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In August 2014, GE unveiled g3 (g cubed) its ground-breaking solution for replacing SF6 in high-voltage equipment. Since then, much has happened. g3 products have been installed successfully in several countries, and g3 life-cycle assessments have underscored the huge environmental benefits of the new gas mixture.
For some 50 years, SF6 has rendered the high-voltage equipment industry good and faithful service, thanks to its exceptional arc-quenching and voltage withstand qualities. Now, with the sustainability of the planet becoming an overriding, universal concern, the power transmission sector is looking for a replacement.
This is because the global warming potential (GWP) of SF6 is 23,500 times that of CO2 and it remains in the atmosphere for 3200 years. There’s more: its atmospheric concentration has increased by 20% over the past five years.
Time to find a substitute. After several years’ research, GE has done just that, with its g3 (g cubed) solution – a gas mixture made up of CO2, O2 and Novec™ 4710 fluoronitrile developed by the 3M™ Company. Remarkably, this revolutionary gas mixture can deliver more than 99% reduction in climate change impact when compared to SF6.
This performance is achieved through only limited modifications to the original equipment. The breaker remains a, single-chamber, self-blast breaker using the same spring drive and the overall footprint is unchanged.
The g3 equipment portfolio to date ranges from 145 kV Gas-Insulated Switchgear (GIS) to 420 kV Gas-Insulated Lines (GIL), a 145 kV Live-Tank breaker and beyond to 123 kV and 245 kV Air-Insulated Switchgear (AIS) Instrument Transformers. All new products are made for g3 and will be released to the relevant industry in their g3 version when there is sufficient demand. This was the case for the latest developments shown at Cigré in 2018: the new 72.5 kV GIS and 145 kV live-tank circuit-breaker.
420 kV gas-insulated line filled with g3 at National Grid’s new Sellindge substation in the U.K.
SF6-free 245 kV current transformers using GE g3 installed in TenneT's substation in Germany
Grid operators in various countries concerned about the environmental impact of their operations have chosen the SF6-free technology from GE, providing the smallest overall physical and economical footprint and converted to g3. Adoption of g3 is growing because, at equal performance to SF6, the environmental benefits are huge.
GE’s Arnaud Ficheux, Grid Solutions GIS Technical Support Manager, emphasizes that “customers will not have to be concerned about mixing the g3 components on site. Delivery of g3 will be just like that of SF6, so the user will not be required to manipulate the different components of the gas mixture.” GE has established partnerships with leading industrial gas suppliers: Air Liquide, DILO and Inventec. “These experts in gas handling have developed equipment and processes for g3 mixing; ensuring the precise percentages of the different elements when transferring to cylinders, which are then shipped either to the customer factory or directly to the site.”
Each large B50-size cylinder contains approximately 22 kilos of the gas mixture in liquid state and is used for large volumes such as GIS and GIL applications. The mixture is also available in gas form in 2 kg cylinders for gas-to-gas operations such as topping up or filling Instrument Transformers. GE’s partners also developed special gas carts (for small and large volumes) for filling and recovery of the gas mixture that are similar to the SF6 standard equipment users are already familiar with. A typical GIL or GIS installation can require anywhere between 10 and 50 B50 bottles.
Air Liquide's g3-filling cart in service at National Grid site, UK
g3 cylinders being delivered at customer site - g3 cylinder with warming system
For the liquid-to-gas transfer, homogeneity of the gas mixture needs to be achieved to ensure the right ratio of the C02, O2 and Novec™ 4710. The service carts achieve this without operator interaction by heating the bottles through an automated process of induction or resistive heating. This is practically the only significant change compared to SF6 gas handling, where heating was required only in the case of cold climate conditions.
The same devices for monitoring gas purity and humidity of SF6 have also been adapted for applications with g3, supplemented with the capability to verify the percentage content of the g3 components. “Operators have been using such tools for the past 40 years,” Michael M. Walter, Electrical Engineering Manager at GE points out, “so, although they have been adapted, the principles are exactly the same, making their operation transparent for customers. It is also important to note that, like SF6, g3 is non-toxic and is safe for equipment handling staff.” For more information about g3 gas filling, look for an upcoming article on www.think-grid.org.
WIKA g3-gas quality analyser - DILO g3-gas quality analyser
"Disposal and recycling are important concerns for grid operators", explains Walter.
In the case of g3, because there are not sufficiently large quantities to be around today to be recycled, the mixture and any decomposition products are disposed after use by a specialized GE partners. Any new gas can be re-used in the mixing plant. Walter draws a parallel with SF6: “in the early days, disposal was the norm. Over time, processes were introduced to separate the components, and even polluted products can now be recycled. The same will happen with g3.” GE is currently investigating two different approaches to recover the NovecTM4710 and they have shown some promise for re-cycling and re-use of such mixtures in the future.
According to the EU green procurement guidelines, the environmental impact of the whole product throughout its complete life cycle should be evaluated in the sourcing and procurement phase (as opposed to only considering the gas global warming impact).
Life Cycle Assessments (LCA) were performed on 420 kV GILs and 145 kV double busbar GIS – both SF6-based and g3-based – to compare their results. The aim was to evaluate
The assessments were carried out according to ISO 14040 and 14044 using SimaPro 8.3.0. and the ILCD (International Life Cycle Data) method covering 16 environmental impacts, including climate change, ozone depletion, resource depletion, toxicity, etc. The following entries were taken into account:
Comparative LCA of 145 kV GIS with SF6 and with g3
Elodie Laruelle, Materials & Eco-design Engineer at GE’s Grid Solutions, explains, “The results for the 145 kV GIS show that, compared to the SF6 version, the SF6-free product has a significant lower impact on climate change (72.5 % reduction) and resource depletion (18% reduction) respectively.”
About the climate change indicator, reduced by 72.5 %, it is worth considering that aluminum, due to its production process, has the main impact in the GIS manufacturing phase. Aluminum represents 65 % of the product using SF6 and only 4.4 % more in GE’s SF6-free version. Actually, one of the main specifications given to GE R&D experts was to keep the same equipment size at equivalent rated voltage. The impact on the climate change result would have been considerably worse with an increase of the GIS size, i.e. of the quantity of aluminum used, for instance.
Laruelle points out: “Other alternative gases on the market lose a large share of the reduced environmental footprint by increased housing sizes, while g3 equipment has identical dimensions as SF6 equipment for a given rated voltage.”
With regards the Global Warming Potential of the gas, g3 has a GWP reduced by 98% compared to SF6. When applied in the GIS, the GWP of the gas is even more reduced and brought down to more than 99% simply because the necessary gas mass of g3 for one bay is half the necessary mass of SF6.
The g3-GIS yields a 15% increased impact on ozone depletion. The latest is due to the greater use of polytetrafluoroethylene (PTFE) material in the circuit breaker to cope with the characteristics of the alternative gas; However, since PTFE quantities are very low, this increase is negligibly small (only 2.8 g of CFC-11 equivalent over the whole life cycle).
On the other 13 indicators, the difference is less than 5%, keeping them in the range of uncertainty of the LCA analysis. In other words, these environmental indicators remain virtually unchanged compared with those of state-of-the-art SF6 GIS.
Comparative LCA of 420 kV GIL with SF6 and 420 kV GIL with g3
For the 420 kV GIL, the results show that the use of g3 brings about a considerable reduction in environmental impact compared to the SF6 product:
Laruelle concludes, “while the ozone depletion shows an increase that is actually minimal, the SF6-free solution using the fluoronitrile C4F7N in combination with CO2 and O2 as background gases has a major and positive impact on the climate change and the resource depletion indicators. In brief, g3 is a revolutionary gas for the electrical transmission industry, offering the same technical performances as SF6 with an environmental impact reduced by more than 99%.”
As this article goes to press, 16 utilities have opted for GE’s SF6-free HV equipment using g3:
UK – England
The first g3 products to be energized (in April 2017) were the 420 kV GIL at National Grid’s new Sellindge substation in south-east England. Around 40 B50 cylinders were used, adding up more than 750 kg of the g3 gas mixture. This represented a total of some 38,000 liters distributed across 15 compartments, using the Air Liquide gas cart. The GIL equipment was filled during the winter time, though ambient outdoor conditions were good at around 10 to 15°C. The process was slightly longer than with SF6, since the gas mixture needed to be heated by the gas cart’s heating belt. However, on-site filling was carried out in parallel with other installation operations and equipment commissioning. Arnaud Ficheux, notes that, “the whole filling activity progressed normally, and the GIL have now been in operation for over a year and a half.”
g3-filling of the 420 kV GIL at National Grid's Sellindge substation, UK
UK - Scotland
Another g3 project in the UK, a 420 kV GIL application for Scottish Power’s Kilmarnock substation, was installed under harsh conditions, with heavy rain, snow, wind and temperatures below 0°C. For Ficheux, “this was a good test of the gas handling process which, with gas carts made for indoor and outdoor conditions, progressed smoothly under the severe climatic conditions. For this specific project, it was necessary to recover the gas mixture in the bushing – down to 0.5 bar – to reduce the pressure in the bushing for connection to the overhead lines. It was returned, intact, to the cylinders in liquid form and, once the connections were made, returned to the GIL at exactly the right mixture percentages. This enabled us to completely validate the process of gas filling, recovery and re-filling in three different compartments.”
420 kV GIL application for Scottish Power’s Kilmarnock substation
This project using g3 was delivered and tested onsite in March 2018 at an Axpo site in Switzerland. It features four bays of 3-phase encapsulated 123 kV GIS and can operate, just like SF6-GIS, down to -25°C. It was the very first project featuring a circuit-breaker using g3 as arc quenching medium.
Axpo's Etzel substation in the Swiss Alps - DILO's g3-service cart used at Axpo's Etzel substation
123 kV GIS with g3 installed and energized at Axpo's Etzel substation in 2018
The gas mixture was delivered onsite already premixed, in cylinders. The filling or evacuation procedure is similar to the procedure with SF6 directly from the cylinder. g3 dedicated gas handling cart is used to fill or recover the gas from/to the GIS compartment. Walter explains, “Gas quality analyzers, densimeters, filling valves, sealing system are adapted to g3 but work the same way. Even the maintenance cycles remain the same.” The site was energized in August 2018.
RTE, the French Transmission System Operator, ordered seven GIS bays at 72.5 kV rated voltage for its substation at Grimaud in France. This fourth project was so far the largest F35 GIS with g3 installed at customer site in September 2018. A dedicated g3 service cart containing a cylinder heating facility was used to fill the GIS. The gas handling of g3 is similar to the gas handling of SF6; it needs an additional step of heating the bottle to bring the mixture from liquid into gaseous phase. After filling, the C4F7N content, O2 content and humidity in each compartment was confirmed using a g3 Analyzer.
72.5 kV F35 GIS using g3 SF6-free alternative installed in RTE's Grimaud substation in France
These first applications demonstrate that the g3 gas handling tools work reliably, even under adverse weather conditions, and that the gas filling process compared to SF6 is practically unchanged.
Various g3 projects are currently in their execution phase.
They confirm how straight forward g3 high voltage products can really be implemented in the field; in every step from manufacturing, engineering, project execution, installation and commissioning phases. GE’s g3 products have already been adopted by 16 different utilities around the world…
Already 16 utilities have adopted SF6-free high-voltage equipment using g3 as alternative gas solution
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GE and the GE monogram are trademarks of General Electric company. 3M, Novec and 3M logo are trademarks of 3M company. Air Liquide's name and logo are trademarks of Air Liquide SA. DILO's name and logo are trademarks of DILO GmbH. Wika's name and logo are trademarks of WIKA Alexander Wiegand Beteiligungs-GmbH. Axpo's name and logo are trademarks of Axpo Grid AG. RTE is a trademark of Réseau de Transport d'Electricité SA. Scottish Power is a trademark of Scottish Power Ltd. National Grid is a trademark of National Grid plc.
Photo Credits: GE's Grid Solutions
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