As discussed earlier circuit
breakers working principle of type of arc quenching medium during breaking of
circuit breaker. In case of Vacuum
circuit breakers 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 circuit breaker 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 electric
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. It
requires minimum maintenance compared to other circuit breaker technologies.
Advantages of Vacuum Circuit Breaker or VCB
1.Service life of vacuum circuit
breaker is much longer than other types of circuit breakers
2.There is no chance
of fire hazard as oil circuit breaker.
3.It is much environment friendly than SF6
Circuit breaker. Beside of that contraction of VCB is much user friendly.
4.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 electric current zero crossing, by establishing high
dielectric strength in between the contacts so that reestablishment of arc
after electric 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 electric
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
electric current zero.
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Vacuum interrupter
At electric current zero this vacuum
arc is extinguished and the conducting metal vapor 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 electric current zero.
Vacuum circuit breaker is shown as above
There are two types of arc shapes.
For interrupting electric current up to 10 kA, the arc remains diffused and the
form of vapor discharge and cover the entire contact surface. Above 10 kA 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.
Preliminary checks before maintenance of Vacuum circuit breaker:-
Following checks must be done before doing maintenance of Vacuum circuit breaker:-
(i) Ensure that Power supply must be disconnected
(ii) By using earthing rod discharge all fixed and moving contacts.
(iii) Check externally for any damages to Circuit breaker
Maintenance of vacuum Circuit breakers:-
Regular and timely maintenance of switchgear always keeps the system healthy and increases life of switchgear. It is recommended to do maintenance related activities such as:-
(i) cleaning and lubrication of fixed and moving parts
(ii) Calibration
(iii) Checking of vacuum by using vacuum checking kit, also known as High pot test.
(iv) Closing and opening mechanism of circuit breaker
(v) Working of motorized operations
(vi) Lubrication of spring charging mechanism
(vii) Working of all trip interlocks
(viii) Checking of working of trippings.
(ix) Working of all mechanical interlocks
(x) Check for all control wiring and all fuses
(xi) Check for all instrument transformers
(xii) Check circuit breaker integrity by using meggar of 5000 V
Frequency of Maintenance of breaker:-
Timely maintenance must be ensured , it is always recommended that maintenance interval for vacuum circuit breaker should be Every two years or 2000 operations whichever is earlier.
Preliminary checks before maintenance of Vacuum circuit breaker:-
Following checks must be done before doing maintenance of Vacuum circuit breaker:-
(i) Ensure that Power supply must be disconnected
(ii) By using earthing rod discharge all fixed and moving contacts.
(iii) Check externally for any damages to Circuit breaker
Maintenance of vacuum Circuit breakers:-
Regular and timely maintenance of switchgear always keeps the system healthy and increases life of switchgear. It is recommended to do maintenance related activities such as:-
(i) cleaning and lubrication of fixed and moving parts
(ii) Calibration
(iii) Checking of vacuum by using vacuum checking kit, also known as High pot test.
(iv) Closing and opening mechanism of circuit breaker
(v) Working of motorized operations
(vi) Lubrication of spring charging mechanism
(vii) Working of all trip interlocks
(viii) Checking of working of trippings.
(ix) Working of all mechanical interlocks
(x) Check for all control wiring and all fuses
(xi) Check for all instrument transformers
(xii) Check circuit breaker integrity by using meggar of 5000 V
Frequency of Maintenance of breaker:-
Timely maintenance must be ensured , it is always recommended that maintenance interval for vacuum circuit breaker should be Every two years or 2000 operations whichever is earlier.
Your post is very nice. Vacuum Circuit Breakers (VCB) is must be carefully operation because Its supply high voltage.
ReplyDeleteThank You For Sharing Great Information. for more details something like visit vaccum circuit breaker types get more informations.
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