Study On Numerical Analysis And Simulation Experiment Of Frequency Characteristics Of Grounding Grid And Impulse Characteristic Of Grounding Electrode Of Tower

The grounding device in the power plant and substation are basic equipment that protect power lines and power apparatus from severe ground faults and lightning currents. The performance of grounding grid affects the safe of power apparatus and staff, and the impulse characteristics of grounding device directly affects the lightning protection performance of power transmission lines. The accurate analysis of the lightning transient performances and the frequency characteristics of grounding devices is the fundament of lightning protection of power system and optimization of the grounding electrode.Based on the quasi-static maxwell equation, a finite-element model which takes the displacement current in to account is presented to calculate the response of grounding devices under the fault current with lower frequency in this paper. In this work, open boundaries of earth environment are processed by introducing a spatial transformation; the ill conditioning of the matrix to be solved because of the marked rise in conductivity between ground and copper are resolved by the weak form of finite element method.Based on the full wave maxwell equation, a finite-element model which takes the displacement current and the induced current in to account is presented to calculate the response of grounding devices under the fault current with higher frequency in this paper. In this work, open boundaries of earth environment are processed by introducing a spatial transformation too, and the application process of spatial transformation in the vector finite element method is deduced in this work. In order to take the skin effect in the grounding electrode in to account in the vector finite element method, the exact impedance boundary condition is derived out by using the general boundary condition in electromagnetic theory, applying Maxwell equations and considering propagation properties of plane wave. And the exact impedance boundary is used in the interface of grounding conductor and the soil, so the function of grounding electrode on the dispersal of fault current in added in the finite element method, the domain of conductor is not participate in the calculation of finite element modeling at the same time. So the introduction of impedance boundary condition dercrease the amount of calculation. And the frequency characteristic of simple horizontal and vertical grounding rod under fault current with higher frequency is analyzed by the vector finite element modeling.Based on the study of grounding performance, this paper makes simulation experimental investigations on the impulse current dispersal regularity of grounding electrodes with various structures, and analyzes the effect of grounding electrode structure, amplitude and soil resistivity on the impulse current dispersal regularity of grounding electrodes.This paper presents a numerical method, combined the finite element method in the spatial domain with the finite difference time domain, to calculate the transient impulse response of grounding systems considering the ionization phenomenon. In this numerical method, space-time variable soil resistivity is used to simulate the soil ionization phenomenon where soil resistivity is controlled according to its relationship with the local instantaneous value of the electric field and no a priori hypothesis on the geometrical shape of the ionized region around the electrodes is necessary. The proposed numerical scheme is validated by comparing computed results with experimental results and simulation results in literature. Based on the measurement and simulation results, the impulse response regularity of grounding electrodes is discussed and the effect of ionization on human and installations safety is reported.According to the impulse current dispersing experimental results, the tower grounding reduction measures for power transmission line in regions with high soil resistivity are proposed. By using the finite element analysis method, the impulse resistance reduction principle of the star-shaped grounding electrode is analyzed. the simulation results show that the impulse leakage currents on different parts of conductor are balanced by adding spicules, and as a result, it can strengthen local electric field intensity in the space between soil particles and promote the ionization breakdown. Impulse grounding resistance measurement test results in the laboratory further verify the effect on reducing impulse grounding resistance of grounding device with spicules.