Differential time-domain spectroscopy technique based on polarization and depolarization current and their applications in oil-paper insulation estimation

To quantificationally apply the results of polarization and depolarization currents (PDC) measurement for condition evaluation of oil-paper insulation in power transformer, the differential time-domain spectroscopy (DTDS) theory and the extraction technique of sensitive characteristic parameters are introduced. A set of oil-immersed paper samples are thermally acceleratedly aged at high temperature (~150 °C) in laboratory based on the developed platform for oil-paper insulation thermal aging and dielectric response test. Under different temperatures (30, 40, 50, 60, 70 °C) and different aging degrees, PDC tests are performed on one of the samples by real-time tracking during the whole aging period. And at each aging stage, the aided tests, such as moisture content dissolved in oil and degree of polymerization (DP) of cellulose paper are measured. The test and analysis results show that aging condition has evident effect on DTDS curves. With sample aging, the DTDS curve shifts upwards. The extracted characteristic parameter, polarization response time τ₁ of the interfacial polarization between oil and pressboard, is regular and sensitive to aging condition changing. The influence mechanisms of aging on the time domain dielectric properties are analyzed. The empirical formula representing the relation among temperature, aging condition, and dielectric response in time domain is developed by means of 3-D surface fitting technique, to preliminarily investigate the application method of time-domain dielectric test.