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Sulphur hexafluoride (SF6) boasts many useful properties, especially for the high voltage electricity sector. It also suffers from a significant disadvantage – it is a powerful greenhouse gas. Hence the need to find a suitable alternative.
SF6 is a versatile gas used in various industries, such as magnesium and aluminium casting and insulated window glazing. But by far its most important application, and one that was first proposed by Vitaly Grosse of AEG (an Alstom Grid ancestor company), is in high voltage power transmission equipment. Some 10,000 tonnes per year of SF6 are used for this purpose (around 80 % of the total). Air- and gas-insulated substations exploit the gas in circuit breakers, instrument transformers, surge arrestors, bushings and busbars, while gas-insulated substations also rely on it in disconnectors and earthing switches.
“SF6 stands out for its remarkable arc quenching capability and dielectric insulation,” says Alstom Grid Materials and Eco-Design R&D Manager, Yannick Kieffel. “Moreover, it is stable, non-toxic and non-flammable. It allows manufacturers to design switchgear that is compact and insensitive to climatic conditions, has minimal environmental impact and requires little maintenance.” However, SF6 is one of the 6 gases included in the Kyoto Protocol aimed at reducing greenhouse gas emissions. Its global warming potential (1) (GWP) is 23,500 times greater than CO2 (IPCC 2013) and it has a lifetime in the atmosphere of 3,200 years, putting it at the top of the Kyoto Protocol list. So, while SF6 switchgear is generally safe for the environment, if just one kilogramme of SF6 leaks into the atmosphere it has the equivalent global warming impact of around 23.5 tonnes of CO2.
(1) Calculation mode of GWP: please refer to the sidebar in Think Grid article “The impact of “F” gas regulation”
Research has been on-going for a long time to find an alternative to SF6 but it has not been successful in finding a suitable solution for transmission networks. Until now.
After 4 years’ research, Alstom, together with the 3M Company, has developed an SF6-free solution that is much more environmentally sustainable yet meets all the very tough specifications for HV switchgear:
It also meets health and safety prerequisites (low toxicity, no flash point) and in particular environmental requirements such as:
“We looked at simple, non-greenhouse gases such as nitrogen and air,” explains Kieffel, “but their dielectric strength is only a third that of SF6. SF6/nitrogen mixtures would have higher dielectric strength, but their GWP is still too high. The same is true of perfluorocarbons (PFCs). On the other hand, trifluoroiodomethane (CF3I) presents dielectric strength greater than SF6, with a GWP of less than 5 and a lifetime in the atmosphere of only a few days. On the down side, CF3I is classified as carcinogenic, mutagenic and toxic; it cannot be used for industrial purposes.”
(2) Reach Annex VII testing
Not a simple gas as SF6 is, but a gas mixture named g3 (green gas for grid) based on 3M™ Novec™ 4710 Dielectric Fluid from the fluoronitrile family specially developed by 3M for that purpose and used as an additive to a complementary gas. Among other candidates, fluoronitriles were studied extensively as the most promising chemical family. Among the different candidates tested, some have a high dielectric strength, but toxicity was an issue. So, new molecules were developed to resolve this problem, chemists and physicists manipulating molecular architecture to combine the advantages of previous candidates while eliminating drawbacks. Finally, one specific molecule, marketed as Novec™ 4710 fluid, was designed and selected as the best compromise for its excellent combination of properties:
“However,” notes Kieffel, “Novec™ 4710 fluid cannot be used alone due to its liquefaction at low temperature. It was found that the best compromise is achieved by mixing it with CO2 (for its arc quenching capability) to create g3, a gas mixture suitable for disconnector and circuit breaker applications.” As a result, the GWP of the g3 mixture of fluoronitrile with CO2 can be brought down by well over 98 % compared with SF6.
But what of the other properties of g3?
Toxicity: g3 is non-toxic (3) and in the same category as SF6.
Insulation: dielectric performance was tested on 145 kV gas-insulated switchgear. The results showed it to be between 87 % and 96 % of SF6 performance. The difference can be compensated by minor design modifications or a dielectric coating on high-stress electrodes.
Temperature rise: the thermal conductivity of the gas mixture is slightly lower than SF6, but significantly higher than pure CO2.Tests show that the temperature rise difference is 5 or 6°K. This can be compensated by adding cooling fins to the enclosure or slots on conductors to improve convection cooling.
Switching: the gas mixture has a good capability of switching bus transfer current and is suitable as a substitute for SF6.
Arc quenching: tests carried out on a 145 kV live tank circuit breaker show very encouraging results. The plan is to use the new gas mixture in the arcing chamber but, due to its different behaviour such as gas flow, the design of the arcing chamber has to be optimised for g3.
(3) Based on GHS and CLP classifications.
The insulation, thermal conductivity and switching performance have now been validated. Current interruption capability also shows positive progress, and g3 is safe for operators. Therefore, g3 is suitable for developing a new generation of clean equipment from 72 kV up to ultra-high voltages with extremely low global warming potential.
* g3: trademark application owned by ALSTOM Grid SAS
* 3M and Novec are trademarks of the 3M Company
Philippe Ponchon, Alstom Grid VP Marketing and AIS Marketing Director, offers a roadmap for its ground-breaking SF6 replacement.
“Our objective of replacing SF6 in our high voltage equipment stems from our customers’ concern for the environment, as well as present or future pressure from the authorities. The breakthrough we have made is to engineer and test a family of gas mixtures for high voltage switchgear applications. We call it g3 (green gas for grid). It uses a molecule, developed by 3M in cooperation with Alstom and according to Alstom’s specifications and guidance, which we have combined with CO2.
“We have already started the development of new products for both GIS and AIS arrangements. In a not too distant future, i.e. from March 2015 we will announce the first specific g3 product.
“Retrofit – to replace SF6 with g3, while adapting some components – is also of major interest for some customers who want to reduce the environmental footprint of their assets as fast as possible. Even if it is only possible in specific cases, we are currently assessing pilot applications with precursor customers.
“The technical foundations are laid; opening the door to the development of new ranges of high voltage equipment comparable to SF6 in terms of performance and footprint, but much more environmentally friendly. It is, from my point of view, the beginning of a major change similar to the move from oil or compressed air to SF6 in the early ’70s. The speed of the move will depend on many parameters – economical, technical and societal.
“On the technical side, in the insulating domain this will enable fast development for all voltages from 72 kV to ultra-high voltages. Circuit breakers are a more complex domain. We have tested some prototypes, with satisfying results. But we are aiming at solutions for all types of circuit breaker on the full range of voltages. This implies modifications to a number of components and exhaustive testing, which will take more time.
“The key parameter will now be how far customers are ready to move to solutions with a global warming potential reduced by up to 98 % and how strong will be the regulatory incentives to protect the planet against climate change!”
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