Experimental Evaluation of the Moisture Effect on the Dielectric Properties of Thermally Upgraded Kraft Paper Impregnated with Mineral Oil and Natural Ester

An essential component of power transformers is the electrical insulation system, composed of cellulose-based materials and usually mineral oil. Currently, this fluid is being replaced by alternative dielectric fluids derived from plants, such as soybean, sunflower, etc. The dielectric system must be kept in prime condition. Moisture affects the dielectric strength and the rate of insulation ageing. Frequency Domain Spectroscopy (FDS) is a promising method to analyze the moisture effect in oil-paper insulation system of the transformer. This study investigates the influence of moisture on the dielectric behavior of Thermally Upgraded Kraft (TUK) paper, both non-impregnated and impregnated with mineral oil and soybean ester. The samples were prepared in the laboratory with different moisture content within 1% to 5%. Measurements were performed over a wide frequency range from 16.62 mHz to 5 kHz. It was found that the impregnation of the paper samples helps to reduce the variation of tan δ with the incrementation of moisture and that the dielectric response is sensitive to moisture content regardless of the impregnation and the type of impregnation liquid.