Experimental Investigation Of Hysteresis Characteristics In The Hump And S-shaped Region Of A Pump-turbine

Hump region in pump mode and S region in turbine mode are typical unstable regions of pump-turbines,which seriously limit the stable and safe operation range of pump-turbines.It is the key problem for the pumped storage units.With the development of pumped storage power plants,pump-turbines as key parts of pumped storage power plants are supposed to higher head,larger capacity,and higher specific speed.Meanwhile,due to the renewable energies such wind energy and solar energy in great amount use in electric grid,which is totally dependent on environment,the loads in the electric grid fluctuate seriously.Pumped storage power plants are required more to balance fluctuations of loads.Consequently,pump-turbines need more frequent switch between different modes.All of these lead to serious problems of hump and S-shaped instabilities,and have become key issues for further development of pumped storage power plants.Recently,an interesting phenomenon-hysteresis characteristic was found in the hump region and S shaped region through model test,which enlarges the unstable regions sharply and leads to unreasonable selection of hump margin and S region margin.However,the generation mechanism of hump and S-shaped characteristic has been not clear.Fewer studies on hysteresis characteristics in the hump region and S-shaped region are presented.In this study,hysteresis characteristics in the hump region and S-shaped region are studied systematically through the experimental and theoretical methods.In order to achieve the experimental studies on hysteresis characteristics in the hump region and S-shaped region,the acquisition system for hydraulic machinery general test in Harbin Institute of Large Electrical Machinery was designed.Based on NI PXI platform,graphical programming language Lab VIEW was adopted to develop general acquisition system of hydraulic machinery.The system features high precision and high automation.It is a high intelligence system,which integrates functions testing,analysis,processing and report.The experiments of energy and pressure fluctuation characteristics were carried out in pump mode of a high head pump-turbine using the designed acquisition system.16 mm,20mm and 24 mm guide vane openings were chosen to validate the existence of the hysteresis characteristics.The hysteresis characteristics in the hump region were analyzed through Euler theory,and the results show that the generation of hysteresis characteristics originate from joint action of hydraulic loss increase and Euler head decrease,where hydraulic loss increases accounts for most.In addition,through the analysis of time and frequency domain characteristics,the hysteresis characteristics are related with the high-amplitude low-frequency pressure fluctuations in the vaneless space.To validate the existence of hysteresis characteristics in the S region in turbine mode of the pump-turbine,the experiments of S-shaped characteristics and pressure fluctuation at 16 mm,20mm amd 24 mm guide vane openings were carried out using the designed acquisition system.Typical operating points were chosen to analyze in detail using the methods of time and frequency domain.The results show that the hysteresis characteristics is caused by low-frequency pressure fluctuations in vanless space(0.1f_n~0.5f_n).