As far as construction is concerned vacuum circuit breaker is very simple in comparison to an air or oil circuit breaker.
A schematic diagram of such a circuit breaker is shown in Fig.
The outer envelope is normally made of glass due to the ease of jointing it to the metallic end caps and also because the glass envelope facilitates the examination of the breaker from outside after the operation.
If it becomes milky white from its original finish of silvery mirror then it indicates that the baffle is losing its vacuum.
A sputter shield, usually, made of stainless steel, is placed between the contacts and the envelope in order to prevent the metal vapour reaching the envelope as it reduces the breakdown strength between the contacts.
Inside the sputter shield, the breaker has two contacts-one fixed and the other moving contact. The moving
contact moves through a short distance of 5 to 10 mm depending upon the operating voltage.
The metallic bellows made of stainless steel is used to move the lower contact. The design of the bellows is very important because the life of the vacuum breaker depends upon the ability of this component to perform repeated operations satisfactorily.
The periphery of the end cap is sealed to the envelope and the fixed contact stem is an integral part of one end cap.
One end of the fixed as well as moving contact is brought out of the chamber for external connections.
Operating Mechanism:
The lower end of the breaker is fixed to a spring-operated or solenoid operated mechanism so that the metallic bellows inside the chamber are moved upward and downward during closing and opening operations respectively.
The contact movement should be such as to avoid bounce. It is noteworthy that the operating mechanism should provide sufficient pressure for a good connection between the contacts.
The pressure in a vacuum interrupter at the time of sealing off is kept about 10-6 torr.
The interrupting rating is between 250 and 1,000 MVA.
The normal current carrying capacity for a single interrupter is 800—3,000 A; 4.2 kV-7.6 kV.
Advantages of Vacuum Circuit Breakers
1. A vacuum circuit breaker is self-contained and does not require the filling of oil or gas. They do not need an auxiliary air system, an oil handling system etc. No need of periodic refilling.
2. Rapid recovery of very high dielectric strength on current interruption so that only half cycle or less
arcing occurs after proper contact separation.
3. Current interruption occurs at the first current zero after contact separation with no restriking, making it exceptionally good for capacitor and cable switching and long line dropping.
4. Very high power frequency and impulse withstand voltages with small contact spacing, allowing
actuation and timing.
5. No emission of gases-pollution free.
6. Non-explosive and silent operation.
7. Breaker unit is compact and self-contained. It can be installed at any required orientation.
8. Larger number of operations on load; or short circuit. Suitable for repeated operating duty. Long life.
9. Usable on any voltage up to 230 kV and higher where exceptionally long life and maintenance-free operation is desired.
10. There are no gas decomposition products in a vacuum and a hermetically sealed vacuum interrupter keeps out all environmental effects.
11. Constant contact resistance. In vacuum the contacts cannot oxidise, therefore, their very small resistance is maintained throughout their life.
12. High total current switched. Because of small erosion of contact piece, rated normal current can be interrupted up to 30,000 times and rated short-circuit rupturing current on average a hundred times.
Because of the above reasons together with the economic advantages offered, vacuum circuit breakers have high acceptance.
Disadvantages of Vacuum Circuit Breakers
1. Requirement of high technology for production of vacuum interrupters.
2. The vacuum interrupter is costlier than the interrupting devices in other types of circuit breakers and its cost is affected by production volume. It is uneconomical to manufacture vacuum interrupters in small quantities.
3. Rated voltage of single interrupter is limited to about 36/√3 i.e., 20 kV. Above 36 kV, two interrupters are required to be connected in series. Thus the vacuum circuit breaker becomes uneconomical for rated voltages exceeding 36 kv.
4. Loss of vacuum, due to transit damage or failure, makes the entire interrupter useless and it cannot be repaired at site.
5. It needs additional surge suppressors in parallel with each phase for interruption of low magnetising currents in a certain range.
Applications of Vacuum Circuit Breakers
1. Because of the short gap and excellent recovery characteristics of vacuum breakers, they are very useful as very high-speed making switches in many industrial applications.
2. There are many applications where a simple load-break switch is not enough and at the same time the devices employed should not be costly. They include shunt reactor switching, transformer switching, line dropping, capacitor bank switching. These applications give a fast RRRV and vacuum circuit breakers are the best solutions.
3. Where voltages are high and the current to be interrupted is low, these breakers have definite superiority over the oil or air circuit breakers.
4. For low fault interrupting capacities the cost is low in comparison to other interrupting devices.
5. The vacuum switches can be employed for capacitor switching which is a very difficult task for oil circuit breakers.
6. These can be used along with static overcurrent relays and given an overall clearance time of less than 40 ms on phase-to-phase faults.
7. Because of the least requirements of maintenance these breakers are very suitable in a country like India, with 11 to 33 kV network extending into a vast rural complex. Even with a limited MVA rating of say 60 to 100 MVA, it should be suitable for a majority of applications in rural areas.