On The Mechanism Of Transient Response And Simulation Of Electromagnetic Field Associated With Lightning Striking To Tall Objects

In this paper, we establish the electromagnetic models and engineering models of lightning stroke a tall tower. By using finite-difference time-domain (FDTD) and electric dipole approach, we analyze the characteristic of current attenuation when current propagating along different shapes of towers. We exam different FCCFs (field-to-current conversion factors) for the initial peak current at the object top, the short-circuit current peak, the largest peak current at the object top and the peak current at the object bottom for the perfectly conducting ground. We also revised FCCFs by considering the propagation effect of finite conductivity on the electromagnetic field radiated by lightning to tall towers. The results of study mainly in the following three aspects:First, supported by three-dimensional finite difference time domain method(3D-FDTD), electromagnetic model of45cm-micro-towers is simulated in order to interpret the attenuation mechanism of lightning current transmitting in the tower. In applying perfectly conducting boundary conditions on tower-air interface, distribution of Gaussian pulse currents along macro-towers can be solved. It is shown that different shapes of towers have different effects on currents. Current pulses suffer almost no attenuation when they propagate downward from the apex of conical tower to its base, but attenuate significantly as they propagate from the base of the tower to its apex. When that excited at one of its extremities of a cylindrical tower, current pulses suffer apparent attenuation both from two directions. Current attenuation becomes more pronounced as thickness of cylindrical tower, the angle of conical tower increases, or the pulse width decreases.Second, on the basis of the transmission line representation of lightning channel and a vertically extended strike object (168m and553m), by using the traditional dipole technique (following the Lorentz condition), the polarity change of vertical electric field in the immediate vicinity is simulated, which is a specific signature of vertical electric field. It is shown that the change of polarity depends on the reflection coefficient at the base of the tower and vanishes when this coefficient is close to unity. Other parameters such as stroke wave front speed, the adopted return stroke model, the reflection coefficient at the top of tower make no difference on the change of polarity. Two theoretical explanations are given to account for such phenomenon. One is based on the general algorithm for calculating the electric field on perfectly conducting ground, the other is the simplified algorithm based on the superposition of field theory. The algorithm of the latter is inferred in the special case when the reflection coefficient at the top of tower is zero, stroke wave front speed is equal to the speed of light. An equation providing an estimation of the critical distance is derived.Finally, we have studied the accuracy FCCFs for currents inferred from far electromagnetic field produced by lightning striking to tall objects, considering the perfectly and finitely conducting ground, respectively. For the perfectly conducting ground, the different FCCFs for the initial peak current at the object top, the short-circuit current peak, the largest peak current at the object top and the peak current at the object bottom have different accuracy ranging from about underestimation of18%to overestimation of10%for the reflective coefficient at the bottom of tower pb=1.0and from underestimation of25%to overestimation of10%for Pb=0.7, and their accuracy decreases with the increase of current risetime RT. For the finite conductivity with0.01S/m and0.001S/m, FCCFs will cause much error if we do not take into account the propagation effect along the finitely conducting ground, and their errors obviously increase with the decrease of the conductivity. For example, when pb=1.0, the errors are about20%when the conductivity is0.01S/m while the errors are about55%when the conductivity is0.001S/m for lightning strike to the168-m-tall tower. Therefore, we revised FCCFs by considering the propagation effect of finite conductivity on the electromagnetic field radiated by lightning to tall towers, and found that our revised FCCFs have much better accuracy for the lossy ground.