Optimization of AC Substation Ground Grid Design and Calculations

The most important purpose of a substation ground grid is to ensure personnel safety, hence a proper design is key. Many IEEE standards provide guidance in ground grid design such as IEEE Std 80 [1], IEEE Std 367 [2] and IEEE Std 142 [3]. Computerized software is available to model ground grids and soil layers using IEEE Std 80 as a guide. While software saves designers significant time compared to hand calculations, they may result in unrealistic values. For example, modeling of ground grids in bedrock without applying a compilation of many grounding standards can result in the ground potential rise exceeding the system voltage and an impractically high touch potential. This paper provides guidance in applying an amalgamation of IEEE Grounding Standards, along with engineering judgement, to achieve realistic models. Application of proper soil models, surface layers, Ufer grounds and split factors will achieve realistic results and a calculated ground grid resistivity that will closely reflect the measured value. The paper will also discuss modeling existing ground grids where design drawings are not available. The applicability of this paper would extend to end users, such as the oil & gas industry, and does not apply to the Utility industry.