SUMMARYThis paper provides the theoretical calculation results of thermodynamic and transport properties of CO 2 , CO 2 -O 2 mixture, CO 2 -H 2 mixture under thermal equilibrium condition at temperatures of 300 to 30,000 K and at pressures of 0.1 to 10 MPa. The gas CO 2 is one of the candidates for the environmentally benign arc-quenching medium in a circuit breaker. Furthermore, the effect of additional gases O 2 and H 2 on the thermodynamic and transport properties of CO 2 was also investigated in this paper. The hydrogen atom included CO 2 is similar to the polymer ablated vapor in switching devices. First, equilibrium compositions of CO 2 , CO 2 -O 2 mixture, CO 2 -H 2 mixture were calculated through the Gibbs free energy minimization method. Second, thermodynamic properties were computed using the calculated composition. Finally, transport properties were calculated by the first-order approximation of ChapmanEnskog method using the collision integrals between species. Inclusion of H 2 increases the electrical conductivity of CO 2 in the range 3000 to 6000 K because CHO molecules produced in this temperature range emit more electrons due to the lower ionization potential of CHO. It also increases the thermal conductivity of CO 2 especially due to dissociation reactions of H 2 around 3900 K and ionization of H around 15,000 K. These properties provided here can be used for CO 2 thermal plasma simulation.

Currently, environmental problems such as global warming are important issues, and SF 6 has been identified as a greenhouse gas with a long atmospheric lifetime. Therefore, in the long term, it is preferable to reduce the amount of SF 6 used as an insulating gas. It is thus important to discuss the possibility of using more environmentally friendly gases as alternative insulation for gas-insulated apparatus. In this paper, we describe the fundamental insulation breakdown characteristics of high-pressure CO 2 gas at gas pressures of up to 1.0 MPa under simulated practical conditions, including the insulation breakdown characteristics of a high-voltage conductor with an insulating coating and the effect of decomposition products on the creeping insulation of a spacer. With the aim of enhancing insulation performance, we discuss the effect of the type of insulating coating on insulation performance. The breakdown electric field was increased by 20% by coating the conductor. It was verified that the application of an insulating coating is a practical method for enhancing the insulation performance of high-pressure CO 2 gas. It was also verified that the decomposition products have only a slight effect on the creeping insulation of the spacer except for when there is heavy pollution on the insulating spacer. However, if a large amount of decomposition products is expected to be deposited on the insulating spacer during operation, which may cause a severe interruption to the current, it will be necessary to consider this factor in the insulation design.