Because of the high global warming potential of SF6 gas, research on alternative gases for electrical insulation with a lower environmental impact is essential. Gas mixtures composed of electronegative gases and N2 gas have the advantage of the reduction of the amount of SF6 gas and of utilizing the synergistic effect in electrical insulation performance. We investigated the partial discharge (PD) and breakdown (BD) characteristics of SF6/N2 and PFC (C3F8/N2 and C2F6/N2) gas mixtures under non-uniform electric field conditions, by changing the dilute content of electronegative gases. As a result, the synergistic effect in SF6/N2 gas mixtures was verified to be higher than that in PFC/N2 gas mixtures. The physical mechanism from PD inception to BD was discussed with consideration of the difference in electronegativity of SF6 and PFC gases. Furthermore, we found that PD inception and PD-to-BD mechanisms changed at a content of 10 ppm for SF6 due to the electron attachment activity of SF6 gas. The change in the PD and BD mechanisms in C3F8/N2 and C2F6/N2 gas mixtures appeared at 0.1% content for C3F8 and at 1% content for C2F6.

We investigated the sequential generation characteristics of partial discharge (PD) pulses in SF6 gas under the application of a positive dc voltage, in order to discuss the PD mechanism based on space charge behaviour and corona stabilization effect. Highly time-resolved measurement of PD current pulses enabled us to discuss the relationship between the PD charge q and the time interval Δt of sequential PD pulses. Experimental results revealed that q-Δt characteristics exhibited negative and positive correlation at small and large Δt regions, respectively. Numerical calculations of ion diffusion and ion drift verified the time-dependent characteristics of the space charge behaviour and the resultant corona stabilization effect.

SUMMARYTo improve GIS insulation specifications, it is important to recognize the insulation characteristics under oscillatory overvoltage waveforms occurring in the field. This paper describes investigations of insulation characteristics for single-frequency oscillatory waveforms with various frequencies and damping ratios. It was found that minimum breakdown voltages (V min ) rose with frequency rising under the same damping condition and V min rose with damping ratio rising under the same frequency condition. From an analysis of actual breakdown voltage characteristics, the probability of breakdown at a valley of oscillation rose with damping increasing. It was found that the insulation characteristics were treated all-inclusively based on the characteristics of V min for rise time or damping time.

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