An examination of high-voltage D.C. testing applied to large stator windings

The merit of a proposed non-destructive method of assessing the breakdown voltage of stator-coil insulation by means of high-voltage direct current is examined. The proposal is to measure the steady-state leakage current at successive increments of applied voltage and predict a breakdown value from the trend of the plotted results. Laboratory tests applied to a range of specimens of sheet insulation, including a number with artificially-formed faults, indicate that the only breakdown which can be predicted by this method is that across air paths external to the specimen. Tests carried out on a complete high-voltage stator winding and on single stator coils give similar results. No assessment of slot-insulation quality is possible: the only increase in current, suggestive of approaching failure, results from end-winding leakage and discharge effects. Surface contamination of the end-winding and even quite extraneous discharges, when present, have a marked influence on the current measurements. It is concluded that the high-voltage d.c. method of testing investigated has no valid basis for non-destructively indicating the service-ability of the insulation of high-voltage machines.