Influence of Moisture Dispersion in Oil-paper Insulation on Dielectric Spectrum of Power Transformer

Dielectric response has the advantages of rich insulation information and no damage to insulation samples. As a non-destructive testing method for oil-paper insulation, it has been widely used in the evaluation of transformer insulation property. In this paper, oil-paper insulation samples with different moisture aggregation states were prepared, and the variation of frequency-domain dielectric spectroscopy (FDS) during moisture dispersion was compared and analyzed. The test results show that one or more loss peaks will appear in the tan delta-f curve when the local moisture content is high, and the location of the loss peaks is closely related to the aggregation or dispersion of moisture in the oil-paper insulation. When the moisture gradually disperses and tends to be evenly distributed, the loss peak gradually moves toward lower frequency until it disappears in the test band range. Different moisture aggregation states in the oil-paper insulation will produce a variety of polarization processes. The increase of locally accumulated water will strengthen the jumping motion of charge carriers and make the interface polarization process stronger. As moisture disperses within the medium, it gradually tends to be evenly distributed. Part of the free water is adsorbed by cellulose, the intermolecular force inside the medium increases, the dipole polarization process slows down, and the loss peak moves to the low frequency. Therefore, it is necessary to consider the influence of the moisture dispersion process on the oil-paper insulation system to avoid large errors in the evaluation results of the FDS method