In this paper, the field dependent characteristics of different stress grading tapes used for high voltage form-wound coils have been examined. High voltage insulation systems require voltage stress grading to suppress the potential gradient and to ensure proper operation. Without these materials, the electric fields in the endarm region will lead to discharge in the surrounding air with potential degradation of the insulation system. The grading materials used in this investigation were woven polyester fabric tape loaded with semi-conductive varnish that included silicon carbide. In order to determine if the vacuum pressure impregnation (VPI) epoxy resin has an influence on grading performance, surface potential within the stress grading regions of sample coils was measured at test voltages of 8kV and 14kV. The measurements were performed prior to VPI (at green stage), after pre-bake at 160°C, and post VPI. The influence of high resistive surface contaminants on the grading was also examined. The experimental results reveal that introduction of the VPI resin can influence the electrical characteristics of the grading systems. In addition, a high resistivity contaminant can influence the performance of the grading system.
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