The study of the flashover characteristics of the aged oil-paper insulation under the AC-DC combined voltage demonstrates that the flashover phenomenon of the aged oil-paper samples along the surface of the insulating paper occurs during the preloading of the higher amplitude DC voltage. Flashover occurs when severely aged oil-paper insulation samples are preloaded with high-amplitude DC voltages. However, flashover is not observed with unaged or “non-severely-aged” samples preloaded with the same DC-voltage amplitude.
Therefore, four types of oil-paper insulation samples, corresponding to different aging times, were prepared by means of the accelerated-thermal-aging method to simulate the oil-paper insulation status of converter transformers with different operational lifetimes. In this study, first, the DC voltage component is boosted to a preset amplitude at a rate of 1 kV/s and maintained for 30 min. Next, the AC voltage component is gradually increased at a rate of 1 kV/s, and the AC component is superimposed on the DC component to obtain the combined AC-DC voltage.
Our results indicate that a flashover occurs upon preloading with a large-amplitude (≥60 kV) DC component only for the older aged insulating paper samples. After the flashover, the surface of the insulating paper is subjected to high-energy ablation caused by discharge, and a watermark-like carbonization trace appears. Therefore, in this study, we examine the space-charge, physical, and chemical characteristics of aged oil-paper insulation, in combination with its electric-field distribution characteristics, in the context of the electron-triggered polarization relaxation theory.
We found that a greater degree of aging of the oil-paper insulation corresponds to a lower DC voltage preloading, causing the flashover. The space-charge behaviors of the unaged and aged oil-paper samples differed under low DC electric fields. The space-charge density of the aged oil-paper insulation samples was 1.9 times that of the unaged samples. We found that aging leads to an increase in the number of traps formed in the oil-paper, and the number of shallow traps formed is relatively greater than that of deep traps, which facilitates the de-trapping of charges from shallow traps. Meanwhile, aging causes an increase in the crystallinity of the oil-paper insulation, along with the acidity and aging-product amount, which eventually leads to a conductivity increase. Moreover, with aging, many impurity ions are generated. All these factors lead to an increase in the number of carriers in the aged oil-paper insulation. For a sufficiently large applied electric field, the rapid migration of these carriers aggravates the disturbance of the space-charge equilibrium of the aged oil-paper insulation. Eventually, the space-charge equilibrium is destroyed, thereby inducing flashover along the surface of the insulating paper.

oil-paper, thermal aging, flashover, direct current (DC) voltage, space-charge