Tetsu Shioiri

Area effect on electric breakdown of copper and stainless steel electrodes in vacuum

The breakdown field strength E/sub b/(= alpha Vb/d) of gaps made with pure copper and stainless steel depends on the effective area S/sub eff/. It can be expressed by the single experimental equation E/sub b/=K/sub 1/S/sub eff//sup -n/, where K/sub 1/ is a constant, n is about 0.24, S/sub eff/ is the electrode area subjected to more than 90% of maximum field strength, alpha is the shape factor, d is the gap length, and V/sub b/ is the breakdown voltage. The field intensification factor beta /sub l/ obtained through the measurement of prebreakdown current can be expressed by a single experimental equation beta /sub l/=K/sub 2/S/sub eff/n, where K/sub 2/ is a constant. This means that the area of effect of E/sub b/ is represented by the area effect of beta /sub l/ and that the vacuum breakdown may be initiated by field emission. >

Influence of electrode area on the conditioning effect in vacuum

Using three types of copper electrodes with different surface areas, experiments were performed to investigate the influence of electrode area on the conditioning effect,which is a characteristic of dielectric breakdown in vacuum gaps. The conditioning process varied with electrode area: the smaller the electrode area, the sooner conditioning ended. Breakdown voltages after completion of conditioning also depended on electrode area: the smaller the electrode area, the higher the breakdown voltage. >

Insulation characteristics of vacuum interrupter for a new 72/84 kV C-GIS

This paper describes insulation technology for 72/84 kV vacuum interrupter for a new cubicle-type gas insulated switchgear (CGIS). The insulation performance has been increased significantly by a multi-gap shield configuration around the ceramic surface, which resulted in a 40% volume reduction of the vacuum interrupter. The CGIS housing the vacuum interrupter also reduced by 40% in volume. In the paper, the authors describe the basic characteristics on surface insulation and the area effect on surface insulation and their application to an new 72/84 kV CGIS.

Insulation technology for medium voltage solid insulated switchgear

The authors have developed solid insulated switchgear that does not use SF/sub 6/ gas at all as an insulating medium. This paper describes the insulating material technology, diagnostic technology for partial discharge, insulation technology of the vacuum disconnecting switch and aerial composite insulation technology which are applied to this solid insulated switchgear. A new epoxy resin, which was dispersedly configured with spherical silica and rubber particles, was developed. The insulation performance of this resin is 50% higher compared with filling of alumina. Utilizing the features of the solid insulated switchgear, a diagnostic technology for partial discharge employing an acoustic emission (AE) sensor was developed. A vacuum disconnecting switch with high insulation reliability was developed applying Weibull distribution to dielectric breakdown in a vacuum. The insulated parts of insulating rods of circuit-breakers and disconnecting switches were miniaturized through composite insulation.

Dielectric breakdown probabilities for uniform field gap in vacuum

To investigate the reliability of the insulation of equipment having vacuum incorporated as an insulation medium, a study was carried out to clarify breakdown probability distributions in the vacuum gap. Experiments were carried out to investigate the effects of test methods, electrode area, and electrode materials on the breakdown probability distributions of uniform-field gaps. The test results show that the breakdown probability distribution of the vacuum gap can be represented by a Weibull distribution using a location parameter, which shows the voltage that permits a zero breakdown probability. When aiming for highly reliable vacuum insulation equipment, an important factor is insulation design with the location parameter just mentioned taken into account. The location parameter depends on electrode area. This is probably because the existence probability of factors as weakpoints for breakdowns such as micro-protrusions and micro-particles depends on the electrode area.

Investigation of dielectric breakdown probability distribution for double-break vacuum circuit breaker

Breakdown probability distribution before and after no-load switching was investigated for a vacuum interrupter made from copper-chromium alloy. Since the multi-break vacuum circuit breaker was considered as a method for realizing a high-voltage vacuum circuit breaker, a double-break vacuum circuit breaker was investigated for breakdown probability distribution. Breakdown probability distribution after no-load switching can be represented by a Weibull distribution in the same manner as before switching. The scatter of breakdown voltage increases when no-load switching is carried out. The shape parameter becomes constant, from 6.0 to 8.5 irrespective of the gap length. If the vacuum circuit breaker uses a double-break, breakdown probability at low voltage becomes lower than single-break probability. Although the double-break vacuum circuit breaker is inequality of potential distribution, its insulation reliability is better than that of the single-break vacuum interrupter even when the inequality of the vacuum interrupters sharing voltage is taken into account.

Influence of electrode area on conditioning in vacuum-gap breakdown

Using three kinds of copper electrodes with different surface areas, experiments were performed to investigate the influence of electrode area on conditioning effect, which is a characteristic of dielectric breakdowns in vacuum gaps. The conditioning process varied with electrode area: the smaller the electrode area, the sooner conditioning ended. Breakdown voltages after completion of conditioning also depended on electrode area: the smaller the electrode area, the higher the breakdown voltage.

The effect of mechanical contact on breakdown characteristics in vacuum

Using copper-chromium electrodes, an experiment was conducted to clarify the reduction of insulation strength by mechanical contact in vacuum. The experiment consisted of prebreakdown current measurements and SEM (secondary electron microanalyzer) observation of the electrode surface. Experimental results revealed that the field intensification factor obtained from the prebreakdown current was increased by mechanical contact. SEM observation showed that mechanical contact caused microprotrusions to be generated. This phenomenon suggests that a reduction of insulation strength by mechanical contact is caused by field emission from a roughened surface. If mechanical contact is made, dielectric breakdown occurs when the field strength of microprotrusions on the cathode surface reaches a critical value (1.0 to 1.7*10/sup 8/ V/cm). >

Effect of annealing and polishing on flashover characteristics of ceramic in vacuum

Experiments were carried out to study the effects of surface polishing and annealing of ceramic specimens on the dielectric breakdown characteristics of their surfaces. It was found that when the samples were annealed at 1000/spl deg/C, allowing the residual stresses in their surfaces to decrease, the breakdown voltage rose, whereas with the polished samples, reductions of residual stresses resulted in a slight rise of the breakdown voltage. Prior to dielectric breakdown, a surface glow was observed, due to luminescence from electron bombardment and insulator surface defects. The starting voltage of luminescence rose when residual stresses were reduced. It is suggested that the mechanical strain due to surface defects contributes to dielectric breakdowns on ceramic surfaces.

Generation of microparticles from copper-chromium contacts in vacuum

Using a scanning electron microscope (SEM) and a laser microscope, we observed micro-flakes and particles. On machined Cu-Cr contacts which were considered to be built-up edges due to machining. A large number of microparticles of 5.7-8.7/spl times/10/sup 3/ from a contact surface of 1 cm/sup 2/, were generated from the machined Cu-Cr contacts during opening and closing operations of normal current. The size of typical microparticles was some tens /spl mu/m and those larger than 100 /spl mu/m were occasionally observed. This implies that the surface of machined Cu-Cr contacts must be treated by any means when they are used for vacuum interrupters. Current conditioning is one of the effective surface treatments. Although the surface roughness of the contacts increases through the current conditioning, the improvement of insulating ability is brought by the elimination of micro-flakes and particles and by the formation of a fine structure layer preventing partial detachment of the contact material. >