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

To investigate the effect of pressboard aging states on its time domain dielectric characteristics, experiments are designed to measure the polarization and depolarization currents (PDC) of different aged pressboard samples. Firstly, new pressboard samples immersed in new transformer oil are thermally aged for different time intervals and then residual oil in aged pressboard samples is removed by de-oiling solvent. Secondly, these pressboard samples are dried in a vacuum oven to remove de-oiling solvent and moisture. The degree of polymerization (DP) is measured to characterize the aging states of each sample. Finally, PDC tests are performed on each pressboard sample respectively in vacuum and new-oil-impregnated ambiences. Experimental results show that badly aged pressboard exhibits larger polarization, depolarization and conduction currents. The deterioration of solid insulation increases its conductivity and enhances its polarization.