The new expression of the lightning current at the striking point is analyzed. The basic four channel-base lightning current quantities are assumed to be known: current peak value, current rise-time, maximum of the current steepness and the charge transfer at the striking point. The algorithm for a fast calculation of the channel-base current parameters is proposed. The numerical example of the current parameters determination is given. The proposed algorithm can be successfully used for the lightning current modelling in power engineering as well as in the research of the radiated lightning electromagnetic pulse and its coupling with the overhead lines and other metallic structures.

Calculation of lightning current parameters

A model of lightning-channel progression towards the Earth is given based on the latest knowledge on the physics of discharge in long air gaps. The physical assessment of the electric field is done with the charge simulation method, which is adopted to input into a computer program all parameters involved in the progression of the negative downward channels towards the Earth and those involved in the inception and propagation of upward positive channels from earthed structures. Starting from the geometrical configuration of the earthed object and taking into account the orographic conditions, the lateral distance may be evaluated as a function of the severity of the lightning stroke in terms of impulse charge and thus of current amplitude. Provided the statistical distribution of the currents is given, the model therefore allows the number of lightning strokes to an object to be estimated, yielding an assumed ground flash density. Reference is made to the critical radius concept acquired with laboratory tests with long front surges. A check of the model with field data relevant to free-standing structures is presented and a reasonable agreement is found. >

Lightning stroke simulation by means of the leader progression model. I. Description of the model and evaluation of exposure of free-standing structures

The paper presents the statistical data of the significant parameters of lightning flash, collected by many researchers over many years around the world. The significant parameters of a lightning flash are: peak current, waveshape and velocity of the return stroke, the total flash charge and /spl int/I/sup 2/dt. Negative first strokes have traditionally been considered to produce the worst stress on the system insulation. The subsequent negative strokes have significantly lower peak current but shorter wavefronts. This may stress the system insulation more. The positive strokes have about the same median current value as the negative first strokes and longer fronts, thus producing less stress. However, their duration is longer than that of the negative strokes. Therefore, the system insulation may be damaged because of the lower volt-time characteristic for long-duration waves. The positive strokes may also cause more thermal damage because of their significantly higher charge and /spl int/I/sup 2/dt. The relationship between the return-stroke velocity and the current peak is a significant parameter in estimating lightning-induced voltages and also in estimating the peak current from the radiated electromagnetic fields of the lightning channel. For better accuracy, the current and the velocity should be measured simultaneously. Better methods to measure the stroke current need to be developed. Correlation coefficient between various lightning parameters is another important parameter which will affect the analysis significantly. Lightning characteristics should be classified according to geographical regions and seasons instead of assuming these characteristics to be globally uniform.

Parameters of lightning strokes: a review

This paper addresses the lightning attractive radius concept and procedures for estimating the number of lightning flashes to power lines. Empirical data - drawn from extensive observations of lightning incidence upon various practical structures (including lines) - are related to a simple analytical model of the striking process, and a generalised expression for estimating strike incidence is presented. Comparison of the new expression with current practice, suggests that the latter may lead to underestimations of more than 50% in the height range 10 - 30 m, and additionally, that expected strike incidence to taller structures could be overestimated by 30% or more.

The Incidence of Lightning Strikes to Power Lines

An improved electrogeometric model - intended for transmission line lightning shielding analysis - is presented, based upon previous empirical and analytical studies of the lightning striking process. In contrast to earlier approaches, the model provides a rational basis for taking account of the influence of structure height and is essentially independent of any assumptions regarding striking distances to ground. Sample applications of the model show comparatively good agreement with line experience, both in the estimation of shielding failure outage rates, and in the calculation of the requisite critical shielding angles.

R. B. Anderson

Papers

Lightning parameters for Engineering Application