Hiromi Koga

Restrike characteristics on capacitive current breaking in vacuum

Restrike characteristics on a capacitive current breaking in a vacuum have been investigated. In a small capacitive current breaking test, double voltage from commercial voltage (peak) would be applied between contacts of a VI. The configuration of contacts and the current conditioning dependence of VI practical samples with axial magnetic field contacts were investigated under a simulated capacitive current breaking test. Minimum restrike voltage decreased and voltage dispersion was high due to the presence of slits on the contacts. On the other hand, the minimum restrike voltage increased and restrike voltage dispersion was low due to current conditioning. By practical tests and standardized tests, a new 72 kV VI and dry air insulated VCB has been developed.

The Effect of contact material composition of AgWC and Axial Magnetic Field intensity on interrupting capability and chopping current

Silver tungsten-carbide (AgWC) has been used for contact materials of low surge type vacuum interrupter because of their properties such as low chopping current and low erosion. However the interrupting capability of AgWC is relatively less than CuCr. Generally, in order to increase the interrupting capability, axial magnetic field (AMF) is applied for large rated currents. On the other hand, it is known that the contact materials greatly affect the interrupting capability and chopping current. In this paper, in order to increase the interrupting capability of low surge type vacuum interrupters, the effect of the intensity of AMF and composition of AgWC under AMF on the interrupting capability were investigated. The contents of Ag and WC were varied and cobalt (Co) was added as a third element. Also the chopping currents were investigated. As a result, it was found that an increase of Co and a decrease of Ag content tended to increase the interrupting capability

Flashover voltage of alumina ceramic contaminated by metal vapor in vacuum interrupters

We investigated the influence of metal vapor contamination of ceramic surfaces on flashover voltage (FOV) in vacuum. First, disk shape alumina (Al/sub 2/O/sub 3/) ceramics with surface resistivity (/spl rho/) of 10/sup 2/-10/sup 15/ /spl Omega/ were produced using deposition phenomena of metal vapor emitted from CuCr contacts. The impulse FOV for the ceramics decreased, as /spl rho/ reduced; FOV, the conditioning effect on FOV, and the scattering of FOV decreased when /spl rho/ was below 10/sup 12/ /spl Omega/. Therefore, the criterion value /spl rho/, which maintains excellent flashover performances of ceramic surface, is 10/sup 12/ /spl Omega/. Second, experimental vacuum interrupters (VIs) were produced to measure breakdown voltage before and after forty short-circuit current switchings with 20-40 kA/sub rms/ and were disassembled to measure the /spl rho/ of their inner ceramic surface. In a VI, which has inside diameters at both ends of the main shield much larger than the contact diameter, /spl rho/ was reduced to 10/sup 4/ /spl Omega/, further decreasing breakdown voltage between terminals.

Vacuum Switch Chopping Current in an Inverter Circuit

When small currents are interrupted by vacuum switch, it is well known that chopping current and the associated overvoltage are observed. Various investigations on the current-chopping phenomena in AC circuits have been reported, but there are no data available on inverter circuits in spite of their recent wide application to synchronous motor systems. In this paper, chopping-current levels and the overvoltages in an inverter circuit under a simple three level pulse width modulated (PWM) control are investigated. As a result, the mean chopping-current level is 3–4 times higher than that in an AC circuit. In particular, high chopping currents occur only when the timing of the current interruption by the vacuum switch synchronizes with that of the inverters switching, which we call ‘double chopping’. High chopping current due to double chopping might be harmful since the high overvoltage proportional to the chopping-current level could damage the insulation of electric equipment. However, we experimentally confirmed that a typical surge suppressor could reduce overvoltage.