A Simulation Research on Diffusion Processes of Discharging Decomposition Products of C4F7N/CO2 in Gas Insulated Transmission Lines

摘要

To research diffusion processes of discharging decomposition products of C4F7N/CO2in gas insulated transmission lines (GIL) and clarify the impact of diffusion on insulation diagnosis, a diffusion model was built up according to Fick's law. 8 decomposition products: CO, CF4, C2F6, C3F6, C3F8, C2F3N, C3F5N, and C2N2 were selected to simulate. Their diffusion coefficients in 9%C4F7N/91%CO2 were predicted by molecule volumes and Fuller-Schuettler-Giddings equation. Molecule volumes were calculated with the level of Becke, Three-Parameter, Lee-Yang-Parr (B3LYP) method and 6-311g* basis sets. The results showed that the diffusion coefficients of C2F6, C3F6, C3F8, C2F3N and C3F5N were similar. And the diffusion coefficients of CF4 and C2N2 were similar. The concentration of decomposition products decayed exponentially with prolong of axial distance. And the delay in variation of concentration would increase with prolong of axial distance. The degree of decay and delay were negatively correlated with diffusion coefficients. The decay and delay in diffusion would impact the accuracy and efficiency of insulation diagnosis. Since the similar diffusion coefficients results in the similar decay and delay characteristics, using the ratio of concentration among C2F6, C3F6, C3F8, C2F3N and C3F5N and between CF4 and C2N2 could weaken the impact of diffusion, and was suitable for the insulation diagnosis of GIL filled with C4F7N/CO2.