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.

The measuring system we used consisted of two UHF sensors with a frequency band of 400 MHZ～1500MHz and a digital oscilloscope with maximum 2 GS/s digital rate in its two channels. The difference in arrival times of UHF signals at two sensors can be measured to assist with locating defects in GIS. The test results in the laboratory indicate that the fault location precision depends on the digital rate of the oscilloscope. In actual site test, if the sample rate is low, it is could not have enough sample plots to recover the real pulse wavefronts, which is difficult to identify the arrival times at the two sensors by the first peak of UHF waveform. In this paper, a new algorithm has been described, which applies cubic spline interpolation to increase points between two real sample points, and improve the location precision of time flight method. And then the distance difference between two waves can be determined by using cross-correlation function. In addition, the application software has been developed to help locate PD source in GIS.

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.

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.

In this paper, the influence of streaming electrification on the creeping discharge of oil-paper insulation system has been investigated, using experimental models. The experimental results indicate that the initial creeping discharge voltage of oil-paper insulation system decreases with oil flow rate. The creeping discharge path develops toward the oil entrance, and there are polarity and position effects on creeping discharge under oil flowing conditions. Reasons of failure due to dendritic discharge on the surface of barrier boards in power transformers have been revealed in these experiments.