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Internal dielectric strength of vacuum interrupters (VIs) is primarily determined by the macroscopic electric field strength and effective area on the electrode surfaces. While internal dielectric strength is usually not an issue for medium-voltage VIs ($U_{m}$ ≤ 52 kV), it requires special consideration when going to higher voltage levels. Generally, the vacuum breakdown is initiated by field emissions from critical enhancement points with higher electric field on a cathode. The most critical point of the macroscopic electric field in a VI is present at the front contact edge between the main contacts. A vapor shield of a VI is one component that determines the electric field strength and distribution. Its floating electrical potential can basically be determined by the geometry of the VI and its surroundings. Since the shield potential can change the maximum electric field strength at the front contact edge in the VI, it seems possible to increase the internal dielectric strength of the VI by giving the vapor shield a well-defined electric potential. The present study focuses on the possibility of electrical potential control of the vapor shield for increasing the internal withstand voltage of the VI. Basically, the potential control of the vapor shield can be realized by connecting a parallel capacitor network. This allows controlling the shield potential arbitrarily between H.V. potential and zero by changing the voltage division ratio. From the field calculation results for a 72.5 kV VI model, the maximum electric field strength can be reduced by up to 23.3% compared to a floating vapor shield. Dielectric withstand tests for the 72.5 kV VI with potential control to determine the internal breakdown voltage with positive impulse voltage were carried out. The experimental results show that the breakdown voltage can be increased by 11.2%, by setting the vapor shield potential to one-third of the applied voltage. The increase in the breakdown voltage of the VI can be explained by the relaxation of the electric field on the critical point as a result of the decrease in the potential difference between cathode and vapor shield. [ABSTRACT FROM AUTHOR]

Surface discharge on solid insulator in vacuum is one of the significant issues for the development of vacuum circuit breakers (VCBs) and vacuum interrupters (VIs). For the development of higher voltage VCBs and VIs, control and suppression techniques of the surface flashover are required. The aim of present study is to suppress the surface flashover, based on physical mechanisms of flashover development. Impulse surface flashover in vacuum, starting with explosive electron emission (EEE) from cathode, develops with the formation of transient charge on the insulator as well as the development of secondary electron emission avalanche (SEEA) along the surface toward anode. To reduce the EEE current from the cathode and the SEEA development, we changed the electrode geometry such as thickness of alumina ceramics and tip radius of cathode rod, respectively, and installed a plane electrode as the back electrode of the cathode rod. As the results, EEE current was reduced by the field relaxation effect of transient charging due to EEE, and SEEA was also suppressed by the perpendicular electric field of the plane electrode. When the flashover was suppressed by using the cathode back electrode, transient charge amount was smaller on alumina dielectrics at the end of the plane electrode area. From the above results, it would be an effective technique for the control and suppression of surface flashover in vacuum to enlarge the perpendicular electric field for the insulator. [ABSTRACT FROM PUBLISHER]

SUMMARY For the development of higher voltage vacuum circuit breakers and vacuum interrupters, surface discharge in a vacuum is one of the most significant issues. We measured the charge activity on alumina dielectrics in a vacuum through ultrahigh-speed optical and electrical measurements. We found that surface flashover in a vacuum starts with an explosive electron emission (EEE) with higher current density, which consists of electron emission from the cathode, and results in surface flashover development. The transient charge on the alumina dielectrics formed by electrons emitted by EEE influences the distortion of electric field distribution around the cathode rod and then the subsequent flashover development. In particular, in the case of a shorter gap between the cathode rod and the alumina dielectrics, the electron emission may be suppressed by the relaxation effect of the electric field due to transient charge accumulation. [ABSTRACT FROM AUTHOR]

Surface discharge on solid insulator in vacuum, which is one of the most significant issues under high electric field, prevents the development of higher voltage vacuum circuit breakers and vacuum interrupters. To clarify the mechanism of the surface discharge in vacuum, charge activities on solid insulators in the development process of the surface discharge should be investigated. By using electrostatic probes behind the alumina dielectrics and measuring fast-framing images of the surface discharge, in the present paper, we clarified that the impulse surface flashover starts in developing with explosive electron emission (EEE) on cathode, which is specific phenomenon on the electron emission site exploding due to extreme field emission current density. Experimental results revealed the characteristics of transient charging by EEE in the surface flash over development as follows: (1) the probe current due to EEE increases with the EEE inception electric field on the cathode, and also depends on the work function of the cathode material, (2) the transient charge distribution on the alumina dielectrics is determined by the EEE spot on the cathode, (3) EEE occurs several times due to the erosion and the regeneration of electron emission sites. (4) the successive EEE makes the transient charge distribution smooth on the alumina dielectrics. [ABSTRACT FROM PUBLISHER]

Surface discharge on solid insulators is one of the significant issues in the development of vacuum circuit breakers (VCB) and vacuum interrupters (VI). We measured the charge activity on alumina dielectrics in vacuum through ultra-high speed and high sensitivity optical measurements. We identified the existence of charge activity with a small light emission on alumina dielectrics before the surface flashover development process, and defined this as a "pre-discharge" and measured its characteristics. We classified the charge activities into three processes: (1) pre-discharge, (2) surface flashover initiation, and (3) surface flashover expansion, in the development of surface discharge in vacuum. Utilizing this classification, we found that the surface predischarge in vacuum is generated by the electron emission from the cathode, and the subsequent flashover initiates with a steep increase in current and develops with the distortion of the electric field due to transiently-formed charging on the alumina dielectrics in vacuum. [ABSTRACT FROM AUTHOR]