Insulation Degradation Assessment of Hydrogenerator Stator Windings Based on Long-Term On-line Partial Discharge Measurements

On-line partial discharge (PD) measurements are conducted on four hydrogenerators to assess the insulation condition of stator windings using the maximum repeatedly occurring PD magnitude (Qm) and its winding-temperature dependence. Repeated measurements over approximately one to two years in different seasons confirm that Qm shows no clear increasing trend, while its temperature dependence can be identified. Qm mainly reflects the size of voids in the insulation, whereas the correlation coefficient between Qm and winding temperature, representing the temperature dependence of Qm, mainly reflects the number of voids. A comparison of the evaluation results shows that the ranking of the machines based on Qm does not coincide with that based on the correlation coefficient. These findings indicate that the two indicators provide different information on insulation condition.