Impulse Breakdown Characteristics of CF₃I–CO₂ at Various Gas Pressure and CF₃I Content

Abstract

Sulphur hexafluoride (SF₆) is currently the preferred dielectric gas used in medium and high voltage power equipment. However, due to the extremely greenhouse effect by SF₆, the search for alternative gas comprising a fluoro-organic gas with low environmental concern mixed with a buffer gas (N₂, CO₂ and O₂) have received considerable attention from the power industry. Trifluoroiodomethane (CF₃I) is identified to have an excellent insulation performance with a very low GWP can replace SF₆. This paper presents the basic lightning impulse performance of CF₃I mixed with CO₂ as an alternative to SF₆ insulation gas. The 50% breakdown voltage (U₅₀) under both positive and negative polarities were measured at various gas pressure and mixing ratio of CF₃I–CO₂. The results show that U₅₀ of CF₃I–CO₂ gas mixtures increases linearly with the pressure, and the difference increases with gap length. Furthermore, an increment of CF₃I content in the mixtures increases the insulation strength. It was also found that U₅₀ with negative polarity is much higher than that with positive polarity under the needle-plane electrode and vice versa under the plane-plane configuration. The V-t characteristics for CF₃I–CO₂ were also analysed under these two different electric fields. The V-t characteristics for plane-plane electrode are more distributed along the wavetail of the lightning impulse under both U₅₀ polarities. As opposed to the V-t waveforms under needle-plane electrode, the instantaneous breakdowns tend to happen earlier. Overall, the results provide a basis for considering the application of CF₃I–CO₂ as an insulating gas in high voltage apparatus.