Instability of large-scale prototype Francis turbines of Three Gorges power station at part load

The rapid increase of renewable energies (e.g. wind and solar energies) requires hydroturbines (e.g. large-scale Francis turbines) to be operated at part load more frequently in order to improve the stability and flexibility of the power supply system. A device named as guide plate is currently being introduced into Francis turbines in order to shrink the size of the unit for the cost reduction. However, the effect of the guide plate on the instability of Francis turbines (especially at part load) is still an open question. This paper aims to elucidate the influence of the guide plate on the instability (e.g. in terms of large pressure fluctuation and its propagation) of the prototype Three Gorges turbines and its generation mechanisms. Computational simulations of the prototype Francis turbine have been performed and validated using on-site measurement. The results reveal that the addition of the guide plate induces an extremely low-frequency and high-amplitude pressure fluctuation at part load (e.g. guide vane opening 16°). This low-frequency pressure fluctuation initially generates in the draft tube, and then further propagates upstream (e.g. runner, guide vanes), acting as the dominant frequency. Our data analysis shows that the above pressure fluctuation is generated by the interactions between the vortex in the spiral casing induced by the guide plate and the swirling vortex rope in the draft tube.