Abstract: The electrical insulation system, usually composed of cellulosic materials and mineral oil, is essential for the reliable operation of the power transformers. The dielectric system must be kept in prime condition to increase the lifetime of the machine. Moisture in these materials reduces the dielectric strength and enhances the rate of insulation ageing. Frequency Domain Spectroscopy (FDS) is a promising method to analyse the effect of moisture in oil-paper insulation system. This study compares the influence of moisture on the dielectric capacity of 3 insulating papers; Diamond Printed Enhanced (DPE), Kraft and Thermally Upgraded Kraft (TUK) impregnated with mineral oil using FDS. 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 dielectric magnitudes measured on the DPE paper samples are higher compared to the other two types of paper samples and that the dielectric response is adversely affected by moisture content, regardless of the type of paper.

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