Experimental study of the aged states effects on pressboard PDC characteristics

To investigate the effect of pressboard aged states on its polarization and depolarization currents (PDC) characteristics, the oil-impregnated pressboard samples were accelerated thermally aged for different time intervals. The pressboard samples of different aged states were sequentially put into petroleum ether as de-oiling solvent for residual oil removal, and dried in a vacuum oven to remove the petroleum ether and moisture. To characterize the aging status of different samples, the degree of polymerization (DP) was measured by viscosimetry and the scanning electron microscope (SEM) was also implemented to observe the surface microstructure. Then PDC tests were performed on each water-removed pressboard sample in vacuum and new oil impregnated ambient, respectively. Experimental results show that, with the aging status heavier and decreasing DP values, microstructure of cellulose fiber is significantly destroyed. Aged samples exhibit higher polarization and depolarization currents, lower insulation resistance and smaller absorptance 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, and hence greatly affects the PDC test results.