Vacuum Circuit Breakers Advantages Disadvantages
Vacuum circuit breakers are employed for switching purposes in High Voltage (HV) applications. Vacuum circuit breakers are more efficient, less bulky and cheaper in cost. It is on par with SF6 circuit breakers which are widely used in power system for high voltage switching applications. The service life of the vacuum circuit breakers are much greater than conventional circuit breakers and almost no maintenance is required. Vacuum circuit breakers are ideally suited for most duties encountered in typical electrical utility and industrial applications.
Every medium that has pressure below atmospheric, ie, 760mm of Hg is known as "Vacuum". Low pressures are measured in torr (1 torr = 1mm of Hg). The pressures below about 10-5 torr are considered as high vacuum. This high vacuum conditions provide high dielectric strength to quench the arc.
High Vacuum has two outstanding properties:
- Highest Insulation Strength - In comparison to various other insulating media in use in circuit breakers Vacuum is superior dielectric medium. It is better than all other insulation media except air and SF6 gas which are generally employed at high pressure
- When contacts are separating in the vacuum, dielectric strength across the contacts builds up at a rate thousands of time higher than that obtained with other circuit breakers
Because of the above properties Vacuum circuit breakers have some advantages compared to conventional circuit breakers
Advantages of Vacuum Circuit Breakers
- Vacuum circuit breakers does not require filling of any gas or oil. They do not need auxiliary air system, oil handling and free from periodic maintenance
- Rapid recovery of very high dielectric strength on current interruption so that only half cycle or less arcing occurs after proper contact separation
- No emission of gases, hence pollution free
- Current interruption occurs at first current zero after contact separation with no re-striking, making it exceptionally good for capacitor and cable switching and long line drooping
- Very high power frequency and impulse withstand voltages with small contact spacing, allowing ease of actuation and timing
- Non explosive and silent operation compared to air blast and SF6 breakers
- Breaker unit is compact and self contained. It can be installed at any orientation
- There is no gas decomposition products in the vacuum and hence hermetically sealed vacuum interrupter keeps out all environmental effects on circuit breaker
- Usable at any voltage up to 230kV and higher voltages where longer life and maintenance free operation is desired
- Suitable for repeated operating duty
- In vacuums the contacts cannot be oxidize, therefore their small resistance is maintained throughout the life
Because of the above reasons with economical advantages added these circuit breakers are widely used. However they have some disadvantages which will be explained below
Disadvantages of Vacuum Circuit Breakers
- Loss of vacuum due to transit damage or failure makes the entire interruption useless and it cannot be repaired at site
- It needs additional surge suppressors in parallel with each phase for interruption of low magnetizing currents in certain range
- vacuum circuit breakers are uneconomical above 36kV and SF6 breakers having equivalent properties is economical. Hence for EHV (voltages above 230kV ) systems SF6 circuit breakers are employed
- It is uneconomical to manufacture vacuum interrupters in small quantities
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