Vacuum interrupter technology was first introduced in 1960, but it is still evolving. A circuit breaker is a device designed to disconnect an electrical circuit in order to prevent excessive current, typically caused by short circuits or overloads. Its primary function is to interrupt the current flow when a fault is detected. This article provides an overview of vacuum circuit breakers and their working principles. For more detailed information on various types of circuit breakers and their importance, continue reading.
A vacuum circuit breaker (VCB) is a type of circuit breaker that uses a vacuum medium to extinguish electrical arcs. The operations of making, breaking, and quenching arcs within the circuit breaker are all performed within the vacuum chamber, which is why it is called a vacuum circuit breaker.
The use of vacuum as the arc-extinguishing medium gives the circuit breaker superior arc-quenching performance and high insulation strength. This makes it suitable for most standard voltage applications. While vacuum technology has been developed for higher voltage systems, it is not yet commercially viable at those levels.
The operation of the current-carrying contacts and the associated arc quenching takes place in the vacuum chamber, which is called a vacuum interrupter. The interrupter consists of a steel arc chamber situated at the center of asymmetrically placed ceramic insulators. The vacuum pressure inside the interrupter is typically maintained at around 10−610−6 bar. The performance of the vacuum circuit breaker is mainly influenced by the material used for the current-carrying contacts, such as copper-chromium (Cu/Cr) alloys.
The working principle of a vacuum circuit breaker involves the generation of an electric arc when the circuit breaker contacts open in a vacuum. The metal vapor released from the contacts ionizes, forming the arc. However, the arc is quickly extinguished due to the rapid condensation of the metal vapor, electrons, and ions outside the contact area, allowing the dielectric strength of the vacuum to be restored quickly. The most important feature of a vacuum is its ability to extinguish the arc rapidly because of the quick restoration of the dielectric strength of the vacuum.
The arc extinguishing characteristics of a vacuum circuit breaker differ from those of other types of circuit breakers. When the contacts separate, metal vapor is released, filling the contact space. The vapor consists of positive ions, and its density depends on the current flowing through the arc. As the current decreases, the vapor release rate also decreases. Once the current reaches zero, the dielectric strength of the medium is restored if the vapor density drops.
The arc extinguishing performance of the vacuum circuit breaker is significantly affected by the contact material, shape, and the handling of metal vapor. Since the arc's path constantly changes, the temperature at any given point remains relatively low.
Once the arc is finally extinguished, the dielectric strength of the vacuum circuit breaker increases rapidly, a feature unique to these breakers. This makes them ideal for capacitor switching, as they prevent re-striking of the arc. Additionally, small currents can be interrupted before reaching zero, potentially causing chopping, depending on the contact material.
Compared to other types of circuit breakers, vacuum circuit breakers require higher insulation standards for arc suppression. The vacuum pressure inside the interrupter is about 10−410−4 bar, which results in a medium with a very low molecular presence. Vacuum circuit breakers have two main advantages:
These features make vacuum circuit breakers more efficient, lighter, and cost-effective. They also have a longer lifespan and require virtually no maintenance compared to other types of breakers.
Vacuum circuit breakers are now considered the most reliable technology for interrupting current in medium-voltage systems. They require minimal maintenance when compared to other types of circuit breakers.
The technology is particularly suited for medium-voltage applications, though high-voltage vacuum technology has been developed, it is not yet commercially feasible. Vacuum circuit breakers are commonly used in metal-clad switchgear and porcelain-enclosed circuit breakers.
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