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The most common mechanical equipment adopted in the new generation of pumped-hydro power plants is represented by reversible pump-turbines, required to rapidly switch between pumping and generating modes in order to balance the frequent changes in electricity production and consumption caused by unpredictable renewable energy sources. As a consequence, pump-turbines are required to extend their operation under off-design conditions in unstable operating areas.\udThe paper presents a numerical analysis of the unstable behavior of a pump-turbine operating in turbine mode near the no-load condition. To study in depth the unsteady phenomena which lead to the S-shape of the turbine characteristic, a load rejection scenario at constant and large guide vane opening was numerically analyzed by running through the flow-speed characteristic up to the turbine brake region.\udThe flow field analysis led to the onset and development of unsteady phenomena progressively evolving in an organized rotating stall (65.1% of the runner rotation frequency) during the turbine brake operation. These phenomena were characterized by frequency and time-frequency analyses of several numerical signals (static pressure, blade torque, mass flow rate in blade passages). \udThe influence of the development of these unsteady phenomena on the pump-turbine performance in a turbine operation was also analyzed, and the potential causes that generated the S-Shaped characteristic curve were also investigated

Analysis of the Unstable Behavior of a Pump-Turbine in Turbine Mode: Fluid-Dynamical and Spectral Characterization of the S-shape Characteristic