Vacuum Circuit
Breaker
A vacuum circuit breaker is such kind of circuit breaker where the
arc quenching takes place in vacuum. The technology is suitable for mainly
medium voltage application. For higher voltage Vacuum technology has been
developed but not commercially viable. The operation of opening and closing of
current carrying contacts and associated arc interruption take place in a
vacuum chamber in the breaker which is called vacuum interrupter. The vacuum
interrupter consists of a steel arc chamber in the centre symmetrically
arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is
normally maintained at 10 - 6 bar.
The material used for current carrying contacts plays an important
role in the performance of the vacuum circuit breaker. CuCr is the most ideal
material to make VCB contacts. Vacuum interrupter technology was first
introduced in the year of 1960. But still it is a developing technology. As
time goes on, the size of the vacuum interrupter is being reducing from its
early 1960’s size due to different technical developments in this field of
engineering. The contact geometry is also improving with time, from butt
contact of early days it gradually changes to spiral shape, cup shape and axial
magnetic field contact. The vacuum circuit breaker is today recognized as most
reliable current interruption technology for medium voltage system. It requires
minimum maintenance compared to other circuit breaker technologies
.
Advantages of vacuum
circuit breaker or VCB
Service life of Vacuum Circuit Breaker is much longer than other
types of circuit breakers. There is no chance of fire hazard as oil circuit
breaker. It is much environment friendly than SF6 Circuit breaker. Beside of
that contraction of VCB is much user friendly. Replacement of Vacuum Interrupter
(VI) is much convenient.
Operation of Vacuum
Circuit Breaker
The main aim of any circuit breaker is to quench arc during current
zero crossing, by establishing high dielectric strength in between the contacts
so that reestablishment of arc after current zero becomes impossible. The
dielectric strength of vacuum is eight times greater than that of air and four
times greater than that of SF6 gas. This high dielectric strength makes it
possible to quench a vacuum arc within very small contact gap. For short
contact gap, low contact mass and no compression of medium the drive energy
required in vacuum circuit breaker is minimum. When two face to face contact
areas are just being separated to each other, they do not be separated
instantly, contact area on the contact face is being reduced and ultimately
comes to a point and then they are finally de-touched. Although this happens in
a fraction of micro second but it is the fact. At this instant of de-touching
of contacts in a vacuum, the current through the contacts concentrated on that
last contact point on the contact surface and makes a hot spot. As it is
vacuum, the metal on the contact surface is easily vaporized due to that hot
spot and create a conducting media for arc path. Then the arc will be initiated
and continued until the next current zero.vacuum interrupter At current zero
this vacuum arc is extinguished and the conducting metal vapour is re-condensed
on the contact surface. At this point, the contacts are already separated hence
there is no question of re-vaporization of contact surface, for next cycle of
current. That means, the arc cannot be reestablished again. In this way vacuum
circuit breaker prevents the reestablishment of arc by producing high
dielectric strength in the contact gap after current zero.
There are two types of arc shapes. For interrupting current up to
10kA, the arc remains diffused and the form of vapour discharge and cover the
entire contact surface. Above 10kA the diffused arc is constricted considerably
by its own magnetic field and it contracts. The phenomenon gives rise over
heating of contact at its center. In order to prevent this, the design of the
contacts should be such that the arc does not remain stationary but keeps
travelling by its own magnetic field. Specially designed contact shape of
vacuum circuit breaker make the constricted stationary arc travel along the
surface of the contacts, thereby causing minimum and uniform contact erosion.
.
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ReplyDeleteThe vacuum circuit has both advantage and disadvantage, it must be evaluated well to consider having one. - WAENZ
ReplyDelete