Control Of Runner Hydraulic Thrust And Mechanism Of Pressure Fluctuation In Transions Of A Pumped-storage Unit

To achieve the goal of "carbon peak and carbon neutral" and build a new power system with new energy as the main body,China urgently needs to develop reliable energy storage technology to absorb the new energy generation in the power grid.Among many energy storage technologies,pumped-storage power technology is the only energy storage technology that can be used on a large scale in the grid.The pumped-storage units experience the transient process as many as ten times a day.During the transient processes,there exist severe pressure fluctuation in pump-turbines.The high-amplitude pressure fluctuation acting on the surface of the flow-through component in the transient process can induce severe fluctuation of runner hydraulic thrust.The pressure fluctuation and runner hydraulic thrust induced by the transient flow may cause fatigue damage of the flow-through parts,and even cause the operation accidents of lifting,chamber sweeping and hydraulic vibration of the rotating parts of the unit.Focusing on the issue regarding the pressure fluctuation and runner hydraulic thrust in transient processes of the operating pumped-storage unit with the largest single capacity in China,this paper carried out the works from following four aspects: multi-dimensional calculation method of transient process considering multi-factors,formation mechanism of the pressure fluctuation in typical transient process,action mechanism of influence factors for the runner hydraulic thrusts,and control of runner hydraulic thrusts based on multi-objective optimization.The research contents and conclusions are as follows:(1)To achievethe accurate simulation of transient process for a pumped-storage unit,this study proposed a multi-dimensional simulation method considering many factors such as the cavitation and water compressibility.The one-dimensional method of characteristics was adopted to simulate the transient flow in long-distance pipeline system.The three-dimensional finite volume method was employed to simulate the unsteady turbulence flow in a pump-turbine.A partly overlapped coupling method was used to achieve the data exchange between the one-and the three-dimensional computational domains.The cavitation two-phase flow was simulated using the mixture multi-phase flow model and Zwart-Gerber-Belamri mass transfer model.The water density was corrected according to the pressure distribution for considering the water compressibility.The study suggests that the proposed simulation method has high simulation accuracy for both the bidirectional transient simulations of prototype turbine and pump modes.The developped method achieves the accurate simulation of transient process for a pumped-storage unit.(2)To explore the formation mechanisms of the pressure fluctuations in typical transient processes of pump-turbines,this study simulated the turbine load rejection process,turbine runaway process,pump power-off and pump runaway process using the one-and three-dimensional coupling simulation method.The temporal and spatial distribution features of pressure fluctuations were revealed in transient process using the time-frequency joint analysis method.The mechanisms of the pressure fluctuations were expounded through analyzing the internal flow field in the pump-turbine.The study suggests that the low-frequency high-amplitude pressure fluctuation components which are lower than the frequency of the runner blade passing,were primarily induced by the interactions of the instable hydraulic phenomena such as the water hammer,cavitation,and backflow etc.The low-frequency high-amplitude pressure fluctuations primarily occur near the characteristic critical transforming conditions such as the zero hydraulic torque etc.in temporal.Monitoring point is closer to the inlet and outlet of the runner in spatial,fluctuation amplitudes of the pressure are higher.(3)Owing to that the runner hydraulic thrusts induced by the pressure fluctuation is one of primary causes which induces the operation accidents of the unit hydraulic excitation,to explore the mechanisms of key factors affecting the runner hydraulic thrusts for proposing the ways to control the runner hydraulic thrusts,this study simulated the turbine runaway process using the one-and three-dimensional coupling method.The influence law of the guide vane opening on the runner hydraulic thrusts was studied through comparing the simulation results of a pump-turbine at three guide vane openings 12°,15° and 21°.The influence law of the rotor inertia on the runner hydraulic thrusts was studied through comparing the simulation results of a pump-turbine for three rotor inertias 1.0J,2.0J and 3.0J at each aforementioned guide vane opening.The study finds that fluctuation amplitudes of radial hydraulic thrusts at large guide vane opening(21°)were approximately three times of those at small guide vane openings(12° and 15°).The fluctuation duration of the runner hydraulic thrust significantly decreases through appropriately increasing the rotor inertia.(4)To control the runner hydraulic thrust during the transition of the pumed-storage unit,the study performed multi-objective optimization of the guide vane closure law and rotor inertia using the genetic algorithm.Meanwhile,the study proposed three optimization methods which are based on the control of global backflow,ratio of rotational speed to hydraulic head,and rotational speed,respectively.The corresponding contraint conditions and objective functions were also established for the proposed three optimization methods.Several improved schemes were obtained after optimization,Additionally,the optimization results were validated using the one-and three-dimensional coupling simulation method.The result shows that the fluctuation amplitudes of the runner radial hydraulic thrust can be reduced by approximately percent fifty and twenty-five using the guide vane closure law optimization methods on the basis of controlling global backflow and the ratio of rotational speed to hydraulic head,respectively.Additionally,the extreme value of the axial hydraulic thrust can also be reduced using the guide vane closure law optimization methods on the basis of controlling the ratio of rotational speed to hydraulic head.The study on the guide vane closure law during transient process of a pump-turbine using ntelligent multi-objective optimization method achieves the control of the runner hydraulic thrusts,which is beneficial to ensure the nornal lifetime of the unit.