Analytical Models for Distributed Grounding Systems

Simple lumped-parameter circuit models for distributed grounding systems have been developed from the truncation of theoretical solutions. The simplest model utilizes two time constants; one for relatively short times and one for relatively long times. The variation of soil conductivity and permittivity with frequency is estimated and taken into account in the determination of the time constants. Estimating curves for the time constants are developed from experience in matching the experimental results of Kawai. Some practical applications of the circuit models are shown in comparing the indicial impedances of different grounding arrangements. The results confirm the fact that, for high resistivity soils, a combination of a concentrated, high-capacitance ground at the tower with a distributed, high-conductance ground of suitable length is superior to either system alone or to a continuous counterpoise.