Flow Phenomena related to the Unstable Energy-Discharge Characteristic of a Pump-Turbine in Pump Mode

Renewable energy sources such as wind and sun have varying and, to some extent, unpredictable production. Pumped storage power plants can play an important role in stabilizing the electric power system when the production is either too high or too low. Subsequently, many new pumped storage power plants have been recently initiated in numerous countries where the need for stabilization is high. The use of reversible pumps turbines are widely regarded as the most cost effective solutions. They can, depending on reservoir size, deliver long term energy storage and/or boost production (turbine) or consumption (pump) in peak power situations. Pump-turbines often involve problematic S-shaped and instability regions in their machine characteristics. Thus, while may solve some problems in the grid, the operation and control can lead to other problems including severe self-excited oscillation in the hydromechanical system. At off design conditions, neither the distributor, nor the draft tube works properly and give awkward boundary conditions to the impeller. In addition there is a strong interaction between the runner and these parts. Flow features such as separation and recirculation occurs heavily in an unsteady manner. One objective of this research is to understand the underlying physical mechanism of the pump-mode instability of a two-stages reversible-pump turbine. The pressure fluctuations will be monitored while the pump-turbine is operating at different flow rates by flush mounted micro pressure transducers in the guide vanes, in the bladed return channel and in the inflow. The flow pattern was also analysed by high- speed flow visualizations, using injected air bubbles. A quite uniform flow pattern in the vaned channels was evidenced at the normal operating range. Whereas at part load the flow is highly disturbed backflow and vortices during the rotating stall. Moreover, the unsteady numerical flow fields were analyzed by the commercial code ANSYS CFX 14.0, to highlight the fluid-dynamical characteristic of the instabilities and investigate their origin. The evolution of rotating stall was identified by these analyses and compared with the experimental results. The spectral analysis of the unsteady pressure, obtained in the diffuser and in the impeller by the numerical results, helped to underline the role of the rotor stator interaction (RSI) in the development of the instabilities.