摘要
雷电电磁脉冲对如今社会中各类电子设备、通讯系统及电力系统的危害越来越大。因此,了解雷电电磁场脉冲在空间的传播特性及其在输电线上的耦合问题对目前的雷电防护工作具有重要的理论和实现价值。首先,文章研究了复杂地表下雷电回击电磁场的传播特性,依据二维分形粗糙模型讨论了光滑和粗糙地表对回击电磁场的影响,并根据Scott提出的土壤模型讨论了土壤湿度引起的电参数变化对雷电回击电磁场的影响。然后,利用时域有限差分法(FDTD)和传输线的耦合模型,讨论了理想地表情况及地面电导率垂直和水平分层情况下,雷电回击电磁场在传输线路上的感应过电压特性。本文研究取得如下结果：
(1)当雷电水平电场沿粗糙陆地表面传播时,电场受地表面的粗糙程度影响很大,且随着粗糙度的增加,水平电场开始阶段的负峰值变小；而沿海-陆交接复杂地表传播时,水平电场受陆地部分粗糙度影响比较明显而受海浪起伏影响很小
(2)电磁场沿土壤湿度为0.2%和10%的地表传播时,土壤湿度为0.2%时对电磁场的影响更加明显,尤其是回击水平电场,其表现主要是水平电场的峰值与湿度成反比；而土壤湿度由0.2%增加到10%时,垂直电场和水平磁场的上升沿时间减小,对垂直电场和磁场的峰值影响不太明显。
(3)地表理想情况下,架空线上感应过电压值随着雷电回击速度、架空线高度的增加而增加,随架空线距雷击点的距离的增加而减小；雷电回击模型的变化对感应过电压的峰值影响不大,主要影响电压波形的上下沿时间,即对其衰减快慢有影响。
(4)电导率垂直分层情况时,感应过电压值受电导率较小段影响更明显,因为电导率越小,电磁场场衰减越大,尤其是对水平电场；电导率水平分层情况下,雷电感应过电压值主要受上层电导率的影响。
The damages of lightning electromagnetic impulse(LEMP)to the social electronic equipments, communication systems and power systems are getting worse. In consequence, looking into the LEMP's transmission property in space and the coupling problem on the transmission line of LEMP is very important in the theories and practical application of lighting protection. Firstly, the paper studies the propagation characteristic of lighting return stroke under complex surfaces, discussing the influence of smooth and rough surface on the return stoke electromagnetic fields based on two-dimensional fractal rough model, meanwhile, on the basis of the soil model proposed by Scott, the paper analyzes the influence of lightning return stroke electromagnetic fields caused by the change of the electrical parameter owing to the different soil moisture. Then, we take advantage of the method of FDTD and the transmission line coupling model to analyze the induced over-voltage characteristic of lightning return stroke electromagnetic field sensor on the transmission line in case of ideal surface and vertical and horizontal stratification of the ground conductivity. The conclusions are as follows:
(1)When the horizontal electric field from lightning spread along the rough land surface, it is of great influence to the electric field according to the surface roughness level. With the increase of the roughness, the negative peak in initial stage of horizontal electric field diminished; But when the horizontal electric filed from lighting spread along the complex surface in the land-sea connecting belt, the influence by the terrestrial part of the roughness is more obvious than the undulate of the waves as to horizontal electric field.
(2)When the electromagnetic field spread along the surface with0.2%and10%of the soil moisture, the impact of the former one(0.2%of the soil moisture) is more obvious to the electromagnetic field, especially to the horizontal electric field from lightning, which reflected in the peak of the horizontal electric field is inversely proportional to the soil moisture. When the soil moisture increased from0.2%to10%, the rise of the vertical electric field and the horizontal magnetic field decreases with time; the impact of the influence to the peak of the vertical electric and magnetic fields is not obvious.
(3) In case of an ideal ground, the induced overvoltage on overhead line increases with the increase of the speed of lightning return stroke and the altitude of the overvoltage on overhead line, decreases with the decrease of the distance from the overhead line to the point of strike. Different lightning return stroke models has little effect to the peak of the induced overvoltage, but mainly affect the rising and falling edge time of the voltage waveform, as that influences the attenuation speed.
(4)In the vertical stratified conductivity situation, Inductive over-voltage values are affected by the conductivity of the smaller segment is more obvious. For the sake of the smaller the conductivity is, the greater the electromagnetic field attenuation, especially to the horizontal electric field. In the horizontal stratified conductivity situation, the lightning induced over-voltage is mainly affected by the upper conductivity, moreover, when the upper thickness is greater than15meters, the lower conductivity of the impact of over-voltage is gradually increasing.
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