40.5KV Vacuum Circuit Breaker / Vcb Breaker Permanent Magnet IEC ZW32

Arc Extinction in Vacuum Interrupters:

The arc interruption process in vacuum interrupters is quite different from that in other types of circuit breakers. The vacuum as such is a dielectric medium and arc cannot persist in ideal vacuum.

However, separation of current carrying contacts causes the vapour to be released from the contacts giving rise to plasma. Thus on separation of current carrying contacts, the contact space is filled with vapour of positive ions liberated from the contact material; the vapour density depending on the current in the arcing. During the decreasing mode of current wave the rate of release of the vapour falls and after the current zero, the medium regains its dielectric strength, provided vapour density around the contacts has substantially reduced.

While interrupting a current of the order of a few hundred amperes by separating the flat contacts under high vacuum, the arc usually has several parallel paths and each arc path originates and sinks in a hot spot of current. Thus the total current is divided in several parallel arcs. The parallel arcs repel each other so that the arc tends to spread over the contact surface. Such an arc is called the diffused arc. The diffused arc can get interrupted easily.

At higher values of currents of the order of a few thousand amperes, the arc gets concentrated on a small region and becomes self-sustained arc. The concentrated arc around a small area causes rapid vaporisation of the contact surface. The transition from diffused arc to the concentrated arc depends upon the material and shape of contact, the value of current and the condition of electrodes. The interruption of arc is possible when vapour density varies in phase with the current and the arc remains in the diffused state. The arc does not restrike again if the metal vapour is quickly removed from the contact zone.

Thus the arc-extinction process in a vacuum circuit breaker is related to a great extent to the material and shape of the contacts and the technique used in condensing the metal vapour. The contact geometry is so designed that the arc root keeps on moving so that the temperature at one point on the contact does not attain a very high value.

The rapid building up of dielectric strength after final arc extinction is a unique advantage of vacuum circuit breaker. There are limitations on oil circuit breakers in switching capacitive currents. In oil circuit breakers after an interruption takes place, the dielectric strength builds up so slowly that delayed restrikes take place often as much as half a cycle later. The vacuum switch is free from this trouble since after half a cycle, corresponding to 2.5 mm contact separation, a dielectric strength of the order of 100 kV (rms) is available. This type of switch has the advantage of restrike-free performance and is ideally suited to capacitor switching. Its operation is also practically independent of contact parting time.

Current Chopping in Vacuum Circuit Breakers:

We have seen that current chopping in air and oil circuit breakers occurs because of instability in the arc column whereas in case of vacuum circuit breakers it depends upon vapour pressure and the electron emission properties of the contact material. The chopping level is also influenced by the thermal conductivity-lower the thermal conductivity, lower is the chopping level. It is possible to reduce the current level at which chopping occurs by selecting a contact material which gives out sufficient metal vapour to allow the current to come to a very low value or zero value, but this is rarely done as it affects the dielectric strength adversely.