An Innovative Approach for Optimization of Earth Mat Design Utilizing Geophysical Resistivity Imaging: A Case Study

Large capacity power plants are now being designed for meeting increasing energy demand and operational scale economy. These plants have large size grounding system and therefore optimization of safe earth mat is essential for economic design. Earth mat of a power plant has two objectives: to carry electric current into earth under normal and fault conditions without exceeding operating and equipment limits and to assure that a person in the vicinity of grounded facilities is not exposed to the danger of electric shock. The soil/rock resistivity is directly related to earth mat design parameters and has a critical role in economic design and in achieving the desired earth mat resistance with respect to ground. Hydro power plants are in general located in rocky areas where higher resistivity values are encountered and denser grid is used for achieving desired earth mat resistance. Therefore finer details of ground resistivity are essentially required to design a safe, economic and effective earth mat.

Geophysical resistivity imaging technique can be used for assessment of finer details of subsurface resistivity at plant site. This technique was utilized successfully at a small hydro power plant located in Eastern Himalaya. At switchyard grounding resistance of Earth mat was higher than the desired resistance of 1 ohm. Earth mat at switchyard was laid as per the design and there was a little scope for placing additional Earth mat. In order to assess the practicable and suitable location, resistivity imaging was taken up at and around switchyard and power house location. In general higher resistivity values from 1700 ohm.m to 8000 ohm.m were encountered near power house and switchyard area. However on the basis of imaging results, two low resistivity zones of about 14–113 ohm.m were identified near power house location and some additional Earth mat was placed at this location. This helped in achieving desired grounding resistance of Earth mat below 1 ohm. Resistivity imaging provided an economic and faster solution of the problem.