In this study, the surface charge distribution on insulators in vacuum has been investigated under an impulse voltage (0.7/4μS) with a non-uniform field electrode system. The insulators studied were 99.9% alumina specimens with the shape of right circular cylinders about 5mm thick and 20mm in diameter. Using an electrostatic probe with shield in the form of capacitive voltage divider that was connected to an electrometer, the surface charge distribution on insulators were measured. The applied impulse voltage, and the corresponding current waveforms, the signal of luminosity were also observed during the voltage application. We investigated the surface charge distributions at different impulse voltage levels, at different shot number with a same voltage level, at the situations after a flashover occurred. Based on the experimental results, we found that the surface charge density on insulators is increased with the increase of the applied voltage, the shot number has a little influence at the same voltage level, and the charge density is decreased a lot after a flashover occurs. It's also found that the increase of charge density is often accompanied by the preflashover events.

Investigations have been carried out on Alumina insulators in vacuum under an impulse voltage (0.7/4μS) in order to study the relationship between surface charging and surface preflashover phenomena of insulators in this paper. The Alumina insulators studied were 99.9% alumina cylindrical specimens and the employed electrode system is a non-uniform field electrode system. The surface preflashover phenomena were observed and the surface charge distribution of insulators was measured under varied impulse voltage levels and different shot number at a same fixed voltage level. Based on the experiments, it was found that the charge density and distribution is not only dependent upon the magnitude of the impulse voltage, but also the preflashover phenomena. The number of voltage shot has little influence on the charge density and distribution at the same voltage level if the surface preflashover phenomena can be observed during voltage shot.

This paper has investigated flashover performance of alumina insulators prepared with different additives in vacuum under an impulse voltage. Four kinds of alumina insulator were studied, which were prepared under the same sintering conditions with different additives. The surface flashover performance of these samples has been tested in vacuum. The microstructure of alumina samples has also been observed using the technique of scanning electron microscope (SEM). Based on the experimental results, the characteristics of flashover are different from the test samples. The analysis shows that the influence of additives may be attributed to the change of microstructure.

Recently surface flashover phenomena of insulators in vacuum were investigated with an impulse voltage (0.7/41as). The test samples studied were 99.9% alumina specimens. The impulse voltage, current and luminosity waveforms were observed whether a flashover occurred or not. Surface charges on insulator were also measured after impulse voltage application. Micro-discharge on the surface of insulators was observed in the experiments. The experimental results show that the micro-discharge occurs frequently as a kind of preflashover phenomenon and can be observed right before the current burst of flashover almost every time. It's considered that the micro-discharge is related to the accumulated charges on the surface, which initiates a flashover.

In this paper, the influence of injected charge on streaming electrification of transformer oil has been investigated using a combined model. The experimental results reveal that there are relations between the injected charge, the charge density and the leakage current, and the polarity effect of injected charge exist in the influence on streaming electrification. The influence of injected charge is attributed to the interaction between the electric double layer and the injected charge. The effect of oil velocity on injected charge has been analyzed also.