Fabio Tossani

Estimation of the influence of direct strokes on the lightning performance of overhead distribution lines

When assessing the lightning performance of distribution lines, major attention is in general devoted to the induced voltages from nearby strikes rather than to direct events hitting the line, as the latter are always expected to cause a fault in the line. This paper proposes a Monte Carlo procedure capable to make it possible the evaluation of the overvoltage probability due to both indirect and direct strokes. The results are presented making reference to specific nodes of medium voltage overhead distribution networks, namely the locations of MV/LV transformers connections. The problem is of interest when a major upgrade of surge arresters is required, for instance when the neutral grounding practice needs to be changed (e.g. from grounded to resonant).

On the Transmission-Line Approach for the Evaluation of LEMP Coupling to Multiconductor Lines

This paper provides some clarification on the subject of the response of multiconductor lines to lightning-originated electromagnetic fields. Specifically, the effect of the presence of more conductors on the line response is analyzed with and without taking into account the ground losses in the surge propagation. The effect of the boundary conditions is also discussed. The presented analysis shows, in particular, that the response of a matched multiconductor lossless line differs from that of a single-conductor line at the same height only in the line currents. The effect of the nearby line conductors on the induced voltages is, indeed, noticeable when the ground losses are taken into account in the surge propagation.

Advancements in insulation coordination for improving lightning performance of distribution lines

The paper is motivated by the results obtained in a joint project between the Brazilian electric distribution utility AES Sul, the Federal University of Itajubá and the University of Bologna. It deals with some aspects relevant to the assessment of the lightning performance of medium voltage (MV) overhead power distribution lines and of MV distribution feeders mainly composed by overhead lines. In particular, the paper focuses on the effects of direct strokes to the line conductors and on the representation of both the AC voltage at the utility frequency and the flashovers of the insulators. Results are presented with reference to lines with standard or compact configuration as well as to a feeder with complex topology.

Influence of the Return Stroke Current Waveform on the Lightning Performance of Distribution Lines

An accurate evaluation of the lightning performance of overhead power distribution lines, that is, the evaluation of the expected number of annual flashovers, can be obtained by the application of the Monte Carlo method. In order to reduce the computational effort, a simplified waveform of the channel base current is generally assumed, for example, a linearly rising current with flat top. This paper proposes a Monte Carlo approach able to take into account the typical functions adopted to represent the waveform of the lightning current at the channel base (i.e., the Heidler function and the CIGRÉ function). Moreover, this paper analyzes the effects of different current waveforms on the lightning performance of distribution lines for both direct and indirect strokes.

An Improved Approach for the Calculation of the Transient Ground Resistance Matrix of Multiconductor Lines

The general integral expressions for the elements of the ground impedance matrix do not have an analytical inverse transform in the time domain. Therefore, the ground transient resistance of a multiconductor line is commonly evaluated in the time domain by means of approximated expressions in order to avoid the numerical inverse fast Fourier transform burden. In this paper, we propose a new analytical approach for calculating the transient ground resistance in the time domain that stands on the very accurate Sunde logarithmic expression for the ground impedance matrix of an overhead multiconductor line. The proposed analytical approach is adopted for calculating lightning induced voltages in a multiconductor overhead line. The results show that the proposed analytical formula is in perfect agreement with the numerical inverse Fourier transform of the general Sunde expression and, therefore, is more accurate than the approximated expressions available in the literature.

Lightning Performance of Overhead Power Distribution Lines in Urban Areas

Buildings nearby urban overhead power lines are expected to reduce the number of direct strikes to the line conductors and also to attenuate the lightning electromagnetic pulse (LEMP) radiated by indirect lightning strokes. The statistical method described in IEEE Std. 1410 for the lightning performance assessment, as the other methods available in the literature, includes only the former effect. In order to take into account also the latter effect, in this paper, the LEMP attenuation due to the buildings is represented by means of specific weighting functions applied to the LEMP analytical expressions valid for open terrain. For both the cases of ideal and lossy ground, the parameters of the weighting functions are identified through the least-square minimization of the differences with the results provided by a finite-element method model that is assumed as reference for the configurations analyzed. The weighting functions can be used for lightning return stroke current waveform and distances between the line and the stroke location different from those used for their identification with reasonable accuracy. Finally, this paper compares the lightning performances of a power line evaluated with and without the presence of nearby buildings.

Lightning induced overvoltages on overhead lines shielded by nearby buildings

In urban areas the presence of buildings is expected to reduce the amplitude of lightning induced voltages on overhead lines with respect to the case of open terrain. This paper presents a method for estimating such a reduction effect. For this purpose weighting functions are applied to the electrostatic, induction and radiation terms of the expressions adopted for the lightning electromagnetic field calculation assuming open terrain. The parameters of the weighting functions are identified by means of a least square fitting procedure using a finite element method model as benchmark for the field calculation. These functions are shown to be rather independent of the lightning return stroke current waveform and of the distance between the line and the stroke location. Although this paper presents only results obtained for the case of a single line, the method is conceived in order to be applicable for the lightning performance assessment of more complex power distribution networks.

Selection of MV/LV transformers to be protected by surge arresters against indirect lightning overvoltages

The papers addresses the problem of locating surge protective devices in a medium voltage distribution network by means of an accurate computer model for the calculation of lightning induced overvoltages and a Monte Carlo statistical procedure. The indirect lightning performance of small network cells of different configuration are evaluated and used as a benchmark for the validation of the surge arrester locations provided by a genetic algorithm procedure. The obtained results appear useful for the lightning protection design of networks with complex topology.

Inverse Laplace Transform of the Ground Impedance Matrix of Overhead Lines

This letter deals with the calculation in time domain of the transient ground resistance matrix of an overhead transmission line (TL). Each element of the matrix is evaluated by solving analytically the inverse Laplace transform of the general integral expressions of the ground impedance in frequency domain proposed by Sunde. The presented expressions are suitable for the direct implementation in an electromagnetic transient program (e.g., EMTP-like ones) based on a time domain solution of the TLs equations.

Calculation of lightning-induced overvoltages on urban overhead lines above a lossy ground plane — appraisal of the shielding effect of nearby buildings

This paper presents an analysis of the reduction effect on the amplitude of lightning-induced overvoltages in an overhead line due to nearby buildings. Such an affect can be appraised by means of a finite element method (FEM) model. A method already proposed by the authors, based on the use of weighting functions applied to the static, induction and radiation terms of the expressions of the lightning electromagnetic field for the case of flat, perfectly conducting terrain, is extended to the case of ground with finite conductivity. Such a method allows to avoid the computational effort associated with the use of the FEM model. Moreover, the paper shows the influence of the cross-section dimension and the exterior faces conductivity of the building on the reduction effect. The analysis is presented for different lightning return stroke current waveforms, line-to-stroke location distances, and return stroke velocities.