Effect of tall instrumented towers on the statistical distributions of lightning current parameters and its influence on the power system lightning performance assessment
Statistical distributions of lightning current amplitude, time-to-peak and other lightning current parameters used in power system insulation coordination are based on experimental data obtained by means of tall instrumented towers. It is, however, generally accepted that these distributions, which we will call 'classical' ones, are affected by the presence of the tower, as the tower ability to attract lightning flashes tends to increase for flashes with larger currents. Current amplitudes are thus biased towards higher values with respect to those which refer to flashes at ground. In this paper, we adopt a procedure recently proposed by the authors to infer the statistical distributions of lightning current parameters at ground level starting from those obtained from data measured using tall instrumented towers. The procedure is first applied to classical statistical distributions of lightning current parameters to quantify the tower bias, and this for various models that have been proposed in the literature to represent the exposure of the tower to direct strokes. Then, the statistical distributions at ground, calculated by applying the proposed procedure, are used to calculate the indirect-lightning performances of an overhead line above an ideal and a lossy ground. The results are also compared with those obtained using the original distributions of lightning current parameters, i.e. by neglecting the bias introduced in the distributions by the presence of the instrumented tower, and a significant difference is found.