Sulfur hexafluoride (SF6) is a synthetic gas that is both recyclable and reusable. It was first produced in the early 20th century in a laboratory in Paris, and first patented to be used in electrical equipment by Westinghouse in the early 1940’s. Since 1960, SF6 has been widely used in all types of electrical switchgear above 52kV as an arc quenching medium. Driven by the need for reduced footprint and more compact designs, outdoor open-air substations have gradually evolved into indoor metal-enclosed or metal-clad switchgear, with all electric components enclosed in one metal housing. Since the 1980’s, the need for more compact switchgear designs has driven leading manufacturers to start using SF6 gas as an insulation medium to help further reduce the footprint. In fact, SF6 is widely used in many industries today, including health care, automobile, semi-conductor manufacturing and the military.
So, what is the myth surrounding SF6? What are the facts? For starters, SF6 is a colorless, odorless, non-flammable gas, and about five times more dense than air. It has 3.7 times higher heat capacity than air and also helps reduce ambient temperature rise. It is a high electro-negative gas, having 2.5 times better dielectric strength than air, which means that the gas molecules tend to catch free electrons and build heavily negative ions, thus preventing electron avalanche that precedes a flashover event.
Additional facts to consider include that commercially available SF6 is not dangerous, and is therefore not subject to the Hazardous Substances Order or Technical Regulations on Hazardous Substances (TRGS). It’s (“N.D.” as of IDLH by NIOSH) and according to NAFA 704. SF6 is also designated as non-ecotoxic; it does not deplete ozone. However, with the world paying more attention to environment issues, SF6 has come under the spotlight because of its high Global Warming Effect (GWE) and long-lasting period in environment. In fact, SF6 is not measured separately for hydrochlorofluorocarbons (HCF) mixed in the “group of F-gases” (HFC, PFC and SF6), in which the HCF gases are by far the most concerned F-gases from an overall climate perspective.
While not trying to diminish the potential environmental impact of SF6 as a greenhouse gas, we must also understand that a lot of rumors spread in the marketplace are driven more by commercial considerations rather than scientific facts. Thus far, there have been no international regulations under consideration to ban the usage of SF6. Several manufacturers are indeed developing and piloting “alternative” gases to SF6, but none of these alternative gases have proven completely non-toxic and stable for their entire lifecycle. As of today, the most effective way to limit the potential environmental impact from SF6 is to handle the gas itself. As mentioned earlier, SF6 is a man-made gas and can be recycled, re-processed and reused. If we handle it within a closed system until the end of its lifecycle, we can dramatically reduce the risk of leakage and thus reduce its environmental impact.
In early generations of medium voltage gas-insulated switchgear (GIS), bus bar gas tanks were filled with nitrogen (N2) before shipment. At the facility, N2 in those tanks had to be evacuated, tanks needed to be vacuumed after the bus bars were connected, and then finally filled with SF6. Furthermore, gas pipes had to be connected between phases. After this labor-intensive process, one still had to manually perform a leakage check point by point. In addition, extra equipment would also be required for site installation and commissioning.
Today, Schneider Electric’s innovative bus bar system designs found in CBGS-0 and GHA, part of the medium voltage GIS lineup, have eliminated gas handling jobs on site. In fact, CBGS-0 does not even have bus bar gas tanks, but instead features Shielded Solid Insulation bus bar technology. In addition, all gas tanks are built, tested, and sealed for their entire lifecycle in a factory setting. Since SF6 is confined to the tank itself for insultation purposes, and the gas itself is only slightly higher than ambient air (4.3psig relative), the leakage risk is minimal.
Schneider Electric’s innovative bus bar system designs have not only eliminated the need for on site gas handling while reducing total cost of ownership through longer maintenance cycles, but have also greatly reduced the potential environmental impact of SF6.
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