Gas insulation properties of sulfur hexafluoride(SF6)

    Sulfur hexafluoride (SF6) gas only began to be used as an insulating medium and arc extinguishing medium in certain electrical equipment (first of all, circuit breakers) in the 1960s. Today, it is the most widely used gas medium besides air.

    The electrical strength of SF6 is about 2.5 times that of air, and its arc extinguishing capacity is more than 100 times that of air. Therefore, in the category of ultra-high voltage and ultra-high voltage, it has completely replaced insulating oil and compressed air and has become the only circuit breaker The arc extinguishing medium is gone.

    At present, SF6 gas is not only used in certain single power equipment (such as SF6 circuit breakers, gas-insulated transformers, etc.), but also widely used to integrate multiple substation equipment and seal them in a container filled with SF6 gas. Enclosed gas insulated switchgear (GIS for short) and gas-filled pipeline transmission lines and other installations. In the field of UHV and UHV power transmission, GIS has shown the advantages that conventional switchgear can't compare with.

    The reason why halide gases including SF6 have particularly high electrical strength is mainly because these gases have strong electronegativity and are easy to trap free electrons to form negative ions (electron adhesion process). After the electrons become negative ions, Its ability to cause impact ionization becomes very weak, thus weakening the discharge development process.

    It should be emphasized that the non-uniformity of the electric field has a far greater impact on the electrical strength of SF6 than on air. Specifically, compared with the breakdown voltage in a uniform electric field, the breakdown voltage of SF6 in an extremely uneven electric field has a much greater drop than that of air. In other words, the excellent insulation performance of SF6 can only be fully exerted when the electric field is relatively uniform. Therefore, when designing various electrical equipment with SF6 gas as insulation, the electric field in the air gap should be as uniform as possible, and shielding should be used. Measures are taken to eliminate the extremely uneven electric field at all sharp corners, so that the excellent insulation properties of SF6 can be fully utilized.

    Although SF6 gas has good electrical characteristics and chemical stability, its price is relatively high, the liquefaction temperature is not low enough, and it is too sensitive to the unevenness of the electric field. Therefore, SF6 mixed gas is currently being studied at home and abroad, with a view to some occasions Use SF6 mixed gas instead of pure SF6 gas.

    Studies have shown that when common cheap gases such as N2, CO2 or air and SF6 are used to form a mixed gas, even a small amount of SF6 can greatly increase the electrical strength of these common gases, but continue to increase the SF6 content. The increase of electric strength will show a tendency of saturation. This is because a small number of SF6 molecules have been able to capture electrons to form negative ions.

(Huiputech)