Experimental Investigation Of S Shaped Characteristics Of Pump-turbines Under Different Heads

Pumped storage power plant as a large-scale energy storage conversion equipment,plays the role of filling valleys and eliminating peaks,as well as frequency modulation and peak regulation in the power grid.With the adjustment of energy structure,the proportion of solar energy,wind energy and nuclear energy in the power grid increases rapidly.In order to promote the consumption of clean energy in the power grid,pumped storage units have obtained a rapid development.Pump-turbine,as the key component of pumped storage unit,has two unique unstable regions,the one is S-shaped region in turbine mode,the other is hump region in pump mode.These two instability regions seriously restrict the rapid development of pumped storage units.Hence,it is important to study the instabilities of pump-turbines.The output is proportional to the water head and flow rate.At the same the output,the higher the head,the lower the flow rate,consequently the smaller the size of the pumpturbine and associated pipeline.In such case,the volume of upstream and downstream reservoirs could be reduced.Therefore,the pump-turbine trends toward higher head.According to the survey of pump-turbines,the maximum head of the pump-turbine rises one step every 10 years.The difficulty coefficient for manufacture of the pump-turbines is usually expressed by the product of the output and maximum head.With the increase of the head,the manufacturing difficulty increases greatly.Meanwhile,the S-shape and hump instabilities as well as associated high amplitude pressure fluctuation become key technical problems.In this study,S-shaped characteristics and associated pressure fluctuation were systematically studied using experimental and theoretical methods.The details are as follows:Firstly,based on the survey of experiments,numerical simulation,theoretical analysis and optimal design about the S-shaped characteristics of the pump-turbine at home and abroad,the existing problems to be solved are summarized.The studies on Sshaped characteristic at different rated head are proposed,including S-shaped characteristic,propagation law and formation mechanism of pressure fluctuation in the S-shaped region.Secondly,the paper mainly studied the S-shaped instability be means of experiments.Therefore,high precision and high reliability test rig is the basis of this study.So,the composition,working principle,performance parameters and main functions of the testrig are introduced in detail.Thirdly,the S-shaped characteristic curves of the pump-turbines with different rated head and different guide vane opening are obtained.Through steady and transient stability criterions,the results reveal that the S-shaped characteristic is obvious with large guide vane opening at the same runner,and it becomes more severe with the increase of the rated head.Fourthly,the experiments of pressure fluctuation in the S-shaped region were carried out at a high head runner with different guide vane openings.The results that the maximum pressure fluctuation occurs at close to runner away operating condition or maximum flow rate operating condition at reverse pump mode.Through the analysis of time and frequency domain,the high amplitude pressure fluctuation mainly comes from the rotor stator interaction.Finally,the pressure fluctuation in S-shaped region with different rated head was studied.The results show that,with the increase of the rated head,the amplitude of pressure fluctuation rises obviously,especially in the vaneless space.Additionally,the splitter blades could greatly reduce the rotator stator interaction in the vaneless space.