Transport and radiation properties of C4F7N-CO2 gas mixtures with added oxygen

Abstract

Owing to global and local legislative mandates pertaining to greenhouse gas emission reduction targets, the focus of industries dealing with electrical power management has shifted towards SF6 replacement within electrical or electro-mechanical equipment since 2010. In particular, fluoronitrile- (C4F7N) or fluoroketone- (C5F10O) based gas mixtures have been identified as the most promising candidates for this purpose in both medium-voltage (MV) and high-voltage (HV) gas insulated switchgear (GIS). The temperature and pressure ranges of interest are 300 K–30 kK and 1–5 bars respectively, which are relevant to the short-circuit current arcing conditions within an MV-GIS. In this work, we focus on a gas mixture with a fluoronitrile mole fraction lower than 20%, with or without O2 having a mole fraction lower than 20% and the rest of the mixture was carbon dioxide (CO2). Throughout this work, we validate our calculation results with published data for 10% and 20% C4F7N-CO2 mixtures at 1 bar and 5 bars and hint at the possible sources of discrepancies. Our local thermodynamic equilibrium based chemical composition results indicate that the addition of 20% O2 to C4F7N-CO2 mixtures significantly reduced CO formation while increasing COF2 formation. However, the addition of 20% O2 induced marginal modifications to the thermodynamic, transport and radiation properties of 10% and 20% C4F7N-CO2 mixtures. Finally, after utilizing the properties database to calculate steady-state temperature profiles for a low-current (10 A) free-burning arc without metallic vapor, we demonstrate that the arc columns of 10% and 20% C4F7N-CO2 mixtures with or without O2 are less diffuse compared to air but more diffuse compared to SF6. We explain the order of diffuse-to-constricted profiles and arc interruption capabilities for different gases in terms of their thermal conductivities and diffusivities.