Partial Discharge Characterisation of Stator Windings Taken from a 50-year-old Norwegian Hydrogenerator

This paper presents results from laboratory measurements of partial discharge (PD) activity in stator bars taken from a 95 MVA hydrogenerator after 52 years in service. The generator has previously been subjected to off-line condition monitoring including partial discharge (PD) testing at both 0.1 and 50 Hz, and on-line PD monitoring at 50 Hz. Stator bars were selected from representative positions along the winding, including both the neutral point and high voltage side. Initial PD testing has been performed at a fixed voltage level (9 kV/1.2 U0) to assess the current state of the stator bars. Based on the PD measurements a comprehensive set of characteristic discharge data has been found for 161 stator bars retrieved from the decommissioned hydro generator. The selection of measured data for presentation in this paper is based on the phase and position of the bars in the generator providing an extensive PD activity mapping of the generator. The test results show a large variation in discharge activity among the stator bars. However, using statistical analysis it is not possible to distinguish between service-aged bars taken from high and low electric stress regions.