Learnings

Outcomes

The starting TRL for the project was 2 – this related to the crystal synthesis work in the project and the methods by which their impact on the dielectric characteristics of the gas might be assessed.  Methods were developed for producing small amounts of crystals for assessment, further work could refine these techniques and there were suggestions at the end of the project for how the testing might be improved.  The ending TRL for the project in this aspect is best described as TRL 3 or 4.

The approach to LCA studies for the circuit breakers was more established but some uncertainties remained about how the methodology needed to be adapted for the situation.  The starting TRL for this workstream could have been described as 6. The successful completion of a report on the LCA comparing technologies suggests that the methodology had been adapted and applied satisfactorily in this instance. The planned ending TRL 7 has been achieved.

SF₆ GIS equipment has higher climate change impacts than the fluoronitrile insulated GIS and technical air insulated GIS due to SF₆ gas leakage for equipment from both suppliers used in this project.

• The Supplier 1 SF₆ GIS has statistically higher acidification impacts as compared to the fluoronitrile insulated GIS. Differences are within 2% of the two GIS systems evaluated. 

• The Supplier 1 fluoronitrile GIS have statistically higher impacts than SF₆ GIS for eutrophication (freshwater, marine, and terrestrial), land use, ozone depletion, photochemical ozone formation, and resource use (minerals and metals, and fossils). 

• The Supplier 2 fluoronitrile and technical air insulated GIS have statistically higher impacts for acidification, eutrophication (freshwater, marine, and terrestrial), ionizing radiation, land use, ozone depletion, particulate matter, photochemical ozone formation, and resource use (minerals and metals, and fossils) as compared to the SF₆ GIS. 

• Electricity consumption, cast aluminium, and copper are primary drivers of impact for Supplier 1 and Supplier 2 GIS systems evaluated. 

• Climate change results for SF₆ GIS are sensitive to gas leakage assumptions. 

• Results across every impact category are sensitive to the background dataset used to capture the impacts of aluminium used in the manufacturing process. 

It has been observed that there are a range of different solid by-products that can form solid structures in C4F7N insulating gas mixtures.  These may be summarised as: 

• Amide – C4F7ONH3 – white powder or needles that sublime at ambient temperature and pressure.
• Ligand (dimer) – C8F14ON2H2 – white powder or needles that may sublime in the range 130°C – 170°C.  This is produced from a reaction of two amides (hence “dimer”) and may have 3 different tautomeric forms (structural isomers).
• Complex – a co-ordination compound of a metal ion and one or two Ligands.  The colour and appearance depend on the metal – violet and brown observed in this project. 

A range of factors influence by-products production within GIS, including temperature, humidity, the gas mixture and material compatibility (e.g. metals, desiccants, o-rings etc.).  Very little information is known about the electrical properties of these materials and if they will have an impact on the long-term stability of equipment.  

In many experiments using realistic equipment/conditions, the likely type of by-product is inferred from its physical appearance as sample collection and analysis is difficult due to the limited amount of sample generated.  In this project crystals were therefore generated from high percentage C4F7N mixtures more rapidly in pressure vessels in an atmosphere of high moisture content with thermal cycling between 13°C and 80°C over a few weeks.  Initially it took 4 months to generate crystals but through experimentation, useful quantities could be generated in a few weeks. 

Once generated, the crystal by-products were handled with care owing to concerns about potential toxicity and then dried in an oven.  During this time the colours were observed to change owing to the loss of water of crystallisation and the texture became brittle and hard, rather than soft and malleable. 

The characterisation tests of the different crystals enabled some clues to be gained about the exact compounds generated with some tests proving more useful than others.  

Further work would be required for formal identification of the by-products formed.  The predicted copper complex of Cu(C8H2F14N2O)2 was not observed by its m/z fragment in any of the mass spectrometry tests, although the by-products are thought to be similar. 

Thermal decomposition tests suggest that at least some by-products would decompose in the event of HV breakdown leaving little or no evidence that they had been the cause of a problem.  To identify if by-product formation is taking place and the potential cause of failure, they would need to be detected and monitored before failure. 

HV testing shows that the presence of by-products may lead to partial discharge, or even flashover in extreme cases.  

Recommendations for further work 

WS2 demonstrated that the presence of by-products in GIS filled with C4F7N gas mixtures can be detected in the gas-phase and there are methods such as FTIR that may offer an opportunity to carry out condition assessment or monitoring of equipment in service. This could be further explored in another project.

While it is evident that the byproducts do not generate strong PD, such as a metallic particle would, some activity occurs.  Further testing would be required to improve understanding of the influence of the electric field on the crystalline by-products.

Total NIA Expenditure on project 

At the time of writing this report the costs for this project are as follows:

Total Project Costs - £460,059 

External - £407,564

Internal/indirect costs = £52,495 

Lessons Learnt

There is limited research available on the life cycle environmental impacts of alternatives to SF6 used for electric transmission and distribution, this presents challenges for utilities attempting to evaluate supply chains and operations to improve sustainability and decrease their environmental footprint. This project demonstrated that an industry standard LCA approach can be applied successfully to produce solutions to complex problems in different areas.

Several observations were made that helped improve the outcomes of this project:

1. Amide crystals generated from C4F7N gas mixtures will sublime readily, making analysis and testing difficult.  The chambers designed for partial discharge testing must take this into account.  
2. In the test vessels used to synthesise crystalline by-products in the laboratory, it was observed that white and green deposits would form on brass fittings. These deposits stopped forming once the brass was replaced with stainless steel.
3. During testing in high pressure vessels, small leak points allowing the gas mixture escape were noted to create a white film on the outside of the vessel which then sublimed. Although these films were not characterised, they agree with previously published work regarding the formation amides at low temperatures which then sublimed at low temperatures. The leaks were repaired, and no more films were formed on the outside of the vessel.

A large number of analytical chemistry techniques were used to examine the various crystals available to the project (synthesised or supplied by others).  Some were found to be more informative than others: 

• FTIR spectra were very similar despite the visible differences, an expected outcome given that the molecular bonds within the different by-products are expected to be somewhat similar.  The presence or absence of water in the by-products was easily seen.
• Raman spectroscopy likewise showed few variations but did indicate the presence of some fluorescence in some samples, likely owing to impurities.
• GCMS is useful for identifying the mass of compound fragments and was particularly helpful in confirming the presence of the amide in the gas phase.
• ICP-MS is a technique for identifying and quantifying metallic elements in a compound down to very low levels of concentration.  The most abundant elements found were those making up the materials in the gas vessel from which the samples were collected.  Also in significant concentrations were metallic elements commonly found in water, suggesting the importance of the water present in the vessel in the by-products formed.
• LCMS proved most useful in identifying compound masses.
• XRD was able to elucidate the crystal structures.
• TGA follows the thermal profile of samples as they decompose and was found to be useful in composition of the compounds.  The different samples separated into three groups (i) a group with low decomposition temperature and no residual mass showing a rapid sublimation at low temperature; (ii) a group where mass loss began around 100°C and a final residue of around 1.5% of the original mass; and (iii) a group where mass loss was above 200°C and a final residue of around 2%. 

Electrical testing of the crystal by-products was focussed on PD monitoring.  Since the PD phenomena releases an array of different energies (i.e. electric charge, electromagnetic pulse, mechanical wave and light) and this was an investigation with no precedent, a mixture of different sensors and acquisition methods were employed during high voltage (HV) testing on the samples. Conventional HVCC and HFCT methods were supplemented with a contact acoustic sensor, an ultrahigh frequency (UHF) sensor and a 4K camera system with live video.  During testing, by-products were seen to move in the electrical field, PD was sometimes observed without particle movement and in a very few cases a full flashover was observed. 

Repeat measurements, which are normally required to get reliable results for inception voltage (PDIV) and extinction voltage (PDEV) of PD, were hampered by the sublimation of some of the samples.  Nevertheless, PD activity was detected across the different samples.  The randomness of movement and behaviour of the solids indicate that they behave like an electrostatically charged insulator. 

Dissemination  
A presentation on the progress of WS2 of this project was given as part of a National Grid sponsored event covering all of NGET’s SF6 and SF6 alternative related research at the University of Manchester, which was attended by UK TOs and DNOs, and invited industry experts. Presentations were also given by GE, Hitachi, RTE, Siemens and Sintef in September 2024 at the Graphene Institute in Manchester. 

The final report was presented to a meeting of subject matter experts from all three UK TOs. 

A paper entitled “Thermal analysis of solid by-products generated by C3F7CN mixture” was submitted to the IEEE Conference on Electrical Insulation and Dielectric Phenomena, presented in Manchester in September 2025.