Gas decomposition of C3F7CN/CO2 mixtures: a chemical kinetics modelling study.

C3F7CN mixtures are more promising environmentally friendly alternatives to SF6 for gas-insulated equipment. Decomposition of C3F7CN mixtures driven by partial discharges during operation can generate toxic by-products, degrade dielectric performance, and pose risks to maintenance personnel. We develop a physics-based chemical kinetic framework to quantify the time-resolved and cumulative concentrations of decomposition products under prolonged partial discharge activity in a C3F7CN/CO2 mixture. A two-region, zero-dimensional model couples Bolsig+ electron-energy kinetics with Cantera gas-phase chemistry, applies a merged nanosecond pulse excitation to represent the partial-discharge excitation and transports long-lived products between the plasma core and the bulk-gas region. The mechanism contains 57 species and 229 reactions. Its predictions are benchmarked against reported measurements obtained under 96 h of AC partial discharge in a 15% C3F7CN/81% CO2/4% O2 mixture. The model shows quantitative agreement with the reported concentrations of CO, CF4, C2F6 and C3F8, while C3F6 is systematically underpredicted. For CF3CN, C2F5CN, CNCN, and COF2, only qualitative agreement in temporal growth can be assessed because the experimental results are reported as uncalibrated peak areas. Overall, the model provides insight into the dominant pathways from electron-impact dissociation to radical product formation. The effects of adding O2 (0-8%) and H2O (100, 500 and 1000 ppm) on the decomposition products are also investigated. The results provide a mechanistic basis for investigating the decomposition behaviour of C3F7CN mixtures to support the long-term reliability and safety of gas-insulated equipment.