This project aims to develop underpinning knowledge on the characteristics of C4F7N gas mixtures, which may be used as alternatives to SF6, with a specific focus on the long-term gas stability to demonstrate performance in service in new equipment. It will consider credible in-service scenarios where the gas may be put under stress that could lead to changes in the chemical make-up of the gas mixture, including any changes that could result from interaction with materials used in the manufacture of the equipment and low levels of moisture and air contamination. The resulting by-products will be assessed for their potential health impacts, their reactivity with other materials and the prospect that they may be use as diagnostic markers for asset health.
Benefits
This research project is intended to deliver benefits through the facilitation of the use of SF6 alternatives and a consequential net environmental benefit and reduction in carbon (CO2 equivalent) emissions.
Learnings
Outcomes
Outcome 1 – New knowledge on decomposition, compatibility and degradation
The project delivered new knowledge on C4F7N gas mixtures in relation to gas decomposition by-products, compatibility with GIS materials, and long-term arcing and degradation. This supported the development of safe handling procedures and the exploration of condition monitoring and estimation of end of life.
Outcome 2 – Effect of operating factors on by-product formation
The factors investigated, including water, air, light and pressure, indicated that no unexpected products were formed during in-service conditions compared with thermal ageing of the gas mixture. The introduction of electrical ageing and the presence of epoxy resin indicated that different by-products might be created under these conditions. This informed the next stage of the work, where the presence of by-products was to be used diagnostically.
Outcome 3 – Handling implications from toxic breakdown products
Despite the non-toxic nature of both SF6 and C4F7N mixtures, both were found to generate small amounts of toxic breakdown products. It was therefore recommended that handling guidelines for in-service C4F7N gas mixtures should be based on IEC 62271-4.
Outcome 4 – Environmental handling implications
Although the GWP of C4F7N by-products was high, causing some concern over environmental impact, the quantities of the by-products were very small. The findings indicated that loss of the gas to the atmosphere should still be avoided through careful handling procedures.
Outcome 5 – Compatibility implications for EPDM components
Compatibility studies involving EPDM with the clean C4F7N gas mixture caused a marked decrease in the C4F7N peak in the GCMS. Other nitriles present in decomposed gas were also seen to decrease, suggesting some degree of incompatibility between EPDM and C4F7N and confirming reports by others. This had implications for retro-filling equipment with EPDM components.
Outcome 6 – Carbon monoxide as a detectable by-product
Carbon monoxide was found to be easily detectable and to appear in high proportion compared to other decomposition arcing by-products
Outcome 7 – Need for handling procedures
Handling procedures were identified as necessary to ensure safety of personnel.
Outcome 8 – Further investigation of solid deposits under high arcing current
Under very high arcing current magnitude, up to fault level on 400 kV systems, the generation of solid deposits was identified as requiring further investigation.
Final TRL achieved: TRL 5 / 6
Overall, the closing TRL level for this project lies between TRL 5 and 6. The acquired knowledge on decomposition and compatibility can be implemented in developing company handling instructions (net benefit is safety of personnel) and can also be implemented in offline/online condition monitoring techniques (e.g., using CO as indicator for early warning of ageing).
The carbon deposit under high current (applicable to circuit breakers) needs to be further investigated. This helps prevent failures in future without the need to open the CB.
Total NIA Expenditure on project
At the time of writing this report the costs are as shown below:
External cost £1,611,619
Internal cost £218,000
Total cost £1,829,619
There is a slight increase of cost due to indirect and internal costs.
Lessons Learnt
Sensor selection for HVAC gas mixture testing was identified as critical
In designing a test set up for HVAC testing of gas mixtures, where the effect of pressure was considered as one of the variables, it was identified that conventional sensors responded too slowly. An integrated-electronics piezo-electric (IEPE) sensor was trialled in combination with another method. Thermal sensors were also slow, and an optical technique was employed.
Different gas detection methods showed different strengths
A commercial gas sensor for C4F7N was used for by-products but had challenges because some potential by-products were not detectable. GC-MS was, therefore, chosen as the best option for identifying them. The commercial detector was better than GC-MS at separating carbon monoxide from other gases (oxygen and nitrogen) and enabling it to be quantified.
Breakdown testing identified key by-products of the C4F7N gas mixture
Repetitive breakdown testing (400 shots) under LI and HVAC conditions in small gas volumes showed evidence of breakdown of the C4F7N gas mixture. CF4 was the first by-product identified and then C2N2. The -CN group was lost readily and combined to produce C2N2, but there were not many other -CN containing by-products, and it may recombine to reform C4F7N. C4F10 was more clearly identified in larger gas testing experiments.
Moisture, air and pressure had different effects on by-product formation
Increasing the moisture content in the breakdown tests had a number of impacts. For example, it reduced the dielectric strength and breakdown voltage and may have affected the recombination process of the gas mixture. However, it did not appear to change the type of by-products present. Adding atmospheric air affected the concentration of by-products but not the compounds produced. Increasing pressure had no effect on by-products either.
Use of external laboratories and partners affected delivery timelines
Where external laboratories and partners were involved, project timelines could be severely affected due to availability, practical arrangements and increasing costs. Such aspects needed to be accounted for in future innovation projects with a defined period of delivery.
Dissemination
The following papers have been published and presented / accepted for publication/presentation
- M. Michelarakis, S. Singh, D. Clark, M. Waldron, G. Wilson and A. Haddad, Gas Analysis of C4F7N Gas Mixture After the Exposure of Epoxy Resin and PTFE to Repetitive Surface Flashover Events, 24th International Symposium on High Voltage Engineering (ISH), Karuizawa, Japan (2025)
- M. Michelarakis, S. Singh, D. Clark, M. Waldron, G. Wilson and A. Haddad, Decomposition of the C4F7N-CO2-O2 Gas Mixture Under Repetitive Lightning Impulse Breakdown Events, 24th International Symposium on High Voltage Engineering (ISH), Karuizawa, Japan (2025)
- S. Singh, M. Michelarakis, D. Clark, M. Waldron, G. Wilson and A. Haddad, Thermal decomposition of C4F7N-CO2-O2 mixture based on molecular dynamics, 24th International Symposium on High Voltage Engineering (ISH), Karuizawa, Japan (2025)
- S. Singh, M. Michelarakis, D. Clark, M. Waldron, G. Wilson and A. Haddad Exploring the compatibility between C4F7N-gas mixture and common GIS Insulator materials using the thermal method., 24th International Symposium on High Voltage Engineering (ISH), Karuizawa, Japan (2025)
- M. Michelarakis, S. Singh, D. Clark, A. Haddad, M. Waldron, G. Wilson, Compatibility of the C4F7N/O2/CO2 Mixture with Epoxy Resin After Long-Term Thermal and Electro-Thermal Tests, IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Manchester, United Kingdom (2025) ·
- R. Sangineni, M. Michelarakis, S. Singh, D .Clark, M. Albano, A. Haddad, G. Wilson, M. Waldron , Effect of ambient light on the SF6 alternative mixture and their decomposition products, 59th International Universities Power Engineering Conference (UPEC), Cardiff (September 2024)·
- S. Singh, M. Michelarakis, R. Sangineni, D .Clark, M. Albano, A. Haddad, G. Wilson, M. Waldron, Effect of Air Ingress on Decomposition of C4F7N Gas Mixture Exposed to Electric Arcing, 59th International Universities Power Engineering Conference (UPEC), Cardiff (September 2024)·
- M. Michelarakis, S. Singh, D. Clark, G. Wilson M. Waldron and A. Haddad, Overview of Long-Term Compatibility and Ageing Tests for Materials Used in High-Voltage Gas Insulated Switchgear, CIGRE Paris Session, France, August 23 to 28 (2026) (Accepted)
