In recent years, a distinctive nonlinear characteristic of the current versus ramped dc voltage test response has been observed which identifies rotating machines having delaminated stator winding insulation. This characteristic response is described and analyzed. Two illustrative examples of hydroelectric generators with delaminated groundwall insulation are presented. In one case, the ramped voltage test curve is compared against power factor and capacitance versus ac voltage measurements taken on the aged stator winding and on unused spare coils. Results of dissection analyses of aged stator coils removed from the winding and spare coils are also discussed. In the second example, ramped voltage test data are compared to on-line partial discharge measurements.
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