This project will focus on developing a full-scale demonstrator retro-filled with a suitable SF6 alternative with condition-based monitoring systems incorporated to perform non-invasively, in-situ monitoring during the long-term energisation. The project will develop an optical test setup which addresses the missing link between long-term safe operation of equipment and traditional academic material testing and characterisation. The project will focus on the applicability of such techniques and critically assess their suitability to give asset managers the information required for retro-filling an SF6 alternative on the transmission network.
Benefits
The benefit of retro-filling assets, where this is possible, is to achieve environmental ambitions to reduce SF6 inventory and therefore the risk of very high emissions of gas with the highest known GWP. This may be achieved through asset replacement but retro-filling represents a more economic and environmentally sound method of doing so as the investment cost is lower and the requirement for raw materials is lower.
Learnings
Outcomes
Partial discharge measurements for a gas that could be used for retro-fill purposes and compared with SF6 in scenarios with an introduced defect suggest that the same detection equipment could be used without modification. Partial discharge monitoring is a common tool for assessing equipment condition.
Understanding the effects of surface roughness on the impulse ratio for SF6 alternatives is important in selecting the gas mixture to be used for retro-fill purposes since it can lead to flashovers. This project is identifying the risk of surface roughness to breakdown as a result of retro-filling with an SF6 alternative.
Recommendations for further work
Further work investigating the impact of crystals formed in C4F7N gas mixtures has started in NIA2_NGET0046 - CrystalClear - Lifecycle Analysis of SF6 Alternative Technologies and Crystal Formation Impacts.
Lessons Learnt
It has been shown that crystal by-products can be produced in conditions that approximate those found in service although with high moisture levels in the chamber. It has been found very difficult to generate enough crystals to enable evaluation of their significance in electric fields. Mass spectrometry was found to be suitable in identifying the crystalline compounds, particularly using soft ionisation techniques. No by-products were generated under dry conditions, unrealistic levels of moisture (for service conditions) were required to produce compounds of note.
The partial discharge characteristics of a 20% mixture of C4F7N in carbon dioxide in a GIS demonstrator with needle defects of different lengths was compared with SF6 in the same equipment. The phase resolved partial discharge (PRPD) patterns are comparable using either of the two detection methods used (UHF and coupled capacitor), thus either detection method can be deemed suitable.
A significant difference was noted in the time to breakdown of the different gases with the same defect type. This suggests the critical defect length is gas dependent and this may vary for different gas mixtures.
Looking again at the 20% gas mixture and SF6, this time under breakdown conditions, observations have been made comparing the two dielectrics under pressure, with particular attention to surface roughness. The following was noted:
- Under AC breakdown conditions, the breakdown voltage increases with pressure. There is more divergence between the values for each gas as the pressure increases. This is an effect of surface roughness.
- The uniformity of the electric field around the electrodes has a significant effect on the breakdown mechanism of the gases.
- The impulse ratio (ratio of peak LI and AC withstand voltage), which is an engineering value that helps with design margin consideration for gas insulated systems, differed for the two gases. The C4F7N mixture was below that of SF6 which maintains the limit at all pressures tested. With increasing surface roughness, the ratio trend with pressure is inconsistent. The variation is largely due to the LI withstand voltage for the C4F7N mixture.
Further testing of gas mixtures has shown that mixtures of C4F7N with nitrogen have higher LI breakdown voltages than equivalent mixtures with carbon dioxide, this for both positive and negative polarities.
The test setup was adjusted to use a longer insulator during breakdown experiments to reduce the risk of flashover and insulator movement. This improved the repeatability of the breakdown testing.
Dissemination
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. Another event was anticipated in 2023/2024 as this was popular, however an event is now expected in September 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.
