- Vacuum Circuit Breaker Definition: A vacuum circuit breaker is defined as a type of circuit breaker where arc quenching occurs in a vacuum, mainly used for medium voltage applications.
- Vacuum Interrupter Structure: The vacuum interrupter, essential for VCB operation, consists of a steel arc chamber and ceramic insulators with a vacuum maintained at 10^-6 bar.
- Contact Material: The material used for current-carrying contacts, typically copper-chromium (Cu/Cr), is crucial for the VCB’s performance.
- Advantages of VCB: Vacuum circuit breakers have a long service life, minimal maintenance, no fire hazard, and are environmentally friendly.
- Arc Interruption: VCBs quench arcs by leveraging the high dielectric strength of a vacuum, preventing arc reestablishment after the current zero crossing.
A vacuum circuit breaker is a type of circuit breaker where arc quenching occurs in a vacuum. This technology is mainly used for medium voltage applications. Although it has been developed for higher voltages, it is not yet commercially viable. The opening and closing of current carrying contacts and arc interruption happen in a vacuum chamber called a vacuum interrupter.
The vacuum interrupter consists of a steel arc chamber in the centre symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is normally maintained at 10– 6 bar.
The material for current-carrying contacts is crucial for the performance of a vacuum circuit breaker. Copper-chromium (Cu/Cr) is the ideal material for VCB contacts. Vacuum interrupter technology was first introduced in 1960 and continues to develop.
Over time, the size of vacuum interrupters has decreased from their original size in the 1960s due to advancements in engineering. The contact shapes have also improved, evolving from butt contacts to spiral, cup, and axial magnetic field contacts. Today, vacuum circuit breakers are considered the most reliable current interruption technology for medium voltage switchgear and require minimal maintenance compared to other circuit breaker.
Advantages of Vacuum Circuit Breaker or VCB
Service life of vacuum circuit breaker is much longer than other types of circuit breakers. There is no chance of fire hazard like oil circuit breaker. It is much environment friendly than SF6 Circuit breaker. Beside that contraction of VCB is user-friendly. Replacement of vacuum interrupter (VI) is much convenient.
Operation of Vacuum Circuit Breaker
The main purpose of any circuit breaker is to quench the arc during the current zero crossing by creating high dielectric strength between the contacts, preventing the arc from reestablishing after current zero.
The dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF6 gas. This high dielectric strength makes it possible to quench a vacuum arc within very small contact gap. For short contact gap, low contact mass and no compression of the medium the drive energy required in vacuum circuit breaker is minimum.
When two face to face contact areas are just being separated to each other, they do not be separated instantly, the contact area on the contact face is being reduced and ultimately comes to a point and then they are finally de-touched. Although this happens in a fraction of microsecond, it is a fact.
At this instant of de-touching of contacts in a vacuum, the current through the contacts concentrated on that last contact point on the contact surface and makes a hot spot.
As it is a vacuum, the metal on the contact surface is easily vaporized due to that hotspot and create a conducting media for arc path. Then the arc will be initiated and continued until the next current zero.
At current zero this vacuum arc is extinguished and the conducting metal vapor is re-condensed on the contact surface. At this point, the contacts are already separated hence there is no question of re-vaporization of the contact surface, for the next cycle of current. That means, the arc cannot be reestablished again. In this way vacuum circuit breaker prevents the reestablishment of arc by producing high dielectric strength in the contact gap after current zero.
There are two types of arc shapes. For interrupting current up to 10 kA, the arc remains diffused and the form of vapor discharge and cover the entire contact surface. Above 10 kA the diffused arc is constricted considerably by its own magnetic field and it contracts.
The phenomenon gives rise overheating of contact at its center. In order to prevent this, the design of the contacts should be such that the arc does not remain stationary but keeps traveling by its own magnetic field. Specially designed contact shape of vacuum circuit breaker makes the constricted stationary arc travel along the surface of the contacts, thereby causing minimum and uniform contact erosion.
