Solid insulated switchgear (SIS) has been developed as a substitution for medium voltage sulfur hexafluoride (SF6) gas insulated switchgears. Its main circuit including a vacuum interrupter is coated with epoxy resin. Therefore, a solid/gas insulation system is composed. For further miniaturization and higher stress design of SIS, improvement of the insulation performance is required. In the solid/gas insulation system, the breakdown strength along the surface is lower than that of solid. Then, the authors investigated the surface insulation performance of the solid/gas insulation system containing an embedded electrode. In the present study, the sizes of the embedded electrode and the solid insulation material were varied and the partial discharge inception voltage and the surface breakdown voltage were measured. As a result, it was found that the surface breakdown voltage varied considerably with the radius of the embedded electrode. From the result, it was deduced that the surface insulation performance could be improved by choosing the size of the embedded electrode appropriately. Moreover, the effects of the thickness of the insulation material and of the surface distance on the surface insulation performance were clarified.
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