Sulphur hexafluoride gas, SF6, is unique in its electrical and thermal performance. However, its Global Warming Potential, GWP, is very high at around 23900, compared with CO2 which has a GWP of one. Such negative impact on the environment led to the designation of the gas as a “Kyoto Gas”. In the UK, it is estimated that SF6 contributes 0.2% of the annual greenhouse emissions.
In recent years, new gas molecules and mixtures of gases have been considered and tested. A number of candidates have been particularly studied, these include dry air, N2 or CO2, polyfluorinated gases especially Trifluoroiodomethane (CF3I), Perfluorinated Ketones, Octafluorotetra-hydrofuran, Hydrofluoroolefins (HFOs), and Fluoronitriles.
This proposal is to extend the work being conducted on the stability of Novec™ 4710 and its mixtures under project NGTO002 to Novec™ 5110 (C5F10O) and its mixtures. Similar techniques, as developed for the Novec 4710™ gas mixtures project, will be adopted here.
Objectives
The aim of this work is to gain a better understanding of the recently proposed alternative gases (C5F10O or Novec™ 5110) gas mixtures to replace SF6 gas. It will also enable obtaining a better understanding of the health and safety implications when using the newly proposed alternative gas mixtures, through by-products analysis. Moreover, it will allow the assessment of the stability of the new gas mixtures and their impact on surrounding equipment.
The objectives of this work are to:
- quantify the impact of electrical discharges and flashover on the dielectric properties of the gas mixtures
- measure the by-products of the gas mixtures following flashover
- determine the impact of the gas mixtures and their by-products on surrounding materials.
- develop an indicator of gas mixture degradation and estimate long term stability.
Learnings
Outcomes
The breakdown tests in a needle-plane electrode system (i.e. non-uniform electric field distribution) showed that the pressurised C5F10O (6% in O2/CO2) mixture undergoes a positive streamer mode breakdown mechanism, similar to the previously identified C4F7N (4% in CO2) mixture but at a higher voltage. Negative PD inception voltage (PDIV) was not detected using current measurement but was visible via emission imaging.
Polarity reversal occurs under AC voltage when the breakdown polarity changes from negative to positive cycle and the polarity reversal electric field (EPR) was quantified during the volume discharge experiments. With the addition of oxygen to the gas mixtures (compared with C4F7N mixtures) the value of EPR was found to be reduced such that:
EPR (6% C5F10O–12% O2–82% CO2) < EPR (4% C4F7N–96% CO2) < EPR (CO2)
Surface discharges under divergent fields using needle-plane electrode systems with different insulation media and different pressures showed that the flashover characteristics of 6%-C5F10O/12%-O2/82%-CO2 is strongly non-linear with pressure (see figure 1 in supporting file showing results for epoxy resin as the insulation).
When compared with PTFE, the epoxy surface flashover of 6%-C5F10O/12%-O2/82%-CO2 gas mixture has different peak pressure values and similar flashover voltages for pressure below 2 BarA. At pressure above 5 BarA, the comparison of flashover voltage is as follows:
V Flashover (PTFE) > V Flashover (Epoxy)
The comparison of inception voltage of the different insulators is as follows:
VInception (Gas gap) > VInception (PTFE) > VInception (Epoxy)
During both LI discharge and partial discharge testing a number of by-products were detected. The following fluorinated compounds were detected but not quantified: CF4, C3F8, C3F6, C4F10, C3HF7 and C5F10O. The toxicity of some of these compounds are known and may be expressed by the LC50 (lethal concentration leading to death of 50% of rats or mice in a population) or the TWA (time weighted average – allowable concentration for 8 hours per day), but for others these values have yet to be established and some compounds were unidentified during these studies either by GC-MS or NMR-19F (Fluorine-19 Nuclear Magnetic Resonance spectroscopy).
Although there are still some unknowns around the toxicity of by-products, this project through literature survey and experimentation has increased the knowledge around gas handling and health and safety implications of C5F10O mixtures especially when they have been in service and may contain breakdown products.
Recommendations for further work
There are areas for further study with C5F10O to understand more about the potential health effects of mixtures in which arcing may have taken place using this gas. However, following completion of the project, the manufacturer of new high voltage equipment designed for use with C5F10O mixtures has announced that it will use C4F7N mixtures at transmission voltage levels.
Lessons Learnt
The project benefitted by following on from a successful project using the same researcher making it easier to carry out the same studies on a different gas. This would be beneficial for further analysis of gas molecules or mixtures proposed as alternatives to SF6.
During the LI breakdown tests, HF gas was not detected, although hydrolysis reactions with trace amounts of water means it is a likely by-product in trace quantities. A specific HF detection tube could be used to detect its present in any future experiments.
Dissemination
A paper is being prepared for publication that will summarise this work similar to the publication for the same studies on C4F7N.
