Answer MET Question 1

Question:  With respect to the High Voltage power systems installation, explain the different types of circuit breaker that are used, comparing them on merits and de-merits. Describe the theory of arc phenomenon and the mechanism fitted to mitigate the arc.

Answer: HV Circuit breakers For HV, the circuit breaker types may be air-break, oil-break, gas-break using SF6 (sulphur hexafluoride) or vacuum-break. Of these types, the most popular and reliable are the vacuum interrupters, which may also be used as contactors in HV motor starters.

Vacuum circuit breaker: Each phase of a vacuum circuit breaker or contactor consists of a fixed and moving contact within a sealed, evacuated envelope of borosilicate glass. The moving contact is operated via flexible metal bellows by charging motor/spring or solenoid operating mechanism. The high electric strength of a vacuum allows a very short contact separation, and a rapid restrike-free interruption of the arc is achieved.

When an alternating current is interrupted by the separating contacts, an arc is formed by u metal vapour from the material on the contact surfaces and this continues to flow until a current zero is approached in the a.c. wave form. At this instant the arc is replaced by region of high dielectric strength which is capable of withstanding a high recovery voltage. Most of the metal vapour condenses back on to the contacts and is available for subsequent arcing. A small amount is deposited on the shield placed around the contacts which protects the insulation of the enclosure. As the arcing period is very short (typically about 15 micro seconds), the arc energy is very much lower than that in air-break circuit-breakers so vacuum contacts suffer considerably less wear. Because of its very short contact travel a vacuum interrupter has the following advantages: compact quiet unit minimum maintenance non-flammable and non-toxic The life of the unit is governed by contact erosion but could be up to 20 years.

SF6 circuit breaker:

In the gas-type circuit breaker, the contacts are separated in an SF6 (sulphur hexafluoride) gas which is typically at a sealed pressure chamber at 500 kPa or 5 bar (when tested at 20 deg. C).