The Advantages of Vacuum Switchgear

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Nowadays, vacuum switchgear getting popularity very rapidly. Since, vacuum circuit breaker is more safely and reliably operated where number of faulty and normal operation is much high. In medium voltage switchgear application, vacuum switchgear ranges from 3 to 36 KV. The performance of a circuit breaker mainly depends upon the dielectric medium used for arc quenching. Another major advantage of this technology, is that vacuum switchgear is nearly maintenance free.

Rapid Arc Quenching

During opening of contacts in current carrying condition, metal vapor is produced between the contacts, and this metal vapor provides a path through which electric current continuous to flow until the next current zero. This phenomenon is also known as vacuum arc. This arc is extinguished near the current zero and the conductive metal vapor is re-condensed on the contact surface in a matter of micro seconds. It has been observed that, only 1% of the vapor is re-condensed on arc chamber’s side wall, and 99% of vapor re-condensed on the contact surface from where it was vaporized.

Low Arc Energy in Vacuum

The energy dissipated during arc in vacuum is about one tenth of that of oil and one fourth of that of SF6 gas. Low energy dissipation mainly due to low interruption time (due to small contact gap) and small arc length (this is also due to small contact gap). Because of this low arc energy dissipation, vacuum switchgear has negligible contact erosion and this gives it nearly maintenance free life span. It is also to be noted that, for breaking certain current, the energy required by vacuum circuit breaker is minimum compared to air circuit breaker and oil circuit breaker.

Dielectric Strength of Vacuum

For a given contact gap, vacuum provides, about eight times more dielectric strength than air and four times more dielectric strength than SF6 gas at one bar. As the dielectric strength is so high, the contact gap of vacuum circuit breaker can be maintained very small. In this small contact gap, arc quenching is safely possible due to high dielectric strength and also vacuum has the fast recovery strength after full arc interruption to its full dielectric value at current zero. This makes vacuum switchgear most suitable for capacitor switching.

Simple Driving Mechanism

In SF6, oil and air circuit breaker, movement of contacts is highly resisted by highly compressed medium of arc quenching chamber. But in vacuum switchgear, there is no medium, and also movement of contacts is quit less due to its small contacts gap, hence driving energy required is much smaller, in this circuit breaker. That is why simple spring-spring operating mechanism is sufficient for this switchgear system, no need of hydraulic and pneumatic mechanism. Simpler driving mechanism gives a high mechanical life of vacuum switchgear.

From above discussion, it is almost clear that, the dielectric strength of vacuum switchgear recovers very fast and contact erosion is almost negligible. 

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