| Wind power is a kind of renewable and clean energy,which first gets the attention and development of European and American countries due to its unique advantages.As a big energy consumer,China started relatively late in the field of wind power,but has developed rapidly in recent years,and its installed capacity has ranked first in the world.At present,there are many accidents in the wind turbine tower on operation,which cause great economic losses.Because of the vibration environment,flange fastening bolts are easy to become loose,leading to the reduction of its fatigue life,or even fracture,so the wind turbine tower in normal operation collapsed accident.When domestic and foreign scholars carry out modeling analysis of wind turbine tower,they often ignore the flange connecting each tower cylinder,so the problem of flange bolt looseness cannot be considered.Based on the 2 MW wind turbine tower of a wind farm in gansu province as the research object,the problem of flange bolt looseness is mainly in the form of numerical analysis research to build whole model considering flange bolt,combined with environment pulsation test data.The static characteristics and dynamic response analysis is used to study bolt looseness on the structure,and provide a reference for assessment of wind turbine tower flange bolt looseness.The main research contents and results are as follows:(1)According to the pulsation test data of the three wind turbines on site,the first two natural frequencies and damping ratios of wind turbines are obtained through spectral analysis,and the natural frequencies and damping of wind turbines before and after maintenance are compared.The results show that the 1st order natural frequency and damping ratio are not sensitive to flange bolt looseness.(2)The three-dimensional body element model of local flange members is established,and the relationship between axial load and displacement of local flange members and equivalent stress of flange bolts and axial load is analyzed under the action of different bolt preloading forces.The results show that the pretension force can reduce the width of flange clearance and enhance the tensile stiffness of flange.After the loss of preload,the fatigue life of bolts decreases sharply,and local components of flange without preload show tension and compression bilinear.(3)On the basis of the mechanical constitutive relation of local flange members,nonlinear spring is adopted to simulate the local flange members.Based on the idea of fiber model,the integral flange model is established,and the whole spring-beam model of the wind turbine tower is composed of tower cylinder and flange.The calculation method of wind load is discussed.The axial thrust of the wind wheel is calculated based on the blade element momentum theory.According to the investigation on the looseness of flange bolts on site,the looseness ratio and Angle of nut are used to quantify the looseness quantity and tightness degree of bolts respectively.The influence of flange bolt looseness on the top displacement and flange clearance width is analyzed by using the whole model of wind tower.When the nut Angle θ>-60o,loose scale η>10%,the displacement of the top of the tower and the flange clearance width is sensitive to bolt looseness.(4)The wind tower integral model and flangeless integral model are used for modal analysis respectively,and the first 6 modes and natural frequencies are obtained.The influence of flange bolt looseness on natural frequency of wind tower is studied.Rayleigh damping parameters are obtained based on the analysis results of ambient pulsation test data.Transient dynamic analysis is carried out on the wind tower subjected to short-time gust.The influence of bolt loosening on the dynamic response characteristics of wind tower structure is studied,and the influence of structural damping on its dynamic characteristics is analyzed.The results show that the natural frequency of the tower changes significantly when nut Angle θ> 0o.The looseness of flange bolts leads to the increase of the displacement amplitude of the tower top,and the structural damping change of the wind tower tested in the field has little influence on its dynamic characteristics. |
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