Comparative evaluation of various grading systems for electric machinery stator windings

The surface potential characteristics of grading systems of various lengths and configurations used for medium voltage form-wound stator coils have been examined. The grading material used in this investigation was woven polyester fabric tape loaded with a varnish containing a semi-conductive silicon carbide filler. Form-wound stator coils for a machine rating of 13.8 kV (line-to-line) were manufactured. Stress grading tape was applied to the coils using two lapping configurations and with two overall lengths. All sample coils then received a vacuum pressure impregnation (VPI) treatment. The surface field distribution in the stress grading regions of the sample coils was measured at test voltages of 8 kV and 14 kVrms. In addition, the temperature profiles of the grading systems were recorded at applied voltages up to 28.6 kVrms. The experimental results revealed that the stress grading tape lapping configuration has an influence on the field distribution to some degree; however, little or no effect was observed on the field dependent property when the grading length in the endarm region was increased.