Insulation properties of C4F7N/CO2 mixtures under non-uniform electric field

The AC insulation properties of C4F7N/CO2 mixtures in non-uniform electric fields were investigated. The optimized interval time between AC breakdowns for C4F7N/CO2 mixtures was determined for a range of electric field non-uniformity f. At high value of f, the test results demonstrated some interesting dielectric behavior. The breakdown electric field strength of C4F7N/CO2 was inversely related to f until the critical non-uniformity f0. Once f reached beyond the f0, the breakdown electric field strength became constant. Also, the gas pressure was positively correlated with f0. In addition, the trend of AC breakdown test in C4F7N/CO2 mixtures and SF6 demonstrated different N-curve phenomenon. The N-curve phenomenon, which has potential to be utilized for the design of gaseous insulation in electrical systems, could be attributed to the influence of corona and to the mean free path of electrons, which competes with each other. The breakdown voltage of 5 similar to 9%C4F7N/CO2 mixtures is more sensitive to the change of non-uniformity than SF6. The AC insulation strength of the mixture 9%C4F7N/91%CO2 is 80% that of SF6.