Research On Key Problems Of Self-Supporting Transmission Tower-Line Systems

With rapid economy development of China, demand of energy has been growing continuously, and more and more capital has been invested into power grid. However, accidents are caused frequently by weakness of power grid which should be attributed to insufficient cognition of working principle of transmission tower. Especially, a large number of towers collapsed because of freezing weather in early 2008. As a result, economic operation and routine life were severely affected. For self-supporting towers are important parts of transmission line system, their economy and security have vital impacts on whole system. Consequently, optimal design of tower can contribute significantly to both economic benefit and security of whole transmission line system. Furthermore, the distribution of resources in China is severly uneven. However, nearly two-third of the land in China is mountainous areas. Hence, more and more transmission lines have to cross mountainous area with continuous construction of power grid. Due to motion of wind in mountainous areas follows special law under impact of micro topography, response of transmission line system in mountainous areas has its own characteristics. Moreover, stress state of iced transmission line systems and their responses after ice shedding in mountainous areas are different from those in flat lands, which result in necessity to conduct deeply research. In this paper, four issues are studied:First of all, the optimal design method for self-supporting transmission tower is put forward, whose procedure is that the tower body form is selected intelligently firstly and then components'types of tower are chosen. The case base of towers is established with application of object-oriented technology. Then, the knowledge of tower body is mined out with case based reasoning technology and data mining. At the same time, the structural regulations of tower in the standard are merged into the knowledge base. So, the intelligent selection system of tower structural scheme based on case reasoning and data mining is explored. A 500kV self-supporting tower is optimized according to above method and a real model test is conducted. Comparison between optimal tower and original tower indicates that weight of optimal tower is decreased by 3% and its ultimate bearing capacity is increased by 19%. Statistical analysis results of 20 optimal towers indicates that the weight is average reduced by 4.7%, the components of the optimal tower are stressed more uniformly and bearing capacity of the tower is savely enhanced 12%.Secondly, the wind characteristics in mountainous areas are researched on. Mean velocity profile and turbulence characteristics of wind in mountainous areas are analyzed. And a comparison is made between the provisions about wind load in mountainous areas in load codes of China, the USA and Japan. The topography factors in the three codes are close to when the hill slope is relatively small. However, the topography factor in China load code is more conservative than USA and Japan as the hill slope increasing.The finite element model of transmission tower-line system(TLS) in mountainous areas is established and its dynamic characteristics are analyzed. Furthermore, the differences of TLS natural frequency with varying of the hill height and width are discussed. These studies verify that the TLS natural frequency at the same order decreases with the increase of hill height and width. The dynamic responses of TLS under wind load in mountainous areas with different roughness, wind velocity, hill hight, hill width and wind direction are presented. As the surface roughness increases, the wind induced vibration responses of TLS decreases slightly. The displacement at tower top, insulator offset, wire displacement, reaction force and unbalanced tension increase with the increase of wind velocity. The top displacement of tower on the hilltop is maximal. However, the offset of insulator which is suspended on the tower located on the hillside is maximal. The unbalanced tension to check the tower in mountainous areas in the code is relative small which is unsafe. The analysis shows that the vertical reaction force reaches the maximum when the wind direction is 60°.Finally, the ice coating and ice shedding model of TLS are established. The capacity of tower carrying ice load which situates on different positions of hill is analyzed. Non-uniform ice coating on wires has a great impact on the TLS capacity of wind resistance. The ability to resistant wind load of TLS decreases evidently when the thickness of ice coating on the wire is non-uniform. Moreover, the dynamic responses of TLS are discussed in different ice shedding situations, taking the different ice shedding span, hill height and non-uniform ice shedding into account. The larger the jumps of the ice shedding span and the adjacent spans are, the closer the ice shedding span is to hill foot. The unbalanced tension of tower locating on the hillside is larger when the ice shedding span is lower in both sides of the tower. The unbalanced tension of tower which locates on the hill summit is larger when the ice shedding span is close to hill foot.