Effects of Frequency and Temperature on Partial Discharge Characterization of Stator Windings

This paper presents results from laboratory measurements of partial discharge (PD) activity in stator bars taken from a 13-kV/95-MVA hydro generator, after 50 years in service. The main purpose of this work is to characterize the stator mainwall insulation by PD testing with the emphasis on the dependencies on the test voltage frequency and temperature. The tested epoxy-mica based stator bars were taken from the high voltage side of the winding. Non-energized (unused) backup bars were also tested to study the effect of aging on the PD characteristics and to set as a point of reference. The PD characterization was conducted at different frequencies (0.1, 1, and 50 Hz) and temperatures (30, 70, and 130°C) at 8.9 kV (1.2 Uo). The observed frequency and temperature dependencies were discussed using theoretical findings regarding the mechanisms of void degradation and surface conductivity. Also, classification using supervised machine learning was briefly applied to discover patterns in the data as a function of frequency and temperature.