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Vacuum Circuit Breaker

Published in: Electrical
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Vacuum Circuit Breaker and Its Details.

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    The vacuum medium ' Every medium that has a pressure below atmospheric (760 mm of mercury) is known as vacuum. ' Torricelli is known to be the first man who succeeded in evacuating a space by building his mercury barometer.
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    Principle When the contacts of the breaker are opened in vacuum, an arc is produced between the contacts. However, the arc is quickly extinguished because the metallic vapors, electrons and ions produced during arc rapidly condense on the surfaces of the circuit breaker contacts, resulting in quick recovery of dielectric strength.
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    Construction of Vacuum Circuit Breaker ' It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak. A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapours falling on the inside surface of the outer insulating cover.
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    Why stainless steel ? ' 1. Excellent corrosion resistance. ' 2. Increased resistance to chloride attack. ' 3. Good resistance to stress corrosion cracking. ' 4. Tensile and yield strength higher
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    Why dielectric strength taken under consideration it decreases with increased sample thickness. it decreases with increased operating temperature. it decreases with increased frequency. for gases (e.g. nitrogen, sulfur hexafluoride) it normally decreases with increased humidity. for air, dielectric strength increases slightly as humidity increases
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    working The working of Vacuum circuit breakers is briefly explained below, When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of arc is due to the ionization of metal ions and depends very much upon the material of contacts. The arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc are diffused in a short time and seized by the surfaces of moving and fixed members and shields. Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in a vacuum breaker occurs with a short contact separation (say 0.625 cm). Vacuum circuit breakers are employed for outdoor applications ranging from 22kV to 66kV.
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    Advantages of vcb Simple construction. Self contained i.e., no need of periodic refilling of gas or oil. Compact size. Low power requirement for making and breaking operations. Pollution free Long life Non-explosive. Suitable for repeated operating duty. High speed of dielectric recovery. Silent operation. Low maintenance. Capable of interrupting highly inductive and capacitive currents without restricting.
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    Limitations of vcb ' Installation cost of vcb is very high. The main working component, the vacuum interrupter is unrepairable and need to be replaced with a few unit. ' (a vacuum interrupter is a switching device that uses electrical contacts in a vacuum. It is the core component of medium-voltage switches.) Generally , current chopping takes place at very low values of current and current chopping leads to overvoItage.(Current Chopping in circuit breaker is defined as a phenomena in which current is forcibly interrupted before the natural current zero. Current Chopping is mainly observed in Vacuum Circuit Breaker )
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    Applications of VCB Capacitor bank switching. Reactor switching. Transformer switching. Line dropping.
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    10 Temperature 0K Figure 16.3: Vapout pressure vs. temperature for various pure metals
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    Transition Vacuum Gas Gap length Pressure Figure 16.5: Schematic Paschen curve and its retatÜt to vacuum as a function of gap length
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    vce The source side of the network is represented by: - a generator with a driving voltage of U - a short circuit impedance of value LS - a T RV network as defined by IEC for short circuit conditions - a capacitor to account for parallel feeders
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    The load side of the network is represented by: - a 8MVAr capacitor bank with floating neutral - single phase cables between CB and the bank The load and the source side of the network are connected to each other by the CB. The CB is treated as an ideal breaker with a NSDD some time after current interruption. The moment of NSDD has been chosen in order to create the worst possible case. The CB is allowed to recover immediately at the first occurring current zero. Parasitic capacitance's on both sides of the CB have been neglected intentionally as they tend to mask the dominant process. [1] At a NSDD, on the middle phase for the case illustrated by fig. 2, the
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    Generally the arc extinction in a circuit breaker take place at natural current zero.


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