Earth grid safety criteria determination with the standards IEEE-80 and IEC-60479 and optimization of installation depth

Major generating or switching stations are normally operated with their neutral points directly earthed (usually an earth grid is buried in the earth). Metallic structures in the yard of generating or switching station, are electrically connected to the earth grid. The dimensions of earth grids are usually up to some hundreds of meters on a side. The design of earthing systems requires a worst-case approach. It ensures that a conductor forming the grid will not fail thermally or mechanically in the worst case of maximum fault current persisting for a fault of maximum duration. Over the life time of the electrical substation and its associated earth grid (30-50 years), it is important to maintain safety. Therefore before installing an earth grid its design is assessed according to the earth grid installation standards. In this paper safety criteria assessment of a design is evaluated with both IEEE-80 and IEC- 60479. In addition, a new method for determining the earth grid´s installation depth has been proposed. It is based upon the soil model of particular substation instead of installing it in 0.5 meters depth for any substation.