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
Environmentally friendly gases have been widely investigated to replace sulphur hexafluoride (SF6) in compressed gas insulated equipment for the power industry. This work presents the experimental investigations on the gas stability of HFO1234ze(E) (hereinafter referred to as C3H2F4) where a unique first breakdown voltage behavior is found for AC, DC and lightning impulse (LI) voltages. A splatter of dark colored soot is observed on the plane electrode after the first breakdown that is followed by consecutive LI breakdowns chopped on the front. The breakdown characteristic stabilizes after the first voltage withstand and the 50% breakdown voltage (U50) is calculated using the subsequent stabilized data series. This first breakdown behavior of C3H2F4 is consistently observed for different tested electrode materials and configurations. The voltage difference between the first breakdown and U50 varies significantly due to the work function of the electrode material and electric field uniformity. For AC breakdowns with comparatively higher discharge energy, the data series is more consistent after the first breakdown but with significant level of soot formation indicating that this is the main influencing factor for the first breakdown effect. Soot is identified as fluorinated carbon through X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses.
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