EARTHING EFFECTIVE LENGTH FOR LOW AND HIGH AMPLITUDES OF LIGHTNING IMPULSE CURRENTS

Earthings of the electrical power systems can achieve wide sizes. Due to equipotential bondings and a presence of main equipotential bar the earthing system can be connected even to a power transformer neutral point. Evaluation of such wide earthings for lightning protection purposes can not be easy to perform because only a part of the earthing system should be taken into account. This useful part of the earthing is described as its effective length. The paper deals with a method of effective length calculations and in order to explain the notion computer simulations and measurements of a model as well as a real earthing have been performed using impulses of 1, 4 and 10 mu s front times. A good convergence investigation results have been obtained.