Learnings

Outcomes

This project has improved understanding of the technical feasibility of using C4F7N -based gas mixtures as lower global warming potential alternatives to SF6 in existing gas-insulated equipment. It has also advanced approaches to condition monitoring, defect detection and gas analysis, providing evidence to support future assessment of retro-fill applications by NGET and the wider network.

Outcome 1 – Partial discharge monitoring of candidate retro- fill gas mixtures 

The project demonstrated that partial discharge and breakdown activity in candidate C4F7N -based gas mixtures can be detected using both IEC 60270 apparent charge and UHF monitoring methods, with behaviour comparable to SF6 under the conditions investigated. This suggests that established monitoring approaches may remain applicable for future retro-fill assessment, subject to further validation.

Outcome 2 – Improved understanding of breakdown behaviour and defect sensitivity 

The project generated evidence on the influence of surface roughness, electric field conditions, pressure and defect geometry on the performance of candidate gas mixtures. This has improved understanding of factors that may affect impulse ratio, flashover risk and breakdown behaviour, strengthening the evidence base for assessing technical suitability and informing future risk assessment for retro-fill options

Outcome 3 – Development of a non-invasive optical monitoring methodology 

The project developed and tested a non-invasive optical monitoring methodology for analysing C4F7N gas mixtures and their decomposition products. The work demonstrated the ability to identify relevant gas species and low-level impurities, and the results were cross-checked against gas chromatography–mass spectrometry measurements, supporting improved understanding of gas condition and degradation processes in candidate SF6 alternatives.

Outcome 4 - Creation of an evidence base for future evaluation

The project has produced a body of laboratory, modelling and analytical evidence, together with supporting data and reporting, to inform future evaluation of SF6 retro-fill options. This provides a stronger basis for follow-on development, comparison of candidate solutions and identification of the remaining technical gaps

Final TRL achieved: TRL 7

The final TRL achieved was TRL 7. This is supported by the completion of all remaining deliverables, development of an analytical approach for characterising C4F7N gas mixtures and their decomposition products, development and testing of a non-invasive optical monitoring methodology, and full-scale energisation and monitoring of an alternative gas system under operational conditions in a laboratory environment. The report states that these outputs provide sufficient evidence to assess retrofill as a viable technical option, while also identifying limitations and risks. Further work on crystal-related impacts has already started in NIA2_NGET0046 – CrystalClear.

Total NIA Expenditure on project

External Cost                       £1,595,261

Internal cost                        £308,000

Total cost                             £1,903,261

Lessons Learnt

2023/2024 update

Moisture conditions and crystal by-product formation

Crystal by-products were produced only under high-moisture conditions, while no by-products of note were generated under dry conditions. Mass spectrometry, particularly using soft ionisation techniques, proved suitable for identifying the crystalline compounds formed during testing. It was also found to be very difficult to generate enough crystals to enable evaluation of their significance in electric fields.

Partial discharge detection methods

The partial discharge characteristics of a 20% mixture of C4F7N in carbon dioxide in a GIS demonstrator with needle defects of different lengths were compared with SF6 in the same equipment. The phase-resolved partial discharge patterns were reported as comparable using either of the two detection methods used, UHF and coupled capacitor, and either detection method was stated to be suitable.

Gas-dependent defect behaviour

A significant difference was noted in the time to breakdown of the different gases with the same defect type. The report states that this suggests the critical defect length is gas dependent and may vary for different gas mixtures

Surface roughness, field uniformity and impulse ratio

Under AC breakdown conditions, the breakdown voltage was found to increase with pressure, with greater divergence between the values for each gas as pressure increased. The report attributes this to surface roughness. It also states that the uniformity of the electric field around the electrodes has a significant effect on the breakdown mechanism of the gases, and that the impulse ratio differed for the two gases, with the C4F7N mixture below that of SF6 at all pressures tested. With increasing surface roughness, the ratio trend with pressure was reported as inconsistent, with the variation largely due to the lightning impulse withstand voltage for the C4F7N mixture.

Effect of gas composition on lightning impulse breakdown

Further testing of gas mixtures showed that mixtures of C4F7N with nitrogen have higher lightning impulse breakdown voltages than equivalent mixtures with carbon dioxide, for both positive and negative polarities.

Test setup and repeatability

The test setup was adjusted to use a longer insulator during breakdown experiments to reduce the risk of flashover and insulator movement. The report states that this improved the repeatability of the breakdown testing

2024/2025 update

Breakdown behaviour and defect observations

The data states that when SF6 breaks down under electrical testing, partial discharge clusters may be seen just prior to breakdown, whereas this was not observed with the retro-fill gas. It also states that the longer energisation time in SF6 appeared to cause sulphite deposition on the needle tip used to create the defect, while this was not seen with the retro-fill gas, although the needle tip was noted to erode during repeated tests.

Increasing the needle tip radius was expected to provide a more uniform electrical field and thereby create a longer time to breakdown. In practice, the largest tip radius (100 micron) reached breakdown first, with the smallest (20 micron) taking the longest.

Decomposition pathway modelling

The decomposition pathways of SF6 were modelled using computer aided chemistry techniques and validated by comparison with literature results for experimental data using packed-bed plasma reactors.  Having validated the modelling approach the techniques was then applied to  C4F7N .  The proposed byproducts included C3F5N, C3F8, CF4, C2F6, C4F10, C6F14, CO, N2, C2F3N, COF2 and C2N2, some of which had been reported in previous work.  The simulation of by-products was found to become more difficult with the addition of water in the starting scenario.  Some rate constants for some reactions were unavailable adding to the uncertainty in decomposition pathways.

2025/2026 update 

Monitoring challenges and optical techniques

The report states that future C4F7N equipment may involve more challenging defects than those found in conventional SF6 equipment, including crystal defects, together with site noise levels. It also states that this project demonstrated partial discharge identification of defects through a unified multi-sensor approach.

For in-situ analysis, optical techniques were reported as capable of providing a direct indication of the amount of contaminants present in the gas phase of operational gas-insulated equipment containing C4F7N. Quantum cascade lasers were also noted as commercially available for monitoring CO, although the report states that the main difficulty is related to compensating for the effects of pressure and temperature broadening, and the mixture composition.

Dissemination 

2022/2023

• A dissemination event was held at the University of Manchester in December 2022 to which representatives of Transmission Operators and Distribution Network Operators were invited.  It was attended by representatives from Eirgrid, ENWL, ESB, NIE, SPEN, SSEN and UKPN.  The progress and aims of this project were presented along with other projects on SF6 and SF6 alternatives.  

2023/2024 

• A paper “Production, Analysis and Identification of Crystal By-products in C3F7CN Mixtures” was presented at the CIGRE Symposium in Cairns, Australia in September 2023.

2024/2025

• A second event was held in September 2024 to which all UK licensees were invited, at the Graphene Institute in Manchester and others were invited to present.  It was attended by representatives of Dilo, ENA, ENWL, ESB, GE Verona, Hitachi Energy, Institute of Science and Technology (Austria), Monitra, NIE, NPG, RTE, Siemens Energy, SINTEF Energy Research, SPEN, SSEN-Transmission, SYSTRA, UKPN, Wika.  Presentations were also given by GE Verona, Hitachi Energy, RTE, Siemens Energy and SINTEF Energy Research.
• A paper “Reducing the Global Warming Potential in Gas Insulated Lines and Busbars by replacing SF6” was presented by Gordon Wilson at the International Conference of Doble Clients in Boston in March 2025 which included work from this project and its T1 predecessor.
• A paper “Breakdown modelling of SF6 and its environmentally friendlier alternatives in quasi uniform fields,” has been published in IEEE Transactions on Dielectrics and Electrical Insulation, doi.org/10.1109/TDEI.2025.3618259
• A paper “Characterization of C3F7CN Gas Mixtures and their Trace Decomposition Products Detection using GC and Photoionization Mass Spectrometry for Electrical Insulation Applications” has been published in Microchemical Journal, doi.org/10.1016/j.microc.2025.116570
• A paper “Partial Discharge Monitoring of C3F7CN/CO2 Mixture Retrofilled in Gas Insulated Busbar,” has been published in IEEE Transactions on Power Delivery, doi.org/10.1109/TPWRD.2024.3454440