CF3I Gas and its Mixtures: Potential for Electrical Insulation

SF6 is an electro-negative gas and has dielectric strength three times higher than that of air. Such outstanding properties of SF6 have resulted in its extensive use as an insulation gas in high-voltage equipment. On the other hand, SF6 is a highly potent greenhouse gas due to its high global warming potential. Research is underway to find alternatives to SF6 and one of the candidates is CF3I gas. SF6 and CF3I share similar properties; both gases are colourless，odourless and non-flammable. The weak C-I chemical bond in CF3I means that it can decompose relatively fast in the atmosphere and therefore the ozone depletion potential of CF3I for surface release is less than 0.0001 [1]. CF3I is therefore considered to be an environmentally friendly replacement to SF6. However, CF3I has a high boiling point –22.5°C at atmospheric pressure, as reported in [2]. This makes the use of pure CF3I unsuitable in HV GIS/GIL equipment, which is normally pressurised at above 0.5 MPa.

This paper begins with a comparison between properties of SF6 and CF3I gas mixtures, which concludes that a 30:70% ratio for CF3I-CO2 is a good gas mixture candidate. Based on the mixture, experimental data was obtained on rod-plane and plane-plane configurations to characterise the breakdown strength of the CF3I-CO2 mixture. Withstand tests were then carried out on standard industrial switchgear unit designed for SF6 gas. To conclude the experimental investigation, withstand tests were conducted on a scaled down coaxial-GIL model. Finally, EMTP simulations of a realistic sized GIL over long distance transmission were carried out to estimate feasible transmission lengths.