| China is rich in hydropower resources,with the world’s largest installed hydropower capacity and a large number of large hydro generator sets,most of which are of 700 MW grade,such as Three Gorges,Xiaowan,Longtan,Xiluodu,Xiangjiaba,Nuozadu and other large hydropower stations.700 MW grade hydro generator manufacturing technology was introduced by China through the Three Gorges Project,and has been digested and absorbed,and has achieved independent innovation to manufacture 1000 MW hydro generator sets.Most of the largecapacity hydro generator sets of 700 MW and above adopt oblique feet structure stator base,which can effectively offset the thermal stress,ensure the concentricity of the stator system and prevent the stator core laminations from warping.However,it reduces the radial stiffness of the stator system and causes the stability problems caused by the mechanical and electromagnetic coupling of the stator system.In this paper,a finite element model and a nonlinear cylindrical shell model are established for the Xiaowan hydro generator unit as an example,and the calculation results are compared with the stability test results,and the calculation errors of the finite element model are 5.68% and 5.23% respectively,and the calculation errors of the nonlinear cylindrical shell model are 1.81% and 11.2% respectively.The calculation results are based on the stability test results,which have practical and guiding significance for the operation and maintenance of hydro generator sets.The specific research contents are as follows.1.The stability test of a unit of Xiaowan power station under variable speed,variable excitation and variable load conditions was carried out,and it was learned that the main reason for the large vibration of the stator system is the unbalanced magnetic pull generated by mechanical-electromagnetic coupling;when the stator system is in stable operation,its axial vibration is very small and can be ignored when building the model;and the horizontal vibration amplitude of the stator seat and stator core is comparable,so when studying the radial vibration,it can be considered as a whole.2.The analytical model of the finite element was established.The calculation process of finite element analysis is refined,and the phase information in the harmonic response coupling can be used to obtain the vibration amplitude time domain map,which greatly reduces the time compared with the transient calculation.3.It is verified that the stator system modal changes when the stiffness of the positioning bar connection changes.When the stiffness is small,the stator seat and stator core modal separation occurs,and when the stiffness is large enough,the stator system modal stabilizes,and the stator system model is in stable operation.The influence of static electromagnetic pull and load torque on the stator system mode under no-load and full-load conditions is analyzed.The static electromagnetic pull reduces the inherent frequency of the stator system and has a greater influence on the low-order inherent frequency,while the load torque has little influence on the inherent frequency.4.A nonlinear thin short cylindrical shell model was developed.The coupling effect between the unbalanced magnetic pull and the radial vibration displacement of the stator system is analyzed.The nonlinear equation of the coupling between the unbalanced magnetic pull force and the cylindrical shell vibration displacement is established.The nonlinear vibration of the stator system is calculated in the rated excitation condition.Under this condition,there is no tangential force and the axial vibration variation is neglected,so the partial differential equation can be simplified to a normal differential equation.Under the same condition,the calculation result of nonlinear model is much larger than the calculation result of finite element model.And in this model calculation,the model stiffness is reduced,the nonlinear vibration frequency is reduced,and the vibration research and concern is mainly for low frequency vibration. |
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