Measurement of Ionization and Attachment Coefficients in C₄F₇N/CO₂ Gas Mixture as Substitute Gas to SF₆

Abstract

As one of the alternative insulation gases, C 4 F 7 N/CO 2 gas mixtures have attracted much attention recently. In this study, the normalized Townsend first ionization coefficient α/N and the normalized attachment coefficient η/N were measured in C 4 F 7 N/CO 2 gas mixtures by using the Steady-State Townsend (SST) method over a range of electric fields E/N from 100 to 550 Td. The concentrations of C 4 F 7 N studied were 4.99, 7.00, 9.00, 13.13 and 20.02%, and the gas pressure were 500 Pa at 20 • C. The effective ionization coefficients and the critical electric field were obtained at 500, 1000 and 2000 Pa to investigate the effect of gas pressure on dielectric strength in 9.00%C 4 F 7 N/91.00%CO 2 respectively. The results indicated that the (E/N) lim have not significant change on the SST experimental conditions. Meanwhile, a comparison with SF 6 gas indicates that 20.02% C 4 F 7 N/79.98% CO 2 has a similar insulation performance with (E/N) lim as 358.82 Td. However, when the liquefaction temperature is considered, the C 4 F 7 N/CO 2 gas mixtures with 9.00% C 4 F 7 N could be considered as an appropriate substitute gas for SF 6 , because the critical electric field of this gas mixture is over 70% of SF 6 gas and could be used under-15 • C at 0.7 MPa. INDEX TERMS C 4 F 7 N/CO 2 , ionization coefficient, attachment coefficient, critical electric field, steady-state Townsend method.

Breakdown Characteristics of a Fluoronitrile Mixture Gas According to Mixing Ratio and Oxygen Content for High-Voltage Power Equipment Application

“…The dielectric strengths of C4F7N 5%/CO2 95% were 18-35% lower in the AC breakdown electric fields and 15-19% in the -LI breakdown electric fields compared to SF6 when the same pressures were considered. It is assumed that C4F7N 5%/CO2 95%, which has a higher effective ionization coefficient than SF6 at the same density normalized electric field (E/N), requires less energy for propagation of the streamer by ionization [27].…”

“…When the fluorine content of the mixture gas is high, decomposition and ionization are difficult because the molecules are more stable. Therefore, with increasing C4F7N content, a high electric field is necessary for the breakdown [27], [29], [30].…”

3D particle-in-cell simulations of negative and positive streamers in C4F7N–CO2 mixtures

“…The decomposition pathways of C 4 F 7 N have been studied in [9][10][11][12][13][14]. Swarm parameters such as ionization and attachment coefficients have been experimentally measured to determine the reduced critical electric field and to estimate the dielectric strength of C 4 F 7 N mixtures [15][16][17][18][19][20]. Furthermore, their breakdown characteristics and dielectric properties have been extensively investigated through experimental measurements [21][22][23][24][25][26][27][28][29][30] as well as theoretical calculations [31][32][33].…”

“…In [36], streamers in C 4 F 7 N mixtures with different buffer gases were simulated at 300 K and 1 bar using an axisymmetric model [76]. Both ionization and attachment processes were considered, with their coefficients interpolated from [16,19,77]. Photoionization was not considered, but instead an initial homogeneous density of 10 14 m −3 for electrons and positive ions was included.…”