Earhthing performance of HV/MV substation and its effect on nearby structures

Abstract

Nowadays, with more of suburban area developed into an urban area, power system for transmission and distribution have become more critical with the growing of load demand in the area. A substation, previously located away from other other structures, are now standing next to residential buildings and other public structures, raising concerns over public safety factor. Increasing energy demands have caused the utility company to increase the capacity of a substation, and hence consequently increases the system fault current level. The existing and installed earthing grid systems had incorporated the safety factor based on the old fault current level, which is now may not be sufficient for the new fault current level capacity. During the propagation of fault current, there exist the earth potential rise (EPR) along the conductor at the surface of the soil, the touch potential (TP), and the step potential (SP), all of which may become lethal when over a certain level. The values of ERP, TP and SP must comply to the safety level as described in the IEEE 80-2000 standard. Several factors that contribute to the ERP value are the fault current level, soil resistivity and grid geometry. The CDEGS (Current Distribution, Electromagnetic Fields, Grounding and Soil Structure Analysis) software was used to analyse the effects of changes in the fault current level, soil resistivity and grid geometry to a substation earthing grid performances as well as to the earthing grids of nearby structures. Simulation results show that TP and SP values at nearby structure are significantly increased with the increasing of fault current and soil resistivity in the worst case, the TP increases to more than 4000% and the SP to more than 5000%.Appropriate improvements to the existing substation earthing grid system, namely, increment in the mesh conductor numbers, increment in the number of installed vertical rods, and incremment in the depth of the vertical rod, were proposed. Simulation results shows the proposed improvement methods significantly improve the TP and SP values.