Vacuum circuit breaker is most reliable among other circuit breakers like oil, Air blast or sulphur hexafluoride circuit breaker.
In this post we will read about vacuum circuit breaker, its construction and working along with advantages, disadvantages and applications of this circuit breaker.
Introduction
In a vacuum circuit breaker, the vacuum of the order of 10-5 to 10-7 torr is used as an arc quenching medium. (1 torr = 1mm of Hg). Basically, vacuum is a pressure below atmospheric pressure where no gas is present.
They are used for switching and protection purpose in power system.
Vacuum circuit breaker has replaced oil circuit breakers in 1970s and then SF6 circuit breakers in 1980s.
These breakers have highest insulating strength than any other medium. Vacuum circuit breaker does the interruption of current at the first current zero. It means it interrupts the current in the half cycle. You will get to know about this line in detail in later half of this post.
Construction
It has an outer envelope which is made up of glass joined with end caps. This glass shield is used to examine the breaker after operation, if the colour becomes milky white then it indicates that the breaker is losing its vacuum.
Two contacts named as fixed contact and moving contact are placed as shown in the figure. The main arcing process is done in between these two contacts.
The moving contact is connected with metallic bellows which are made up of stainless steel. These bellows are used to move the moving member up and down and a spring mechanism is connected with the bellows to operate them.
Sputter shield made up of stainless steel is present to prevent metal vapour from reaching outer envelope.
The ceramic envelope is also present to insulate the chamber.
Now, we will see the working of this breaker below.
Working of vacuum circuit breaker
Initially, the fixed contact and moving contact both are connected to each other and the current flows through them. But as soon as any fault occurs in the system, the moving contact start separating from the fixed contact and arc phenomenon occurs between them.
Vacuum circuit breakers have a different way of arc generation. The arc is generated due to vaporisation of metal vapours from the contact surface.
A contact surface has large number of microscopic projections and when current reaches to these projections.
Now, we know that, after disconnecting the contacts the last point of current will be the contact surface which results in high current density at these points and due to which resistive heating occurs.
Due to excessive heating, the metal ions starts vaporising from the contact surface
When these ions come in between of two contacts, then they provide a path to the current and current starts flowing which we see as an arc. This is how arc generates in these breakers.
How arc extinguishes in vacuum circuit breakers?
Now as we all know that AC has sinusoidal nature. So, the current wave has two zeros in one cycle.
When this arcing current reaches its first zero (at half cycle) the dielectric strength of the vacuum rapidly builds up and it interrupts the current at first half cycle. The arc will not restrike because of very high insulating strength of vacuum.
After the current interruption at fist half cycle, the metallic ions will condense again on the contact surface.
In this way, the arc and extinguishment and current interruption is done in a vacuum circuit breaker.
Now, we will see the advantages, disadvantages and applications of these breakers.
Advantages
- There is no fire hazards or risk of fire.
- These breakers are compact in size.
- They require less maintenance and it increase their life span.
- These are highly reliable. It means that they have very high chances to perform arc extinction process successfully every time.
- Noiseless or quiet operation
- Arc energy is low as compared to other circuit breakers.
- They can also withstand lightning surges.
- Quick operation (interrupts current at its first half cycle.
Disadvantages
- Their voltage range is limited and they can’t be used for higher voltage levels like SF6 circuit breaker.
- If they are used for voltage level more than 36kV, then some units are to be connected in series which increases complexity and cost.
- Extra care for leakage should be done.
Applications
- These circuit breakers are used both for indoor and outdoor applications.
- They are very useful for remote areas like hilly areas or rural areas because of less maintenance and longer life.
- Vacuum circuit breakers are employed for Reactor switching, capacitor bank switching ,.and Transformer switching.
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