A sulfur hexafluoride circuit breaker is a circuit breaker that uses SF6 (sulfur hexafluoride) gas as the arc quenching medium. Sulfur hexafluoride gas has good dielectric, arc quenching, chemical, and other physical properties compared with oil or air, which makes it effective for high power and high voltage applications. Sulfur hexafluoride is an electronegative gas with a strong tendency to absorb free electrons. When the contacts of a breaker are opened in a high-pressure flow of sulfur hexafluoride gas, an arc is struck between them. The gas captures the conducting free electrons in the arc, forming relatively immobile negative ions. The loss of conducting electrons quickly generates enough insulation strength to extinguish the arc.
In comparison with sulfur hexafluoride, the use of air or oil as an insulating medium produces a relatively slow arc extinguishing force after the movement of contact separation. High voltage circuit breakers require quick arc extinction properties and less time for quick recovery, voltage builds up. Sulfur hexafluoride circuit breakers are used in substations for voltages ranging from 144 to 765 kV or higher, continuous currents up to 8000 A, and symmetrical interrupting ratings up to 63 kA at 765 kV and 80 kA at 230 kV.
- Toxicity—in its pure state, SF6 is odorless, colorless, tasteless, and nontoxic. However, it can exclude oxygen (reducing the normal oxygen percentage of air) and cause suffocation without warning.
- Dielectric strength—SF6 has about three times the dielectric strength of air at standard atmospheric pressure for a given electrode spacing. Its dielectric strength increases with pressure roughly equalling transformer oil at three atmospheres.
- Thermal conductivity—while SF6 has a lower thermal conductivity than air its overall heat transfer capability (in particular convection) is similar to that of hydrogen and helium gas and higher than that of air.
- Arc quenching—SF6 is approximately 100 times more effective than air in quenching spurious arcing. This is due to fast recombination after the source energizing spark is removed.
- Electrical properties—its electronegative characteristics give SF6 excellent dielectric properties, capturing free electrons and forming heavy ions with low mobility, preventing electron avalanches.
- Chemical properties—SF6 gas is extremely stable and inert and has density five times that of air. It fully satisfies the valence requirements of the sulfur molecule, with an octahedral structure (fluorine molecule at each apex). SF6 is non-flammable and hydrogen, chlorine, and oxygen have no action on it. SF6 is also insoluble in water and is not attacked by acids.
- Toxicity of arc products—when SF6 gas is subjected to an electric arc, toxic decomposition products (metal fluorides and toxic gases) are formed.
Sulfur hexafluoride circuit breakers consist of two main components:
The interrupter unit consists of moving and fixed contacts enclosed in a chamber containing SF6, known as the arc interruption chamber. When the contacts are opened, a valve mechanism permits a high-pressure sulfur hexafluoride (SF6) gas from the reservoir to flow towards the arc interruption chamber. The fixed contact is a hollow cylindrical current-carrying contact fitted with an arcing horn. The moving contact is also a hollow cylinder with rectangular holes in the sides. These holes permit the sulfur hexafluoride gas to let out through them after flowing along and across the arc.
A closed-circuit gas system is employed in SF6 circuit breakers. The SF6 gas is expensive, so it is reclaimed after each operation. This unit consists of low- and high-pressure chambers with a low-pressure alarm along with warning switches. When the pressure of the gas is very low, it causes the dielectric strength to decrease, and the system gives the warning alarm as the arc quenching ability of the breakers is endangered.
Sulfur hexafluoride circuit breakers operate similarly to normal circuit breakers. In the closed position, the contacts are surrounded by SF6 gas. When the breaker operates, the moving contact is pulled and an arc is produced between the contacts. The moving contact is synchronized with a vale that permits high-pressure SF6 gas from the reservoir to the arc interruption chamber. The gas absorbs the free electrons in the arc path forming immobile negative ions that are ineffective charge carriers. The medium between the contacts builds up high dielectric strength, quenching the arc.
The original sulfur hexafluoride circuit breaker design used a double pressure type system, where gas from the high-pressure system is released into the low-pressure system through a nozzle during the arc extinction process. This is similar to the air blast design but modified for a closed-look system. After quenching the arc gases in a low-pressure reservoir are filtered, compressed, and stored in the high-pressure reservoir for future use. This type of circuit breaker has become obsolete.
Single-pressure puffer circuit breakers superseded double pressure systems in the 1970s when HV switchgear became popular. In this type of circuit breaker, the SF6 gas is compressed by a moving cylinder system and released through a nozzle to quench the arc.
Both the double and single pressure types use either dead tank design or live tank design SF6 switchgear.
- Live tank, or modular design, is used for indoor installations. The interrupter chamber is mounted ion insulators at line potential.
- Dead Tank means the interruption occurs in a grounded enclosure.
The following are advantages of sulfur hexafluoride circuit breakers:
- SF6 gas has excellent insulating and arc extinguishing properties.
- SF6 gas is non-inflammable and chemically stable and also generates non-explosive decomposition products.
- SF6 gas has high dielectric strength.
- Performance is not affected by variations in atmospheric condition.
- Reduced maintenance requirements and no costly compressed air system are needed.
There are also disadvantages of sulfur hexafluoride circuit breakers:
- SF6 gas can become suffocating to some extent. In the case of a breaker tank leak, the SF6 gas settles in the surroundings due to it being heavier than air potentially suffocating operating personnel.
- Moisture in the SF6 breaker tank is harmful to the breaker, potentially causing failure.
- Internal parts require cleaning during periodic maintenance.
- SF6 gas requires special facilities for transportation and maintenance to ensure the quality of the gas.
This dead tank circuit-breaker can interrupt 41.8 kA under 138 kV (10,000 MV·A) and 37.6 kA under 230 kV (15,000 MV·A), however, the three chambers per pole and the high-pressure source required for the blast (1.35 MPa) became a constraint that had to be avoided in subsequent developments.
The relative movement of a piston and a cylinder linked to the moving part generates the pressure rise necessary to blast the arc via a nozzle made of insulating material.
The circuit breaker could interrupt 5 kA under 115 kV, but it had six interrupting chambers in series per pole.
High-voltage 15 kV to 161 kV load switches were developed with a breaking capacity of 600 A.