oil and increased in the cellulose, beneficial as the oil tends to have the lower dielectric strength. This paper reports the ac and impulse dielectric strengths of ester impregnated paper and pressboard. It was found that the ac withstand voltages of cellulose impregnated with ester are at least equal, or better, than cellulose impregnated with mineral oil. The lightning impulse dielectric strengths of cellulose impregnated with ester were found to be comparable to mineral oil. An analysis is given on the breakdown process of cellulose, as cellulose can fail due to either a direct puncture or discharges created in the oil wedge.