Types Of SF6 Circuit-Breakers

 Types Of SF6 Circuit-Breakers

The development and types of SF6 circuitbreakers can be represented as follows:

Double-pressure SF6 circuit-breakers : 

This type is developed by using principles similar to air-blast circuit-breakers. The contacts are located inside the compartment filled with SF6 gas. During the arc interruption, the arc is cooled down by compressed SF6 from a separate reservoir. After the interruption, SF6 gas is pumped back into the reservoir. This reservoir must be equipped with heating equipment to ensure that the SF6 will not liquefy. However, failures of heating equipment can result in this type being unable to operate as circuit-breakers. This type of SF6 circuit-breaker
is rarely use in the market nowadays because of its high failure probability

Self-blast SF6 circuit-breakers : 

The interrupting chamber of this type of circuit-breaker is divided into two main compartments with the same pressure (around 5 atm). During the arc interruption, the gas pressure in the arcing zone is heated resulting in high pressure. This high pressure gas from the other compartment then blasts into the arcing zone and in the meantime cools the arc column. Finally, the arc is extinguished. This type of circuit-breaker is normally used in high voltage levels up to 123 kV. The interruption principle and structure are shown in Fig. below

Arc interruption principle of self-blast circuit-breakers
(Source: SIEMENS)
1. terminal plate                                            5. nozzle
2. contact carrier                             6. contact cylinder
3. main contact                                                7. base
4. arcing contact                                8. terminal plate

Puffer-type SF6 circuit-breakers : 

The principle of this type is to generate compressed gas during the opening process. The moving contacts move the piston and thus compressing the gas in the chamber. As a result, the compressed gas flows along the arc channel and thereby extinguishing the arc. The development of puffer-type SF6 circuit-breakers can be divided into two generations: first and second generations. The principle of arc interruption of both generations is similar but the improvements of the second generation concentrate on the better design, improvement of
short-circuit rating, arcing contact lifetime and the material of the nozzle

Since the gas has to be compressed, the puffer-type SF6 circuit-breaker must have a strong operating mechanism. For example, when large current such as three-phase fault is interrupted, the opening speed of circuit-breakers is slowed down because of thermal pressure. The operating mechanism should have adequate energy to move the contacts apart. Consequently, the reliable operating mechanisms dominate the costs of circuit-breakers.

At present, SF6 circuit-breakers are predominant in high voltage levels with the high shortcircuit capability up to 63 kA. They can be used as dead tank circuit-breakers, live tank circuit-breakers and in gas insulated substation (GIS).