Investigation of the added mass effect and bearing stiffness on the rotordynamics of pump-turbine shaft systems

Pumped-storage hydropower ( PSH ) is an essential technology for efficient energy storage and plays a vital role in achieving net-zero emission targets. Pump-turbine ( PT ) units are the most commonly used configuration due to their flexibility and rapid transition between storing and generating energy. These units are typically designed as rigid rotors, meaning their rated and runaway speed is below the first critical speed. To guarantee this condition, it's important to accurately calculate the natural frequencies of the shaft line in operation. In this paper, the natural frequencies of a PT shaft line are thoroughly investigated through numerical simulation models and validated by experimental modal analysis on a PT prototype which determines that the added mass of the pump-turbine runner also has a significant impact on the entire axis. Furthermore, it is predicted that a low stiffnesses in any of the radial or axial bearings can lead to a substantial reduction in the first critical speed of the rotor. Furthermore, through wavelet analysis applied to radial displacement sensor data during the start-up phase, the first natural frequency in operation can be identified. This frequency can serve as a health indicator for the entire unit and be integrated into advanced monitoring strategies. • Natural frequencies of pump-turbine shafting analyzed via numerical modeling and prototype modal testing. • Added mass of surrounding water significantly influences shafting natural frequencies. • Lower radial or axial bearing stiffness markedly decreases the first critical speed. • Wavelet analysis of start-up displacement data enables identification of operational natural frequencies. • Operational natural frequencies can serve as health indicators for advanced condition monitoring.