Experimental study on oil aging effects on time-domain dielectric characteristics of oil-paper insulation

To investigate the effects of oil aging on the measurements of time-domain dielectric characteristics of transformer oil-paper insulation and to accurately estimate the state of oil-paper insulation, accelerated thermal aging experiment was performed on oil-paper. Three oil samples of different aging states were obtained and their chemical and physical properties were analyzed. The new cellulose paper samples were dried in vacuum oven and then sufficiently impregnated respectively by the three oil samples to prepare three oil-impregnated paper samples. The polarization and depolarization currents (PDC) test was performed on each oil-paper sample to analyze oil aging effects on time-domain dielectric characteristics. Experimental results show that the moisture content, acid value and dissipation factor (tan δ) become larger in aged oil, while the pH value and resistivity decrease. When the oil gets aged, the polarization and depolarization currents of the oil-impregnated paper samples increase. The conduction current also becomes larger and takes more time to reach a steady value. The aged oil-paper sample also exhibits smaller insulation resistance (R₆₀) and absorption ratio (K_a).