Thermal-aging-dependent electric-field redistribution and interfacial charge dynamics in oil-pressboard insulation under DC and polarity reversal voltages

Abstract During the operation of converter transformers, the aging of insulating oil and pressboard materials is accelerated by prolonged exposure to high temperatures, thereby increasing the risk of insulation degradation or failure. However, the evolution of interface charge in oil-pressboard insulation structures during thermal aging has not yet been fully clarified. In this paper, typical transformer insulating oils and pressboard materials were subjected to thermal aging, and the space electric field distribution and interface charge behavior were investigated using a Kerreffect electric field measurement system. It was found that the charge accumulated at the oil-pressboard interface increased with aging time, with a maximum increase of 136.98% being observed after 35 days of aging. Under DC voltage stress, the steady-state electric field was found to decrease gradually with aging time, whereas increasingly pronounced electric-field distortion was observed under polarity-reversal conditions. Furthermore, under identical aging conditions, greater interface charge accumulation and more significant variations in the space electric field distribution were observed in naphthenic-based insulating oil than in paraffinic-based insulating oil. These findings provide further insight into the charge behavior of oil-pressboard insulation systems during thermal aging and offer support for the optimization of insulation design in converter transformers.