Evaluation of breakdown characteristics of gas insulated switchgears for non-standard lightning impulse waveforms - breakdown characteristics under non-uniform electric field

To lower the lightning impulse withstand voltage of gas insulated switchgear (GIS) while maintaining the high reliability of its insulation performance, it is important to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In the preceding researches, the dielectric breakdown voltage-time characteristics were measured under several different conditions on the quasi-uniform SF₆ gas gap and partly the cone-shaped insulating spacers that represent an insulation element of GIS for six kinds of non-standard lightning impulse waveforms associated with lightning surges and disconnector switching surges. This paper describes obtaining through experiments the insulation characteristics for single-frequency oscillation waveforms as a representative waveform when there is a non-uniform electric field represented by metallic particles in the equipment while changing the applied voltage polarities, frequencies, and damping factors. As a result, it was found that the dielectric breakdown voltages are lower in the positive polarity and it was confirmed appropriate to conduct experiments mainly in positive-polarity in order to consider dielectric strengths. Furthermore, changing the frequencies and damping factors under positive-polarity waveforms did not cause major changes in the feature of the dielectric breakdown voltages, which remained constantly higher (by 1.41 to 2.58 times) than those under standard lightning impulse waveforms.