What is a Vaccum Circuit breaker?
It is high voltage electrical equipment that is used in high voltage engineering work to break the circuit in either overload or short circuit conditions. In this breaker, the Vacuum is used as an arc extinction medium, hence it is called Vacuum Circuit Breaker. The arc is extinct as the contacts are separated in a high vacuum. It is mainly used for medium voltage ranging from 11 kV to 33 kV. As compared to other circuit breakers it has a high insulating medium for arc extinction. During the interruption of the circuit breaker, a few airs and other gas molecules are present inside the circuit breaker and the pressure inside the CB is around 10-4 torr
Working Vacuum Circuit Breaker
When a high current start flowing through the Circuit breaker due to either overload or fault in the system contacts of the circuit breaker are moved apart. Since Contacts are working at high voltage and due to sudden movement of the contacts, a high voltage gradient appears inside the gap between the contacts, and this voltage gradient induces electric stress on air molecules present around the contact. and ionized the air molecule. This incident happened so fast that ionized air molecules form a big arc that is looking like a flame and due to this flame temperature around the contact increased rapidly. Due to arcing around the contact whole space is filled by an ionized air molecule and all ionized air molecules are discharged by a contact material. The density of ionized molecules depends arcing current and it decreases as the current decreases and finally regains its dielectric strength when the arcing current becomes zero.
Construction of Vacuum Circuit Breaker
As compared to other circuit breakers vacuum circuit breakers are simple in construction. To Understand the working and construction of a Vacuum Circuit breaker We can divide the circuit breaker into three-part:
- Fixed Contact
- Movable Contact
- Arc Shield
Fixed and moving contacts are used to provide the path for the current flow while the Circuit breaker is operational. Fixed Contact is fixed and movable contacts are able to move 5 to 10 mm depending on the voltage range of the Circuit Breaker. The Arc shield is kept in the arc interrupting chamber.
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The outer envelope of the vacuum circuit breaker is made up of glass because the glass envelope help in the examination of the breaker from the outside after the operation. If the glass becomes milky from its original finish of silvery mirror, then it indicates that the breaker is losing the vacuum.
Inside the Arc Shield moving and fixed contacts are placed and during the sealing of contacts interrupter pressure is maintained at torr.
How do Vacuum Circuit Breakers Work?
When the fault occurs in the system, the contact contacts of the circuit are pulled, and therefore an arc forms between them. When the current-carrying contacts are separated, the temperature of their junction is very high due to the ionization that occurs. As a result of ionization, the contact space is filled with positive ion vapor that comes out of the contact material. The vapor density depends on the current in the arc. Due to the reduced mode of the current wave, their rate of vapor evolution falls and when the current is lost it refuses to regain its dielectric strength if the vapor density around the contact is reduced. Therefore, the arc does not re-ignite because the metal vapor is quickly removed from the contact area.
What is Current Chopping?
Current cutting in the air circuit gap usually occurs in oil and air circuit breakers due to arc column instability. In open circuit breakers, the internal current mainly depends on the vapor pressure as well as the electron emission properties of the contact material. HANTH is then affected by Hanh and making it possible to discourage, and then frustrated, then it will be low. It is possible to reduce the current level of the Hass-select contacts to provide the right hand to give up the power but does not do this because it affects the strong dielectric power.
Advantages and Disadvantages of Vacuum Circuit Breaker
The advantages and disadvantages of vacuum circuit breakers are given below:
Advantages
- Current interruption takes place at the first current zero after contact separation with no restriking makes it very suitable for capacitor and cable switching.
- The vacuum circuit breaker has a long life
- The vacuum circuit breaker does not require any additional filling of oil or gas. They do not need periodic refilling.
- The breaker unit is compact and self-contained. It can be installed in any required orientation
- They are pollution-free, non-explosive, and silent operations.
- They provide a large number of operations on load or short circuits.
Disadvantages
- High-tech plants are required for their production, and their production becomes economical only on a large scale.
- Loss of vacuum due to either transit damage or failure makes the entire interrupter useless, and it cannot be repaired on the application site.
- its upper working voltage is limited to 36KV