SOLUTION

Types of Circuit Breakers

The different types of high voltage circuit breakers include the following

• Air Break Circuit Breaker
• SF6 Circuit Breaker
• Vacuum Circuit Breaker
• Oil Circuit Breaker
• Airblast Circuit Breaker

SF6 gas circuit breakers are widely used for EHV applications today, as Sulphur hexafluoride(SF6) gas is an electronegative gas having excellent dielectric and arc quenching properties resulting in many advantages such as compactness and less maintenance of EHV circuit breaker

Circuit breakers are mechanical device designed to close or open contacts members, thus closing or opening of an electrical circuit under normal or abnormal conditions.

Circuit breakers are made in varying sizes, from small devices that protect low-current circuits or individual household appliances, up to large switchgear designed to protect high voltage circuits feeding an entire city.

Properties of SF₆ Gas

The properties of SF₆  gas are divided as

1. Physical properties
2. Chemical properties
3. Dielectric properties

Physical Properties

• The gas is colorless, odorless, non-toxic, and non-harmful to health.
• The gas is non-inflammable.
• It is heavy gas having a high density.
• Liquification starts at low temperatures which depend on pressure.
• The heat transferability is high as compared to air at the same pressure.
• The heat content property is much more which will assist cooling of the arc after current zero.
• The gas is electronegative.

Chemical Properties

• The gas is chemically inert and stable upto 500 CC. The life of the metallic part, contacts is longer in SF^6 gas. The components do not get deteriorated. Hence the maintenance requirements are reduced.
• During arc extinction, SF₆ is decomposed to SF₄,  SF₂. The products of decomposition are toxic and hence proper care must be taken for their disposal.
• Metallic fluorides are good dielectric materials. Hence are safe for electrical equipment.

Dielectric Properties

• The dielectric properties of SF₆ are about 2.5 times that of air at atmospheric pressure.
• This depends upon the nature of the field existing between electrodes, the shape, and configuration of electrodes, and the gap between the electrodes.
• The dielectric strength increases rapidly with an increase in pressure compared to air and oil.
• SF₆ gas is strongly electro-negative which means that free electrons are quickly removed from discharge by the formation of negative ions by the process of attachment of a free electron to a neutral gas molecule. The resulting ions are heavy and immobile and so ineffective as current carriers and thus the ionized SF₆  gas possesses a very high dielectric strength like unionized N₂ gas at equal density.