What is sf6 switchgear

The SF 6 gas absorbs the free electrons in the arc path and forms ions which do not act as a charge carrier. These ions increase the dielectric strength of the gas and hence the arc is extinguished. This low-pressure gas is pulled back to the high-pressure reservoir for re-use. Now a day puffer piston pressure is used for generating arc quenching pressure during an opening operation by mean of a piston attached to the moving contacts.

Its main function is to avoid arc by releasing SF6 gas which form ions with the current moving across the contacts. Hence, achieving zero current flow after cut out.

Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Skip to content A circuit breaker in which SF 6 under pressure gas is used to extinguish the arc is called SF 6 circuit breaker. The moving contact is also a hollow cylinder with rectangular holes in the sides to permit the SF6 gas to let out through these holes after flowing along and across the arc.

The tips of fixed contact, moving a contact and arcing horn are coated with copper-tungsten arc-resistant material. Since SF6 gas is costly, it is reconditioned and reclaimed by a suitable auxiliary system after each operation of the breaker. When the breaker operates, the moving contact is pulled apart and an arc is struck between the contacts.

The high-pressure flow of SF6 rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers. The result is that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc.

After the breaker operation i. Due to the superior arc quenching properties of SF6 gas, the SF6 circuit breakers have many advantages over oil or air circuit breakers. Some of them are listed below :. Since SF6 gas is costly, it is reconditioned and reclaimed using the suitable auxiliary system after each operation of the breaker. Since the gas pressure is very high, a lot of care is to be taken to prevent gas leakage at joints by providing perfect scaling.

The low and high-pressure systems are fitted with low-pressure alarms and set of lockout switches which give a warning the moment the gas pressure drops below a certain value because otherwise there will be a reduction in the dielectric strength and arc quenching ability of the breaker is endangered.

If the danger limit is reached the safety devices immobilize the breaker. The overriding safety devices see to it that a fault in the control circuit does not permit the compressor to build up excessive pressure in the high-pressure reservoir or continue to pump gas into the atmosphere in the event of a major leak.

The gas is stored in the high-pressure chamber at 16 atmospheres whereas the gas pressure on the low-pressure side is 3 atmospheres. The working of an SF6 circuit breaker is similar to the working of a normal circuit breaker we discussed in previous articles. In the closed position of the breaker , the contacts remain surrounded by sulphur hexafluoride gas SF6 gas at a pressure of about 2.

When the breaker operates , the moving contact is pulled apart and an arc is struck between the contacts. For more details visit arcing phenomenon in a circuit breaker.

The movement of the moving contact is synchronized with the opening of a valve. The high-pressure flow of sulphur hexafluoride gas SF6 rapidly absorbs the free electrons in the arc path. It forms immobile negative ions which are ineffective as charge carriers. The result is that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc. After the breaker operation i. The practical opening and closing of an SF6 circuit breaker are animated in this video.

The SF6 circuit breaker has been developed by several manufacturers and several designs have emerged. The types of SF6 circuit breakers can be broadly identified as. In double pressure type SF6 circuit breaker , the gas from the high-pressure system is released into the low-pressure system through a nozzle, during the arc extinction processes.

This design has become obsolete. The double pressure type system worked on a very similar basis to the air blast design , modified to make a closed-loop system for the exhaust gases. After the arc was quenched, gases in a low-pressure reservoir were filtered, compressed and then stored in the high-pressure reservoir for further use.

Heaters were also fitted to ensure the gas did not become a liquid in low temperatures, which would make the medium unusable as an interrupter. In single pressure type SF6 circuit breaker , the SF6 gas is compressed by the moving cylinder system and is released through a nozzle while extinguishing the arc. This design is most popular over a wide range of voltages from SF6 breaker designs were first successfully manufactured by Westinghouse in Initial designs used a double pressure system, which was superseded in the s by single-pressure puffer type design.

In both the double pressure and single pressure types, SF6 switchgear is of either the dead tank design or live tank design. The dead tank design is for outdoor installations and live tank or modular design is for indoor installations. Increasingly dead tank breakers are integrated into SF6 insulated substations for indoor or outdoor installations.

The SF6 Circuit Breakers are available as in the following designs. The live tank means the interruption happens in an enclosure that is at line potential.

Such SF6 CB has an interrupter chamber that is mounted on insulators and is at line potential. An interrupter with such a modular design can be connected in series to operate at higher voltage levels. The dead tank means that interruption takes place in a grounded enclosure. Current Transformers are located on both sides of the break i. In such circuit breakers, the interruption maintenance takes place at ground level. However, they require more insulating gas in order to provide the proper amount of insulation between the interrupter and the grounded tank enclosure.

The modular dead tank CB has been specially developed for integration of SF6 insulated substation systems. The grounded tank means that interruption happens in an enclosure that is partially at line potential and partially at ground potential.

The evolution of the grounded tank CB design is the result of installing a live tank CB interrupter into a dead tank CB design. The important properties of SF6 circuit breaker are:. In its pure state SF6 gas is colorless, odorless, tasteless and non-toxic. At room temperature and pressure, it is gaseous and has a density of 6. Since its critical temperature is Due, to the superior arc quenching properties of sulphur hexafluoride gas SF6 gas, the sulphur hexafluoride gas SF6 circuit breakers have many advantages over oil circuit breaker and air blast circuit breakers.

The important advantages of SF6 Circuit Breaker are :. In general, the important disadvantage of the SF6 Circuit Breakers compared to other circuit breakers is their relatively high costs.

The cost has been somewhat coming down in recent years. Some other disadvantages of the SF6 Circuit breaker are:.

The maintenance of the sf6 circuit breaker is necessary because of their importance for routine switching and for the protection of other equipment. Electric transmission system breaks up and equipment destruction can occur if a circuit breaker fails to operate due to lack of preventive maintenance. Breakers that remain idle for 6 months or more should be made to open and close several times in succession to verify proper operation and remove any accumulation of dust or foreign material on moving parts and contacts.

Although pure SF6 is not toxic, it contains no oxygen and will therefore not support life.