Investigation on dielectric response characteristics of thermally aged insulating pressboard in vacuum and oil-impregnated ambient

Dielectric response measurements are non-invasive and promising diagnostic methods that are being gradually used for assessing the insulation and aging condition of transformer oil-paper insulation system. It is known that moisture content in oil-impregnated insulation has significant effect on its dielectric response phenomena, and whereas at present there is few research about the characteristics of aged cellulose without the impact of water. In this work, a series of experiments are designed and carried out in well-controlled laboratory. Accelerated thermal aging is performed on oil-immersed pressboard samples with different time intervals (0, 120, 250 and 400h). Then these pressboard samples with different aging states are put into petroleum ether to remove the residual oil inside, and dried in a vacuum oven to remove the petroleum ether and moisture. The degree of polymerization (DP) and scanning electron microscope (SEM) are implemented to characterize the aging status of pressboard samples. Two kinds of dielectric response tests, i.e., the polarization and depolarization currents (PDC) and frequency domain spectroscopy (FDS), are performed on each oil/water-removed pressboard sample in vacuum and new-oil-impregnated ambient, respectively. Samples with different aging states show quite different dielectric response characteristics in both ambients. With the aging status more serious, the pressboard sample exhibits higher values of polarization/depolarization currents and complex capacitances. It is confirmed that, 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 and FDS test results. Dielectric response diagnostic techniques have a potential to reflect the aging information of solid insulation.