Hydraulic instability of pump-turbine during fast pump-to-turbine transition under different control schemes: Changing guide vane pre-opening angles

Pumped storage power plants are widely known for their ability to flexibly transition from pump to turbine operating mode and vice versa, in line with the power system requirements. Therefore, it is of great importance to accurately predict the hydraulic instability during such a transient process to enhance operational stability and the associated safety. In addition, an appropriate understanding of the effect of key control parameters is essential to optimize control strategies. In line with this, based on numerical simulation method, this study seeks to investigate the fast pump-to-turbine transition process. To achieve this, three different control schemes with different guide vane pre-opening angles were studied from three perspectives; namely, external parameter changes, pressure fluctuation characteristics, and the evolution mechanism of flow patterns. The research results indicate that during the transition, pre-opening the guide vanes can significantly accelerate changes in rotational speed and flow rate, thereby facilitating a faster transition. Different control schemes exhibit similar evolutionary trends in external parameters, pressure fluctuations, and flow patterns, but they differ in timing and amplitude. Increasing the pre-opening angle from 38.5% to 53.8% of the no-load position can effectively reduce the transition duration by 11.1% and decrease the peak-to-peak pressure fluctuations in the vaneless region by 2.2%. Although the maximum positive axial force increases by 16.0%, the increase remains insignificant in magnitude. Notably, the results suggest a decreasing trend in the effectiveness of further increasing the pre-opening angle in minimizing the transition duration. Simultaneously, the positive axial force and pressure fluctuation intensity show an accelerating growth. Therefore, increases in the pre-opening angle should be moderate. Therefore, increases in the pre-opening angle should be moderate. This study provides theoretical guidance for accelerating the fast pump-to-turbine transition process and optimizing control strategies.