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The structure and shape of the coils in high-voltage apparatus have a pronounced effect upon their electricallife expectancy because of the dielectric stress concentration at the edges and corners of the conductors. In these regions, as a result of the high dielectric stress, accelerated damage to the insulation can occur due to the partial discharges on the surfaces and in minute voids created during manufacture or in service. The extent of the influence of these increased stresses on the electricallife expectancy of several types of epoxy-mica insulation for large electrical machines was investigated.The partial discharges on surfaces and artificially-created voids were applied to a combination of mica splittings and mica paper with various backing fa bries and types of epoxy-resin bonding materials. The discharge intensity was amplified by means of sharp pin-electrodes, and the resulting life-expectancy data were compared with those obtained from tests on actual machine insulation. The test results on generator coil replicas insulated with epoxy-bonded mica splittings demonstrate that the electrical strength can be improved by means of favorable geometry of the coil cross section. A specially designed coil-edge contour is shown to assure a considerably longer life expectancy of this type of mica insulation for large turbogenerators.

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