Influence of dielectric stress concentration on voltage endurance of epoxy-mica generator insulation

Abstract

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.

Proposal of Physical-Statistical Model of Thermal Aging Respecting Threshold Value

“…Problems with both models can occur due to extrapolating the measured data in actual operating conditions. By assuming that an electric field will not cause aging at low levels of stress (e.g., nominal voltage) [10,11], the service life extrapolated from the measured aging curves will be significantly lower than reality. This fact can be removed by introducing a threshold value, as shown in Figure 2, and various models of electric aging are therefore adjusted.…”

“…However, a more concrete model of aging can be designed when all degradation processes are fully understood. Certain degradation factors acting in a concrete EIS can be identified, such as electric field intensity [10][11][12][13], temperature [14][15][16][17], mechanical stress [18], radiation [19], moisture [20], dust [21], chemical stress [22], etc. and including these degradation mechanisms into a single model would be impossible.…”

Applications

“…Wichmann and Gruenewald (131,134) show that temperature increase sometimes increases life under alternating mechanical stress, by softening bonding resins of machine insulations and thereby extending the elastic limit. From partial-discharge endurance tests they show that rounding of conductor edges to about one millimeter radius significantly improves electric-stress life, although ideal conformations for minimum stress are uneconomic.…”