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To study the dc insulation design of gas insulated switchgear (GIS), the insulation characteristics under lightning impulse (LI) voltage with a superimposed dc voltage (superimposed voltage) must be clarified. The paper experimentally examined the GIS breakdown characteristics under this superimposed voltage. The test models simulating an insulator creepage surface were used for which consideration of the influence of dc voltage among various other GIS insulating elements is particularly important. To be specific, a cylindrical model made of epoxy resin or fiber-reinforced plastic (FRP) as the material and a conical epoxy spacer model were tested. For the cylindrical model, a cap-shaped electrode was placed on the insulator and a small gap was established between the end of the electrode and the insulator. When the dc breakdown voltages were measured using these samples, they were higher for the applied voltage of positive polarity than that of negative polarity for all samples. The post-test observation of the electrification condition revealed greater electrification on the insulator surface for the applied voltage of positive polarity. The electrification charges are considered to have relaxed the electric field and increased the breakdown voltage. Subsequently, the breakdown test was conducted using a superimposed voltage, whereby a foregoing dc voltage was applied to samples for a certain period, whereupon a LI voltage was applied with the dc voltage continually applied. The breakdown voltage when the LI voltage and dc voltage had equivalent polarity was approximately same to the LI alone breakdown voltage. Conversely, when they were opposite in polarity, the breakdown voltage under the superimposed voltage obviously tended to decrease from the LI alone breakdown voltage. It is considered attributable to the fact that the insulator surface was electrified by the foregoing dc voltage and applying LI voltage opposite in polarity to this electrification intensified the electric field where the breakdown started to occur. Accordingly, it emerged that the GIS breakdown characteristics changed significantly depending on the polarity combinations of LI and dc voltages. The influence of these polarities must be taken into consideration when studying the GIS dc insulation design.

Now that gas insulated switchgear (GIS) for ac systems is becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. The insulating portions for which the influence of dc voltage must be taken into consideration are solid insulators, such as insulating spacers. Under dc voltage, since the electric field distribution in an insulator differs from that under ac or impulse voltage and is governed by the resistance characteristics, clarifying its characteristics is crucial to study the GIS dc insulation design. As a solid insulator, focusing on fiber-reinforced plastics (FRP) used for GIS, for example, insulating rods, as well as partially treated epoxy resin; this paper experimentally investigated the bulk and surface electric conductivity under dc voltage, using the electric field, temperature, and other factors as parameters. As a result, the bulk electric conductivity of FRP in an edgewise direction exceeded that in the penetrating direction by one digit. It emerged that the electric conductivity of an insulating material with orientation like FRP varied depending on its direction. It was also found that, despite the fact the bulk and surface conductivity depended on the electric field for both FRP and epoxy resin, the variation width was relatively narrow within the range of the actual GIS operating electric field. The bulk and surface electric conductivity were also temperature-dependent, which meant the variation width was relatively wide. Furthermore, the surface electric conductivity was measured in SF6 gas and in the air to investigate the influence of the ambient atmosphere, whereupon it emerged that the electric conductivity was higher in air due to the adherence of moisture. As mentioned above, the electric conductivity of an insulator varies due to various factors, such as the influence of the material orientation, electric field, temperature, and moisture. Consequently, the electric field distribution inside the insulator also changes, meaning these electric conductivity characteristics must be taken into consideration to study the GIS dc insulation characteristics.