Now days, vacuum switchgear is getting popularity very rapidly. In medium voltage switchgear application, medium voltage vacuum switchgear ranges from 3 to 36 KV. Now days, vacuum interruption technology, in medium voltage application, dominates, air, SF6 and oil technologies. Since, vacuum circuit breaker is more safely and reliably operated where number of faulty and normal operation is much high.
Vacuum as an Interruption Medium
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. Now we will discuss one of the different features of this technology, which makes it so popular-
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.
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. Law 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.
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 the movement of contacts is quite less due to its small contacts gap, hence driving energy required is much smaller, in this circuit breaker. That is why the 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.
Rapid Arc Quenching
During opening of contacts in current carrying condition, metal vapour is produced between the contacts, and this metal vapour provides a path through which electric current continuous to flow until the next current zero. This phenomenon is also known a 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 microseconds. 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.
From the above discussion, it is almost clear that the dielectric strength of vacuum switchgear recovers very fast and contact erosion is almost negligible.
It has been observed that, up to 10 KA, the arc remains diffused. It takes the form of vapor discharge and covers the entire contact surface. But above 10 KA, the diffused arc is concentrated at central point of the contact surface due to its own magnetic field. Due to this phenomenon, the center of the contacts overheated. This problem can be solved by providing a specially designed contact surface so that, the arc can travel throughout the surface area, instead of being stationary at a certain point. Different manufacturers use different contact surface designs to achieve this traveling of arc due to its own magnetic field. This causes minimum and uniform contact erosion.