Research On Thunderstorm Downburst's Wind Field Characteristics And Its Effect On Transmission Line

Downburst is a transient localized strong wind which is induced by a crash of the high altitude air flow onto ground,and is named due to the fact that it often occurs in thunderstorm weathers.Recently,the tower failures caused by downburst have been reported frequently,including some tower's together failures.Moreover,the site survey and weather report of the transmission line's wind-induced flashover,has revealed that downburst might be the main cause.Due to the yearly increase of the power system's construction scope,the lengthened spans will raise the possibility to encounter extreme weathers.Thus,it is necessary to develop the experimental research and theoretical analysis towards the downburst wind field,and study its action towards transmission line.It has a significant benefit for the planed lines'design optimization(including routine selection and weather condition combination,et.al)and the determination of completed lines' locally strengthened schemes.The paper's major works are given as follows:1.The experimental research of stationary downburst's three dimensional turbulence field and its simulation method:based upon the impinging jet model,an indoor jet tunnel was built.With a suitable point arrangement,the Cobra Probes were used to measure the stationary downburst's three dimensional turbulence field.Classical turbulence theory and mathematical statistics method were adopted to deal with the wind speed samples.The spatial distribution characteristics of the turbulence field's key parameters,including turbulence intensity,turbulence integral]ength,power spectrum,point coherence function,spatial coherence function,were concluded.With the use of least square method and genetic algorithm,some empirical formulas were fitted.On this basis,the fluctuating wind speed could be computed quickly with a proper random flow generation method.2.The experimental research of moving downburst's unsteady wind filed characteristics and its simulation method:a moving impinging jet device was used to simulate the moving downburst.With a similar test method with the former chapter,the moving downburst's unsteady wind speed time history is sampled.A triggered sampling system was built to obtain the mapping relation between the distance from the point to jet center and the sampling time,with the real-time monitored device's motion speed.Accordingly,the relationship between the wind speed and the point's location could be establishe.A suitable signal processing method was selected to deal with the unsteady wind speed.Then a long period time-varying mean part,as well as an unsteady high-frequency fluctuating part,could be separated reliably.The key parameters were fitted,and a practical model was raised for the time-varying mean part.The fluctuating part was uniformly modulated to be changed into a nearly-steady signal with its statistic characteristics and power spectral density function well concluded.In the basis,a practical simulation method for the moving downburst was developed,whose results will be compared with the simulation results from the classical method.3.Study of the downburst-induced conductor's dynamic swing and unbalanced tension:with the consideration of the linkage mechanism between adjacent spans,a precise finite model of 8-span transmission line section was established,including tension insulator string,suspension insulator string and conductors.Conductor is a long-span flexible structure,and a discussion towards its fundamental frequency and vibration mode under different load distributions was raised.Based upon the quasi-steady assumption,the conductor's wind load was simulated with the models raised in the earlier part.Then a nonlinear transient finite element analysis was held to study the transmission line's wind-induced swing and unbalanced tension.Meanwhile,a parametric analysis was improved.For the stationary downburst,the main variables were the storm parameters,such as jet diameter and jet velocity,and the relative location between the transmission line and downburst;for the moving downburst,the focus might be on the motion path.Eventually,the most unfavorable condition for either wind-induced swing or unbalanced tension was given,which will be beneficial for the design.4.Calculation method study of transmission line's downburst induced design wind load:based upon the most unfavorable conditions raised in the earlier part,the chapter provides a simplified calculation method for the transmission line's downburst-induced load.According to the practical representation meanings of the transmission line's wind load calculation parameters in the current codes,the formulas of all the mentioned parameters,including the 3-sec gust wind speed at 10-meter,wind pressure height coefficient,wind pressure uneven factor and wind load adjustment factor,were given under downburst.Meanwhile,a compensation factor was presented to take the wind angle into consideration.5.Study of the tower-line system's downburst-induced response:the finite model of a long arm tower with four-circuit lines was built,together with two spans of line.Based upon the recommended working conditions in the current tower design code and the load translated from conductors,some necessary calculation conditions were selected to analyse the tower-line system's wind-induced response and the maximum response is obtained.Then the contribution of the string wind load to the tower's wind-induced response was mainly discussed.