Diagnosis of Moisture Content in Oil-immersed Paper Based on Dielectric Loss with Variable Voltage at Power Frequency

this paper studies the dielectric property of oilpaper in high electric field, especially the effect of moisture on the dielectric loss under variable voltage at power frequency in oil-immersed paper with different moisture content. Oil-immersed paper insulation model samples with different moisture content are prepared in laboratory. Afterwards, increasing voltage and decreasing voltage tanδ-U curves of samples with different moisture content were tested under different experiment condition and a non-linear phenomenon was observed. Test results show that the tanδ-U curve is influenced by the moisture content of oil-immersed paper and test temperature. When the moisture content in the oil-immersed paper is high, the tanδ-U value is gradually increasing with the increase of the voltage. And in the process of decreasing the voltage, the tanδ-U value gradually decreases but is greater than the value at the same voltage in the process of increasing the voltage. The decreasing voltage tan tanδ-U curve is above the increasing voltage tan tanδ-U curve while these two curves overlap at high voltage and have great separation at low voltage. Moisture content changes the overall shape of the curve and the area between the two curves by influencing the transfer of charge carriers in oil-immersed paper. As water is polar, energy level and density of the trap in the oil-immersed paper with different moisture content are different. Water provides more paths for free moving charge carriers so that the accumulation and dissipation velocity of the charge carriers in Oil-immersed paper vary. The temperature doesn’t change the overall shape of the tanδ-U curve. Based on the dielectric response, the tanδ-U curve of samples with different moisture content could be expounded and the moisture Content in oil-paper insulation equipment could be evaluated by the tanδ-U curve.