Calculation Analysis Of Hydraulic Transition Process On Pumped Storage Hydropower Station With Multiple Units Per Penstock

Layout ways of multiple units per penstock for water conveyance system are extensively applied in pumped storage power stations because of its good effect in save investment and space.Closely hydraulic interaction characteristics exist among the two or more units connected to the same run penstock,in the circumstances,significant fluctuations take place in water level of surge shaft and piezometric tube water head at bifurcation point owing to sudden load rejection or load increasing of one hydraulic unit,and the parameters such as water head,output,rotating speed and guide vane opening will inevitably alter for the rest units which are still operating in normal.Furthermore,unit output volatility caused by hydraulic disturbance will not adapt to load requirement,which ultimately leads to variation in rotational speed and automatically adjust the speed governor by force.In this case,the modes and strength of hydraulic disturbance are different from the load interference,so hydraulic transition process is distinct from the slight fluctuation within load interference,as well as the great fluctuation induced by emergency shutdown.This paper aims to perform a calculation analysis and safety evaluation of hydraulic transition process for multiple units per penstock in pumped storage hydropower power station on the basis of serious project practice.This thesis mainly includes the following works:Maximun pressure at the inlet of volute,minimum pressure at the inlet of draft tube,maximum rise rate of rotating speed and the highest water level of intake surge shaft and tailrace surge tank were calculated by transient process for serious stations.In terms of the calculation results and engineering experience,recommended operating conditions suitable for the transition process of pumped storage power station were given,which can provide evidence for engineering design and guarantee of engineering safetyThe study on the effect of runner characteristics and parameters of water conveyance system with or without surge shaft,size of impedance hole diameter and location of bifurcated penstock on transition process was presented,it could provide guidance for plant operation.In the region of positive flow capacity,variation tendency is more gentle in large opening curve,less influence of unit speed on corresponding unit flow is observed.The maximum pressure at volute inlet can be greatly improved with the equipment of intake surge shaft,and the tailrace surge tank could raise minimum pressure at draft tube inlet.On the other hand,minimum pressure at draft tube inlet drops with the increment of ratio of impedance hole area with sectional area of headrace f/s,the minimum pressure of the draft tube is reduced by 2m if the value of f/s increases by 10%.It reveal adverse consequence in transient process where anti-bending of the "S" area is sharp,especially at the situation of guide vane moving conditions,the transition process subject to disturbance more easily.Sensitivity analysis for key factors of the closure law of guide vanes and rotational inertia are performed during transient process of pumped storage power station.Both pressure and rise of rotational speed must be taken into account in the process of optimizing the closure law of guide vanes.When it cannot meet the limitation requirements of regulation and guarantee calculation for linear closure law,it is necessary to consider the method of stage closing.Enhance the rotational inertia have effects on preventing rise of rotational speed,and it can control the maximum pressure at the inlet of volute.But it has to be combined with the optimization of guide blade closure rules when the rotational inertia exceeds a certain critical valueIn case of double units per penstock in water conveyance system,load rejection or increasing for one unit has obvious impact on inlet pressure of volute for the other unit and inlet pressure waveform for the two volutes is similar.In addition,the load rejection has more influence than the load increasing.Load increasing for one unit gives rise to output fluctuates for another unit operating in rated condition,and the rise of maximum output is light,while the minimum value drops sharply.Correspondingly,load rejection for one unit also brings about output fluctuates for another unit operating in rated condition,and the rise of maximum output is evident while the minimum value of output drops slightly.Simulation results of power interruption process at runaway condition shows that dominant frequency for pressure at inlet of volute and draft tube is about 0.27 times that of runaway frequency,and is is 9 times that of runaway frequency in the vaneless region between guide vane and runner.It can observe vortex at the inlet of runner at braking conditions,and it appear in runner at runaway condition.The braking condition could induce vortex rope in draft tube,which firstly rotates on the wall counterclockwise and forms a cavity vortex rope because of separation from wall,then the cavity vortex rope evolves into a clockwise columnar vortex rope owing to diameter reduction of vortex rope and change of rotation direction,and finally it expresesses in the form of helical vortex rope with clockwise and eccentricity.