Dynamic Behavior of Reversible Pump-Turbines in Turbine Mode of Operation

Abstract

Reversible pump-turbines, with their steep and s-shaped characteristics, have proven throughout history to be a challenge with regards to both stability and transients. The aim of this thesis is to increase our understanding of the dynamic behavior of these machines in turbine mode of operation. The work comprises of three objectives. The first is to investigate and verify stability criteria for hydraulic systems equipped with reversible pump-turbines. An investigation of the influence of the characteristics on the system transients forms the second objective. The third objective is to improve and verify an analytical one-dimensional reversible pump-turbine model for transient calculations. The research methods utilized in this work includes laboratory and field measurements combined with transient calculations. Measurements from the laboratory and prototype are used to investigate and verify stability criteria, while the prototype measurements are also used to explore the characteristics impact on the transients. Transient calculations are used to better explain phenomena revealed in the measurements, and to further explore the dynamic behavior of the hydraulic systems. In addition, the laboratory measurements combined with transient calculations are used to verify the improved one-dimensional reversible pump-turbine model. The findings from the research show that the theoretical stability criterion consists of two inequalities; one for a negative Ted-Ned-gradient at runaway and one for a positive. The inequality for a negative gradient gave a correct prediction for the laboratory system, while the criteria for stable behavior with a positive gradient could not be verified for the prototype. Transient calculations revealed that the system was at the stability limit. The investigation of the impact of the characteristics on the system transients showed that, for the prototype, the turbine was the major contributor. In the research presented the importance of transient analysis at an early stage of a project has been emphasized. This has motivated further development of a one-dimensional reversible pump-turbine model improving the correspondence with measured characteristics. Based on these findings it is recommended that future work includes measurements of the dynamic characteristics on a prototype for comparison with predicted theoretical behavior. Further improvement of the one-dimensional model is recommended, suggested work includes both analytical studies and measurements.