In this topic, you study Vacuum Circuit Breaker Working Principle, Construction, Diagram, Advantages & Applications.
Working Principle of Vacuum Circuit Breaker
When two contacts of this circuit breaker are separated in vacuum, an arc is struck and hot spots are formed on the surface of contacts. These hot spots produce metal vapour and plasma. The amount of vapour in the plasma depends on how rapidly the vapour is emitted from contact surface which indeed depends on arc current. The current is of alternating nature, it passes through zero several time, so the rate of vapour emission also becomes zero, and the vapour already emitted gets condensed. During this process the dielectric strength builds up rapidly and restriking of the arc is prevented.
Construction of Vacuum Circuit Breaker
Fig. 1: Vacuum Circuit Breaker
It consists of a enclosure made up of insulating material like glass. The vacuum retained inside the container is about 10-7 torr. The enclosure is supported by means of end flanges made up of non-magnetic material as shown in Fig. 1. The contacts are disc shaped. The contact tips are made up of copper-chromium or copper-bismuth alloy. The contact region is surrounded by vapour condensing shield. It does not allow the condensed vapour to pass on to the glass enclosure. The metallic bellows are used which can permit the movement of moving contact without disturbing the vacuum. Metal ceramic seals are used at respective points to be sealed.
Working of Vacuum Circuit Breaker
Under normal working conditions the contacts are closed. On the occurrence of fault, the moving contact moves upwards. An arc is produced between the contacts due to ionisation of metal vapours. The arc gets extinguished. The arc extinction process is already explained in principle of working of vacuum C.B.
Advantages of Vacuum Circuit Breaker
- Vacuum C.Bs. are compact in size.
- Reliable and longer life.
- Heavy fault current can be interrupted effectively.
- No gas is generated after arc extinction operation.
- The operation is not noisy.
Disadvantage of Vacuum Circuit Breaker
- Vacuum has to be maintained at a desired level always.
Applications of Vacuum Circuit Breaker
Outdoor application where maintenance required is minimum. In the high voltage system from 22 kV to 66 kV power circuits. This breaker is also useful in rural area with limited rating 60 to 100 MVA.