Time-Domain Modeling of Grounding Systems’ Impulse Response Incorporating Nonlinear and Frequency-Dependent Aspects

At lightning discharge, the grounding systems' transient behavior is influenced by time-dependent nonlinear behavior and frequency-dependent effects. These effects are related to complex phenomena arising from the soil ionization surrounding the conductors at high current and to the fast risetime of the impulse current, respectively. Most of the performed investigations were either focused on soil ionization effects or frequency-dependent effects. This paper proposes a model that incorporates both aspects including the mutual coupling and air-soil interface effects, giving a more realistic representation of the grounding systems behavior. Based on the transmission line approach, the model can compute the grounding systems transient response buried in uniform and nonuniform soil. The model is applied to horizontal, vertical electrodes, and grounding grid subjected to impulse currents of variable magnitudes and shapes. The model results are validated against published measurement results obtained with low-current and high-current impulses. The time-domain transient potential and transient impedances show good agreement with experimental results. Other impulse impedance parameters used for the grounding systems analysis are reproduced quite accurately. As an application, the impulse impedance of a horizontal electrode and a grounding grid buried in two-layer soil is discussed when the upper layer depth and the resistivity of soil layers are varied.

I. Introduction

The use of grounding systems of various geometries such as grid, vertical and horizontal conductors, and/or a combination of these in electrical systems, is essential to ensure the safety of people and equipment. They are designed to provide a low impedance path for fault currents or transient currents flowing into the ground [1]. Under transient high-frequency and impulse currents, such as those caused by switching or lightning strokes, the response of grounding systems is significantly different from their response under steady-state low-frequency currents [2]. The reason is that, under transient conditions, their behavior is affected by numerous factors such as the magnitude and shape of the impulse current, and the frequency-dependent soil parameters, in addition to the structure of the soil layers [3].