There are five Types of Circuit Breakers and In this article we will speak about
- Air break circuit breaker
- Air-Blast Circuit-Breaker
- Bulk Oil and Minimum Oil Circuit Breaker
- Sulphur Hexafluoride (SF6) Circuit Breaker and SF6 Insulated Metalclad switchgear
- Vacuum circuit breaker
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 mili-meters only. The movement of the contacts within the sealed interrupter unit is permitted by metal-bellows.
Figure 30 Construction of vacuum circuit breaker1
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).
Figure 31 12 kv indoor vacuum circuit breaker
Figure 32 12 KV outdoor vacuum circuit breaker
The structural configuration of the circuit breakers of two 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.
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.
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 10-3 Pa. Experience shows that during the process of switching, the level of vacuum slightly rises due to the condensed metal steam absorbing the residual
Figure 34 Model of contact system AEC
Merits of Vacuum Circuit Breakers
The vacuum switchgear has been successfully developed and is gaining rapid popularity. The vacuum switches are likely to be popular for wide range of applications. These switches devices have several merits such as :
- VCB is self contained and does not need filling of gas or oil. They do not need auxiliary air system, oil handling system, etc. No need for periodic refilling. 2. No emission of gases, pollution free.
- Modest maintenance of the breaker, no maintenance of interrupters. Hence economical over long period.
- Breakers forms a unit which can be installed at any required orientation. Breaker unit is compact and self contained.
- Silent operation.
- Large number of operation on load, or short circuit. Suitable for repeated duty.
- Long life of the order of several hundred operations on rated normal current.
- Constant dielectric. There are no gas decomposition products in vacuum and the hermetically sealed vacuum interrupter keeps out all environmental effect.
- Constant contact resistance. In vacuum, the contacts cannot be oxidized, a fact which ensures that their very small resistance is maintained through their life.
- High total current switched. Since contact piece erosion is small, rated normal interrupted current is up to 30.000 times; and rated short circuit breaking current is on the average of a hundred times.
The above reasons, together with the economic advantages offered, have boosted acceptance of the vacuum arc quenching principle.
Demerits of Vacuum Circuit Breakers
- The vacuum interrupter is more expensive than the interrupter devices in other types of interrupters and its cost is affected by production volume.
- It is uneconomical to manufacturer vacuum interrupters in small quantities.
- Rated voltage of single interrupter is limited until very recently to about 36√3 = 20 KV above 36 KV, two interrupters are required to be connected in series. This makes the breaker uneconomical for voltage rated above 36 KV.
- Vacuum interrupters required high technology for production.
- In the event of loss of vacuum, due to transient damage or failure, the entire interrupter is rendered useless. It cannot be required at site.
- For interrupter low magnetizing currents, in certain range, additional surge suppressors are required in parallel with phase of a VCB.