Effect of stress grading systems and guarding techniques on measured power factor and power factor tip-up of electric machinery stator windings

This study compares the effects of voltage grading systems having various lengths and configurations on the measured power factor and power factor tip-up. The effectiveness of different guarding techniques is also compared. 13.8 kV form-wound stator coils were manufactured and a grading tape with two different lapping configurations and two overall lengths were applied to the coils. The stator coils then received a vacuum pressure impregnation (VPI) treatment. The power factor and power factor tip-up of the coils were measured and compared. In addition, three different guarding techniques were used to minimize the measurement errors caused by the electrically graded portions of the overhang, and the results of the power factor measurements were compared.