Naoki Asari

Breakdown characteristics of CuCr electrode after electron beam irradiation in vacuum

For development of high-voltage equipment such as vacuum circuit breakers, it is very important to improve their dielectric breakdown characteristics in a vacuum. It is thus necessary to clarify the breakdown mechanism in a vacuum. It is well-known that these characteristics are influenced by the electrode material and its surface condition[1][2], but the breakdown mechanism has not been clarified. This paper presents measurements of breakdown characteristics and observations of a cross-section after electron beam irradiation. The results are compared with those for no electron beam irradiation. It is thus shown that the breakdown voltage of a CuCr electrode after electron beam irradiation is higher than that for no electron beam irradiation. This is because a fine Cr layer is formed on the electrode surface.

Influence of heat treatment in atmosphere on surface flashover characteristics of alumina in vacuum

Alumina Ceramics are widely used for vacuum insulation in devices such as vacuum interrupters. In development and design of vacuum interrupters, it is very important to improve the flashover characteristics on alumina ceramic surfaces in a vacuum. Thus, it is necessary to clarify the flashover mechanism on alumina ceramics in a vacuum. From past research, it is well-known that these characteristics are influenced by the insulation material, its surface condition, surface charging of the insulator and so on.(1) This paper presents the results of measurements of surface flashover characteristics and the cathode luminescence (C.L.) spectra after heat treatment in the atmosphere. These results are compared with those without heat treatment in the atmosphere. It is thus found that oxygen defects on an alumina ceramic surface decreases and the flashover voltage is improved by heat treatment in the atmosphere.

Breakdown characteristics of CuCr Electrode after electron beam treatment and current conditioning treatment in vacuum

For development of high-voltage equipment such as vacuum circuit breakers, it is very important to improve their dielectric breakdown characteristics in a vacuum. It is thus necessary to clarify the breakdown mechanism in a vacuum. It is well-known that these characteristics are influenced by the electrode material and its surface condition and the breakdown voltage is increased by current conditioning and electron beam irradiation [1] [2] [3] . This paper presents breakdown characteristics and observations of a cross-section after electron beam treatment and current conditioning treatment. The results are compared with those for only electron beam treatment and only current conditioning treatment. It is shown that the breakdown voltage of a CuCr electrode both treatments is higher than that achieved by only one of them. Treatment combining electron beam and current conditioning forms a CuCr layer with small Cr grains. The breakdown voltage after both these treatments is increased by the formation of this layer.

Effect of chromium oxide coating on surface flashover characteristics of ceramic in vacuum

Experiments were conducted to determine the conditioning effects of a chromium oxide coating on a ceramic surface. Chromium oxide is known to have a small secondary electron emission coefficient. With a chromium oxide coating, conditioning effects were completed early and flash over voltage increased. While luminescence was observed on a bare ceramic surface under a high applied voltage, none was observed on the sample coated with chromium oxide. One cause of increasing flashover voltage due to coated chromium oxide is depression of the secondary electron avalanche. Another is considered to be depressed surface charge due to decreasing surface resistance

Basic study on surface insulation design of solid/gas composite insulation system with embedded electrode

Solid insulated switchgear (SIS) has been developed as a substitution for medium voltage SF6 gas insulated switchgear. Its main circuits including a vacuum interrupter are coated with epoxy resin and a solid/gas insulation system is formed. For further miniaturization and higher stress design, the authors investigated the surface insulation performance of the solid/gas insulation system containing an embedded electrode. In the previous study, the authors investigated the surface breakdown characteristics. The authors also investigated the surface discharge propagation characteristics and proposed an estimation method of the surface breakdown voltage. In the present study, the authors investigated the surface breakdown characteristics numerically. The calculation was done based on the estimation method proposed previously. The divided voltage ratio, the radius of the embedded electrode, the enhancement factor of the surface insulation strength and the peak value of the surface breakdown voltage are examined. As a result, the characteristics of the parameters and the criteria for the optimum insulation design were clarified.

Fundamental research of uniform vacuum arc control by magnetic field and its application to VCB

The vacuum arc behavior and high current interruption ability of electrodes applied in the uniform vacuum arc control method are presented. The axial magnetic field (AMF) distribution of this method is unlike that of the conventional AMF distribution. For extremely high interruption current, the arc of the conventional AMF electrode concentrates, causing local heating of the electrode, but the arc of this method diffuses over the whole contact area thus avoiding local heating. The interruption characteristic is investigated. The relationship between the arc energy and the electrical field after the current zero was estimated. The relation among the electrode structure, the arc behavior and the magnetic field distribution between the electrodes is investigated. The vacuum interrupter with this vacuum arc control method is applied for Solid Insulated Switchgear (SIS). These vacuum interrupters are demonstrated to be higher interruption ability than the conventional AMF vacuum interrupter.

Influence of coating to surface flashover characteristics of alumina ceramics in vacuum

Alumina ceramics are widely used for vacuum insulation in devices such as vacuum interrupters. In development and design of vacuum interrupters, it is very important to improve the flashover characteristics on alumina ceramic surfaces in vacuum. Thus, it is necessary to clarify the flashover mechanism on alumina ceramics in vacuum. From past research, it is well-known that these characteristics are influenced by the insulation material, its surface condition, surface charging of the insulator and so on. This paper presents the results of measurements of surface flashover characteristics and observation of the surface and cross-section surface of Al₂O₃ coated Cr₂O₃ by SEM. These results are compared with those without coated Cr₂O₃. It is thus found that Cr₂O₃ coating decreased the surface resistance and improved the surface flashover characteristics of Al₂O₃ in vacuum.

Characteristics of vacuum breakdown field and comparison of field enhancement factor β in CuCr electrodes with slit configuration

A contact electrode of vacuum interrupter (VI) has slit parts on the electrode surface to control the vacuum arc during the current interruption. The edge configuration of the slit is important for the withstand performance of high voltage application for VI. We investigated the effect of the curvature radius of slits configuration on the field enhancement factor β and the characteristics of vacuum breakdown for the slit electrodes made of CuCr alloy simplified simulating the AMF-electrode. As the results, the round chamfered configuration at the edge of slit increases the breakdown field, because of the reduction of pre-breakdown current flowing, that is, small field emission current. This is due to be small β-value by rounding chamfering of the edge at the slit. The BD field of the slit electrode can be improved by rounding the edge of the end of the slit more than 1 mm curvature radius, and the curvature of 0.5 mm radius for R0.5 slit electrode had almost little effect on improving the BD field owing to manufacturing inadequate curvature at the edge of slit.

Estimation of surface breakdown voltage of solid/gas composite insulation with embedded electrode

Solid insulated switchgear (SIS) has been developed as a substitution for medium voltage SF6 gas insulated switchgear. Its main circuits including a vacuum interrupter are coated with epoxy resin, therefore, a solid/gas insulation system is formed. For further miniaturization and higher stress design of SIS, the authors investigated on the surface insulation of the solid/gas insulation system containing an embedded electrode. In the present study, the surface breakdown voltage was estimated based on the surface discharge propagation characteristics. From the measurement of the surface flashover voltage on a dielectric against the surface distance, the relation between the surface flashover voltage and the surface distance was found to be expressed with an equation derived from the empirical equation presented by Toepler. Then, the surface breakdown voltage was estimated based on the obtained equation. The estimated values were compared with the experimental data obtained varying a parameter of the radius of the embedded electrode, thickness of the epoxy resin plate or the radius of the plate. As a result, a good correlation was obtained. Therefore, the estimation method of the surface breakdown voltage on the solid/gas insulation with embedded electrode was established.

Investigation of relation between miniaturization of Cr and breakdown characteristics of CuCr electrode in vaccum

CuCr alloys are widely used in vacuum interrupters. For development of vacuum interrupters, we need to improve the breakdown characteristics of CuCr electrodes, which are influenced by the electrode surface condition. It has been found that both the electron beam and current conditioning treatment modify the CuCr electrode surface which improves the breakdown characteristics. However, the relation between Cr particle size in this melting layer and the breakdown voltage is not clear. Therefore, we measured the breakdown voltage and Cr particle size after several kinds of surface treatment in order to investigate the relation between Cr particle size in the treatment layer and the breakdown voltage. Results show that miniaturization of Cr particles tends to improve the breakdown characteristics of CuCr electrodes in a vacuum.