Construction of Vacuum Circuit Breaker

5. Description of different Circuit Breaker

5.5 Vacuum circuit breaker

5.5.3 Construction of Vacuum Circuit Breaker

The vacuum circuit breaker comprises one or more sealed vacuum interrupter units per pole (Fig. 30). The moving contact in the interrupter is connected to insulating operating rod linked with the opening mechanism. The contact travel is of the order of a few millimeters only. The movement of the contacts within the sealed interrupter unit is permitted by metal-bellows.

Vacuum circuit breakers can be classified in the following two categories:

Vacuum interrupters installed in indoor switchgear and kiosks rated up to 36 KV (Fig. 31).
Vacuum circuit breakers suitable for outdoor installation and having two or more interrupters in series per pole (Fig. 32).

The structural configuration of the circuit breakers of tow categories mentioned above is quite different as it can be seen, though the basic interrupter unit is base on same principle of operation.

The multi-unit vacuum circuit breakers rated 72.5 KV and above have been developed and installed in England and U.A.S. However, they are not very popular and are not likely to be preferred to other types of circuit breakers.

For voltage up to 36 KV, vacuum circuit breakers employing a single interrupter unit have become extremely popular for metal enclosed switchgear, arc furnace installation, switchgear in generating stations and industrial applications.

Figure 30 Construction of vacuum circuit breaker1

Figure 31 12 kv indoor vacuum circuit breaker

Figure 32 12 KV outdoor vacuum circuit breaker

The construction of the vacuum chamber is relatively simple. As it can be seen in Fig. 33, it consists of a pair of contacts (4; 5), one of which is mobile (5), enclosed in a vacuum dense shield, soldered to ceramic or glass isolators (3; 7), upper and lower metal covers (2; 8) and a metal screen (6).The movement of the mobile contact in relation to the immobile one is provided by means of using a bellows element (9). Chamber outputs (1; 10) serve to connect it to the main current circuit of the breaker. It is necessary to state that only special metals that are vacuum dense and cleaned of dissolved gases are used in vacuum chamber shield manufacturing : copper and special alloys as well as special ceramic composition (usually it is 50%-50% copper-chrome) that provides high breaking capacity, low deterioration and resistance to the appearance of welding points on the surface of the contacts.

Figure 33 Construction of vacuum interruption chamber

Cylindrical ceramic insulators together with the vacuum space when the contacts are open provide insulation between the chamber outputs when the circuit breaker is in the opened position. To prevent metal steam condensation on the surface of the ceramic insulators, which causes damage to the electric strength of the insulators, a metal screen (Fig. 34) is used that "intercepts" and absorb metal steam formed during the switching. By doing this it prolongs the durability of the chamber (electrical endurance). The level of vacuum in the modern arc extinction chambers equals 10^-⁷-10^-⁶ Pa, which provides the durability resources for the chambers for their entire term of use because the necessary insulation parameters of the vacuum space are reached at 10^-³Pa. Experience shows that during the process of switching, the level of vacuum slightly rises due to the condensed metal steam absorbing the residual gases.