Experimental and DFT study of decomposition characteristics of C3F7CN/CO2 mixture under overheating fault

Abstract

The heptafluoro-isobutyrontrile(C 3 F 7 CN) performs well in dielectric properties and environmental protection, which has the potential to replace SF 6 gas. The decomposition characteristics of C 3 F 7 CN mixture under fault conditions directly affect the possibility of its replacement for SF 6 gas. However, the studies on the decomposition characteristic of C 3 F 7 CN mixture under overheating fault is less. In this paper, the decomposition characteristics of C 3 F 7 CN/CO 2 mixture under overheating fault were studied by gas chromatography-mass spectrometry (GC/MS) and quantum chemistry calculation. Comparing with standard gas, it is found that insulation medium C 3 F 7 CN contains an impurity, which is CF 3 CHFCF 3 . Experimental results show that CF 3 CHFCF 3 reduces the decomposition temperature of C 3 F 7 CN/CO 2 mixture. By-products CO, C 3 F 6 and HF were detected at 200 °C. At 550 °C, the C 3 F 7 CN molecule begins to decompose and produce CHF 3 , C 2 F 4 , CF 3 CN and CNCN. The decomposition by-products of C 3 F 7 CN/CO 2 mixture at 650 °C are CO, HF, CHF 3 , CF 4 , C 2 F 6 , C 2 F 4 , C 3 F 8 , C 3 F 6 , i-C 4 F 10 , CF 3 CN, CNCN and C 2 F 5 CN. Theoretical calculation shows that the decomposition of CF 3 CHFCF 3 into C 3 F 6 is easier than that of C 3 F 7 CN into C 3 F 6 . The by-products C 3 F 6 and HF mainly come from dehydrogenation reaction of CF 3 CHFCF 3 molecule.