On the Impact of Soil Resistivity Measurement and Modelling on Grounding Performance

This study highlights the heterogeneity impact of fields on the grounding system modelling and the uncertainty of soil resistivity measurements/models caused by this reason. Using Wenner method, six sounding profiles are considered over a square shape area within the same site. Test arrangements and associated results, including soil equivalent models, are presented in order to discuss the soil heterogeneity aspect. The impact of this latter on grounding modelling is determined by simulations over a wide range of frequencies using HIFREQ module of CDEGS software. The results show that soil resistivity measured results can be different with respect to the selected profile, providing different equivalent models of the same soil. Grounding modelling process is affected by this variety of models in a dependent manner with the frequency range of the injected current. Additional analyses are also considered for a second site in a different location. The same conclusions are obtained - arbitrary selection of survey profile and/or soil equivalent model may significantly influence the grounding design, affecting the associated protective systems. Such findings lead to extending relevant standards to cover this aspect, especially for large-scale ground networks or grounding systems for surge arresters.

I. Introduction

Soil resistivity measurement and modelling represent a key step, not only in lightning protection system and grounding designs, but also in many other engineering applications such as transmission lines, corrosion and cathodic protection. Soil resistivity measurement and interpretation along with grounding analysis are a necessity for safety assessments for faults associated with power plants or electrical substations. Indeed, effective field measurements followed by an accurate modelling process of soil resistivity are crucial to provide an optimal solution, a safe design, or effective analysis and studies for the considered applications [1].