A Theoretical Method For Quasi-static Response Of Transmission Lines Under The Moving Downburst

Downburst is a local extremely strong downdraft that causes catastrophic winds on or near the ground.Transmission lines have the characteristics of large lateral flexibility,small structural damping and sensitivity to wind load.Compared with strong wind in the atmospheric boundary layer,downburst tends to cause the collapse of transmission lines in inland areas.Therefore,it is urgent to study the wind-vibration response and load effect of transmission lines under downburst.Considering the existing field measurements,wind tunnel tests and numerical simulations,it is shown that the downburst fluctuating wind response is dominated by the background component,and the resonance component can be neglected.Therefore,this paper mainly focusses on quasi-static(background)response analysis on the dynamic response of transmission line under the moving downburst wind.This paper mainly conducts the following research:(1)Firstly,the computational fluid dynamics(CFD)numerical simulation of downburst flow field is carried out by using the Reynolds averaging method,and compared with the existing mathematical model of downburst average wind,the most appropriate theoretical model of mean wind is selected.Then,the theoretical model of fluctuating wind is determined.Considering the moving wind speed,the moving downburst flow field of transmission line is generated by vector synthesis.Using the quasi-stationary assumption,the moving downburst load of the transmission line is determined.(2)Based on the static equilibrium equation and deformation coordination equation of transmission line structure,the mechanical model of hinged transmission line and transmission line-insulator system under arbitrary wind load is derived.Furthermore,the nonlinear equations of the transmission line-insulator system under downburst are derived theoretically,the responses of the reaction forces and the vibration displacement of the insulators are obtained by solving the nonlinear equations using the iterative method,the precision and efficiency of the results by different strategy for the iterative methods and initial value are investigated.The results of the proposed theoretical framework in this paper are valid by the comparison of the results with those of the existing numerical algorithm and the nonlinear finite element method(FEM)in the time history,extreme value and wind vibration coefficient.(3)On the basis of the above nonlinear theoretical methods,the linearized theoretical equations of the transmission line-insulator system under the time-varying mean wind of the downburst are derived.Compared with the nonlinear solution and the finite element calculation results,the accuracy of the linear solution is investigated.Then,the influence line function of the longitudinal tension,longitudinal displacement and other responses of the insulators in the transmission line-insulator system under the fluctuating wind(concentrated force)are derived and analyzed.The quasi-static response of transmission line-insulator system under fluctuating wind load is calculated by means of the influence line function,and compared with the results of finite element calculation,the accuracy of the influence line solution method in calculating the fluctuating wind vibration response of transmission line is verified.In this thesis,a theoretical framework for calculating the wind-induced vibration response of transmission lines under the moving downburst wind is systematically established from three aspects: nonlinear method,linear method of time-varying average wind response,and quasi-static method of fluctuating wind response.