Lightning Impulse Breakdown Characteristics of SF6 and 20% C3F7CN / 80% CO2 Mixture under Weakly Non-Uniform Electric Fields

This paper provides a comparative study on the impulse breakdown performance of SF6 and 20% C3F7CN / 80% CO2 gas mixture under weak non-uniform electric fields. Two geometrically distinct electrode configurations, with similar field utilization factor, were subjected to standard lightning impulse waveform (1.2/50 μs) and the effects of pressure, gap distance and impulse polarity on breakdown characteristics were studied. The results show that SF6 has a higher breakdown voltage than the C3F7CN/CO2 mixture and the positive 1.2/50 μs breakdown voltages were found to be higher than the negative counterparts. These results show that the effect of polarity on breakdown voltage does not exclusively depend on electrode geometry but also on pressure and gas type. The results of this paper identify the weaker polarity for 20% C3F7CN / 80% CO2 tested with hemispherical rod-plane and coaxial electrode configurations and the results contribute to the design of SF6-free gas insulated equipment.