Thermal Aging Influence on Relaxation Time of Transformer Insulation Paper Impregnated in Natural and Synthetic Ester Oils

For many centuries, mineral oil was used as an insulating fluid with power transformers. However, mineral oil is produced from limited petroleum products and is not biodegradable. These reasons motivated researchers to investigate the usage of alternative insulating liquids such as natural and synthetic esters. But, for practical application of ester oils, there is a need to study the influence of the thermal aging on the paper insulation impregnated with such oils, which is the main aim in this paper. The aging of kraft paper in natural, synthetic esters and mineral oil was compared. The oil-paper insulation was aged at 120°C for 72 hrs, 144 hrs and 240 hrs. The dielectric behavior was investigated via using frequency domain spectroscopy analysis over a frequency range from 10HZ to 1 MHZ. The dielectric relaxation times were calculated for unaged and aged oil impregnated paper samples. The results indicated the better behavior of synthetic ester over natural ester and mineral oil.