Impulse Impedance of Grounding Grids

A method and an empirical formula to calculate the inductance of square grounding grids fed at either centre or one of the corners is presented. Empirical formulae for the effective area and impulse coefficient for centre fed and corner fel square grids are determinel. The valility of the analytical methol has been verified experimentally with model tests. It is shown that the effect of soil ionization on the impulse impedance of grounding grils can be ignored for grounding grids in high voltage sub-stations.