Sensitivity of Dielectric Strength of C4F7N Binary Gas Mixture to Electric Field Distribution under Lightning Impulse

This study explores the breakdown properties of three C4F7N binary gas mixtures under lightning impulse and the reduction of dielectric strength in the presence of insulation defects. The 50% breakdown voltage (U-50%) of three C4F7N binary gas mixtures in a quasi-uniform electric field is observed to determine the basic dielectric strength, which has the ranking C4F7N/Air approximate to C4F7N/CO2 > C4F7N/N-2. A coefficient SE is introduced to quantitatively describe the sensitivity of dielectric strength to electric field inhomogeneity. The SE of different gas medium can be ranked as SF6 < C4F7N/CO2 < C4F7N/Air < C4F7N/N-2, indicating that C4F7N mixed with nitrogen, carbon dioxide, or dry air is less capable of tolerating insulation defects than SF6, which merits consideration in insulation designs.