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 [5].
 
    
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 Novec^TM4710 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 reduction of impact on climate change considering not only the gas itself, but the complete product over its whole life cycle, from raw material extraction to end-of-life;
• the environmental impact of the g³ solution on the other environmental indicators compared to SF₆.

 
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:

• manufacturing including all materials contained in the complete switchgear bays including surface treatments and painting
• transportation (1000 km on the road and 6000 km on the sea)
• 40-years of service use considering both power losses and gas leakages
• the end of life phase taking into account that the gas is taken out from the GIS and treated separately

 
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:

• on climate change, a reduction of 96% (this is an overall improvement taking all parameters into consideration.  The gas global warming effect being reduced by 99.3%)
• on all other environmental indicators, with an average reduction of 14%.

 
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:

• 5 sites with 420 kV GIL, for over 2000 meters of gas-insulated lines;
• 12 sites with 145 kV GIS, totaling 70 bays with circuit-breaker;
• 2 sites with 245 kV AIS Current Transformers, with 6 CTs and one site with 3 combined metering units at 123 kV.

 
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
 
Switzerland
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 [6].
 
France
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
 
Conclusion
 
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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