Cavitation mechanism and effect on pump power-trip transient process of a pumped-storage unit

Backflow vortexes (BFV) and cavitation are the main sources of pressure fluctuations (PF) in pump-turbine (PT) transitions. However, their interaction mechanism and effect on the transitions of pumped-storage power systems remain unclear. In the present work, the guide vane closing process (GVCP) after the pump power-trip (PPT) of a pumped-storage power system was investigated adopting the one- and three-dimensional coupled two-phase cavitation flow simulation approaches and time-frequency combined signal processing approach. Based on the study findings, large-scale vapor cavities in the draft tube and flow-separated vortexes in the guide/stay vane blocked the PT flow passage. Further, their dynamic evolution induced instantaneous flow rate pulses and further caused pressure pulses, which are equivalent to the water hammer effects. Large amounts of BFV and vapor cavities were found on the impeller high-pressure (HP) side. The unsteady evolution of BFV and vapor cavities and their interactions induced complex-frequency PF near the maximum reverse discharge (Q_{max_r}) condition in the pump brake (PB) mode. Herein, the flow-separated vortexes were related to the low matching degree between the guide vane (GV) and stay vane. Overall, our findings can provide an important theoretical basis for avoiding the vibration of pumped-storage power systems.