Learnings

Outcomes

WS2 

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 has shown 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. 

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.

Lessons Learnt

In the process of carrying out the work for WS2, several observations were made that helped improve the outcomes of this project and should be noted for similar work in future. 

• 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.   
• 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. 
• 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” has been submitted to the IEEE Conference on Electrical Insulation and Dielectric Phenomena for presentation in Manchester in September 2025.