High-voltage switches with arc preventing or extinguishing devic – Arc preventing or extinguishing devices – Vacuum
Reexamination Certificate
2000-02-01
2002-02-12
Donovan, Lincoln (Department: 2832)
High-voltage switches with arc preventing or extinguishing devic
Arc preventing or extinguishing devices
Vacuum
C218S130000
Reexamination Certificate
active
06346683
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a vacuum interrupter that performs interruption/conduction of current in vacuum, and to a vacuum switch wherein this vacuum interrupter is mounted, more particularly, it relates to improvements in the contact resistance characteristic and restriking characteristic of the contacts of the vacuum interrupter.
2. Description of the Related Art
In order to maintain or improve, apart from the three fundamental requirements typified by the anti-welding characteristic, voltage withstanding characteristic and interruption characteristic, the current chopping characteristic, erosion characteristic, contact resistance characteristic and temperature rising characteristic etc., the contacts of vacuum interrupters mounted in a vacuum switch or vacuum circuit breaker are constituted of various base materials. However, it is considered to be impossible for these to be fully satisfied by a single element, since the above required characteristics often demand mutually contradictory material properties.
Accordingly, contact materials have been developed for specific applications such as large current interruption applications or high withstand-voltage applications, by use of composite materials or by base material cladding etc., and these exhibit excellent characteristics in their own way. For example, as contact materials for large current interruption satisfying the three fundamental requirements, there are known Cu—Bi alloys, or Cu—Te alloys containing 5 weight % or less of anti-welding constituents such as Bi or Te (Issued Japanese patent Sho. 41-12131, and Issued Japanese patent number Sho. 44-23751).
Cu—Bi alloy has excellent large-current interruption characteristics, since a low welding separation force is achieved by the embrittlement of the alloy itself which is produced by the presence of brittle Bi segregated at grain boundaries. Likewise, Cu—Te alloy has excellent large-current interruption characteristics, since a low welding separation force is achieved by the embrittlement of the alloy itself which is produced by the presence of brittle Cu
2
Te segregated at grain boundaries and inner grains.
In contrast, Cu—Cr alloy is known as a contact material for high withstand-voltage/large current interruption use. This alloy has a smaller vapor pressure difference between its structural constituents than have the aforementioned Cu—Bi alloy or Cu—Te alloy, and so has the advantage that it can be expected to exhibit uniform performance, and indeed is excellent, depending on the application. Cu—W is also known as a high withstand-voltage contact material. These alloys exhibit excellent anti-arcing characteristics, on account of the effect of the high melting point materials.
In a vacuum circuit breaker and/or vacuum switch, the phenomenon may be induced that, after current interruption, flashover occurs within the vacuum interrupter, causing a conductive condition between the contacts to be re-established (subsequent discharge does not continue). This phenomenon is called the restriking phenomenon, but the mechanism of its occurrence has not yet been elucidated. Abnormal over-voltages frequently occur on account of the rapid change to a conductive condition after the electrical circuit was first put in the current-interrupted condition. In particular, in tests wherein restriking was produced on interruption of a condenser bank, the occurrence of extremely large over-voltages and/or excessive high-frequency current was observed. The development of a technique for lowering the probability of restriking is therefore sought.
Although, as described above, the mechanism of occurrence of the restriking phenomenon is not known, according to the experimental results of the inventors, restriking occurs with fairly high frequency between one contact and another contact or between the contacts and the arc shield within the vacuum interrupter. Accordingly, the inventors succeeded in greatly reducing the number of occurrences of restriking by discovering that techniques for suppressing abrupt gas that is discharged for example when the contacts are subjected to arcing and techniques for optimization of the contact surface condition are extremely effective in lowering the probability of restriking.
In recent years, however, to meet demands for improving the voltage withstanding performance and demands for improving the large current interruption performance of vacuum interrupters, in particular demands for miniaturization, further reductions in restriking of the contacts are required. Specifically, in recent years, severity of the conditions of use demanded by users and of the variety of loads have increased. A marked recent trend is increasingly frequent application to reactor circuits and capacitor circuits. The development and improvement of contact materials for this has become an urgent task.
In the case of capacitor circuits, about two or three times of the usual voltages are applied, so the surface of the contacts is severely damaged by arcing during the current interruption or current switching, and, as a result, surface roughening and exfoliative erosion of the contacts is promoted. Such surface roughness and/or exfoliation increases contact resistance, and is believed to be a factor causing restriking. Thus, although it is unclear which is the initial trigger, cause and effect are repeated, with the result that the frequency of occurrence of restriking and the contact resistance both increase. However, notwithstanding the importance of the phenomenon of restriking from the point of view of product reliability, and neither a way of preventing it nor its direct causes have yet been elucidated.
When the inventors observed in detail the correlation with occurrence of restriking of the total quantity of gas discharged in the heating step of Cu—W alloy or Cu—Mo alloy, the type of gas and its mode of discharge, they discovered that, in the case of contacts where there was considerable abrupt discharge of gas in pulse fashion in the vicinity of the melting point, albeit for a very short time, the rate of restriking was also high.
Accordingly, the restriking phenomenon was reduced by subjecting the Cu, W raw material or Cu, Mo raw material or Cu—W contact alloy or Cu, Mo contact alloy beforehand to heating in the vicinity of the melting temperature or above the melting temperature, or removing beforehand factors causing the discharge of abrupt gas in the Cu—W alloy or Cu, Mo contact alloy, or high temperature aging of the Cu—W contact surface layer or Cu—Mo contact surface layer or by improving sintering techniques so as to suppress pores and/or structural segregation in the Cu—W alloy or Cu—Mo alloy.
However, with the further demands for suppression of restriking in recent years, the need for further improvements has been recognized and in particular development of other strategies has become important.
As described above, for high withstand-voltage contact materials, Cu—W alloy or Cu—Mo alloy were used in preference to the Cu—Bi alloy, Cu—Te alloy or Cu—Cr alloy described above, but in fact they cannot be described as contact materials that can fully meet the increasingly severe requirements for reduction of restriking. Specifically, even in the case of Cu—W alloy or Cu—Mo alloy which have been preferentially used hitherto, occurrence of restriking in more severe high voltage regions and in circuits where there is rush current, or the existence of instability of the contact resistance characteristic caused by the material properties of the Cu—W alloy or Cu—Mo alloy have been identified as problems.
Accordingly, the development of contact material for vacuum interrupters having in particular excellent restriking characteristics and contact resistance characteristics, while still maintaining a certain level of the aforementioned fundamental three requirements, is desired.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a novel vacuum interrupter and vacuum switch in which this is
Homma Mitsutaka
Kusano Takashi
Nishimura Takanobu
Ohshima Iwao
Okutomi Tsutomu
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