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Increasing demands towards a more flexible and reliable operation of backbone systems for electrical grid stability has been leading to an upsurge of pumped-storage in the last years. As technological requirements concerning reversible pump-turbines have been undergone substantial transformations with respect to operational stability and noise emission, hydraulic designs are prompted to change according to market needs and requirements. One of the main factors threatening the mechanical structure of components represents the unstable operation in pump and turbine mode. Instabilities are typically accompanied by an intense increase in pressure pulsations that can be a main reason for vibration issues and resonance excitations in reversible pump-turbines. Avoiding instabilities during operation is, therefore, one of the major targets for competitive designs. This paper emphasizes the relevance of a thorough analysis of S-shape instabilities and provides an explanation of their underlying mechanisms based on analytical, numerical and experimental methods. It also addresses conflictive mechanisms behind a varying rotor-stator distance that could occur in terms of operational stability and rotor-stator interaction. The introduction of a new 3D-shaped guide vane design represents a further key outcome of this study. With respect to a varying rotor-stator distance, special attention has been paid to the description of influencing factors for turbine stability and pressure pulsations using three rescaled versions of an existing guide vane design. Series of model tests have been conducted on a test rig at ANDRITZ in Linz, Austria, revealing whether a measure is competitive based on the layout of a 2x330MW variable-speed pump-turbines plant.

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