Power factor tip-up reduction in high voltage coils

The author reviews the coil design and processing factors that are required to obtain a well-consolidated high-voltage stator coil with minimum void content. The coils use a thermosetting epoxy-mica-tape groundwall insulation system. Quality control electrical tests are performed on new coils to verify conformance to design requirements. One of these electrical tests is the tangent delta measurement at various test voltage levels. The calculation of delta tan delta, or coil power factor tip-up, is used as a relative indication of the void content in the coil´s insulation system. The locations and major causes of the voids are described. Methods that have been developed to reduce the void content or the ionization of remaining voids, thereby causing a reduction in power factor tip-up in high-voltage coils, are described. Coil power factor data that illustrate the effectiveness of void reduction is presented