Hiroyuki Aoyagi

Estimation of fast transient overvoltage in gas-insulated substation

The authors measured FTO (fast transient overvoltages) at various points of a commercial 550 kV gas-insulated substation (GIS) by operating energized disconnectors. Based on the results of these measurements, the possible levels of FTO at various points of the GIS and the probability of occurrence of large levels are discussed. Laboratory tests were also performed by using a recently developed 1 GHz surge sensor to check on the insignificance of high-frequency components due to short-surge impedance discontinuities such as spacers, disconnectors, and short bus branches. Waveforms calculated by rough simulation that ignored the short surge impedance discontinuities are compared with measured waveforms at the commercial substation, proving that simulated FTO levels agreed well with measured values. >

A study on effects of conducting particles in SF/sub 6/ gas and test methods for GIS

The effects of conducting particles in gas-insulated switchgear (GIS) on insulation performance were investigated. The objective was to recognise the relationship between the length of conducting particles and AC impulse flashover voltages. The data showed that the minimum harmful length of conducting particles at various voltages applied to GIS in service is governed by lightning surge amplitude corresponding to the basic impulse insulation level (BIL). The size of minimum harmful conducting particles was also estimated for various GIS ratings. To further improve the reliability of GIS, factory and on-site test methods are suggested for preventing insulation failures caused by conducting particles. >

Development of detection and diagnostic techniques for partial discharges in GIS

This paper describes detection and diagnostic techniques for partial discharges in gas insulated substations (GIS). Partial-discharge current waveforms of SF/sub 6/ gas and external noise characteristics of actual substations were investigated, and a measuring system having sensors in GIS and employing frequency range of VHF band was developed. By this system, detecting very small partial discharges of several 10 pC level has become possible. Also, a neural network system, focusing attention as a method for recognizing patterns, was applied for determining causes for partial discharge in GIS, and its applicability has been proved. >

Non-Uniform Field Flashover Characteristics in SF6 Gas under Negative Steep-Front and Oscillating Impulse Voltages

The predischarge phenomena in compressed SF6 gas have been investigated to clarify the flashover mechanism under the very fast transient overvoltages. Experiments were carried out for non-uniform electric field gaps with negative non-oscillating and oscillating impulse voltages. Predischarge current measurements and high-speed streak photographs showed that the back discharges under 2.2МНz oscillating impulse voltage occurred in the space charges, and leader propagation velocity was slower than that under non-oscillating impulse voltages. Flashover voltages under the oscillating impulse voltages were higher than those under the non-oscillating impulse voltages. As a result, the volt-time characteristic was affected by leader propagation velocity. The test results show that the lightning impulse test for GIS is sensitive to failure detection.

DETERIORATION OF EPOXY MOLD INSULATIONS DUE TO VOLTAGE ENDURANCE

ABSTRACT This paper describes experiments to determine the cause of epoxy mold insulation breakdown under sustained applied voltage. It can be readily understood that epoxy mold insulations, which are widely used as solid insulation in gas insulated switchgear, break down under sustained applied voltage when the mold insulations contain significantly large voids. The working electrical life of such insulations can be expressed as a gradient plotted against sustained applied voltage. However, epoxy mold insulations which are practically void-free also deteriorate under sustained applied voltage. The experiments described in this paper show that the deterioration is caused by small discharges occurring in the minute space at the embedded electrode/epoxy resin interface.

EFFECTS OF ELECTRODE SURFACE ROUGHNESS ON DARK CURRENT AND SWITCHING IMPULSE BREAKDOWN VOLTAGE IN COMPRESSED SF6 GAS

ABSTRACT Effects of an electrode surface roughness on the dark current, the switching impulse breakdown voltage and its dispersion were examined under the condition of 0,7MPa SF6 gas pressure and the maximum surface protrusion height between 0.6 and 12 μm. Smoother electrode surfaces give the lower dark current which varies irregularly at each time of voltage application and they give the larger breakdown voltage dispersion. On the contrary, rougher electrode surfaces give the higher dark current which varies regularly and the smaller dispersion. Many light emission spots were observed by an image intensifier, and the light emission behavior changed significantly depending on the applied voltage polarity. From the investigations it is concluded that the breakdown voltage and its dispersion depend strongly on the statistical behavior of dark current which is affected by electrode surface roughness and applied voltage polarity.