Computation of current-division factors and assessment of earth-grid safety at 161/69 kV indoor-type and outdoor-type substations

The paper computes current-division factors and assesses earth-grid safety at 161/69 kV indoor-type and outdoor-type substations in the system of Taiwan Power Co. For computation of current-division factors, a method based on the grid current is proposed and the results obtained by the proposed method are compared with the Endrenyi method presented in IEEE 80:2000 as well as the current-distribution electromagnetic-grounding and soil-structure analysis (CDEGS) program. On the other hand, mesh and step voltages are computed and compared with minimum touch and step voltages for assessing earth-grid safety, respectively. The values of current-division factor obtained by the proposed method and the CDEGS program are smaller than those from the Endrenyi method, since coupling effects between overhead earth wires and phase conductors are taken into account. Nevertheless, the design of earthing systems at both types of 161/69 kV substations is safe for humans with 50 kg and 70 kg body weight.