Effect of Earth Wires on Station Earth Impedance

Earth impedance of a substation, with earth wires of lines connected to its earthing system, may be considerably smaller than the true earth impedance and it may have a sjgnificant reactive part. Thus, measured value of earth impedance with earth wires connected to the station earth may, be appreciably different from the design value of earth impedance (generally assumed purely resistive). Also the measured value of earth impedance with earth wires tied to the station earth cannot be used to compute the Ground Potential Rise of the station as the same depends on the true earth impedance of the station. In this paper, a simple model of earthing system with a number of earth wires connected to it is proposed. The model is used to develop an algorithm for computing effect of earth wires on earth impedance of the station. Earthing systems of two typical substations are analyzed to demonstrate the utility of the algorithm in computing possible quantum of the effect of earth wires on earth impedance.