Response of ramp test to delamination in stator groundwall insulation

High direct voltage ramp tests incorporating a continuous record of current versus voltage are known to be a sensitive indicator of groundwall cracks and fissures. Such defects often result in an obvious avalanche in the leakage current. Recently a different current versus voltage pattern has been associated with internal delamination. This paper reviews the evidence of this condition using ramp tests and partial discharge measurements on stator coils that are known to be delaminated between the groundwall and the turns to a greater or lesser extent.