Research on the energy loss characteristics in the double-hump region of a pump-turbine

With the large-scale integration of renewable energy, pumped storage, as a globally recognized reliable peak-shaving power source, has become increasingly prominent. However, frequent operating condition transitions inevitably force pump-turbines to pass through the hump region, or even the double-hump region during start-stop or load regulation processes, which seriously affects the stable operation of the unit. In this study, to reveal the formation mechanism of double-hump region, the energy loss characteristics of a pump-turbine are thoroughly investigated by a combination of experiment and unsteady simulations. The characteristics of energy conversion, pressure fluctuation, and internal flow in the double-hump region are also elaborated in detail. The results show that the decline rate of unit hydraulic loss energy curve increases significantly under the double-hump conditions. The runner work energy curve initially shows a downward trend in the first hump region, and then the energy coefficient continues to increase rapidly with the decrease in flow rate. The characteristic frequencies in the double-hump region exhibit complex evolutionary patterns. In the first hump region, the pressure fluctuation frequencies in the runner disperse across multiple frequency bands with enhanced nonlinear superposition effects. In the second hump region, the pressure fluctuation amplitudes in the guide vane increase significantly. Additionally, the formation of the first hump region is primarily attributed to the emergence of a large-scale flow separation region near the band side of the runner inlet. This leads to a sharp increase in runner's energy losses, acting as the primary factor, and a decline in its work capacity, both of which contribute to the formation of the first hump region. The formation of the second hump region is mainly due to the significant increase in rotating stall intensity within the guide vane. The dynamic evolution of Dean vortices in the draft tube also has a certain promoting effect on its formation.