SF6 Gas and its Uses Within Electrical Applications

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 in 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. The 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.

Regular analysis of SF₆ gas is essential as part of a maintenance schedule. In order to protect personnel and equipment, it is advised that periodic servicing is carried out throughout the lifetime of the equipment. Identification of elevated decomposition levels will ensure unnecessary inspections, outages and failures in addition to reducing maintenance expenditure and ensuring the effectiveness of the SF₆ gas.

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.

The Rapidox uses high precision infrared (IR) sensor technology for the analysis of SF₆ gas. Infrared sensor technology is based on 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. For further information regarding gas analysis products, please contact Cambridge Sensotec on +44 (0)1480 462142, or email us at sales@cambridge-sensotec.co.uk.