Investigation of dielectric response characteristics of transformer oil-paper insulation - Dielectric spectroscopy of acceleratedly thermal aging

For achieving the characteristic parameters of dielectric response phenomena of transformer oil-paper insulation aging, the oil-impregnated pressboard samples were acceleratedly thermally aged. The dielectric response tests, including polarization and depolarization current (PDC) and frequency domain spectroscopy (FDS), were performed on the samples with different aging degree at the same temperature. The time-domain dielectric spectroscopy theory was introduced to investigate the aging characteristic parameters of dielectric response based on the PDC data. The results reveal that, with sample aging, its polarization and depolarization currents shift upwards to higher value, the real and imaginary capacitance and dissipation factor shift upwards and rightwards at lower frequencies, and the time-domain dielectric spectroscopy shifts upwards and leftwards. It is considered that aging process induces the increment of water and organic acids content and the rapid degradation in amorphous regions of cellulose paper, which greatly impacts the oil-paper interfacial polarization and enhances its intensity and response speed. It is confirmed that time-domain spectroscopy is sensitive to the aging condition of oil-paper insulation, and can quantitatively distinguish the change of aging condition to some degree. The peak value and peak time constant of time-domain spectroscopy can be considered as the characteristic parameters to evaluate the aging condition of oil-paper insulation.