Insulation characteristics of gas insulated switchgear under lightning impulse and ac superimposed voltage

The insulation performance of gas insulated switchgear (GIS) against lightning surges is verified by applying a lightning impulse (LI) voltage alone. However, the surge voltage generated in an actual system is superimposed over the ac operating voltage to form a waveform that differs from the LI voltage waveform. For UHV- and 500 kV-class GIS in particular, where the ratio of ac operating voltage to the LI withstand voltage is high, the insulation characteristics for this LI and ac superimposed voltage are important factors to study the insulation design. Accordingly, this paper researched the insulation characteristics for the superimposed voltage of element models simulating the SF6 gas gap, creepage surface of the insulating spacer and inside of the insulating spacer, all of which are the main insulating parts of GIS. The ac component ratio of the superimposed voltage ranged from 30 to 70% of that of breakdown (BD) voltage when ac voltage was applied alone to each model, wherein conditions for superimposed phase or similar served as parameters. Subsequently, these results were evaluated from perspectives of an insulation coordination and GIS insulation test. As a result, the BD voltage showed results almost equivalent to those obtained in the case where the LI voltage was applied alone; even when the ac component of superimposed voltage was varied at three different insulation parts. Accordingly, within the 30 to 70% range of superimposed ac voltages, it emerged that the BD voltage was almost determined by the peak value of the applied voltage and was unaffected by the ac component. When comparing with GIS test voltages in the 66 kV to UHV-class, this range of 30 to 70% covers all the ratios of power-frequency and LI test voltages for operating voltages in an actual system. Consequently, the LI voltage test was considered adequate to verify the insulation performance of GIS against surge overvoltage generated during operation whereas superimposition of ac voltage need not be considered. These results are meaningful findings for rationalizing GIS while maintaining its insulation reliability.