Search

A model for the calculation of lightning-induced voltages is presented allowing the assessment of the effect of corona when power distribution lines are illuminated by lightning electromagnetic fields. It is shown that, as far as corona can be described by means of an increase of capacitance after the voltage has reached a threshold value, corona can produce an increase in the peak amplitude of the induced voltage. An explanation of this result is given. The need for experimental results to test this theoretical finding is stressed in the paper. It is also shown that for lightning-induced voltage calculations, the ground resistivity plays a more important role than the corona in modifying the wave shape of the induced surges. This is different from the direct-strike behaviour, and is due to the fact that the ground resistivity affects more strongly the lightning-radiated fields rather than the surge attenuation along the line, while corona affects only surge propagation

Considerable progress has been achieved in the last 10-15 years on the modeling of the lightning phenomenon and on the electromagnetic-field-to-transmission line coupling. The aim of this tutorial is to emphasize the latest developments which took place in the last 2-3 years in particular concerning protection efficiency evaluation using these models and to remind some problems which are still under discussion

The paper presents a general procedure for the evaluation of the indirect lightning performance of a distribution network, which takes into account the actual network topology and the presence of transformers, surge arresters and line poles. The procedure is an extension of a Monte Carlo-based approach developed for a single straight overhead line to the case of a more complex distribution network consisting of a number of lines (main feeder and laterals) and typical power components. The procedure is based on the accurate evaluation of the induced voltages along the network, achieved by using the LIOV-EMTP computer code, which combines a model for the calculation of lightning-induced overvoltages (LIOV) on overhead lines with the electromagnetic transient program (EMTP). In order to reduce the required computational effort, a heuristic technique is proposed and its accuracy is verified. Finally, the paper presents a comparison between the lightning performances of distribution networks characterized by the same overall length of overhead lines but with different topologies. The effects of the presence of line poles, transformers and surge arresters are also analysed.

The lightning return stroke model by Master, Uman, Lin, Standler (1981) (MULS), a modification of the Lin, Uman, Standler (1980) (LUS) model, can be viewed mathematically as having a specified channel-base current, although in the original formulation it was the channel current sources that were specified. The specification of the MULS channel-base current is independent of the time dependence of the corona current sources which characterize the MULS model. Further, when starting from the same channel-base current, the MULS return stroke model, except for the uniform current component, is mathematically equivalent to the Modified Transmission Line (MTL) model. For both the MULS and the MTL models the authors present a single analytical expression relating the far fields and the channel-base current. This formulation provides a simple procedure to infer MULS channel-base currents from remote electric and magnetic field measurements and hence to test the MULS model

The authors present an analysis in the frequency domain of lightning electromagnetic fields making use of a computer code developed by the authors. The analysis is made for different distances from the lightning channel, for two different current waveshapes corresponding to the first and the subsequent negative return stroke, and evidencing the contribution of the various terms (static, induction and radiation) to the total field. The calculated frequency spectra match satisfactorily with the average spectra obtained by others by means of measured oscillogram. Features of the calculated spectra, such as the peak near 4 kHz, the decrease with 1/f from about 104 to 106 Hz and with 1/f2 above 106 Hz are in reasonably good agreement with the experimental spectra. Dips in the electromagnetic field spectrum at large distances from the lightning channel and at high frequencies have been justified

This paper presents experimental results obtained at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida during the smnmers of 2002 and 2003. Currents induced by triggered and natural lightning events were measured at the terminations of a buried power cable, in the cable shield, and in the inner cable conductor. Measurements of the horizontal component of the magnetic field above the ground surface for both natural and triggered lightning are also presented. For distant natural lightning events, locations of ground strike points were determined using the U.S. National Lightning Detection Network (NLDN). Based on the theoretical developments presented in Part I of this paper, the field-to-buried cable coupling equations are solved in both the time domain and in the frequency domain. The obtained experimental results are then used to validate the numerical simulations provided by the relevant developed codes. © 2005 IEEE.

The paper focuses on the solution of the minimum loss reconfiguration problem of distribution networks, including embedded generation, by means of a mixed integer linear programming (MILP) model. The proposed model takes into account typical operating constraints of distribution networks (radial configuration, branch ampacity limits and bus voltage requirements). The accuracy of the results and the computational performances of the proposed MILP model are evaluated by making reference to test networks already adopted in the literature for the problem of interest.

In this paper, an analysis of electric and magnetic fields radiated by lightning first and subsequent return strokes to tall towers is presented. The contributions of the various components of the fields, namely, static, induction, and radiation for the electric field, and induction and radiation for the magnetic field are illustrated and discussed. It is shown in particular that the presence of a tower tends, in general, to increase substantially the electric and magnetic field peaks and their derivatives. This increase is mainly caused by the presence of two oppositely propagating current wavefronts originating from the tower top and by the very high-propagation velocity of current pulses within the tower (practically at the speed or light), and depends essentially on the wavefront steepness of the channel-base current. Because of the last factor, the increase of the field magnitudes is found to be significantly higher for subsequent return strokes, which are characterized by much faster risetimes compared to first return strokes. Furthermore, the presented results are shown to be consistent with recent experimental observations of current in lightning strokes to the Toronto CN Tower and of the associated electric and magnetic fields measured 2 km away. These findings partially explain the fact that subsequent return strokes characterized by lower current peaks but higher front steepnesses and return stroke speeds may result in higher field peaks. The results obtained in this study haste important implications in electromagnetic compatibility. It is found that lightning strikes to tall metallic objects (towers, rods, etc.) lead to increased electromagnetic field disturbances. Also, subsequent return strokes are to be considered an even more important source of electromagnetic interferences than first return strokes. Indeed, electromagnetic fields from subsequent strokes are characterized by faster fronts and additionally, they may reach greater peaks than first strokes. Lastly, findings of this study emphasize the difficulty of extracting reliable lightning return stroke current information from remote electromagnetic field measurements using oversimplified formulae.

Measurements of return stroke currents on instrumented towers reveal the existence of reflections at the tower bottom and top. In the paper, the effects of the reflections on both measured currents and the original lightning electromagnetic fields are discussed. To this purpose, an extension of the modified transmission line (MTL) return stroke model is given. The model takes into account multiple reflections at the bottom and at the top of the object and allows for the `decontamination' of measured lightning currents from reflections due to the presence of the strike object. The authors' results show that even though the reflections might not be discernible in experimental recordings for tower heights as those used in Berger et al.'s (1975) and Garbagnati and Lo Piparo's (1982) investigations, the peak current might be significantly affected. Therefore, consideration of the reflections in the elaboration of meaningful statistics on lightning current is required, as well as an accurate evaluation of the tower reflection coefficients. Radiated fields calculated considering and neglecting the presence of the strike object differ. However it is shown that relatively short strike objects, such as triggered lightning launching structures, do not appreciably affect the calculated field peak values and overall wave shapes, but only the field derivatives

The paper illustrates a procedure based on the continuous-wavelet transform for the analysis of voltage transients due to line faults and discusses its application to fault location in power distribution networks. The analysis carried out allows inferring that correlation exists between some characteristic frequencies of the transformed signal and specific paths in the network covered by the traveling waves originated by the fault. This correlation provides useful information for the location of the faulted section in a MV distribution network starting from the recorded voltage transients at one or more busses. The presented approach can find a useful application in the realization of a distributed measurement and fault locating system. A brief description of such a system and the estimation of the relevant uncertainty are also provided in the paper.

The ground impedance matrix elements of a multiconductor overhead transmission line do not have analytical inverse Fourier transforms in the time domain. Thus, in general, the ground transient resistance matrix elements are to be determined numerically. Using the low-frequency approximation analytical expressions are available for the ground transient resistance. These expressions present however a singularity at t = 0 and require a careful treatment in a direct time domain analysis. In this paper, we show that the singularity in the ground transient resistance is due to the low frequency approximation. Also, we show that at early times, the ground transient resistance elements tend to an asymptotic value, which depend on the line geometrical parameters and ground relative permittivity. Finally, we propose new expressions which are analytical, non-singular, and which describe, within the limits of transmission line theory, both the early-time and late-time behavior of ground transient resistance in a more accurate way. © 2002 Elsevier Science B.V. All rights reserved.

We present a review of theoretical methods to compute lightning-induced currents and voltages on buried cables. The evaluation of such induced disturbances requires the calculation of the electric field produced by lightning along the cable path. We show that the Cooray's simplified formula is capable of predicting accurately the horizontal electric field penetrating the ground, at distances as close as 100 m. Regarding the parameters of the buried cable, a comparison of several approximations of the ground impedance is presented. It is also shown that, within the frequency range of interest, the wire impedance can be neglected, due to its small contribution to the overall longitudinal impedance of the line. The ground admittance, however, can play an important role at high frequencies (1 MHz or so) especially in the case of poor ground conductivity. The ground admittance needs to be taken into account in the calculation of lightning-induced currents and voltages on buried cables. This is in contrast with the case of overhead lines in which its contribution is generally negligible even in the MHz range

Presents an analysis in the time and frequency domains of the electromagnetic field originated by negative lightning return strokes, calculated at ground level. The analysis is performed on the basis of the transmission line model modified by the authors, which gives results in good agreement with experimental data. The influence of several parameters which deeply affect the time waveshape and the frequency spectra of the electromagnetic field has been investigated. These parameters are: the rise time and the peak value of the lightning current pulse, its velocity of propagation and the rate of decreasing of the intensity of the current pulse propagating along the channel. The influence of other parameters such as the lightning channel height and the distance of the observation point is also analyzed. It is shown that the height of the channel does not affect the electromagnetic field. Two simplified expressions of the electromagnetic field used for the calculation of overvoltages induced on transmission lines are discussed in order to assess their validity limits

The paper presents a procedure for parameter identification along with its application to the model of a combined cycle power plant that includes the surrounding electrical network, built for the analysis of islanding maneuvers transients. The paper illustrates both the power system computer model, implemented within the EMTP-RV simulation environment, and the developed identification procedure based on the interface between the developed model and Matlab. The parameter identification procedure is applied to experimental transient recordings that make reference to a similar power plant.

We discuss in this paper the influence of the presence of an elevated strike object on the peak of the lightning return stroke current determined from remote field measurements. We develop analytical expressions relating the lightning return stroke channel-base current and the far electromagnetic field for different specific cases, namely, (1) ground-initiated return strokes (classical transmission line (TL) model), (2) ground-initiated return strokes including possible reflections at ground level, (3) tall strike objects for which the current's zero-to-peak time is smaller than the travel time along the object, and (4) electrically short strike objects. It is shown that for tall structures, the field enhancement relative to a return stroke initiated at ground level is expressed through a factor equal to ktall = [1 + c / v (1 - 2 ρt)] / (1 - ρt), where v and c are the return stroke front speed and the speed of light in vacuum, respectively, and ρt is the top reflection coefficient. For very short towers and/or very slow return stroke current wavefronts, when the condition tf very much greater than h / c applies, expressions relating the far electromagnetic field and the return stroke current were also derived. For case (2), return strokes initiated at ground level, the field enhancement relative to a return stroke initiated at ground level, case (1), is expressed through a factor equal to kshort = (1 + (c / v) ρch - g) / (1 + ρch - g), where ρch-g represents the reflection coefficient between the lightning channel and the grounding impedance.

The presence of an elevated strike object markedly affects both the measured lightning return stroke current and the radiated electromagnetic fields. The MTL and the TCS lightning return stroke models have been extended to take into account the presence of elevated strike objects. In this paper, we compare theoretical results obtained using these two models with a set of simultaneously recorded data consisting of the current measured at the top of the Peissenberg tower and the associated electric and magnetic fields at about 200 m. It is shown that both the extended MTL and TCS models are able to reproduce with a reasonably good accuracy the magnitude and waveshapes of the electric and magnetic fields. However, the computed field peak derivatives are found to be much higher than the measured values. Possible reasons for this discrepancy are discussed in the paper

In this paper, we present a review of theoretical methods to compute lightning induced currents and voltages on buried cables. The evaluation of such induced disturbances requires the calculation of the electric field produced by lightning along the cable path. We show that the Cooray's simplified formula is capable of predicting accurately the horizontal electric field penetrating the ground, at distances as close as 100 m. Regarding the parameters of the buried cable, a comparison of several approximations of the ground impedance is presented. We show that the Pollaczek expression corresponds to the Sunde general expression, when the displacement current is neglected. The analysis shows also that all the proposed approximations provide very similar results for the considered range of frequencies (up to 30 MHz). Most of the approximate formulas neglect the contribution of the displacement current and, therefore, predict values for the ground impedance which tend to infinity at higher frequencies. This corresponds in the time domain to a singularity of the ground transient resistance at t = 0. By analogy to the Sunde approximation for the ground impedance of overhead lines, we propose a logarithmic approximation for the ground impedance of a buried cable. In addition, unlike most of the considered approximations, the proposed formula has an asymptotic behavior at high frequencies; therefore, the corresponding transient ground resistance in the time domain has no singularity at t = 0. It is also demonstrated that within the frequency range of interest, the wire impedance can be neglected, due to its small contribution to the overall longitudinal impedance of the line. The ground admittance, however, can play an important role at high frequencies (1 MHz or so) especially in the case of poor ground conductivity. The ground admittance needs to be taken into account in the calculation of lightning induced currents and voltages on buried cables. This is in contrast with the case of overhead lines in which its contribution is generally negligible, even in the MHz range. We also investigate the time-domain representation of field-to- transmission line coupling equations. The coupling model includes the effect of ground admittance which appears in terms of an additional convolution integral. An analytical expression for the ground transient resistance in the time domain is also proposed which is shown to be sufficiently accurate and nonsingular. Finally, we present a time domain solution of field-to-buried cable coupling equations using the point-centered finite difference time domain (FDTD) method, and a frequency domain solution using Green's functions. In our companion paper (Part II), we compare both solutions to experimental waveforms obtained using triggered lightning. © 2005 IEEE.

Chlamydia Trachomatis (C.T.) most commonly causes cervicitis in women, often without overt symptoms; when untreated, it produces a clinically inapparent but persistent mild infection or an evident salpingitis. Either processes may lead to scarring of the falloppian tubes and fertility problems. A group of 30 women 20-35 years aged has undergone an ophthalmological routine examination; 7 of them presented a chronical follicular conjunctivitis and history of yellow-white cervical discharge, while the remaining 23 had a mild chronical follicular conjunctivitis but not genital symptoms. Each patient has undergone an immunofluorescent test with monoclonal antibody specific for C.T. on the samples collected by genital and conjunctival scraping and cytological evaluation of the above samples with Giemsa staining. 6 out of 7 patients with genital discharge were positive for C.T., one of which even for eye samples too; all eyes samples showed lymphomonocytes and neutrofilic cells in their eye samples. An additional 3 cases negative for C.T. showed this kind of cells in the eye scraping. The preliminary results of this study on mild follicular chronical conjunctivitis suggest that the Ophthalmologist could play an important role in order to make an early diagnosis in genital and eye infection by Chlamydial Trachomatis.

The collaborations between universities and companies is a useful mechanism for the improvement of the technical performance of companies and, furthermore, for funding research in the universities. An example of transnational collaboration between EDF, EPFL and the university of Bologna, is presented here. This example points out that despite cultural differences of each country and different methods for working the results of the collaboration are positive for each partner

On the basis of a distributed-source representation of the lightning channel, the mathematical formulations of the so-called engineering lightning return stroke models are generalized to take into account the presence of a vertically extended strike object. The strike object is modeled as a lossless uniform transmission line, and the reflection coefficients are all assumed to be constant. The distribution of current along the lightning channel for each model is expressed in terms of the "undisturbed" current, object height, and current reflection coefficients at the top and the bottom of the object. The undisturbed current is defined as the current that would flow in the channel if the current reflection coefficients at the extremities of the strike object were equal to zero, that is, the characteristic impedances of the lightning channel and the strike object were equal to each other and equal to the grounding impedance of the strike object. The distributed-source representation of the lightning channel adopted in this study allows for a more general and straightforward formulations of the generalized return-stroke models than the traditional representations implying a lumped current source at the bottom of the channel, including a self-consistent treatment of the impedance discontinuity at the tower top

Two corona models, applicable to the calculation of lightning-induced overvoltages in multiconductor overhead lines, are considered in this paper. The first model represents corona by an increase in each conductor self capacitance after the voltage has reached a threshold value, and in the second model, the influence of corona on the mutual coupling between conductors is also taken into account. Results show that corona (using any of the models) produces an increase in the overvoltage amplitudes, confirming that found in a previous work concerning single-conductor lines. It can also be seen that the results obtained by each model are quite different from each other. The effect of corona in mutual coupling should not be disregarded

Return-stroke lightning models and coupling models permit a fairly good estimation of overvoltages induced by indirect lightning strokes. However, some discrepancies that still exist between measured data and calculations stress the necessity to incorporate leader effects into the models, to improve our understanding of ground conductivity effects on the formation of the fields, and to obtain more experimental data

The lightning discharges are one of the main causes of interruptions of medium voltage overhead distribution lines, being the reason of great concern for the utility companies. Its destructive effects frequently extends to equipments and connected installations, with the possibility to cause personal injuries and material damages, beyond economic losses, due to out of income and the possibility of indemnities, penalties and fines. With the beginning of the deregulation of the electric energy supply, some actions were been taken by the utilities for the prevention and minimization of the damages associated to the lightning discharges. However, as the lightning discharges are random events, consequently, difficult to predict, the majority of these actions does not follow a study or a detailed analysis of the problem. By this way, in the majority of the cases the actions were taken based on the knowledge of the engineer in charge, or based in rules defined without any effective evidence, by means of studies or by laboratory tests. As a result, many of them besides of presenting high cost are not effective. Among others, the distribution network reliability depends directly on its exposition to the lightning discharges. To determine the exposition level of the line, the designer should know the number of discharges to the ground per unit of area per unit of time. The aim of this paper is to present the results of the performance study of medium voltage overhead distribution lines against lightning discharges, in the way to define methodologies to reduce the system failures. The results were obtained within the partnership among the High Voltage Laboratory of the Federal University of Itajubá, AES Sul Utility Company and the University of Bologna. Direct discharges and induced surges were simulated into real networks to identify the major factor of influence for network failures. Then commentaries on the relative performance and comparisons of different construction configurations of overhead lines are presented. Once that the atmospheric discharges phenomena are random, this work considers that the parameter generation of the discharges follows the statistical data proposed by Anderson and Eriksson. The Monte Carlo Method is used for the incidence distribution of the discharges and the Electro Geometrical Model for the interception point of the discharge.

The statistical assessment of the lightning-performances of distribution lines must take into account the presence of protection devices such as surge arresters and shielding wire/neutral groundings. Indeed, the presence of those devices does modify the indirect lightning-response of the line. A statistical investigation based on the Monte Carlo method is carried out on a distribution line with realistic configuration, aimed at assessing the indirect lightning-performance of the line. Accurate models are used to evaluate the lightning performance of the line equipped with surge arresters, in order to estimate the effects of arresters spacing and characteristics. A pre-processing procedure based on the simplified Rusck formula for a first evaluation of the lightning-induced voltages of the randomly generated events is implemented in order to reduce the computational time.

Lightning strikes emerge as one important cause for the Loss Of Containment (LOC) of storage vessels containing hazardous materials if past accident databases are analyzed. The release of these substances may cause severe accidents in refineries, storage sites, processing sites and other industrial facilities that may involve both workers and nearby residential areas. Although lightning hazard is well known and it is usually considered in the risk analysis of chemical and process plants, well accepted quantitative procedures to assess the contribution of accidents triggered by lightning to industrial risk are still lacking. In particular, the approaches to the assessment of lightning strike probability and to the damage caused by lightning strike are mainly qualitative or semi-quantitative and they are mostly based on expert judgment. In the present study, a quantitative methodology for the assessment of the risk due to major accidents triggered by lightning is presented. The methodology takes advantage of the relevant progress made in recent years in the assessment of the expected frequency and of the expected structural damage due to lightning. The starting point of the methodology is the assessment of the frequency and severity of the lightning event. The standard parameter for the frequency is the isokeraunic number given by meteorological studies. The lightning severity is quantified by means of probability distribution functions of two parameters: peak current intensity and lightning charge. Through the application of a Monte Carlo simulation of lightning strikes on different process equipments the overall frequency and intensity of the LOC is thus determined. These results are then adapted in order to obtain an analytical relationship between the equipment features and the release probability. The application of the conventional risk recomposition procedure allows the calculation of individual and societal risk figures.

In the determination of lightning-induced voltages on overhead power lines, ground losses play a role in both the evaluation of the lightning radiated electromagnetic fields and of the line parameters. The aim of this paper is to assess the effect of these losses through each of these two mechanisms. The authors show, in particular, that for lines whose length does not exceed a certain `critical' value, the surge propagation along the line is not appreciably affected by the ground finite conductivity, and, therefore, it can be neglected in the computation process. Further, it has been shown that, for typical power lines, the wire impedance and the ground admittance, both frequency-dependent, can be neglected in the computations for typical overhead power lines

The paper deals with the analysis of the possible correlation between cloud-to-ground lightning discharges and relay operations inferred by using the experimental data obtained means of a measurement set-up, installed on an Italian MV distribution network, and a statistical procedure based on the coordinated use of the Italian Lightning Location System (LLS) CESI-SIRF, the monitoring systems of protection manoeuvres and an advanced simulation tool for the calculation of lightning-induced overvoltages. The procedure takes into account the uncertainties related to the estimation of lightning stroke location and current amplitude provided by the LLS. The experimental set-up, composed by a distributed monitoring system, is able to measure a-periodic voltage transients characterized by a frequency content up to 4 MHz. Three of these units have been installed in the feeder of a three-phase overhead distribution network with a total length of 21 km. The same feeder is protected with overcurrent and zero-sequence relays whose operations are recorded by a monitoring system with a 10-ms sampling time. The data collected during one year of operation of the experimental set-up have been analyzed together with the data collected by the LLS for a 144 km2 area covering the equipped feeder and its surrounding area, in order to account for all direct and indirect lightning events that may have caused a relay intervention. The data have been also used to test the calculation model. The paper eventually reports the results of the correlation analysis carried out using the mentioned procedure, which is used to evaluate the probability that a specific event, detected by the LLS and time-correlated to relay operation, may have been the cause of a flashover in the network.

Purpose Retinal ischemic phenomena occur in several ocular diseases that share the degeneration and death of retinal ganglion cells (RGCs) as the final event. We tested the neuroprotective effect of azithromycin, a widely used semisynthetic macrolide antibiotic endowed with anti-inflammatory and immunomodulatory properties, in a model of retinal ischemic injury induced by transient elevation of intraocular pressure in the rat. Methods Retinal ischemia was induced in adult rats with transient elevation of intraocular pressure. RGCs were retrogradely labeled with Fluoro-Gold, and survival was assessed following a single dose of azithromycin given systemically at the end of the ischemia. The expression of death-associated proteins and extracellular signal-regulated kinase (ERK) activation was studied with western blotting. Expression and activity of matrix metalloproteinase-2 (MMP-2) and -9 were analyzed with gelatin zymography. Results Acute post-injury administration of azithromycin significantly prevented RGC death. This effect was accompanied by reduced calpain activity and prevention of Bcl-2-associated death promoter (Bad) upregulation. The observed neuroprotection was associated with a significant inhibition of MMP-2/-9 gelatinolytic activity and ERK1/2 phosphorylation. Conclusions Azithromycin provides neuroprotection by modifying the inflammatory state of the retina following ischemia/reperfusion injury suggesting potential for repurposing as a drug capable of limiting or preventing retinal neuronal damage.