Simulation Of Electromagnetic Fields Due To Lightning Strikes To Tall Objects Based On FDTD Method

Lightning electromagnetic pulse(LEMP),as a random and high-power electromagnetic radiation source with wider influence,can cause serious interference or damage to transmission or communication cables and electronic and electrical systems.In order to optimize lightning protection effectively,it is essential to accurately estimate the lightning electromagnetic environment.Compared with ground or short objects,tall objects can attract lightning more easily,which provides a good radiation source for the study of lightning electromagnetic fields.So far,lots of experimental data has been obtained by the observation experiment due to lightning strikes to tall objects,which enhances our understanding on LEMP characteristics.However,because of the constraints of objective factors,there are still some laws which can not be obtained directly from the observation results.Some factors,such as the surrounding building environment,topography,the parameters of tall objects and soil conductivity,can influence lightning electromagnetic fields,and the effects of the above factors are difficult to assess directly by experimental observation.Therefore,employing electromagnetic simulation technique is still necessary for enriching and improving the propagation law of lightning electromagnetic fields in order to study the influence of these factors on lightning electromagnetic fields.In this paper,we accomplish the simulation research of electromagnetic fields due to lightning strikes to tall objects based on FDTD,and obtain following results:(1)The lightning electromagnetic simulation code of tall objects has been established based on 3D-FDTD method.The accuracy of the simulation code is tested by comparing our simulation results with the observation results and the simulation results in relevant literature.The electromagnetic simulation code can provide rich features.For the lightning current source,the simulation of lightning transient process can be accomplished for the case of tall objects with different heights.For the propagation path of LEMP,the simulation can be realized including horizontal or vertical stratification ground and the complex mixed path.For the building environment,different numbers of buildings with various sizes can be modeled.In theory,most of the numerical calculation about lightning electromagnetic fields can be solved.(2)The effect of the tall object parameters on the propagation characteristics of lightning electromagnetic fields is studied in this paper.By employing the simulation code established in this paper,the effects of the above parameters on the waveform characteristics and peak values of lightning vertical electric field,horizontal electric field and azimuthal magnetic field have been studied by adjusting the current reflection coefficients and the height of the object.The simulation results show that the variation of the current reflection coefficient at the top(or bottom)of the tall object can significantly affect the multi-peak characteristics and the peak values of lightning electromagnetic field waveforms.In addition,the lightning vertical electric field,horizontal electric field and azimuthal magnetic field are also sensitive to the height of tall objects,and the change of the height can significantly affect the peak values and the pulse width of lightning electromagnetic field waveforms.(3)The effects of horizontal stratified soil and land-sea mixed path on lightning electromagnetic fields have been studied.For the case of horizontal stratified soil,the change of the conductivity of the upper layer(or the lower layer)has a significant effect on the lightning horizontal electric field,when the thickness of the upper layer is constant.The horizontal stratified soil has a significant effect on the horizontal electric field and azimuthal magnetic field,when the thickness of the upper soil changes.For the case of land-sea mixed path,the effects of distance d between the observation point and wind turbine and distance dl between the observation point and coastline are investigated.Considering lightning strikes to the onshore wind turbine,the effect of mixed path on the azimuthal magnetic field and vertical electric field is obvious for a close distance d,while the effect on the horizontal electric field is obvious for a large distance d.For lightning strikes to the offshore floating wind turbine,no matter how d and dl change,the effect of land-sea mixed path on lightning electromagnetic fields is significant.(4)In order to understand the influence of urban buildings on the propagation process of lightning electromagnetic fields,the distribution law of lightning electromagnetic fields near non-isolated buildings is studied by adding several buildings.The results show that the vertical electric field can be significantly enhanced by the building for the field point(observation point)located at the top of the non-isolated building,and it can be reduced due to the shielding effect of other nearby buildings(shields)at the same time.The shielding effect of the shields mainly depends on their heights,and the dependence of shielding effect on the distance,relative distance and cross-section length of the shield is weaker than the height.The dependence on conductivity of the shield is weakest among all the factors.In addition,for the field point located at the ground surface near the non-isolated building,the horizontal electric field and vertical electric field are significantly affected,while the influence on azimuthal magnetic field is negligible,considering lightning strikes to flat ground or tall objects.