Unsteady vortices and system instability of pump turbines: An investigation of the S-shaped region

Under the background of extensive access to a high proportion of new energy (wind/solar) in China, the traditional power grid has been transformed into a new system with new energy as the main body, in which pumped storage has become the key regulating power source due to its advantages of fast, flexible, and large capacity. Under such circumstances, pump turbines are prone to operate within the S-shaped characteristic region, which leads to unstable vortices and flow separation in the flow passages of hydraulic components, inducing complex hydraulic excitations and pressure fluctuations that significantly compromise the safe and stable operation of the unit. To gain a deeper understanding of the hydraulic excitation characteristics induced by unstable flow in the S region, this study investigates a high-head model pump turbine through numerical simulations and experimental studies on the unsteady dynamic behavior within the S-shaped characteristic region. Based on Nielsen's theory, the analysis was extended to investigate the stability of high-head pump turbines, supported by unsteady flow field simulations, ultimately revealing the mechanism of flow instability. The main conclusions are as follows: The stability of the pump-turbine system is closely related to the slope of its characteristic curve. When the slope of the characteristic curve is positive, the unit becomes unstable. Moreover, the inflection point of the S characteristic marks the onset of backflow on the hub side, and its relative position to the zero-torque point determines the sign of the slope. If the zero-torque point lies below the inflection point, the slope is positive, and the system may become unstable. A comparative evaluation of the velocity distribution at the runner inlet reveals a regular pattern in the flow evolution around the inlet, and the transition process from turbine mode to braking mode is divided into four distinct regions. Under varying flow conditions, the internal unsteady flow characteristics of the pump turbine indicate that large-scale flow separation at the runner inlet induces vortex structures, leading to flow losses in the unit. As the guide vane opening increases, the energy exchange at the runner inlet intensifies, resulting in a more distorted S-shaped characteristic curve at larger openings.