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We have proposed a concept of an all solid insulated substation system as a next generation substation from a viewpoint of environmental protection. In order to make the system compact and its layout flexible, a compact connection system with a soft insulating material and a hard one has to be developed. There is a high possibility that the dielectric strength of such a connection system is decreased when a metallic defect such as a wire exists at its interface. Thus, we prepared samples that imitate the interface of the connection system and measured partial discharge inception voltages (PDIV) in both cases that a metal wire was present or not at the interface. When we put the metal wire along the direction of electric field, the PDIV decreases significantly with an increase in the length of the wire, while it decreases a little when the wire is along the equipotential line. The presence of air at the interface also reduces the PDIV.