An epoxy molded equipment is installed in various high voltage circuit. It is because epoxy molded equipment is nonflammable and can be made in small. And more, it has been thought that an epoxy molded equipment is not need to maintenance. However by over time, when cracks are formed in the epoxy resin, a partial discharge occurs at the position. As a result, dielectric breakdown is caused by continuous partial discharge occurring. So partial discharge diagnosis method has been hoped to be developed. Therefore we performed a partial discharge characteristics test for epoxy resin, then we could develop a diagnosis method detecting electromagnetic waves associated with a partial discharge. A frequency analysis is applied to the diagnosis method, the method can identify a state and a position of partial discharge. It is considered that our method is useful because it can be applied to a dielectric degradation diagnosis for an epoxy molded equipment in service.

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