Survey of the equivalent substation earth current and earth impedance for transformer stations

The paper deals with the clarification of basic terms related to the calculation of the earth potential rise (EPR), transferred potential and earth resistance of the earthing grid. In case of earth fault occurring in a substation, significant fraction of the fault current is flowing in the passive conductors (cable sheath, earth wire) of the lines feeding to the fault. These passive conductors are referred to as “return conductors”. Only the remaining fraction of the fault current, i.e. the “equivalent earth current” is passing through the earthing grid entering into the earth. Furthermore, the substation earth is composed of not only from the earth resistance of the grid itself but also the driving point impedance of the passive earthed conductors (other than the return conductors). The parallel resultant of these two is referred to as “equivalent earth resistance” of the substation. The EPR of the substation is given as the product of the equivalent earth current and the equivalent earth resistance.