Abstract
Frequent start-stops in hydrogenerators impose increased thermal, mechanical, and electrical stresses on stator winding insulation, accelerating degradation and potentially leading to failure. To emulate such operating conditions, a multi-stress, single-phase stator-slot model was built to cycle stator coils between 40 and 110°C while simultaneously applying 1.5 times the rated phase-to-ground voltage (U0). Service-aged and backup coil groups taken from the same hydrogenerator were tested, with partial discharge (PD) recorded during cycling (on-cycle) and off-cycle diagnostics at room temperature, including PD inception voltage, PD measurements at 1.5 U0, and loss tangent. The on-cycle results show distinct aging trajectories: the service-aged coils entered a stable-degradation regime with limited, similar discharge magnitudes during heating and cooling, whereas the backup coils exhibited larger, more frequent discharges, with PD markedly higher during cooling than during heating at the same temperature. Off-cycle measurements indicate similar trends but with lower magnitudes, suggesting that room-temperature tests alone cannot probe multi-stress degradation. The study demonstrates that reliable assessment requires the use of combined on-cycle and off-cycle diagnostics under representative loading conditions.