Monday, September 15, 2014

Vacuum Circuit breaker working principle and advantages of VCB

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.

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.