闪电脉冲电磁场在地下空间的分布特性

摘要

为了研究闪电脉冲电磁场(LEMP)在地下传播的过程与空间分布特性,本文采用一种改进的时域有限差分法(FDTD)计算了LEMP在整个地下空间的传播过程.与以往发表的研究只局限在个别观察点的讨论不同,本文通过计算得到了空间中所有网格位置上的电磁场、时间导数、功率密度和能量密度及它们的峰值,并将它们按照空间坐标表示在相应的分布彩图中.发现在靠近地表和远离闪电通道的区域内,LEMP的各个分量同时在平行和垂直地面的两个方向上呈指数型衰减.其中磁场与水平电场的峰值在整个空间极性统一,且呈现类双指数形分布.而垂直电场在地下的峰值分为极性相反的两个区域,并且靠近闪电通道的区域呈类球状分布.其他特性参量也有类似以上的空间分布特点.并且本文通过对不同大地电导率、电容率、放电通道模型和基电流情况的计算,发现虽然在个别情况下LEMP在地下空间的数值整体增大,但整体分布特征并未改变.即使是在水平或垂直分层大地的情况下,同一层内部仍然具有相同的分布规律.甚至是在击中避雷针情况下,LEMP在地下空间的分布特点也仍然未变.而这些分布规律和特征的发现和利用,将为地下管线、设备、线缆的电磁防护设计与位置规划提供重要的支持.%In order to repeat the propagation and its distribution characteristics, the lightning electromagnetic field (LEMP) in the entire space is calculated by a modified finite difference time-domain (FDTD) approach in this paper. Different from issued results, in which the electromagnetic field operates only at some discrete points near the ground, the LEMP and its time derivatives, power density and energy density at each unit in the whole space under the ground are calculated and expressed as groups of pictures. We find LEMPs attenuate exponentially in the horizontal and vertical direction, in the area near the ground and far from the discharge channel. The peak values of horizontal electric field and azimuthal magnetic field each have an unaltered polarity and their contours are similar to double exponential functions in the entire space under the ground. However the peak values of vertical electric field can be divided into two opposite-polarity parts in the whole area under the ground, and the contours in the area near the strike channel are spherical in shape. The other components have similar features. We also calculate the LEMPs with different values of ground conductivity, ground permittivity, return model and base current, and the characteristics of distribution are not changed, although the values are enhanced integrally in some cases. And in the layered earth, the LEMP has similar contours inside the layers. Even in the case of striking to the lightning rod, the characteristics of distribution are not changed. These patterns and distribution characteristics of LEMP can provide support and reference in shielding design and route planning under the ground.