Polarization and Depolarization Current Analysis on Hydrogenerators

Polarization and Depolarization Current measurements (PDC) are well-known in the industry. HydroQuébec has been using PDC measurements as a diagnostic tool for more than 25 years. Since 2012, systematic PDC measurements are conducted every 6 years on the stator windings of all the hydrogenerator fleets. When the tool was first implemented, thresholds were defined to convert insulation resistance values into a condition health index. These limits were decided based on preliminary results obtained from a few hydrogenerators and laboratory measurements on accelerated-aged samples. However, with more than 500 PDC measurement results, the database can now help refine the limits for each type of insulation system (asphalt, epoxy and polyester). The purpose of this paper is to present the evolution of the insulation resistance value as global aging occurs and to observe the trending of insulation resistance over time. The influence of factors such as humidity, and the nature of the stress grading coating are also presented to describe how the limits can be managed for diagnostic purpose with statistical uncertainty.