Francis turbine instabilities during off-design and transient operations: A comprehensive review of the state-of-the-art and recent research and development

Modern Francis turbines play a crucial role in stabilizing power grid through flexible operation. However, flexible operation often requires running under off-design conditions and undergoing frequent transient operations. These regimes expose the turbine to instabilities, primarily associated with cavitation vortex ropes and Rotor–Stator Interactions (RSI), which lead to pressure fluctuations, vibrations, and even resonance. These issues can cause erosion, fatigue failure, and structural damage, and may even destabilize the power grid. Accordingly, achieving flexible operation relies on two complementary approaches: assessing and avoiding unstable operating conditions, and mitigating instability to acceptable levels. In recent years, significant advances have been achieved through experimental investigations and numerical simulations with reduced-scale model turbine, and collaborative projects aimed at extending the stable operating range. This review provides a comprehensive overview of state-of-the-art understanding of instabilities during off design and transient operations in Francis turbines, as well as technologies and methods proposed to mitigate these phenomena, and it finally outlines future research directions.