Novel characteristic parameters for oil-paper insulation assessment from differential time-domain spectroscopy based on polarization and depolarization current measurement

For achieving the aging characteristic parameters of transformer oil-paper insulation, oil impregnated pressboard samples are acceleratedly thermally aged at high temperature (~150°C) in laboratory, and on the same sample before and after being aged, polarization and depolarization current (PDC) tests are performed at different temperatures (30, 40, 50 and 60°C), respectively. Besides analyzing the PDC results, the differential time-domain spectroscopy (DTDS) technique is introduced to investigate the aging status of oil-paper insulation and the influence of measurement temperature. The DTDS parameters such as polarization component Q_n, polarization response time τ_n and line-shape parameter a_nare extracted with DTDS unscrambling method based on PDC data. It is revealed that DTDS represents similar polarization process and mechanism to PDC, and all the DTDS curves calculated with the obtained parameters (Q_n, τ_n,α_n) are in good fit with those measured. By analyzing the effect of temperature and aging, two types of relaxation polarization mechanism are observed, i.e., the interfacial polarizations between oil and paper insulation and inside the pressboard. The polarization components, response speeds and line-shape parameters of both the polarization mechanisms increase evidently on the whole with temperature rising or sample aging. It is demonstrated that, the DTDS theory is suitable for oil-paper insulation material, the DTDS unscrambling method is effective and objective, and such DTDS parameters are sensitive and can quantitatively reflect the changing of temperature and aging condition with regularity. Consequently, the parameters (Q_n, τ_n, α_n) can be considered as the novel characteristic parameters for aging condition estimation of oil-paper insulation to upgrade the application effect of PDC curves.