What is Vacuum Circuit Breakers?
A Circuit Breaker that uses a vacuum as an arc extinction medium for arc extinguishing purposes is called a vacuum circuit breaker. in this type of circuit breaker fixed and moving contacts are sealed inside the high-pressure vacuum bottle is called Vacuum interrupters. In this type of circuit breaker, a high-pressure vacuum is used as an arc extinction medium. Principle of Vacuum Circuit Breaker
When the contacts of the circuit breaker are separated in the vacuum (Vacuum pressure in the vacuum bottle is 10-7 to 10-5), an arc is produced between the fixed and moving contacts by the ionisation of metal vapours of contacts. However, the arc is quickly extinguished because of the metallic vapours on the contacts surface, electrons and ions produced during the arc rapidly which condenses on the surfaces of the circuit breaker fixed and moving contacts, resulting in a quick recovery of dielectric strength between the circuit breaker contacts.
The vacuum circuit breaker has the salient feature of vacuum as a fast arc quenching medium, As soon as the arc is produced in a vacuum between the contacts, it is quickly extinguished due to the fast recovery rate of dielectric strength in a vacuum.
Construction of Vacuum Circuit Breaker
1. Moving Contacts
2. Fixed contacts
3. arc Shields
4. Stainless steel bellows and bellows shield
5. Insulating envelop
The moving contacts, arc shields, and fixed contacts are sealed inside the high-pressure vacuum bottle called vacuum interrupter. the arcing process taking place between fixed and moving contacts.
The Fixed and moving contacts are made from copper bismuth or copper chrome. The moving contacts of the circuit breaker move from fixed contacts a distance of 5 to 10 mm depending upon the voltage range. the moving contacts are connected with the metallic bellows made from stainless steel through the breaker mechanism. these metallic bellows are used to move moving contacts up and down and the spring mechanism is connected with the bellows to operate that bellows with moving contacts. the contacts have disc-type faces that dissipate large amounts of heat energy produced by arcing current.
The arc shields which are also made of stainless steel are used to prevent metal vapour from reaching to the outer envelope. the outer envelope is made from glass or porcelain material. The Vacuum pressure inside the vacuum chamber is 10-6 bar. the possibility of vacuum leakage is completely eliminated by sealing glass or porcelain or ceramic bottle. the ceramic bottle is used as the outer insulating body.
Working of Vacuum Circuit Breaker
During normal conditions fixed and moving contacts are connected to each other and normal current passes through them. During fault conditions moving contacts are start separating from fixed contacts and an arc produced between the fixed and moving contacts as shown in the figure.
Due to arc current, a large amount of heat energy is produced between the fixed and moving contacts. this heat energy vaporizes the contact's surface and produced metal vapour, electrons and ions on the contacts surface. the vacuum has a high dielectric strength than air and sf6 gas.
AC has sinusoidal nature and two times current zero takes place in its one full cycle.
When this arcing current reaches its first current zero in the first half cycle the dielectric strength of the vacuum instantly builds up and interrupts the heavy arc current in the first half cycle. Due to the high dielectric strength of vacuum arc can not restrike again in vacuum circuit breaker.
The vapour density depends upon arc current. when arc current increases the rate of vapour release increases and when arc current decreases rate of vapour release is reduced. Metallic vapour, electrons, ions are condensed on the contacts surface and the arc is extinguished. In these ways, Arc extinguishes and current is interrupted in vacuum circuit breakers.
Current choping in Vacuum circuit breaker depends upon the vapous pressures and electron emission properties of contact material.
Advantages of Vacuum Circuit Breaker
1. The vacuum circuit breaker is compact in size, longer service life and is more reliable.
2. The vacuum circuit breaker has no fire hazards because the arc is extinguished in a vacuum.
3. In vacuum circuit breaker gas is not generated during and after breaker operation.
4. The vacuum circuit breaker interrupts a very high fault current when its moving contacts travel a short distance from fixed contacts.
5. The Vacuum circuit breaker requires less and little maintenance.
6. The vacuum circuit breaker easily withstands lightning surges.
7. The vacuum circuit breaker has very low arc energy.
8. The vacuum circuit breaker has low inertia so small power is required for the control mechanism.
9. The vacuum circuit breaker capable of large number of load switching without maintanace.
10. Contact gap is small generally 6 to 10mm.
11. Arc extinguish time is short,Arc voltage is low and arc energy is small, so contact loss is small.
12. The circuit breaker is capable of handling severe recovery transients associated with short circuit line fault.
Disadvantages of Vacuum Circuit Breaker
1. The vacuum circuit breaker is uneconomical for voltage more than 33kv. the cost of breaker become higher for higher voltages.
2. For production of vacuum circuit breaker high technology required.
3. The vacuum circuit breaker required external surge supressers for interruption of low magnetising currents.
5. Loss of vacuum from vacuum intrupter due to failure make entire vacuum intrupter useless and it can not repaired at site.
Applications of Vacuum Circuit Breaker
1. Vaccum circuit breakers are used in remote areas.
2. vacuum circuit breakers are employed for outdoor and indoor application voltage ranging from 11Kv to 33Kv.
3. The Vacuum circuit breaker has ratting of 60 to 100MVA.
4. The Vacuum circuit breaker can intrupt large capacitor current or charging current without restrike and rate of restriking voltage is negligible, so vacuum circuit breaker used for capacitor bank switching or transformer switching.
5. When voltage is high and current to be interrupted is small this breaker are very useful than other breakers.