Chemical reaction network and kinetic analysis for natural origin gases, C 4 F 7 N and their mixtures

Abstract For more than half a century SF 6 has been the predominant gas insulation medium. Due to its high global warming potential it is now being replaced with mixtures of natural origin gases (NOG), namely N 2 , O 2 and CO 2 , either alone or in combination with the strongly electron-attaching gas C 4 F 7 N. Information about the thermodynamic properties and reaction rates of these mixtures and their decomposition products is essential for predicting their electrical insulation performance. However, currently available reaction rate data is lacking. In this work, we employ an automated chemical network exploration tool to derive an internally consistent reaction mechanism for these mixtures. The mechanism captures the kinetically significant reactions between neutral species and includes the reaction rates for the decomposition of C 4 F 7 N, its reactions with its decomposition products, and their reactions with NOG. Our reaction mechanism contains all fluorinated by-products experimentally observed after arcing tests and reveals that the barrier-free decomposition rates of C 4 F 7 N reported in the literature appear to be significantly overestimated based on comparison with available experimental decomposition temperatures. We carry out chemical kinetics calculations to investigate the decomposition by-products of two different C 4 F 7 N, CO 2 , O 2 mixtures under different conditions and show that the presence of C 4 F 7 N promotes CO generation, whereas higher O 2 concentrations suppress it. We report the volumetric heat capacity ( ρ C p ) based on chemical kinetics rather than chemical equilibrium, as done in existing literature, and show that the peaks in ρ C p that are associated with high thermal interruption performance shift to lower temperatures when the gas is cooled. The full reaction mechanism file is provided to support and enable further studies. In the future, we aim to use this reaction mechanism to model the stepped leader dielectric breakdown process in these mixtures.