Computation of ground resistances and assessment of ground grid safety at 161/23.9-kV indoor-type substation

This paper computes ground resistances and assesses ground grid safety at three 161/23.9-kV indoor-type substations in the system of the Taiwan Power Company. For computing the ground resistance, a method based on two kinds of two-layer models (i.e., grid buried in the lower-layer of two-layer soil, two-layer soil simplified with one-layer structure of the Sverak's and empirical equations) is used and compared with the method proposed by the one-layer model of the Schwarz's equation in IEEE Standard 80, 2000 edition. As a result, the ground resistance obtained by the two-layer soil simplified with one-layer structure of the empirical equation is near the measurement. For assessing the ground grid safety, the ground potential rise is computed based on the calculated ground resistance and is compared with the minimum touch voltage. As a result, the design of the ground grid at all three 161/23.9-kV indoor-type substations is unsafe under a one-layer model of the Schwarz's equation for humans with 50-kg body weight.