Comparative Analysis of Discharge Sensitivity by the Free Metal Particles in C4F7N/CO2 and SF6/N2 Gas Mixtures Under DC Electric Field

The electrical performance of DC GIL using SF6 gas mixture or alternative gas as insulation medium will be deteriorated by free metal particles. The analysis of the factor affecting the insulation strength of SF6/N-2 and C4F7N/CO2 gas mixtures is mainly performed in this paper. The breakdown experiments with particle were conducted with the ball-bowl electrode under the DC electric field, which included different gas compositions such as C4F7N/CO2(4%/96%) gas mixture, SF6/N-2(volume ratio of SF6 includes: 20%, 30%, 50%, and 70%, respectively) gas mixtures and pure SF6 gas. The concept of Discharge Sensitivity by Particles(DSP) were proposed to evaluate the sensitivity of the insulation strength of different compositions gas to the electric field distortion caused by metal particles. The experiment results show that in the pressure range of 0.1 to 0.5 MPa, the dielectric strength of 4% C4F7N/96% CO2 is equivalent to that of 30% SF6/70% N-2 in the absence of particle, while due to the presence of particle, the DSP of 4% C4F7N/96% CO2 gas mixture is higher than that of 20% SF6/80% N-2 gas mixture and lower than that of 30% SF6/70% N-2 gas mixture, accompanied by the appearance of discharge current of double peak. The high dielectric strength and low sensitivity to particle discharge of C4F7N/CO2 gas mixture is closely related to the strong electronegativity and high attachment coefficient of C4F7N. Combined with the physical model of moving particle-triggered discharge, the emergence of the double peak characteristics of gap breakdown current is also elaborated in this paper.