Impulse Characteristic Of Grounding Devices And Its Improvement Measures

As for the impulse characteristics of the grounding devices,impulse grounding resistance is a key factor for evaluating the electrical performance of grounding devices.To calculate the impulse grounding resistance,it is necessary to analyze the soil structure where the grounding devices are buried and the geometry of the grounding electrodes.In the existing researches,the measurement of the soil structure is carried out mainly at the power frequency,which only provides the constant soil resistivity.However,both actual measurement and simulation show that the soil parameters are variable due to the wider frequency spectrum of lightning current dissipated in the soil.The soil parameters in this case are not constant values,but will vary with the frequency of the injection current.Moreover,the existing calculations for grounding electrodes are mainly focused on the study of cylindrical geometry,but now more and more grounding electrodes use the flat or other shapes.Therefore,it is of great practical significance to study the influence of the frequency-dependent characteristics of soil parameters and the electrical performance of grounding devices.With the above situation in the view,this paper intends to carry out systematic research on experiment and modeling calculation,which are specifically implemented in the following aspects.An experimental system of frequency-dependent characteristic of soil parameters is established,including variable frequency power supply,grounding electrodes and measurement modules,which can measure soil parameters at different frequencies.The moment method is used to analyze the interference between the measuring leads at high frequencies,and the influence of capacitance and inductance in the system on the measurement results is analyzed,and different influencing factors in the actual wiring are also considered.Experiments have shown that soil parameters exhibit significant frequency-dependent characteristics,which is the main difference from the traditional soil parameter measurements.Based on the measured soil parameters,a program is written using MATLAB to invert the actual soil structure.The measured and inverted structures show that as the frequency increases,the soil resistivity and relative permittivity will decrease in varying degrees.According to the existing type of grounding electrodes,the theoretical impedance frequency characteristics of the cylindrical grounding electrodes are first calculated,and then the corresponding electromagnetic field simulation model is built.The model can calculate the skin effect by discretizing the space,and then evaluate the impedance frequency characteristics of different grounding electrodes.Compared with the theoretical calculation model in this paper,the accuracy of the model is verified.The calculated results show that the flat strip grounding electrodes has a larger dissipating area and a relatively smaller grounding resistance than the cylindrical ones with the same cross-sectional area.The impulse characteristics of grounding devices in substations and line towers are analyzed.The impulse characteristics and optimization measures of independent lightning rod and frame lightning rod are studied,respectively.Two new methods for resistance reduction of the tower grounding devices are proposed,and the feasibility of the two methods is analyzed.Finally,based on the outdoor grounding grid,the impulse resistance,the potential distribution and the current-dissipation at each point of the grounding grid are analyzed,which reveals that the steeper the waveforms,the greater the impulse grounding resistances.