Seasonal variation of soil resistivity and corrective factor for optimal substation earth grid design in Eastern Cape

Abstract

Optimal Substation Earth Grid Design is a vital aspect of the electrical power system protection. The Seasonal Variation of Soil Resistivity mostly influences Earth Grid Design. Due to seasonal changes, there is an annual variation in the Soil Resistivity with no known correction factors that can be utilized. This has led to a need to ascertain annual 'Seasonal Soil Resistivity Correction Factors' for utility application. The objective of this paper is to develop seasonal Soil Resistivity Correction Factors for Optimal Substation Earth Grid Design in a power distribution substation focusing on meeting the minimum safety requirements, which are the step and touch potentials. A case study site was used for this study in the Eastern Cape region based on a study carried out in the Gauteng province. Results obtained show that in winter, (June/July), the highest soil resistivity was reached and lowest value obtained in autumn season (March). The upper soil layer resistivity noticeably varies more than the lower layer. The corrective factors for the upper layer with probe spacing (0.5m-1m) is multiplied by 1.16; for the second upper layer probe spacing from (2m-5m) is multiplied by 1.02; while the lower layer with probe spacing (5m-50m) is multiplied by 1.01. The corrective factors are focused on July, which is the highest, for an Optimal Substation Earth Grid Design.