Abstract
Three types of resin-coated papers were investigated; kraft papers and heat-resistant kraft papers partially covered with epoxy resin, and a kraft paper covered with phenol resin; those were laminated to certain thickness. They were thermally degraded at 120°C for 240 to 1320 hours, and their mechanical characteristics such as tensile strength and average polymerization degree were measured. As a result, it was found that the tensile strength of the first and second resin-coated papers was larger than that of the pressboard, but the tensile strength of the third one was smaller. As the effect of the heating time, it was found that the tensile strength of the first resin-coated paper decreased down to that of pressboards after 500 hours of heating time while those of the second and third ones almost retained the initial values 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 a resin-coated paper and a plane electrode to induce partial discharges (PDs) at the same location. PDs occurred before breakdowns and it was found that their PD inception electric field strength was almost as high as that of the pressboard and the effect of the heating time was negligible. It was also found that the electrical breakdown field strength has similar characteristics to those of the PD inception field strength; negligible effects of the type of resin-coated papers and the heating time. Electrical breakdown occurred at the oil-filled gap and the edge of a high voltage electrode.
