The vacuum interrupter technology was first introduced in the year of 1960 but still is a developing technology. A circuit breaker is a device that interrupts an electric circuit to prevent unwarranted current caused by short circuit, typically resulting from overload. It’s basically functionality is to interrupt current flow after a fault is deducted. This is kind of circuit breaker where arc quenching takes place in a vacuum medium. The operation of switching on and closing of current carrying contacts and interrelated arc interruption takes place in vacuum chamber in a breaker is called vacuum interrupter. Vacuum offers high insulating strength.
The vacuum circuit breaker consists of fixed contact, moving contact and an arc shield mounted in a vacuum interrupter. The outer insulating body is made with glass and has high vacuum capacity. The movable part is connected to control mechanism by stainless steel bellows. Arc shield prevents deterioration of internal dielectric strength by preventing metallic vapors from falling on inside surface of outer insulating body. To reduce possibility of leak, permanent or tight sealing is done. Vacuum has high dielectric strength as compared to air so vacuum circuit breaker is preferred where arc interruption need high in place of air circuit breaker.
When breaker operates, the moving contact separates from fixed contact and an arc is struck between the contacts. The production of arc is due to ionization of metals ions and depend very much upon materials of contact. The arc is quickly extinguished because metallic vapours, electrons, and ions produced during arc are diffused in a short time and seized by surface of moving and fixed member and shields. Since vacuum has very fast rate of recovery of dielectric strength because dielectric strength of vacuum is eight times greater than air and four times greater than sf6.
Contact materials must have following properties:
- High density.
- Contact resistance must be less.
- High arc resist capability.
- A boiling point must be high to diminish arc erosion.
- Thermal conductivity is high to dissipate quickly the large heat produced throughout arcing
Ratings and specification of vacuum circuit breaker
The rating of circuits breakers is generally given in MVA. VCB comes a wide variety ranging from 250 to 1500MVA. According to European standard, the commonly available preferred values for rated current are 25A, 32A, 40A, 50A, 63A, 80A, 100A, 125A
- It does not require any additional filling of oil or gas.
- The VCB is fast in operation so ideal for fault clearing and suitable for repeated operation comfortably
- Vacuum circuit breaker is almost maintenance free and minimum maintenance required as compared to other circuit breakers.
- No exhaust of gas to atmosphere and noiseless operation.
- No fire hazards
- The dielectric strength of vacuum is greater than air and sf6 circuit breaker. High dielectric strength makes possible to quench a vacuum arc within very small gap.
- The main disadvantages of this circuit breaker is that it become uneconomical at voltage exceeding 38kv because at more than 38kv, two number of vacuum circuit breaker are required to be connected in series.
- VCB production is uneconomical if produced in small quantities.
Now a days, vacuum circuit breakers applications to not only medium voltage power systems but also for in high voltage transmission systems. It is because it has so many advantages characteristics such as high interruption capability, long operation life, safety and high-cost performance. After Kyoto protocol in 1997, sf6 circuit breaker is one of major problem of global warming gases, since then vacuum circuit breaker uses become widespread.