Impact of pressboard aged states on the time domain dielectric characteristics of oil-paper insulation

To investigate the effect of pressboard aged states on its time domain dielectric characteristics, a series of experiments are designed and carried out to measure polarization and depolarization currents (PDC) of different aged pressboard samples. New pressboard samples immersed in new transformer oil are accelerated thermally aged for different time intervals. The aged oil-pressboard samples are sequentially put into petroleum ether as de-oiling solvent to remove residual oil. Then these pressboard samples are dried in a vacuum oven to remove the petroleum ether and moisture. The degree of polymerization (DP) is implemented to characterize the aged states of each sample. PDC tests are performed on each water-removed pressboard sample in vacuum and new oil impregnated ambient, respectively. Experimental results show that, with the aged states heavier and decreasing DP values, aged samples exhibit higher polarization/depolarization currents, lower insulation resistance and smaller absorption ratio in both ambient. It is confirmed that, for oil-paper insulation, besides the influence of water induced by aging, the deterioration of cellulose itself also causes the increasing of pressboard conductivity, decreasing of absorption ratio, and hence greatly affects its time domain dielectric characteristics.