Experimental and Theoretical Studies of C₃F₇CN/CO₂ Mixture Decomposition Under Overheating Fault

Heptafiuoroisobutyrontrile (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 number of studies on the decomposition characteristics 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 an overheating fault are studied by gas chromatography-mass spectrometry (GC/MS) and quantum chemical calculations. By-products CO, C ₃ F ₆ and HF are detected at 200°C. At 550°C, the C ₃ F ₇ CN molecule begins to decompose and produces CHF ₃ , C ₂ F ₄ , CF ₃ CN and CNCN. Moreover, the results of GC/MS show that the insulation medium C ₃ F ₇ CN contains an impurity, which is C ₃ HF ₇ (HFC-227ea). Combined with the results of GC/MS and quantum chemistry calculations, it can be concluded that the decomposition by-product C ₃ F ₆ at 200°C mainly comes from the decomposition of C ₃ HF ₇ . C ₃ HF ₇ reduces the decomposition temperature of the C ₃ F ₇ CN/CO ₂ mixture. Therefore, it is very important to improve the purity of the insulation medium C ₃ F ₇ CN.