Breakdown characteristics of SF₆ in quasi-uniform field under oscillating lightning impulse voltages

The field impulse voltage withstand test plays one of the most important roles to ensure the safety of the gas insulated equipments. It is hard to carry out the standard impulse voltage test for the equipments because the generator is very large and difficult to transport. The oscillating impulse voltage waveform has high efficiency and has been recommended by the IEC 60060-3 as the field test waveform. SF₆ gas is the insulation medium of gas insulated equipments. Its insulation characteristics under different types of impulse waveforms have great influence to the design, operation and test amplitude. This paper focuses on the breakdown characteristics of SF₆ gas in a quasi-uniform field under oscillating impulse voltages (OIV). Four types of oscillating impulse lightning voltage waveforms with oscillating frequency of 100, 200, 300 and 400 kHz were adopted as the test waveforms. The standard lightning impulse (SLI) voltage was also adopted as the reference waveform. A calculation model is established to explain the results. The experiment and calculation results show that the breakdown waveforms in time domain drop quickly and then have a recovery with low amplitude to zero under OIV for the influence of circuit inductor. 50% probability breakdown voltages (BDV_50%) under OIV are higher than that under SLI. The minimum breakdown voltage and BDV_50% both increase with oscillating frequency. The voltage-time distributions show that breakdowns under OIV occur in the vicinity of each wave peak of the applied voltage. OIV have a longer time lag than SLI due to the oscillation of the waveforms.