| The development of ultra-high voltage (UHV) electric power grid in China is imperative because it is an effective way to achieve energy distribution optimization in more extensive geographical regions, relieve tense contradiction between electric power supply and demand,reduce construction cost and raise transmission efficiency. However, with the enhancement of public awareness of environmental protection, the electromagnetic environment problem has become a critical restriction for UHV AC transmission project. In order to develop a resource-saving and environment-friendly UHV project based on our own actual national conditions, these are key issues in the prophase of the project to exactly forecast and evaluate the electromagnetic field intensity produced by UHV transmission lines, and propose the transmission lines optimization design. However, some previous methods for calculating the electromagnetic field of transmission lines are not appropriate for UHV any more, because there are distinct differences in the voltage level, the structure, the conductor type and the geographical and meteorological circumstances between extra- and ultra- high voltage transmission lines.This thesis focuses on developing theoretical models and arithmetics to study the power frequency electric field (PFEF) and the power frequency magnetic field (PFMF) under UHV transmission lines, the surface PFEF around bundled conductors and the radiated electromagnetic field of corona discharge. Moreover, the influence factors and protective measures of the electromagnetic field are explored with the aim to provide theoretical basis and reference for UHV transmission lines optimization design and eliminate the public worries.The main contributions of the thesis are as follows:①3D model for calculating the PFEF under UHV transmission lines is proposed on the basis of the character of overhead catenary. The analytical expressions are then derived by combining with charge simulation method (CSM) and the moment method (MOM). The results of simulating and measuring show that the 3D model improves the accuracy with respect of conventional 2D model. The influence of transmission lines'structural parameters and conductor types on the PEFE are analyzed and compared. Especially, the quantitative relationship between the length of span, the meteorological parameters and the electric field intensity are firstly founded. ②The line currents which cause the PFMF under transmission lines are determined by using the transmission line theory. Based on Biot-Savart law, the 3D model for the PFMF under UHV transmission lines with sag are established. The influence factors, such as meteorological conditions, configuration parameters, conductor types, etc, are systematically researched. The results are helpful for lower the PFMF.③Motivated by the main ideas of charge simulation method, the moment method and the successive image method, a modified model is established to calculate the surface PFEF around UHV bundled conductors with sag. The optimum location algorithm for simulating charges is adopted to improve the accuracy. The proposed method can truly describe the electric field intensity and its spatial distribution on the surface of each of the bundled conductors. The study results about the influence factors, such as meteorological conditions, configuration parameters, conductor types, etc, on the surface electric field would be used as reference for the UHV transmission lines optimization design to prevent the corona discharge.④A novel model for the corona radiated electromagnetic field is proposed by taking account of the time varying corona current pulses. Then approximately analytical expressions for the radiated electromagnetic field are derived based on antenna radiation model and uniform transmission line theory. The technique can be used analyze the transient process of the radiated electromagnetic field. The concept of radiated electromagnetic interference (EMI) level is defined and its influence factors also discussed and compared.⑤The shielding lines are effective to weaken the electric field intensity. The thesis analyzes the influence of the number of shielding lines and their position with respect to the transmission lines on the shielding effectiveness. On the other hand, the metal plane is usually used as shield for the PFMF. The algorithm for calculation the PFMF shielded by a finite width plate is presented. The simulation results show that the shielding effectiveness is dependent upon not only the physical properties and the dimension of finite width plate shields, but also the structure of the shields and transmission lines.
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