Abstract: In a pumped-storage power station, complex layout of its water transfer system and frequent change in its working condition often cause hydraulic oscillations or even hydraulic resonances. This paper summarizes unstable characteristics of the flows in pump-turbines and clarifies their relationship with the hydraulic oscillations of the transfer system. First, we examine the pump-turbine working conditions for the occurrence of unsteady flows and their typical frequency ranges related to draft tube vortices, rotating stalls, unstable oscillations in the S region, rotor stator interaction, vane tip Karman vortices, etc. Then, natural frequencies of a typical water transfer system are calculated using the transfer matrix method and compared with the frequencies of unstable flows in the pump-turbine. We found out that the frequencies of draft tube vortices and rotating stalls overlap with those of the hydraulic system, indicating the possibility to cause hydraulic resonances. Finally, the forced oscillations of the transfer system are simulated with given head disturbances to draft tube flows. The results show that draft tube vortices and rotating stalls are more likely to cause hydraulic resonances, which could lead to fluctuations in the power output. Also, oscillations in the S region could induce hydraulic oscillations during transient process. On the other hand, the high frequency characteristics of rotor-stator interactions and Karman vortices could originate the structural resonances of runners but not the hydraulic resonances in water transfer systems, for they mainly affect high-frequency pressure pulsations.

Key words: pumped-storage power station, water transfer system, flow patterns, pressure pulsations, hydraulic oscillation, hydraulic resonance