| In recent years, HVDC projects have been developed rapidly in China. The related issues about HVDC technology, like the electromagnetic environment of transmission line and insulation problems of HVDC equipment, have become the focus of scientific researchers. Taking the key equipment of HVDC system such as transmission line, dc voltage divider and converter transformer as the research object, this essay studies steady state distribution and transient movement of space charge in the air medium and oilpaper insulation respectively. Based on the upstream finite element method (FEM), the paper solves Poisson's equation coupling with current continuity equation, and the simulation research on the distribution of space charge is achieved.When calculating the ionized field of HVDC transmission line by traditional upstream FEM, the ionized field will not converge in solving process if the initial value of wire's surface charge density or space charge density is set unreasonably. This paper introduces an adaptive iterative controlling factor, which can improve the stability of the algorithm, without affecting the computing speed. In addition, the influence of wind speed on the ionized field is studied in the actual transmission line project. Based on the improved upstream FEM, computing platform is achieved for the ionized field engineering calculation of the HVDC transmission line.On the basis of the two-dimensional (2-D) upstream FEM, three-dimensional (3-D) upstream FEM is achieved to calculate the ionized field. When finding the upstream element for updating the space charge density using the traditional upstream FEM, the nodes whose space charge density are known in the stack are compared one by one to determine whether they are in the same upstream element, which will cost a lot of time. A new method that only adjacent nodes rather than all the nodes in the stack are required to find the upstream element is introduced, which can improve the space charge density updating efficiency. A simple example is used to verify the space charge density updating efficiency when using the new method to find the upstream element, thus this method is the base for solving the3-D large scale ionized field. The ionized field around the building model which is placed near the DC experimental line section is calculated, and the result agrees well with the experimental data, which can improve the accuracy of the3-D upstream FEM when solving the ionized field around the complex structures. The ionized field of the UHVDC transmission line when considering the human body nearby on the corridor is calculated, and the conclusion is that the existence of the space charge will enhance the electric field around the human body significantly.The ionized field near the DC voltage divider is the research foundation of the external insulation problem and the structural optimization design of the divider.2-D axisymmetric upstream FEM is achieved to calculate the ionized field of the axisymmetric model. Transient upstream FEM is deduced which can be used in the simulation of the charge movement in the air medium. Space charge accumulation characteristics on the dielectric surface is proposed in this paper, as well as the steady-state criterion of the above transient process. Based on the2-D axisymmetric transient upstream FEM, and the accumulation theory, the needle plate electrode corona charge accumulation process on the dielectric surface is simulated. Surface charge density on the dielectric surface obtained by the simulation presents a bell-shaped distribution, which is consistent with experimental result. The ionized field distribution of the DC voltage divider is calculated, and the calculation result shows that the existence of the space charges around the divider can improve the electric field distribution.The simulation of charge movement inside a single-layer oil-paper insulation is realized by solving the transport equations of bipolar charges using transient upstream FEM. Through the adjustment of material microscopic parameters in the equations, simulation result which is consistent with the experimental data is reached. Based on the simulation parameters above, adjusting parameters such as the applied electric field strength, electron injection barriers, carrier mobility, trap capture coefficient, trap concentration, and recombination coefficient of positive and negative carriers, the influence of the applied electric field strength, temperature gradient, and material property on the movement characteristics of space charges inside the oil-paper insulation are investigated. The conclusion of the investigation can be applied to the research of breakdown mechanism of oil-paper insulation. |
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