Design of optimal grounding mats for high voltage substation

Earthing is essential for electric supply system to ensure safety and proper operation. An effective earthing system depends on various factors like resistivity of surface layer of soil, duration and magnitude of fault current, maximum safe current that a human body can tolerate and the permissible earth potential rise that may take place due to fault current. There are basically six grounding systems in use. The soil resistivity can be determined with the well known three electrode method. An electrolytic tank of dimension 1×1×0.5m is fabricated in this department to experimentally compare the results with analytical values. There are empirical formulae given [1] to calculate the ground resistance at power frequency and under impulse current injection. The analytical values for the step and touch potentials for 220KV & 400KV are given in table II for grid size of 2m for a mat size of 60m×60m and 100m×100m. This data is used for optimal sizing of the grounding grid. The performance of this grid is then studied under impulse current injection using the method described in [2]. Based on the results, the discussions and conclusions are reported.