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.