Influence Study Of Soil Nonlinear Breakdown Effect On Lightning Impulse Characteristics Of Grounding Grid

Grounding grid is the discharging channels of lightning current, and its dispersing ability under the impulse current determines the lightning protection level of grounding grid.Therefore, it is of great significance for the reasonable design of lightning protection performance to study impulse characteristic of grounding grid under the impulse current. The soil nonlinear breakdown phenomenon of grounding grid in the process of impulse dispersing is the important factor for the accuracy study of impulse characteristics of grounding grids,which is in connection with the structure size of grounding grid, soil conductivity and the impulse current waveform and so on.Hence, based on soil nonlinear ionization model proposed by L-D(Liew and Darveniza), this paper uses three-dimensional finite-difference time-domain method(3-D FDTD) to develop grounding electrode grid simulation model and then it is easy to analyze the impulse dispersing rules of grounding grid from the structure size of grounding grid, soil conductivity and the impulse current waveform. The main conclusions are as follow:(1) Firstly, the influence of soil nonlinear breakdown effect on the dispersing characteristics of single vertical grounding electrode is studied .The results indicate that the vertical grounding electrode has the obvious transverse and longitudinal direction discharging characteristics, and the transverse discharging radius of vertical grounding electrode is roughly 0.5m and 1.2m and the longitudinal discharging radius along the grounding electrode top is roughly 0.3 m and 0.9 m, for the soil electric conductivities in low frequency of 0.02 S/m and 0.005 S/m. The ground potential rise is closely related with the current wave front time and peak value and the soil electric conductivity; When considering the soil nonlinear breakdown effect ,the smaller the soil conductivity is ,the greater the current magnitude is and the current wave head of time is, the bigger the ground potential rise is.(2) Secondly, the influence of soil nonlinear breakdown effect on the dynamic dispersion of the grounding grid is studied.The results indicate that 1) The smaller the soil conductivity is and the smaller the grounding grid area is,the more obvious the "edge effect" and the "end effect" of the grounding grid is, the larger the transient ground potential and the bigger the transient impulse impedance is; 2) The larger the current amplitude is, the more obvious the "edge effect" and the "end effect" of the grounding grid is, the larger the transient ground potential and the smaller the transient impulse impedance is;3) The transient ground potential and impulse impedance when considering the soil breakdown effect is smaller than that without considering it, especially in the situation where the smaller the soil conductivity and the smaller the grounding area and the lager the current amplitude is, the greater the difference of ground potential rise and transient impulse impedance are.(3) Then this paper study the relationship of the soil electrical conductivity, the current amplitude , the grounding grid area with the surface potential and the step voltage of grounding grid.The results show that the smaller the soil conductivity is the larger the amplitude of current is, the smaller grounding area is, the greater the step voltage is, the larger the peak value of surface potential, the peak value of surface potential decreases quickly with the increasing distance r. Therefore, in the study of the impulse characteristics of the grounding grid, the influence of the nonlinear breakdown effect of the soil can not be ignored. The research methods and results have certain theoretical significance for the design of the grounding grid.