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 Influences of electrical field on boiling-condensation heft transfer system have been investigated using a cylinder beat transfer model Freon-ll is selected as working fluid. The condensation heat transfer coefficient, the boiling heat flux the saturation pressure are meassured in this investigation. According to the experimental results, it is found that the electrical field can influence best transfer system. The boiling heat transfer is enhanced by the applied voltage, and the saturate vapor of working fluid is condensed on the high voltage electrode directly when the applied voltage is higher than 6kv. The experimental results have been discussed, and it is considered that the high electrical field strength change the thermal properties of working fluid.

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.

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.

A measuring system for on-line detecting partial discharge of generators has been designed with the ultra high frequency (UHF) and high frequency (HF) method. It not only has the capability of calibrating the PD level, but also has the excellent ability of anti-interfering on site. The test data shows this technique is very promising.