Electrical characteristics of natural ester impregnated pressboards under different periods of impregnation

This paper represents electrical characteristics of natural ester impregnated pressboards under different impregnation period. Each pressboard specimen was prepared as a rectangular shape having the width of 100 mm and the length of 100 mm as well. Two thickness dimensions of the pressboard for the experiment were 1.6 mm and 3.2 mm respectively. All pressboards were dry up in the oven with temperature at 60°c for 1 hour. Next, these pressboards were divided into three groups according to the impregnation periods. The first group was impregnated with natural ester (FR3) for 3 hours with temperature at 60°c and 200 mbar pressure controlled condition. The second and third group were impregnated for 6 and 9 hours respectively with the same temperature and pressure condition. Then, the breakdown voltage characteristic of the impregnated pressboards were investigated according to IEC 60243-1. Besides, the capacitance and dielectric dissipation factor of the natural ester with various conditions were examined in accordance with IEC 60247. According to the test results, the pressboard impregnated for longer period provided higher breakdown voltage than the pressboards impregnated for shorter period by which the breakdown voltage of the pressboard impregnated for 9 hours was higher than the pressboard impregnated for 6 hours about 8-24% and the breakdown voltage of the pressboard impregnated for 6 hours was higher than the pressboard impregnated for 3 hours about 5-19%. Whereas the dielectric dissipation factor of the natural ester utilized for pressboard impregnation process for 3 hours was higher than the natural ester utilized for such process for 6 and 9 hours about 5% and 12% respectively. Moreover, the electrical characteristics of pressboard impregnated with natural ester was compared with that of the pressboard impregnated with mineral oil. The breakdown voltage of natural ester impregnated pressboard was higher than the mineral oil impregnated pressboard about 5-11% but the dielectric dissipation factor of natural ester was 30 times higher than mineral oil. It is clear that the time for impregnation significantly affects the breakdown characteristic of the natural ester impregnated pressboard. Furthermore, impregnation of the pressboard with natural ester provides better breakdown strength than the pressboard impregnated with mineral oil.