Analysis of the Coupling of Overhead Lines Excited by Indirect Lightning Considering Dispersive Soil Water Percentage

This article proposes a method to analyze the coupling of overhead lines excited by indirect lightning above the dispersive soil, considering the change of soil water percentage. The whole process is achieved through analytical equations. First, the two-dimensional finite-difference time-domain method with convolution perfectly matched layer boundary based on the Debye representation of frequency-dependent ground is used to calculate the lightning electromagnetic field. In the frequency domain, the vector fitting method is introduced to deal with the frequency-dependent part of the governing equations. The recursive convolution is then combined with the integration by parts to process the convolution part of the governing equations of overhead lines in the time domain, enhancing the robustness of the solution. The resulting iterative equation is in turn derived, allowing to solve the problem at hand, for line loads defined by either R–C parallel or R–L series loads. The results highlight that, with respect to purely resistive loads, inductive loads increase the coupling voltage and reduce the coupling current, whereas capacitive loads do the opposite. In addition, the coupling value is verified to be more sensitive to changes in water percentage at low soil water quantity.

I. Introduction

Direct lightning is a visible hazard to the power system, which has a very strict protection system [1]. However, the damage caused by indirect lightning to electrical systems is difficult to detect, as indirect lightning may induce voltage or current on surrounding conductive objects. The indirect lightning has abundant harmonic energy in the frequency band whose wavelength is comparable to the length of the overhead lines, which are usually on the order of kilometers [2]. Lightning and overhead lines have a large coupling area, and the induced voltage or current can pose potential risks to terminal loads. Therefore, accurate calculation and evaluation of the coupling effect of indirect lightning and overhead lines have positive effects on power system protection.