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SF6 gas, an insulation medium used for gas insulated switchgear (GIS), is known for its high global warming potential, hence the search for an effective alternative from an environmental perspective. As a potential alternative, the authors are focusing on N2 gas. To use this N2 gas for actual GIS, the insulation characteristics for actual overvoltage waveforms generated in the field (non-standard lightning impulse waveforms) must be obtained. To do so, the earlier study experimentally obtained and evaluated the insulation characteristics for single-frequency oscillation waveforms as a representative among nonstandard lightning impulse waveforms under the quasi-uniform electric field, which was considered to be a basis for the insulation design of GIS. The present study obtained the insulation characteristics for double-frequency oscillation waveforms to investigate surge waveforms generated in substations in more detail. Consequently, it emerged that negative polarity waveforms were more severe in terms of insulation, as was the case for single-frequency oscillation waveforms. The insulation performance was consistently higher than that for the standard lightning impulse waveforms, which were 1.08 to 1.09 times. In other words, it was confirmed that the insulation specification could be rationalized by about 10%, even for complex oscillation waveforms, such as those of double-frequency oscillation. Furthermore, a study was conducted on the influence of the gas pressure and gap length for single-frequency oscillation waveforms. Consequently, it emerged that the results under the basic experimental conditions were likely to be applicable to actual GIS under wider-ranging conditions.