Experimental Study of the Effect of Moisture and Temperature on the Dielectric Properties of Kraft Paper Impregnated in Biodegradable Transformer Liquid

Power transformers' electrical insulation system, usually composed of cellulosic materials and mineral oil, is critical for their reliable operation. The dielectric system must be kept in prime condition to increase the lifetime of the machine. High moisture content (m. c.) and temperature in these materials reduce their dielectric strength and enhance the rate of insulation aging. Currently, mineral oil is being replaced by alternative dielectric fluids derived from plants. Thus, this study assesses the influence of moisture and temperature on the dielectric capacity of a traditional transformer Kraft paper, impregnated with a biodegradable ester using Frequency Domain Spectroscopy (FDS). The samples of impregnated Kraft paper were prepared in the laboratory with different m. c. within 1% to 5%. Measurements were performed over a wide frequency range from 16.62 mHz to 5 kHz, at 20 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{oC}$</tex> and 50 °C. It was determined that soybean ester, when used as an impregnation liquid, significantly reduces the variation of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\tan\delta$</tex> with increasing m. c. and temperature compared to mineral oil. The existence of synergies between temperature and moisture has also been detected.