Research On Wind-induced Dynamic Response Of Transmission Lines Under Tornado

Tornado is a funnel-shaped vortex caused by strong air convection in extremely unstable weather,and has the characteristics of locality,strong destructiveness and high wind speed.Transmission lines are tall and flexible structure with the characteristics of low stiffness and low damping,and are particularly sensitive to wind loads.The characteristics of tornado wind field are different from those of conventional strong wind in the atmospheric boundary layer.As high-intensity wind,it is very easy to cause failures of transmission line and even serious damage and collapse of the tower.As a result,Investigation on the mechanism of wind-induced damage of transmission lines under tornado is quite necessary.In view of that the current research at home and abroad mainly concentrates on the structural wind effect of the tower.However,the research on the law of wind-induced dynamic response of the lines is rare.For that,a parametric analysis for tornado-induced dynamic response of the lines is carried out in this paper.The main research conducted is as follows:(1)Based on the available literature,a comprehensive summary of the existing tornado wind field analytical or empirical models is presented.In conjunction with the conclusions of others,the characteristics and wind speed profiles of each tornado wind field model were compared and analyzed.The Baker-Sterling analytical model was finally selected,which is used to analyze the wind-induced dynamic response of the lines under tornado.Furthermore,the main parameters of wind field were discussed,and the range of the corresponding parameters of value is given,which lays the foundation for subsequent research.(2)Taking the six-span transmission line-insulator system as a typical object in the strain section,the three-dimensional spatial wind field of tornado was established based on Baker-Sterling analytical model,and the quasi-steady wind load distribution characteristics of the lines under moving tornado were investigated.By the nonlinear analytical method and finite element numerical method,the swing deflection displacement of insulator and reaction of support on the tower under tornado are investigated.By analyzing the time history and amplitude,the results calculated by analytical and finite element method are compared,which verifies the validity of the analytical method.In the meantime,the influence of vertical wind speed and moving wind speed of tornado on the response of the lines are investigated.The results indicate that the vertical wind speed has less influence on the horizontal response and can be disregarded,but the moving wind speed has a significant effect and should be taken into account.(3)Using the validated analytical method above and by changing the impact angle and the impact tower,the changes of wind field parameters including swirl ratio,vortex core radius and structural parameters including the number of span,insulator length,suspension height and span on the law of the longitudinal and transverse swing displacement of insulators and reaction of supports mainly concerned in wind-resistant design of transmission lines are investigated.At the same time,the most unfavorable position of tornado and tower,the most unfavorable impact angle and impact tower corresponding to the maximum value are determined.Based on this,the three spans of the tornado design wind speed profiles perpendicular to the lines in the horizontal direction are developed.The span reduction factor is calculated,and it is found that it is smaller than that calculated in the ASCE code.The parametric analysis carried out in this paper for the multi-span transmission line-insulator system provides essential databases and reference for developing tornado-resistant design of transmission lines.