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.

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.

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.

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.

A new set of facility for accelerated aging test of composite insulators has been developed at CEPRI (China Electric Power Research Institute). This paper mainly described the measurement and parameters monitoring system of this facility. This system can monitor total six channels current of insulator in real time, and it can also collect the corresponding information at the same time, such as the applied voltage, the environment parameters. Based on the collected data, the system can analyze and statistic the characteristics of the aged leakage current in time, and it can also save the important test data during the whole test procedure. In addition, this system has the capability of auto alarm, auto reset for the unexpected accidents during the test procedure. This system is a powerful system for the research on the aging performance of composite insulator.

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 reports on the infuence of sintering conditions on the flashover/preflashover characteristics of alumina insulators in vacuum under an impulse voltage. Four kinds of alumina insulator were studied, which were prepared using the same formula with different sintering conditions. The surface pre-flashover and flashover phenomena of different samples in vacuum have been observed under impulse voltage. Based on the experimental results, the characteristics of pre-flashover and flashover vary with different kinds of test samples. It was found that the insulators with a higher sintering temperature had a lower flashover voltage, and more active pre-flashover. From a scanning electron microscope (SEM) analysis of alumina insulators, we conclude that the methods of preparing alumina samples can affect their microstructure resulting in a change in the pre-flashover and flashover performance as alumina insulators.

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.

This paper provides some experimental investigation on the influence of the surface roughness on the flashover performance of ceramics in vacuum. The test samples were prepared with four kinds of ceramics (99.9% Alumina) which were polished by four different SIC abrasives. The flashover performances of 20 kinds of test samples have been tested with an impulse voltage in vacuum. According to the experimental results, it was found that the roughness of the test sample was quite different among different ceramics although the same grinding procedure was used with the same abrasive. It was also found that the flashover performance was influenced by the surface roughness, and there is a trend that the samples with a lower surface roughness have a higher breakdown voltage.