Experimental and Theoretical Studies of C3F7CN/CO2Mixture Decomposition under Overheating Fault

Abstract

Heptafiuoroisobutyrontrile (C3F7CN) performs well in dielectric properties and environmental protection, which has the potential to replace SF6 gas. The decomposition characteristics of C3F7CN mixture under fault conditions directly affect the possibility of its replacement for SF6 gas. However, the number of studies on the decomposition characteristics of C3F7CN mixture under overheating fault is less. In this paper, the decomposition characteristics of C3F7CN/CO2 mixture under an overheating fault are studied by gas chromatography-mass spectrometry (GC/MS) and quantum chemical calculations. By-products CO, C3F6 and HF are detected at 200°C. At 550°C, the C3F7CN molecule begins to decompose and produces CHF3, C2F4, CF3CN and CNCN. Moreover, the results of GC/MS show that the insulation medium C3F7CN contains an impurity, which is C3HF7 (HFC-227ea). Combined with the results of GC/MS and quantum chemistry calculations, it can be concluded that the decomposition by-product C3F6 at 200°C mainly comes from the decomposition of C3HF7. C3HF7 reduces the decomposition temperature of the C3F7CN/CO2 mixture. Therefore, it is very important to improve the purity of the insulation medium C3F7CN.