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This paper is described on the charging characteristics of floating electrode modeled on Vacuum Interrupter (VI). The experimental sample has a floating electrode between two ceramics cylinders. We prepared two kinds of samples which are equipped with or without shield rings inside of the floating electrode to relax an electric field around the triple junction of floating electrode. A charging signal was measured by a surface potential meter as a charging potential.As a result, in case of low voltage application, only an induced voltage in the floating electrode was measured depending on the capacitive dividing voltage of the sample and grounded objects around the sample. In case of high voltage application, not only induced voltage but also a positively charging potential was observed in the floating electrode regardless of sample configuration or polarity of applied voltage. This is due to the field electron emission from the floating electrode itself. The positive charging potential was increased almost linearly to the applied voltage. Comparing the samples, it was found that the sample without shield rings began to form a positive charge at a lower voltage application rather than the sample with shield rings.

In a vacuum interrupter (VI), surface flashover events are one of the obstacles for improving its higher voltage performance. To suppress the vacuum flashover along the insulator surface, field relaxation rings apply to it around the connection point between the metal and alumina insulator, which is called as Triple Junction (TJ) in order to reduce the high field around the TJ point. To obtain higher voltage performance in vacuum interrupter, it is necessary to optimize the shape of field relaxation rings for example, its length, tip radius and gap length etc. In this study, we investigated the vacuum flashover voltage in model vacuum interrupter samples which were equipped with the field relaxation rings inside cylindrical alumina insulator. We carried out the flashover tests for the model samples changing the length of field relaxation rings. As a result, it was confirmed that the install of field relaxation ring increased the vacuum flashover voltage in vacuum interrupter. But in case of the short ring length, the flashover performance was inferior to that of sample having a certain length ring. On the other hand, in case of applying a long ring length, the flashover voltage also decreased rather than the certain appropriate ring length. Therefore, the design of ring length is necessary to select the appropriate length considering the field relaxation for TJ point and the tip of shield rings.

This paper deals with charging characteristics on the insulator surface in vacuum after the AC voltage application. In designing Vacuum Interrupters (VIs) which is current interruption parts of Vacuum Circuit Breakers (VCBs), one of the consideration points is the control of surface flashover events in vacuum. It is believed that such a flashover occurs via charging on the insulator surface. In this study, we measured the charge distributions of cylindrical alumina insulators in vacuum after the AC voltage application. Those insulator samples are equipped with electric field relaxing shield rings (shields) at both ends of the insulator to relax electric field around the triple junction (TJ), and we prepared three kind of shield rings which have different gap length from the alumina insulator surface. As a result, positive charges were formed on the insulator surface of all samples by voltage application. The amount of charge was increased almost linearly to the applied voltage. In addition, when repeating high voltage application test under same condition, the amount of charge was gradually decreased by conditioning effect of shields, and finally reached a stable value by repeating voltage application tests. After that, when a higher voltage rather than above conditioning voltage was applied to the experimental sample, the charge amount was slightly increased.

In vacuum devices such as vacuum interrupters (VIs), surface flashover events are one of the obstacles for developing their higher performance. It is believed that such a surface flashover in vacuum has a close relationship with a charging phenomenon on the insulator surface. Therefore, it is important to grasp the charging characteristics on the inner surface of the VI. In this study, we have measured the charge distribution characteristics inside a cylindrical type insulator employed as a model insulator of VI after the AC voltage (50 Hz) application. This cylindrical insulator has shield rings to relax the electric field at triple junction at both ends of the insulator. We investigate the charging characteristics for samples having different gaps between the shield and insulator. As a result, in the case of first AC voltage application to each sample, the vacuum side of the cylindrical insulators were charged to a certain value. As repeating the AC voltage application several times, the amount of electric charge on the insulator surface was gradually reduced in each sample. However the reduction rate of the sample with the narrow clearance is larger than wide one. In addition, the sample with the wide clearance was charged on a larger surface area than that of the narrow clearance.