| Global warming is a major challenge that the world has to face and needs to be solved urgently.The main reason is that humans overuse of fossil fuels leads to excessive carbon dioxide gas,and other gases produced by fuel combustion pollute the atmosphere and worsen the environment.Under this background,renewable power generation,such as wind power generation and photovoltaic power generation,has been continuously promoted and developed rapidly due to its environmental and economic characteristics.Because of intermittency and uncertainty of renewable energy generation,China has adopted the development mode of large-scale centralized and established a number of large power generation bases in order to make full use of sustainable energy.At the same time,in China,due to the obvious imbalance and regional differences in the energy distribution and power demand distribution of wind and solar energy,long-distance overhead transmission lines are usually used to transfer power from power generation to load center.Therefore,electrical power received by the main grid is not only restricted by the capacity of the power plant,but also by the capacity of the overhead transmission line.It is of great significance for the sustainable development of new energy generation and power system to study the influence of overhead transmission line capacity on the main grid.Traditionally,the capacity constraint of an overhead transmission line refers to the power or current value through the line.This value can be obtained based on conservative environmental factors such as wind speed,wind direction,ambient temperature and light intensity,which ignores dynamic changes of temperature in the calculation process.Nevertheless,the environmental factors in actual operation are usually far lower than conservative values,so that the transmission capacity of overhead lines can't be fully utilized and lines can't be used economically and efficiently.In order to fully exploit the transmission potential of lines,electro-thermal coupling takes temperature-resistance correlation as a link by introducing environmental factors into the transmission simulation of the line,so as to ensure safe operation of power system without exceeding the thermal stability limit.Although application of electro-thermal coupling theory can increase the transmission capacity of the line,it also causes a significant increase in the temperature of transmission lines.According to operating reliability theory of power components,annealing of high temperature conductors is the main reason for the loss of tensile strength of wires.The higher the temperature of overhead lines,the greater the loss of tensile strength after annealing.When the tensile strength loss achieves a certain level,service life of the conductors can be considered to be over,which is a gradual accumulation and irreversible process.In the theory of life-cycle cost,initial capital investment of transmission lines remains unchanged,however,the higher failure rate of overhead lines is,the huger operating and maintenance costs are.Electro-thermal coupling theory can improve the utilization efficiency of lines,enhance the ability to absorb renewable energy and increase the return on investment of lines.Nevertheless,it will shorten the service life of overhead lines and increase operation and maintenance costs.Therefore,a comprehensive and detailed assessment for average profits in terms of lines and the maximum allowable temperature is essential for the further application of electro-thermal coupling theory.According to this,the paper comprehensively considers electro-thermal coupling and life cycle cost,proposes a transmission line investment income and cost evaluation model based on the timing simulation of renewable energy generation and line current synchronization under the influence of various environmental factors.Thus,the maximum allowable value of operating temperature can be determined.The main work can be summarized as follows:The second chapter introduces linearly changing environmental variables into the thermo-dynamic equilibrium equation to simulate the temperature change process by continuous solution.And simulation analysis can provide data support for subsequent chaptersThe third chapter provides the expression of power flow of overhead lines considering electro-thermal coupling by combining heat balance equation and power flow equation.Based on the background of connection large-scale wind power with grid via long distanced overhead lines,capacity of wind assessment under the influence of various environmental factors is evaluated compared with the traditional thermal rating.Then,the amount of wind power abandon and network loss in terms of two thermal criteria are analyzed to confirm effectiveness of electro-thermal coupling theory in improving power transmission of overhead lines.The fourth chapter takes the large-scale wind farm connecting with grid via long distanced transmission as research background,allows for the synchronous effect of environmental factors on current carrying characteristics of renewable energy transmission lines and the degradation of their mechanical properties,combines theory of electro-thermal coordination with model of line aging failure and proposes an evaluation model of the maximum allowable temperature of overhead lines based on electro-thermal coupling and full life cycle cost,thus reveals the mechanism effect of operating temperature on overhead transmission lines under electro-thermal coupling theory. |
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