Application of flat glass backed mica paper tape to VPI’ed high voltage stator coils

Abstract

The trend to boost MVA capabilities with smaller machine size of large electrical generators, has concentrated on the reduction of stator coil groundwall insulation thickness to obtain a better heat flow to maintain efficiency. This approach has about reached its limits using the presently commercially available glass backed mica tapes on VPI'ed stator coils. The major limitation that has been reached is the dielectric performance of the groundwall. To improve the dielectrics, one approach initiated by a leading tape manufacturer is to fabricate a glass backed mica tape using a thinner carrier or glass backing. This thinner glass is more commonly referred to as flat glass which describes its physical makeup. A number of papers have described and compared this new flat glass mica paper tape with mica tape that uses a standard glass carrier for backing. The advantage of using the flat glass is that a higher density of mica paper can fit into the space allocated for the high voltage coil groundwall insulation. In this way, the dielectrics can be increased and by using an overall thinner groundwall thickness, enhanced thermal performance can be realized. Significant improvement in void reduction is also realized because of the reduction in free space between tape layers. However, with this reduction in free space, paths for the resin impregnation are also reduced. There are many significant challenges associated with the practical application of the flat glass mica tape to the insulation of high voltage stator coils when using a VPI process. Fabrication and testing of both test bars and full size stator coils, impregnated with an epoxy resin, have resulted in many significant differences when compared to the use of conventional glass backed mica tape. Excellent electrical performance can be realized when applying the flat glass mica paper tape provided manufacturing adjustments are incorporated. This paper will present the findings of extensive testing of test bars and full size stator coils, insulated with the flat glass mica tape and VPI'ed with an Epoxy resin.