Study On Simulation Experiment And Finite Element Method On Impulse Characteristics Of Grounding Device

Power system requires low grounding resistances to provide protection for the safe operation of equipment, and the impulse characteristics of grounding device directly affects the lightning protection performance of power transmission lines. It is very important to build up accurate simulation models and carry out experimental research on the impulse characteristics of grounding devices for the safety and reliability operation of the power system and the personal safety of the staff. At present, the shortcomings of the research on impulse characteristics of grounding devices are as follows: little literature proposes effective method to successfully measure the distribution regulation of leakage current, while this regulation is of great meaning to theoretical analysis of impulse characteristics of grounding devices; on the other hand, the nonlinear characteristics of the soil under impulse current has not been taken into account accurately, especially the irregular distributed spark area in the soil formed when soil ionization breakdown happens usually is ignored. Therefore, it is necessary to carry out experiments to analyze the distribution regulation of impulse current on the grounding devices and to propose new method to consider the nonlinear characteristics of soil and its breakdown ionization.Based on the analysis of the literatures on experimental study and theoretical analysis on the impulse characteristics of grounding devices, in this paper, simulation experiment is proposed to analyze the impulse current distribution rules on the grounding devices and based on the finite element analysis method, the finite iterative calculation analysis method on grounding device impulse characteristics is put forward. The main conclusions obtained in this paper are:â‘ Based on electromagnetic field theory and the similarity theory, the relationships of the relevant physical quantities in the laboratory impulse grounding flow simulation test are derived and the simulation test platform is designed and established in laboratory;â‘¡On the test platform, a lot of measurements are performed. The axial current distribution in grounding devices is gained and the distribution regulations of impulse leakage current on different structure grounding devices are analyzed. The impact of several factors, such as, the injected impulse current amplitude, soil resistivity, current injection position and buried depth is taken into account. The experimental results show that: the impulse leakage current distribution on a single horizontal electrode is uneven and the nouniform extent increases with soil resistivity and inject current amplitude further; the leakage current on the limited horizontal radiation type grounding device increases with the distance from the measured point to the injection point and the trend becomes more obvious when the branch conductor number increases around the current injection point because of shielding effect; the leakage current distribution on each branch conductor of the four mesh grounding grid displays that the middle part of the conductor disperses more impulse current than the end parts; the leakage current on the end parts of vertical grounding branch conductor increases with the soil resistivity and parallel branch number of conductor.â‘¢Based on the finite element method and combined with the finite analysis software, by setting the unit soil resistivity as a function of its unit electric field strength to take soil nonlinearity and its ionization breakdown characteristics into consideration, the finite iterative calculation analysis method on grounding device impulse characteristics is developed. The simulation results of this method are more consistent with the actual physical impulse current dispersing process in soil. Using this method to analyze impulse characteristics of single vertical grounding electrodes, it shows that the method can be convenient to get relationship of the grounding transient resistance changing with time. According to impulse test conditions, a single horizontal electrode is simulation computed and the impulse current dispersing mechanism is analyzed from the view of electric field strength distribution.â‘£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.