Monday, April 4, 2011

Vacuum circuit breaker

5. Description of different Circuit Breaker
5.5 Vacuum circuit breaker
5.5.1 History
       In 1972 Professor Sorensen and his assistants at the California Institute of technology achieve significant in the research on switching of currents in a vacuum. The developed theoretical knowledge greatly preceded the level of current technology and couldn't offer a simple and reliable piece of equipment to register micro-leaks and to control the level of vacuum in the chambers. Problem such as copper contacts' tendency to form cold welds in vacuum and generation of over voltages due to premature arc extinction of the refractory contacts could not be resolved. The above mentioned technical problems led to the stagnation of vacuum switching technology development.
       Almost twenty years passed before vacuum technology saw a new shift in its development. In the middle of 1950's a technical breakthrough took place in the development of semiconductors, which allowed developing an industrial method of cleaning gas from copper contacts. The invention of sensitive mass spectrometers allowed registering even the smallest leaks in vacuum chambers. Based on fundamental and applied research, construction bureaus manufactured and successfully tested a whole line of vacuum arc extinction chambers. Figure 26 show one of the first vacuum chambers.
Figure 26 On of the first vacuum chambers with wolfram contacts with a diameter of 1.25 cm, contact spacing of 0.63 mm, nominal current 2000 Amp

       As a result of implementation of current achievements, it became possible to create a test vacuum arc extinction chamber (AEC) that successfully performed approximately 90 current breaking operations in the 10-40 KV voltage which was impossible for oil circuit breakers. The research of switching current in vacuum became interesting in Russia in the 1960's. It was at this time that GE introduced the first commercial series of vacuum circuit breakers. It is necessary to state that in the 60's only tow countries, Russia and the USA possessed the required technology to manufacture AECs. The specific technologies were the manufacture of oxygen free copper, creation of metal ceramic alloys and the manufacture of metal alloys with a coefficient of heat expansion analogical to glass ceramics.

5.5.2 The theory of Current Switching in Vacuum
       If contacts carrying current disconnect, an electric arc is created between them, which supports a high conductivity of the space between the contacts and allow the current leaking between the contacts as if were still closed (Fig. 27). This cause the element between the contacts to heat a temperature high enough to disconnect (break-up) its molecules an d ionize the space. The existence of an electric arc is characterized by the quasi-equilibrium state, when the arc voltage self regulates at a level sufficient to maintain the conductivity of the formed plasma, where the diameter of the arc column shrinks or expands depending on the changes of the value of the passing current.
Figure 27  Photograph of an electric arc in the space between the contacts at 15-20 k Amp current1
       The energy produced at this moment goes into the surrounding environment due to thermal conductivity, convection and radiation. This energy balance must be destroyed if the arc is to be suppressed and current is to be disconnected. When the current reaches natural zero, the energy stops coming from the network. By intensively cooling the space between the contacts at this moment, it is possible to break the arc, disturb the space conductivity and restore its electric stability and by doing this to accomplish the disconnection.
Figure 29   Photograph of an arc looking as a braid

1 التعليقات:

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