Partial Discharge in Stator Winding Under Harmonic Voltage Superimposed on Generator Voltage

Partial discharge (PD) magnitude and phase-resolved PD (PRPD) patterns are key indicators for diagnosing stator winding insulation. Nonlinear loads in power systems induce harmonic voltages that can alter the PD characteristics, and this may subsequently affect insulation diagnostics. This study investigates the impact of harmonic voltage superimposition on the PD behavior in stator windings. We measure the PDs in the coil bar extracted from the stator winding under harmonic voltages superimposed on the AC voltage. Further, we analyze the effects of the harmonic order and ratio on the PD characteristics. We conduct PD measurements on an actual hydrogenerator exposed to a high-order harmonic voltage. The findings show that while harmonic superimposition can alter PRPD patterns, its effect on the PD magnitude is negligible. Therefore, when using PD-based insulation diagnostics, special attention should be given to the impact of harmonic voltages, particularly when PRPD patterns are used to identify insulation defects.