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

Abstract

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, N2 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.