Condition Assessment of Hydrogenerator Stator Bar Insulation Using Partial Discharge Measurements

This paper presents results from laboratory measurements of partial discharge (PD) activity in 50 cm long samples cut from the mainwall section of old hydrogenerator stator bars. All stator bars were manufactured in 1976 and samples were taken after 35 years in service from both the low and high voltage sections of the generator, as well as non-energized back-up bars. The PD activity, using a phase resolved (PRPDA) measuring system, was investigated at different test voltages up to 9.6 kV (1.5 U 0 ), frequencies and temperatures in the range 20–155 °C and 0.1-50 Hz, respectively. The service-aged and the unaged reference samples showed a clear difference in voltage frequency dependence. It was, however, not possible to distinguish between service-aged bars from high and low electric stress. The observed frequency and temperature dependences are discussed with respect to theoretical assumptions regarding possible void degradation and surface conductivity.