Random Modeling Of Typhoon Wind Loads And Wind-resistant Reliability Assessment Of Electric Towers And Poles

Transmission and distribution lines used to transmit the electrical energy are important parts of the power networks,the important lifeline infrastructure.The transmission towers and poles are tall and flexible structures,which are sensitive to dynamic wind load,and damages to them by strong wind are the main reason for widespread power outages.Typhoons frequently attack the southeast coast of China and are the main cause of damages to the electric power structures in this region.Therefore,the development of theories for quantifying the typhoon risk of power networks can serve as the basis for improving the wind-resistance design of electric power structures,and also provide the technical support for the regional risk management and early warning of the typhoon wind hazard.However,the development of typhoons is consequence of many factors,the wind field near the ground is complicated,and wind loads have strong uncertainty.The design and risk analysis of electric power structures under typhoon conditions thus should be based on the probabilistic theory framework.In this study,based on the typhoon meteorology,aerodynamics,statistics and structural reliability theory,the random wind load models for transmission conductors and towers under typhoons are established,the wind-resistant reliability of electric towers are systematically studied,and the inherent reliability of transmission towers designed in accordance with the China design codes for electric towers is calibrated.Meanwhile,the risk assessment method for typhoon wind damage assessment of transmission networks is developed.The outcomes of this thesis provide scientific basis and tools for the optimization of the wind-resistant design of electric towers and the decision-making for disaster prevention and mitigation of power networks.The main contents are as follows:Firstly,taking the southeast coastal region of China as the study area,the probabilistic models of mean wind loads of typhoons are established.The models are developed by introducing the randomness of the yearly extreme wind speed of typhoons,the typhoon wind fields and the wind loads.The probabilistic distribution of yearly extreme wind speeds for typhoons is obtained by statistical analysis of10,000-years wind speed samples generated by the full track synthetic tropical cyclone model,and the fitted probabilistic distributions are tested by the K-S test.Then,the probabilistic models of turbulence profiles of typhoons are developed.By statistical analysis of the collected typhoon measurements from published literatures,probabilistic models of the three-dimensional turbulence intensity and turbulence integral scale profiles are established using the first-order second-moment method.The results show that the mean of typhoon probabilistic turbulence intensity profiles is generally greater than the recommendation in current codes,and the mean of typhoon probabilistic turbulence integral scale profiles is nearly consistent with the suggested value by codes.In addition,the recommended turbulence profiles in codes almost fall within the one standard deviation of the developed probabilistic profiles.The turbulence characteristics of typhoon winds are more stochastic than that of synoptic winds.Compared with the existing probabilistic turbulence models,the developed models for typhoon turbulence characteristics demonstrate that the turbulence intensity decreases with the increase of mean wind speed,and the turbulence integral scale increases with the increase of mean wind speed.Based on the developed probabilistic models of the typhoon mean wind load and the typhoon turbulence characteristics,probabilistic models of the equivalent static wind loads(ESWLs)of transmission conductors and towers are developed.The aeroelastic wind tunnel tests of transmission conductors are conducted.The tests confirm that the dynamic wind loads of conductors under strong winds are mainly contributed by the background responses.In particular,the tests examine the reduction of dynamic responses caused by the shielding and the asynchronous pulsation of different conductors.As for the wind loads on transmission towers,improved methods for determining the global drag coefficient,the skewed wind load factor and the wind load distribution factor are proposed based on the wind tunnel tests on tower section models.Furthermore,the wind tunnel tests on the rigid tower model are performed to determine the distribution of fluctuating wind loads on the transmission tower,and the gust response factor of the transmission tower is identified by dynamical analysis using FEM.By introducing the randomness of mean wind loads and turbulence characteristics,the random models of ESWLs on transmission conductors and towers under typhoon winds are developed.The developed random models consider the randomness of mean winds,the random process of fluctuating winds and its dynamic effects on structures,and the randomness of parameters used to characterize the fluctuating winds.In the design reference period of 50 years,the maximum ESWLs on transmission conductors and towers under typhoon winds follow the generalized extreme value distribution,and their mean values and standard deviations are significantly greater than those under synoptic winds.The mean of the maximum ESWLs under synoptic winds is close to the standard wind loads recommended by the China load code for overhead transmission lines(i.e.,DL/T 5551-2018).The random models of ESWLs on transmission conductors and towers developed for East China(represented by Shanghai)and South China(represented by Shenzhen),are applied to examine the component reliability of transmission towers designed by the current China codes for electric towers.Further,a typical 220 k V transmission line is selected to check the reliability of actual transmission towers under typhoon wind loads using the moment method.The component reliability of the actual transmission tower is significantly greater than that implied in the design limit state formula by the China design codes.Taking the target reliability index of structures when they assume the ductile failure of category 2 and 3(called the secondary and tertiary safety level),as defined by the reliability standard for building structures(i.e.,GB 50068-2018)for comparison,it shows that the component reliability of the 220 k V transmission tower is lower than the tertiary safety level at 0° and 30° wind directions,exceeds the tertiary safety level at 60° wind direction,and generally reaches and exceeds the secondary safety level at 90° wind direction.Finally,the fragility models of electric towers are obtained by the structural reliability analysis when the random models of ESWLs of transmission condutors and towers under typhoon winds are applied.Taking the 10 k V distribution networks in the coastal region of Guangdong,China as an example,the typhoon risk assessment method for power networks is established by combining the geographical location of power networks,the structural geometries and the numerical modeling of typhoon wind field.The method is validated using three typhoon cases which cause damages to the distribution networks.Comparison with the post-disaster survey shows that the estimated number of failed poles by the proposed method is within 20% deviation from damage survey for most of the severely damaged counties.