Breakdown characteristics of CF₃I/N₂/CO₂ mixture in power frequency and lightning impulse voltages

Trifluoroiodomethane (CF₃I) and its mixtures are believed to be prospective alternatives to sulfur hexafluoride (SF₆ ), which has been included as a greenhouse gas. In this paper, the breakdown properties of a CF₃I/N₂ /CO₂ mixture with the volume fraction of CF₃I fixed at 10% are investigated under power frequency and lightning impulse voltages. The experimental result shows that N₂ possesses higher power frequency and negative lightning impulse breakdown voltages than CO₂, but the power frequency and more negative lightning impulse breakdown voltages of the CF₃I/N₂ /CO₂ mixture do not increase with the content of N₂. For the purpose of explaining this abnormal phenomenon, the ionization energies and excitation energies of CF₃I, N ₂ and CO₂ are calculated. The computation results indicate that the ionization energy of CF₃I is lower than the first excitation energy of N₂, but higher than the lowest excitation energy of CO₂, which means that CF₃I molecules are easily ionized by metastable N₂ molecules. The first excitation energy of N₂ is too high, which hinders its application as the buffer gas of CF₃I.