Cost effective design of grounding grid using ground rods — A case study

For safe, reliable and economic design of grounding grid, ground rods play a vital role. It is mandatory to use two separate and distinct connections with two different ground rods as per Indian Electricity Rule of section 61. Another purpose is to inject the grid current into deep low resistivity soil strata, in order to reduce ground potential rise (GPR), surface potential, grid resistance, and far more stringent safety criterion such as step and touch voltages. This paper presents grounding grid design of 400/220 KV, transmission substation with vertical ground rods. The soil resistivity is modeled as a two layer model with highly resistive top layer and bottom layer of low resistivity. The ground rods along with equally and unequally spaced horizontal grid conductors are analyzed using Autogrid Pro SES grounding software. The results show that use of ground rods with unequally spaced horizontal grid conductor is very sound and cost effective solution.