Measurement and computation of ground resistances at 161/23.9-11.95-kV indoor-type substations

The purpose of this paper is to compare the calculated and actual ground resistances at two 161/23.9-11.95-kV indoor-type substations in the system of Taiwan Power Company. For the calculated ground resistances, one- and two-layer soil models based on Sverak and Schwarz equations, the method of images and the current distribution electromagnetic grounding and soil structure analysis (CDEGS) program are performed. As for the actual ground resistances, they are found by subtracting the measured ground resistances which are obtained by the fall-of-potential method from the earth mutual resistances. As a result, the calculated ground resistance based on the method of images as the observation point located at the lower layer of the two-layer soil model is approximately equal to the actual ground resistance measured by the fall-of-potential method with the potential electrode buried at the lower layer. In addition, the ground potential rises (GPRs) at both substations are computed and compared with the minimum touch voltages for assessing the grounding system safety. The result shows that the design of the grounding system at both substations is unsafe under the one-layer soil model for humans with 50 and 70 kg body weight.