Lightning impulse breakdown characteristics of SF₆ and 20% C₃F₇CN / 80% CO₂ mixture under weakly non-uniform electric fields

Abstract

This paper provides a comparative study on the impulse breakdown performance of SF 6 and 20% C 3 F 7 CN / 80% CO 2 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 SF 6 has a higher breakdown voltage than the C 3 F 7 CN/CO 2 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% C 3 F 7 CN / 80% CO 2 tested with hemispherical rod-plane and coaxial electrode configurations and the results contribute to the design of SF 6-free gas insulated equipment. Index Terms-heptafluoro-iso-butyronitrile (C 3 F 7 CN), sulphur hexafluoride (SF 6), lightning impulse, non-uniform field, polarity effect, breakdown, gas insulation

Negative DC Breakdown Characteristics of C3F7CN / CO2 Gas Mixture for Application in High Voltage Accelerators

“…Thus, for the mixture of 20% C 3 F 7 CN / 80% CO 2 , a lower applied field is sufficient to initiate the avalanche formation, which results in a lower breakdown voltage than SF 6 . Similar characteristic was observed under AC [13] and LI [14]. The effective ionization coefficient of 20% C 3 F 7 CN / 80% CO 2 gas mixture increases with the electric field at a greater rate than that of SF 6 [13,24], which indicates a higher sensitivity towards varying field uniformity.…”

“…Both SF 6 and C 3 F 7 CN have a high electron affinity and are expected to behave similarly. Based on existing literature, for uniform and quasiuniform field, 20% C 3 F 7 CN / 80% CO 2 gas mixture is comparable or slightly better than SF 6 [14,16], as observed in Figures 7 and 8.…”

“…Recent investigations on breakdown characteristics of 5-20% C 3 F 7 CN and CO 2 mixtures under AC [13] and LI [14] voltages have shown a comparable performance as SF 6 . In the limited research on SF 6 alternatives research under DC, uniform field breakdown tests were conducted using low concentrations of C 3 F 7 CN (4% and 8%) in the pressure range of 3-7 bar absolute [15].…”

Anomalous First Breakdown Behavior for HFO1234ze(E)

“…A digital gauge was used after each gas handling procedure to verify the pressure reading against the analogue gauge. Detailed handling procedures are reported in [16]. All the gas pressures discussed in the present work are in absolute.…”

“…Starting from a lower voltage than the pre-determined U 50 with a step voltage, U of 5 kV and a time interval of 6 minutes between consecutive voltages. U 50 and standard deviation, σ was calculated using Equations ( 1) and (2) [16] respectively:…”