Two types of resin-coated papers were investigated; kraft papers and heat-resistant kraft papers partially covered with epoxy resin; those were laminated to some thickness. They were thermally degraded at 120°C for 240 to 1320 hours, and their mechanical strength, i.e. tensile strength was measured. The tensile strength of the resin-coated kraft paper decreased with the heating time and became smaller than that of the pressboard after 500 hours while the tensile strength of the resin-coated heat-resistant kraft paper almost retained the initial value after 1320 hours of the heating time. Then, electrical breakdown characteristics of composite insulation systems with a resin-coated paper and insulation oil were investigated. In the system, an oil-filled gap was artificially introduced between the resin-coated paper and a plane electrode to induce partial discharge (PD) initiation at the same location. PDs occurred before breakdowns and their PD inception electric field strength was almost as high as pressboards and the effect of the heating time was negligible. Characteristics of electrical breakdown field strength were similar to the PD inception field strength; negligible effects of the type of the resin-coated papers and the heating time. Electrical breakdown occurred at the oil-filled gap and the edge of a high voltage electrode.
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