Development trends in the field of reversible pump-turbines – Study of pumping and generating mode off-design conditions

The role of pumped storage power plants (PSP) in the electrical grid systems is changing in last years. Demands for switching from pumping to generating mode are becoming more and more frequent. Moreover, the operating ranges of the reversible pump-turbines used in PSP systems are getting wider in order to use PSP as a regulator and a stabilizer of the electrical grid. Main challenges in the pump-turbines development are hydraulic instabilities that occur in both pumping and generating modes. The paper focuses on the cavitation and the rotating stall that occurs at pumping mode partial load and can cause uncontrollable switching of the operating point, strong pressure oscillations and can damage the pump-turbine. Moreover, at the generating mode, the characteristics form an S-shaped curve in the turbine, turbine break and reverse pump quadrants. This S-shape leads to unstable behavior of the turbine when coupling to the grid or to surge transient phenomena in case of an emergency shutdown. The paper presents results obtained by Computational Fluid Dynamics (CFD) tools for both types of instabilities. Additionally, pumping mode rotating stall has been compared to the experiment performed in the hydraulic laboratory. Three rotating stall cells with rotational frequency 2,5% of nominal pump-turbine frequency have been identified at the operating point 65% of nominal discharge. Cavitating vortices related to the rotating stall have been found in the guide vanes region. S-shaped curve has been identified in the generating mode and reproduced by CFD. Unstable flow patterns have been analyzed at various unstable operating regimes and compared to the stable ones. Understanding the causes of the instabilities leads to the improved pump-turbine design that enable safer, more flexible and more reliable operating with less unwanted instabilities. Moreover, together with mechanical and manufacturing optimization, the acquired knowledge helped to develop pump-turbine with significantly wider operating ranges in pumping and generating modes. (paper)