SF6 is an inert chemical gas synthesized by sulfur and fluorine, colorless, odorless, non-toxic, non-flammable, and 5 times heavier than air. Sulfur hexafluoride molecule has strong negative electronegativity and can adsorb electrons to form inert ions. Because of this characteristic, it is difficult to exist free electrons in sulfur hexafluoride gas, so it is a kind of gas medium with high insulation strength and good arc extinguishing performance, its arc extinguishing ability is 100 times higher than that of air, and it can restore the insulation very soon after the arc is extinguished.
The main advantages of SF6 circuit breaker are:
(1) large breaking capacity of sulfur hexafluoride circuit breaker breaking current has reached 40 ~ 63 kA, the maximum current reaches 80 kA.
(2) Long life of electrical appliances, the repair cycle can be as long as 10 to 20 years.
(3) good breaking performance not only breaking short-circuit performance is good, but also has the advantages of breaking no-load long lines and no-load transformer does not re-ignite, low over-voltage and so on.
(4) high interrupting chamber fracture voltage has reached the level of 245 kV and 50 kA for a single fracture, therefore, the structure of circuit breaker is simple, compact and occupies a small area.
(5) Safety There is no danger of fire and explosion.
Disadvantages of SF6
Pure SF6 gas is a good arc extinguishing medium, if used in frequent operation of low voltage electrical appliances, due to frequent operation of the arc, metal vapor and SF6 gas decomposition reaction, combined with the generation of insulating fine powder (hydrofluoric acid salts, sulfuric acid salts, etc.), deposited on the surface of the contact, and serious corrosion of the contact material, thus the contact resistance increases dramatically, so that the sealing of the contacts filled with SF6 gas Can not work reliably. Therefore, for frequent operation of low-voltage appliances are not suitable for SF6 as an arc extinguishing medium.
Because SF6 gas in the discharge of high temperature will decompose corrosive gases, aluminum alloy has a serious corrosive effect on the phenolic resin laminate materials, porcelain insulation is also damaging. If the SF6 and N2 mixed use, when the SF6 content of more than 20% to 30%, its insulation strength has been the same as the full charge of SF6 insulation strength, and corrosiveness is greatly reduced, so SF6 is often mixed with the use of N2, in the SF6 circuit breaker, SF6 gas water content must be strictly stipulated not to exceed the standard. Water will produce hydrofluoric acid with SF4 in the arc decomposition material (H2O + SF4 → SOF2 + 2HF) and corrosion of materials. When the water content reaches saturation, but also in the insulation surface condensation, so that the insulation strength is significantly reduced, and even cause discharge along the surface. Operation experience and the above analysis shows that: SF6 circuit breaker due to the insulation structure of the smaller volume, if the SF6 gas moisture content is higher, the insulation level will be greatly reduced, the contact resistance increased dramatically, easy to damage or explosion in operation. Therefore, the manufacturing plants and operating departments are required to have a strict sealing process, while the water content of SF6 gas shall not exceed the standard. China's standard is SF6 gas water content should be less than 300ppm (volume ratio). SF6 circuit breakers use SF6 gas as the arc extinguishing medium. Under normal circumstances, SF6 is a non-combustible, odorless, non-toxic inert gas, the density of about 2 times the air. However, under the action of arc, a small part of SF6 gas will be decomposed, generating some toxic low fluoride, such as SOF2, SF4, SOF4 and SO2F2, etc., which has an effect on the health of the body, and also has a corrosive and deteriorating effect on the metal parts. Therefore, in the SF6 circuit breaker, generally are equipped with adsorption device, adsorbent for activated alumina, activated carbon and molecular sieve. Adsorption device can completely adsorb SF4 gas in the arc of the high temperature decomposition of toxic substances.
Conclusion:
SF6 circuit breakers have many advantages over traditional circuit breakers, such as high dielectric strength, reduced maintenance, and increased reliability. However, there are also some disadvantages, including the environmental impact, high cost, and safety concerns associated with SF6 gas. Overall, SF6 circuit breakers are an effective solution for high voltage systems and are widely used in the electrical industry. However, efforts must be made to reduce the environmental impact of SF6 gas emissions, and safety measures must be followed to avoid potential risks.
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