In this paper, the influence of electric field on C4F7N/CO2 mixture with different gas pressure and mixing ratio are tested, analyzed and compared with SF6 by experiment (Fig.1). According to Fig. 2 and Fig. 3, it can be found that the electric field sensitivity of CO2 is higher than that of pure SF6 after mixing C4F7N, and the sensitivity is obviously improved with the increase of C4F7N content or mixed gas pressure. At the pressure of 0.1MPa or more, the ratio of breakdown voltages in highly non-uniform electric field and quasi-uniform electrical field of C4F7N/CO2 is lower than 1. When the gas pressure was 0.6 MPa and the mixing ratio was 12%, the breakdown voltage in highly non-uniform electric field is even close to half that of quasi-uniform electrical field. It can be predicted that if the mixing ratio or the gas pressure is further increased, the breakdown voltage ratio will continue to decrease, and the difference of the breakdown voltages in the two electric fields further expand. In order to further explain the breakdown characteristics of C4F7N/CO2 mixture under the extremely heterogeneous electric field and verify the effectiveness of the experimental results, this paper theoretically analyses the dielectric insulation strength under different proportions and pressures from the perspective of calculation. (Fig.4)
Fig.1 Experiment platformFig. 2 The breakdown characteristics in quasi-uniform electrical field and highly non-uniform electric fieldFig. 3 The ratio of breakdown voltages of different gases in highly non-uniform electric field and quasi-uniform electrical fieldFig.4 Sensitivity of mixed gases to electric field non-uniformity
The electric field sensitivity of CO2 is higher than that of pure SF6 after mixing C4F7N, and the sensitivity is obviously improved with the increase of C4F7N content or mixed gas pressure. It is very easy to discharge in the non-uniform electric field commonly induced by the rough metal surface inside GIE. Its sensitivity is higher than that of SF6 and CO2, which makes equipment more likely to suffer quality problems and affects the security, reliability and power quality of power systems. Therefore, the requirements for equipment manufacturing and maintenance are higher, and it is necessary for related fields to pay enough attention. Future research will be required to study the influence from gap spacing and other shape of electrodes.