Vacuum Circuit Breaker

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What is a Vacuum Circuit Breaker?

A vacuum circuit breaker is a current interruption device that uses vacuum as the arc-extinguishing facility. When the contacts open, the arc is quickly extinguished due to the absence of proper medium in the vacuum. We commonly refer to a vacuum circuit breaker as a VCB.

We’ll cover here how they work, why they’re used, and their key technical benefits.

Vacuum circuit breakers are widely used in low and medium-voltage levels. The main advantage of VCBs is their ability to extinguish arcs very quickly.

Vacuum as Interruption Medium

Vacuum has a unique property. Due to the absence or lack of an ionized medium, it does not support an arc. In a VCB, the separation of the moving and fixed contacts takes place in a vacuum atmosphere inside a shielded vacuum chamber. This chamber is commonly known as the vacuum interrupter. When the contacts separate inside the vacuum interrupter, the arc extinguishes rapidly due to the absence of ionizable gas. Although some vapour of contact metal is created during contact separation under fault conditions, it instantly condenses, allowing the dielectric strength to recover almost immediately.

Basic Construction of Vacuum Interrupter

The vacuum interrupter is the key part of a vacuum circuit breaker. It is enclosed in a ceramic capsule. The fixed and moving contacts are usually made of copper-chromium. An arc shield surrounds the contacts. The moving contact is fitted with a bellows to allow movement while maintaining the vacuum inside. An external spring-loaded mechanism drives the moving contact inside the interrupter.

Working Principle of VCB

During normal operation, the contacts are closed. Upon fault detection, a relay activates the trip coil. The plunger of the trip coil strikes a latch, which releases the deformed tripping string. Then, the moving contact—attached to the lever mechanism and the spring—moves back and opens the breaker. An arc forms briefly and collapses within milliseconds due to the vacuum conditions.

The arc in vacuum behaves differently based on the fault level. At lower currents, it is diffuse, while high currents cause a constricted arc due to the contact metal vapour. The formation of vapour during contact separation is unique to a vacuum interrupter. When the moving contact detaches from the fixed contact, the last point of contact becomes extremely hot due to the high current concentration. This extremely high temperature creates metal vapour. However, during the first two or three zero crossings of the fault current, the vapour fully condenses, leaving no more vapour to sustain the arc. Hence, the insulation strength recovers and the arc extinguishes instantly.

Advantages of Vacuum Circuit Breakers

VCBs offer multiple advantages. They provide very fast operation. Due to their simplicity in design, they require lower maintenance compared to oil and SF6 circuit breakers. They also offer a longer lifespan if the vacuum interrupters are replaced as per the manufacturer’s recommendations. Moreover, vacuum circuit breakers are very compact in design and can easily be accommodated in indoor switchgear even at the 33 kV level. Vacuum circuit breakers do not use greenhouse gases like SF6, and the absence of oil makes them clean. These advantages make vacuum circuit breakers a sustainable choice.

Disadvantages of Vacuum Circuit Breakers

Despite their benefits, VCBs are not suitable for very high-voltage systems. To optimize the very fast operating time, the travel distance of the moving contact is kept quite short in VCBs compared to SF6 or oil circuit breakers. The smaller contact gap in the open condition may not be able to sustain higher voltages. This is why vacuum circuit breakers are still used only up to medium-voltage levels.

In summary, vacuum circuit breakers are a robust, clean, and high-speed protection solution for medium-voltage power systems. Their role will only grow as power grids modernize.

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  1. Vacuum Circuit Breakers – An Introduction
  2. Working Principle of Vacuum Circuit Breakers
  3. Construction and Design of a Vacuum Interrupter
  4. Comparison Between VCBs and Other Types of Circuit Breakers
  5. Arc Interruption in Vacuum: Physical Mechanism Explained
  6. Role of Copper-Chromium Contacts in VCBs
  7. Effect of Contact Gap and Velocity on Arc Quenching
  8. Current Chopping and Its Consequences in Vacuum Interruption
  9. Vacuum Arc Behavior at Different Current Levels
  10. Design of Vacuum Interrupters
  11. IEC and IEEE Standards Governing Vacuum Circuit Breakers
  12. Type, Routine, Acceptance and Special Testing of VCBs
  13. Mechanical Endurance and Life Expectancy of VCBs
  14. Failure Modes in Vacuum Circuit Breakers and Diagnostic Methods
  15. Condition Monitoring and Predictive Maintenance of VCBs
  16. Application of VCBs in Medium-Voltage Power Systems
  17. Recent Advances in Vacuum Interruption Technology
  18. Design and Simulation of VCBs Using FEM Tools
  19. Solid-State and Hybrid Vacuum Breakers: The Next Generation
  20. IoT-Enabled Smart VCBs for Real-Time Monitoring
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