VCB (Vacuum Circuit Breaker)
VCB (Vacuum Circuit Breaker) is a type of electrical circuit breaker that uses a vacuum as the medium to extinguish the arc when the contacts open. It is designed to protect electrical circuits from overloads, short circuits, and other faults by automatically disconnecting the faulty section of the circuit. The vacuum environment effectively suppresses the electrical arc that forms when the circuit is interrupted, ensuring fast and reliable operation with minimal wear on the contacts. VCBs are commonly used in medium-voltage electrical systems.
Main Components of a Vacuum Circuit Breaker:
Vacuum Interrupter (Arc Interruption Chamber):
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- The vacuum circuit breaker consists of fixed contact, moving contact and an arc shield mounted in a vacuum chamber, typically made of metal. Fixed Contact that is connected to the supply line and a moving contact that moves when the breaker is actuated to open or close the circuit. It is usually connected to a mechanical drive. The mechanism drives spring-loaded, motor-operated, or hydraulic, depending on the design of the VCB. The mechanism drives the movement of the moving contact, ensuring the proper operation of the circuit breaker. Here spring or hydraulic mechanism ensures that the moving contact is opened or closed with sufficient force. It also controls the speed of operation to prevent damaging arcs. Arc chutes are used to divert and quench the arc formed during the opening of the contacts. However, in VCBs, arc chutes are less necessary because the vacuum environment itself helps in arc extinguishing.
When a fault occurs, the contacts within the interrupter open, and the arc is extinguished inside the vacuum, as vacuum has high dielectric strength and can prevent the formation of arcs. The vacuum interrupter is insulated from the outer environment by solid insulating materials, often ceramic or epoxy, to ensure that no external currents or faults interfere with the operation.
Operation Procedure:
- When a fault occurs in the circuit (such as a short circuit or overload), the circuit breaker is triggered to open and disconnect the faulty section.
- In the VCB, the contacts open in a vacuum, which is one of the key elements that enable it to break the circuit. A vacuum is a very effective medium for quenching electrical arcs because it lacks free electrons to sustain the arc. When the contacts separate, the arc that forms is rapidly extinguished.
- The vacuum interrupter, which is the main component of a VCB, ensures that the arc is extinguished almost instantaneously, thereby preventing any damage to the circuit and allowing for safe disconnection of the fault.
1. Normal Operation (Closed Position):
- VCB in Closed Position During normal operation and allowing current to flow through the circuit. The contacts inside the vacuum interrupter are tightly closed, and the circuit is live.
2. Detection of Fault Condition:
- Fault Detection Mechanism: When a fault occur such as a short circuit or overload, the protection relay or the fault detection mechanism triggers the operation of the VCB. The protection relay continuously monitors the current in the system. If the current exceeds a preset threshold (due to a fault), it sends a signal to the VCB to open the contacts. The breaker typically uses either a spring-loaded mechanism (mechanical) or an electromagnet (motorized) to operate the contacts.
3. Arc Extinguishing:
- Arc Formation: Initially, when the contacts begin to separate, a high-energy arc forms between them. In typical air or oil circuit breakers, this arc would continue to burn, potentially causing damage. However, in a VCB, the arc is quickly extinguished in the vacuum. The vacuum inside the interrupter chamber has extremely low pressure, which means there are very few gas molecules. This makes it difficult for the arc to sustain itself. The absence of free electrons in the vacuum environment ensures that the arc is rapidly extinguished once the contacts open, usually within a few milliseconds.
- Current Interruption: Once the arc is extinguished, the current flow is completely interrupted. The VCB remains open, isolating the faulty section of the circuit.
The VCB stays in the open position until the fault is cleared and the system is ready to be restored.
4. Resetting the Breaker (Post-Fault):
- Manual or Automatic Reset: After a fault has been cleared (either manually or automatically), the VCB can be closed again.
- Manual Reset: In many systems, the VCB can be manually reset after the fault has been identified and resolved. This involves the operator physically closing the breaker.
- Automatic Reset: In some systems, the VCB may automatically close after a set time delay, once the protection relay determines that the fault is no longer present.
5. Re closing (if Applicable):
- Automatic Re closure: In certain applications, such as power distribution systems, the VCB may be set to automatically reclosing (i.e., attempt to close after a fault has cleared) to restore service, especially in transient faults. This process involves a time delay after which the breaker closes again if the system conditions are normal.
Operating voltage range:
The operating voltage range of a Vacuum Circuit Breaker (VCB) typically spans from 1 kV to 72.5 kV, making it suitable for medium-voltage applications. VCBs are commonly used in electrical distribution systems, industrial power systems, and substations where voltages are within this range. For higher voltage applications (above 72.5 kV), other types of circuit breakers, such as SF6 circuit breakers, are generally used.
Advantages:
- Compact and reliable
- Longer life
- No risk of fire
- Small switching stroke
- High dielectric strength
- Easy maintenance
- Arc extinction is very fast
Disadvantages:
- The cost of VCB will increase if the voltage exceeds 38kV
- Loss of vacuum due to failure makes the entire current interruption useless
Applications of VCB
The uses of the vacuum circuit breaker are:
- High voltage circuits
- Used in substations and generators
- Interrupts the double earth faults and out of phase currents
- Applications that require high switching sequences use VCBs
- Railway applications use this circuit breaker for traction current and electric supply switching.
- To switch motor drives.
