EFFECT OF RESISTANCE ON LIGHTNING RETURN STROKE CURRENT

Abstract

As is well known, lightning is responsible for millions of dollars of damage to buildings and forests, starting bush fires and causing the deaths of many people in the world every year. Many lightning return stroke current models have been developed to meet different requirements. However, these models all assume infinite ground conductivity although return stroke currents may be influenced by the finite conductivity of the propagation path. In order to study the effect of resistance on lightning return stroke current, a new method of calculation to determine the return stroke current is developed in this thesis. It is convenient for this method to reproduce curves of return stroke currents and then calculate return stroke currents with various resistance values at ground level. The return stroke current in this thesis is viewed to consist of two regions: (1) a breakdown current from the leader core due to the release of charge stored in the channel core, and (2) a corona current from discharging the corona sheath surrounding the leader core. The core channel and the corona channel are represented by several resistance-capacitance circuits. Each circuit begins to discharge toward the earth when the return stroke front reaches its height. Hence, the return stroke current at ground level is obtained by integrating all discharge currents along the whole channel, with the current from each section arriving after an appropriate time delay. Time delay constant for each section relates to the velocity of return stroke front, the velocity of discharge current into the channel and the length of each section. The return stroke current at any height is the sum of all discharge currents arriving from all the sections above this height. Two preliminary calculations were carried out with our new method. By proper choice of model parameters, it was found possible to reproduce with high accuracy the output of the DU model and the typical return stroke currents reported by Uman [1]. Then, various resistance values are added at the base of the channel. The calculation shows that peak value, risetime and current-time derivative of the return stroke current are strongly influenced by this added resistance element. In other words, return stroke currents can be restricted by added resistance elements at the base of the lightning channel. In practice, the design of a suitable resistor for the purpose of limiting direct lightning stroke currents poses problems and difficulties. The voltage across this resistance element may be too large, exceeding the withstand voltage of this element, and then flashover along its surface will result in failure of the current limiting. Hence, this thesis also analyzes and simulates a novel method of current sharing following the termination of a lightning stroke onto a resistive lightning protection terminal. Such a system of resistance elements is essential to ensure that surface flashover does not occur across lightning rods and current limiting is successful.