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Table of Contents. Gas System 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. Working of SF6 Circuit Breaker The working of an SF6 circuit breaker is similar to the working of a normal circuit breaker we discussed in previous articles.
Double Pressure Type SF6 Circuit Breaker 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.
Live tank and Dead tank circuit breaker The dead tank means that interruption takes place in a grounded enclosure. It can, however, exclude oxygen and cause suffocation. If the normal oxygen content of air is reduced from 21 percent to less than 13 percent, suffocation can occur without warning. Dielectric strength SF6 has a dielectric strength about three times that of air at one-atmosphere pressure for a given electrode spacing.
The dielectric strength increases with increasing pressure; and at three atmospheres, the dielectric strength is roughly equivalent to transformer oil. Thermal conductivity The thermal conductivity of SF6 is below that of air but its overall heat transfer capability, in particular when convection is taken into account is excellent being similar to that of gases such as hydrogen and helium and higher than that of air.
At high temperatures, the thermal conductivity curve of SF6 reveals one of the exceptional qualities of the gas, which allows it to be used for extinguishing arcs by thermal transport. Arc quenching SF6 is approximately times more effective than air in quenching spurious arcing. SF6 also has a high thermal heat capacity that can absorb the energy of the arc without much of temperature rise.
Electrical properties The excellent dielectric properties of SF6 are due to the electronegative character of its molecules. It has a pronounced tendency to capture free electrons forming heavy ions with low mobility making the development of electron avalanches very difficult. Because of the arc quenching ability of SF6, corona and arcing in SF6 does not occur until way past the voltage level of onset of corona and arcing in air.
SF6 will slowly decompose when exposed to continuous corona. Chemical properties SF6 fully satisfied the valence requirements of the sulfur molecule. Its molecular structure is octahedral with a fluorine molecule at each apex.
The six bonds are covalent which accounts for the exceptional stability of this compound. SF6 is non-flammable and hydrogen, chlorine, and oxygen have no action on it. SF6 is insoluble in water and is not attacked by acids. Toxicity of arc products T oxic decomposition products are formed when SF6 gas is subjected to an electric arc. The decomposition products are metal fluorides and form a white or tan powder.
Toxic gases are also formed which have the characteristic odor of rotten eggs. Due to this SF6 circuit breakers have very short arcing time. Dielectric strength of sulphur hexafluoride SF6 gas is 2 to 3 times compared to air. Thus SF6 breakers can interrupt much larger currents. The compact design of SF6 gas CBs substantially reduces space requirements and building installation costs.
The SF6 gas circuit breakers can handle all known switching phenomena. SF6 gas circuit breakers perfectly can adapt to environmental requirements. They have completely enclosed gas system that eliminates any exhaust during switching operations. Contact separation is minimum in SF6 gas circuit breakers due to dielectric strength provided by the high-pressure SF6.
Disadvantages of SF6 Circuit Breaker In general, the important disadvantage of the SF6 Circuit Breakers compared to other circuit breakers is their relatively high costs. Some other disadvantages of the SF6 Circuit breaker are: Heavy-duty switching operations can generate harmful by-products from decomposition.
During typical sealed operation, these by-products will recombine into SF6. However, during arcs or in the event of switchgear failure, SF6 and its by-products can be released into the environment. A circuit breaker in which SF 6 under pressure gas is used to extinguish the arc is called SF 6 circuit breaker. SF 6 sulphur hexafluoride gas has excellent dielectric, arc quenching, chemical and other physical properties which have proved its superiority over other arc quenching mediums such as oil or air.
The SF 6 circuit breaker is mainly divided into three types. The circuit breaker which used air and oil as an insulating medium, their arc extinguishing force builds up was relatively slow after the movement of contact separation. In the case of high voltage circuit breakers quick arc extinction properties are used which require less time for quick recovery, voltage builds up.
SF 6 circuit breakers have good properties in this regards compared to oil or air circuit breakers. So in high voltage up to kV, SF 6 circuit breakers is used. Sulphur hexafluoride possesses very good insulating and arc quenching properties.
These properties are. 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 :.
SF6 circuit breakers Applications: A typical SF6 circuit breaker consists of interrupter units each capable of dealing with currents up-to 60 kA and voltages in the range of 50—80 kVA number of units are connected in series according to the system voltage.
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