Effect of moisture on dielectric properties of oil-paper insulation at low temperature

Oil-paper insulation is main insulation type for HV, EHV and UHV large transformer. The operation reliability of transformers is largely determined by dielectric properties of oil-paper insulation. At present, research on dielectric properties of oil-paper insulation is mostly carried out based on room temperature and high temperature, and the dielectric properties of oil-paper insulation at low temperature is not clear. Therefore, AC dielectric properties of oil and oil-paper insulation at low temperature were researched through experiments in this paper. The results show that relative dielectric constant of oil and composite increases with decrease of temperature, but the trend of loss factor is different. The loss factor of oil presents the trend of first increase then decrease with the decrease of temperature, and the composites presents opposite trend, and the extreme point of loss factor moves to high temperature with the increase of initial moisture content. It is found that dielectric properties of oil and oil-paper change with spatial location. These phenomena are related with the moisture content of oil-paper insulation. With decreasing temperature, the water dissolved in oil is precipitating from oil and forming the microscopic droplets of water. The droplets of water play the important role to the dielectric properties of oil-paper insulation.