Sulphur hexafluoride (SF6) is a man-made gas comprising of one sulphur and six fluoride atoms. SF6 is extremely chemically stable, non-flammable and highly electronegative, with an excellent dielectric property of approximately 2.5 times more than air.
Therefore it is commonly used within electrical switchgear, transformers and substations as an electrical insulation, arc quenching and cooling medium.
Electrical arcs are a discharge of light and heat and can be caused within electrical power systems such as switchgear, when conductive channels facilitate their formation. The high electronegativity and heat transferring properties of SF6 means that it is able to absorb the electron energy and lower the arc temperature through the capture of free electrons. Combination of the SF6 gas and free electrons results in big, heavy ions with low rates of mobility, which effectively prevents electron avalanches and flashovers.
While under the influence of electrical discharges SF6 partially decomposes, separating into its atomic components. During this time oxygen, moisture and decomposition products such as sulphur dioxide may be created. Unlike other insulating mediums, SF6 gas has the unique ability to ‘self-heal’ with the atoms recombining after the discharge, this is one of the reasons for its widespread use within the electrical industry.
The Kyoto Protocol is an international agreement between nations that came into force in 2005. It commits its parties by mandating country-by-country reductions in greenhouse gas emissions. Due to the chemical stability of SF6 gas it has the potential to remain in the atmosphere for thousands of years. Under the Kyoto Protocol, SF6 gas has been identified as a greenhouse gas with a limitation as to the amount that can be released into the atmosphere.
Stricter legislation has seen changes to the requirements of the electrical power industry in relation to the handling of SF6 gas. As a result Cambridge Sensotec has developed the new Rapidox SF6 6100 gas analyser featuring a pump back gas return function. The analyser is able to extract the SF6 gas from equipment such as Gas Insulated Substations (GIS) and simultaneously analyse all measured gases. It will then pump it back at the same pressure as used within the electrical equipment by using a powerful 10-bar compressor. A vacuum purge cycle and gas storage system ensures that no air is allowed to contaminate the gas sample and that no SF6 gas is allowed to escape during the testing period.
High precision infrared (IR) sensor technology is used by the Rapidox for the analysis of SF6 gas. Infrared sensor technology is based upon the principle that some gases absorb IR radiation at specific wavelengths. It is advantageous in that it supplies the user with a rapid response, is immune to poisoning and contamination and has the ability to operate in the absence of oxygen.
Decomposition products sulphur dioxide (SO2) and moisture (H2O) may also be measured by the Rapidox SF6 6100 gas analyser, using electrochemical and advanced ceramic sensor technology respectively. Additional sensors for gases such as carbon monoxide (CO), hydrogen sulphide (H2S) and hydrogen fluoride (HF) are also available.
Cambridge Sensotec has also been carrying out field tests, which have highlighted the requirement for quick and easy to use instruments for onsite SF6 gas maintenance procedures. Therefore the Rapidox SF6 6100 pump back gas analyser has been developed with an inbuilt microprocessor operated by a 7” colour touchscreen, with the ability to store 12 months of continuous data. Pre-loaded IEC and CIGRE tests are available, alongside the ability to customise test parameters. Housed within a robust IP66 Peli case, the Rapidox is lightweight and features an 8 hour lithium ion battery in addition to an in-built printer, making it a truly portable field instrument.
Due for release in Autumn 2014, the Rapidox SF6 6100 pump back gas analyser is to be showcased in October 2014 at the EP China exhibition http://www.epchinashow.com