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 |
