Transient Performance of Grounding Grids

Understanding the behaviour of grounding systems to transient currents is important to establish the efficacy of protection offered by these systems to personnel and to the electrical installations against lightning. In this paper, an analytical method to determine the transient performance of grounding grids is presented. The validity of the proposed methodology has been verified by extensive field test results. Most published studies have been aimed at improving the knowledge of transient behaviour of ground electrodes of various forms. Some researchers have proposed methods to determine the impulse impedance of grounding grids also, by considering square grounding grids and by assuming the grid to be an equivalent circular plate. In actual practice however, the station grounding grids can have complex configurations. In this paper, a generalised model of grounding grid is considered for analysis under transient conditions. The two factors of importance in such an analysis are 1) the ground resistance (or conductance) and 2) self and mutual inductances of each element of the grid. In the proposed model, the distributed nature of ground conductance of each element has been lumped and represented as a nominal-pi. In this way, the leakage current is better represented than by considering the grid as an equivalent circular plate. In addition, the model proposes formation of susceptance matrix for the grid in a similar manner as the bus-admittance matrix for the load flow solution.