A Vacuum Circuit Breaker is such a kind of circuit breaker in which arc quenching takes place in the vacuum. The technology is suitable for generally medium-voltage applications. The operation of opening and closing of contacts and associated arc interruption take place in the vacuum chamber to the breaker which is called the vacuum interrupter. The vacuum interrupter consists of a steel arc chamber in centrally arranged ceramic insulators. The vacuum pressure inside the 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. Copper & Chromium is the ideal material to make VCB contacts.
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| 33 KV VCB Panel |
Advantages of Vacuum Circuit Breaker or VCB:
- The service life of a vacuum circuit breaker is much longer than other types of circuit breakers.
- There is no chance of fire hazards like an oil circuit breaker.
- It is much more environment-friendly than the Sulphur Hexafluoride (SF6) Circuit Breakers.
- Besides, the contraction of VCB is many users friendly.
- Replacement of a vacuum interrupter (VI) is much more convenient.
Operation of Vacuum Circuit Breaker:
The main purpose of any circuit breaker is used to quench the arc during fault conditions or any instant of fault by establishing high dielectric strength between the contact. The dielectric strength of a 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 a very small contact gap.
The vacuum arc results from the neutral atoms, ions, and electrons emitted from the electrodes themselves. As the current-carrying contacts are separated, cathode spots are formed depending upon the current to be interrupted. For low currents, a highly mobile cathode spot is formed and for large currents, multiple numbers of cathode spots are formed. These spots constitute the main source of vapour in the arc. The processes involved in drawing the arc will be due to the high electric field between the contacts or resistive heating produced at the point of operation or a combination of the two. The cathode surfaces, normally, are not perfectly smooth but have many micro projections. When the contacts are separating, the current flowing in the circuit will be concentrated in these projections as they form the last point of contact. Due to their small area of cross-section, the projections will suffer explosive evaporation by resistive heating and supply sufficient quantity of vapour for the arc formation. Since in the case of vacuum breakers the emission occurs only at the cathode spots and not from the entire surface of the cathode,
the vacuum arc is also known as the cold cathode arc. In a cold cathode the emission of electrons
could be due to any of the combinations of the following mechanisms:
(i) Field emission;
(ii)Thermionic emission;
(iii) Field and Thermionic emission;
(iv) Secondary emission by positive ion bombardment;
(v) Secondary emission by photons; and
(vi) Pinch effect.
It is known that current chopping in air and oil C.Bs. occurs due to instability in the arc column whereas, in the case of vacuum breakers, it depends upon the vapour pressure and the electron emission properties of the contact material. It is possible to reduce the current level at which chopping takes place 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 it is normally not done as it affects the dielectric strength adversely. Since gas pressure is low in a vacuum switch, the main criterion to limit current chopping is the proper selection of contact material. It has been found that no single metal gives all the desirable properties. A high vapour pressure and low conductivity metal are more desirable to limit the current chopping whereas low vapour pressure metals are more desirable from the arc extinction point of view. Materials having high boiling and melting points have a low vapour pressure at high temperatures but are poor conductors whereas metals having low boiling and melting points have a high vapour pressure at high temperatures, low electron functions and good thermal and electrical conductivities. Therefore, to combine these contradictory properties in one single material, composites of two or more metals or a metal and a nonmetal have to be made. Copper-bismuth, silver-bismuth, silver-lead, and copper-lead are some of the alloys used as contact materials.
When the arc interruption is over, the space between the surrounding electrodes is filled with vapour and plasma. The presence of this residue affects very much the ability of an interrupter to withstand high voltages. The process by which this residue decays and by which the vacuum gap regains its dielectric strength is known as the arc recovery phenomenon. At current zero the cathode spot extinguishes within 10–8 seconds and after this, the original dielectric strength is established very soon. This quick build-up of dielectric strength is due to the condensing, and quick diffusion of metal vapour to the glass walls in absence of gas molecules. After the arc is interrupted, the recovery strength during the first few micro-seconds is 1 kV/ micro-sec for an arc current of 100 A, as compared with 50 V/micro-sec in the case of the air gap.
