Analysis of Underlying Mechanisms of S-shape Instabilities in Reversible Pump-Turbines and Development of Countermeasures for Enhancing Operational Stability During Transient Maneuvers in Turbine Mode

The quintessence of this work is a comprehensive analysis of underlying mechanisms of S-shape instabilities based on analytical, numerical and experimental methods. Special attention is given on turbine brake and reverse pump mode at which S-shape instabilities typically intensify hydrodynamics. The introduction of countermeasures being capable of improving operational stability during transient maneuvers in turbine mode represents a key outcome of this work. A model conception developed to identify the causality of such instabilities provides a better understanding of physics and acts as a basis for the development of a flow-stabilizing guide vane design. The analytical consideration refers to a pump-turbine model machine and is supported by unsteady CFD simulations under steady and transient operating conditions. Corresponding model tests are executed on a test rig at ANDRITZ Hydro in Linz, Austria, providing experimental data of integral quantities at off-design operating conditions. In order to draw conclusions on the prototype, transient studies are carried out for different load rejection scenarios based on the layout of a 2x330MW variable speed pump-turbines power plant. The level of stabilization, efficiency and further performance parameters demonstrate whether a measure is competitive and capable of fulfilling requirements. The use of misaligned guide vanes acts as the reference for a final comparison with the elaborated guide vane designs. As such misaligned guide vanes have been proven to stabilize characteristics both at synchronization and in case of load-rejection, they are suited to assess the outcome of the given guide vane variants.