The Importance of Multi Layer Soil Model in Substation Grounding System Design

Soil characteristic is the most important parameter to determine the performance of a substation grounding system. A grounding system design that lacks proper characterization of the soil will not be successful. The success criterias for a grounding design are the Ground Potential Rise caused by the current flowing through the impedance of this earthing system during a short circuit, as well as the related touch and step voltages are below the allowable limits. The Wenner method is the most popular method in determining the soil apparent resistivity and the model of the soil structure. Switching from the data obtained from field measurements to the number of layers, layer thickness and resistivity of the layers is only possible with optimization-based computer software. Using the average of the measurement results taken from the field with the assumption that the soil has a homogeneous structure is used as an alternative method. However, finding a homogenous soil structure in the real world practical purposes is almost impossible. Therefore, this assumption will cause the design to be inaccurate in cases where the soil is not homogeneous. In this study, the importance of the layered soil model in the design of a substation grounding system will be presented with numerical data by analyzing a soil structure created in computer environment with the number of layers, thickness and resistivity controlled and the grounding system placed in this soil structure.