Electrical characteristic comparison of mineral oil and natural ester for transformer applications

In this paper, the electrical characteristics of mineral oil compared with natural ester were studies. AC breakdown strength, dielectric dissipation factor, and permittivity of both liquids were investigated. Both new liquids were heated in the oven at 60°C with 200 mbar for 6 hours before starting all investigation, the water contents of mineral oil and natural ester were 18 and 178 ppm respectively. Moreover, the mineral oil and natural ester were used to impregnate the pressboards which were 1.6 mm and 3.2 mm thick respectively. These pressboards were impregnated for 3-12 hours. Then, the AC breakdown voltage of the impregnated pressboards was examined. AC breakdown strength of the liquid insulations was tested according to IEC 60156. The dielectric dissipation factor and permittivity of the liquids with the temperature of from 35°C - 100°C were investigated as well. Moreover, the breakdown voltage strength of the impregnated pressboards was examined with the electrodes in accordance with IEC 60243-1. From the test results, mineral oil sample provided lower AC breakdown voltage than natural ester sample. However, mineral oil generated approximately 77 times lower dielectric dissipation factor compared with the natural ester. Besides, natural ester impregnated pressboards had a bit higher breakdown strength than mineral oil impregnated pressboards. Furthermore, breakdown strength of the liquid impregnated pressboard depended clearly the function of time and the pressboard thickness.