The impact of recent advances on lightning measurement and detection on the protection of transmission and distribution lines

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1 Current [ka] Current [ka] 1 The impact of recent advances on lightning measurement and detection on the protection of transmission and distribution lines Silverio Visacro, IEEE Member Abstract Recent technological advances have been used to assess and improve the lightning performance of transmission and distribution lines, notably the dissemination of modern lightning detection systems used to determine the historical flash density in very small areas (less than 1 km 2 ), the use of advanced instrumentation to characterize in detail the waveform and amplitude of lightning currents of both first and subsequent strokes and the application of advanced electromagnetic models to simulate accurately the overvoltage across line insulators developed from direct strikes to the line or induced from nearby strikes. Such tools have been allowing to develop systematic sensitivity analyses to determine realistic expectations of lightning flashes over insulators of individual structures (towers and poles) using the disruptive method (DE). The results of such evaluations are used to determine improved practices and recommendations to decrease the number of lightning outages to recommended values. Index Terms Lightning; Lightning Performance of Transmission and Distribution Lines; Modern Techniques to Improve the Lightning Performance of Transmission Lines. L I. INTRODUCTION IGHTNING is a transient phenomenon that occurs in atmosphere, responsible for transferring electrical charges from thunderclouds to the earth by means of a short duration and high intensity return stroke current flowing along discharge channels established between cloud and ground. Different mechanisms can operate such transference either of negative or positive charges. The most common mechanism is the negative downward lightning that corresponds to around 9% of cloud-to-ground lightning events. Once a negative downward lightning occurs, after the flow of the return stroke current, which lasts a few milliseconds, the event practically ceases, but only in around 2% of cases. In most cases (~8%), after several tens of milliseconds the lightning channel is crossed by one or more subsequent return stroke currents. The return stroke can use the entire channel or can use it partially in the cases a new termination is reconstructed from the tip of a dissipating lightning channel connecting it to the ground nearby the original striking point S. Visacro is the Head of the LRC - Lightning Research Center of Federal University of Minas Gerais (UFMG - Av. Antonio Carlos Belo Horizonte, MG, Brazil - Lrc@cpdee.ufmg.br) and is a Full Professor of Electrical Engineering Department of UFMG. (typically distant up to around 2.5 km). Flash is the designation of a lightning event, comprising either a single stroke (and corresponding single discharge current) or multiple strokes, each of them with a return current. It is important to note that the current of single stroke flashes and of first strokes in multiple stroke flashes are similar and quite different from those of subsequent strokes, as illustrated by typical measured currents in Fig. 1. Time [ms] Time [ms] Fig. 1. Typical currents of first (a) and subsequent (b) negative downward strokes measured at the instrumented tower in Morro do Cachimbo Station Brazil. Adapted from [1]. This subject, which is introduced only briefly in this section, is considered in detail in chapter 2 to 4 of [1]. II. BASIC CONSIDERATIONS ON LIGHTNING EFFECTS Direct strokes to phases or to shield wires of transmission lines might cause high overvoltage and electrical discharges across insulator strings, leading to short-circuits and, eventually, to line outages. Lightning induced overvoltage caused by nearby strikes is the main source of faults and damages to distribution lines [1,2]. Regarding these occurrences, the severity of lightning effects depends on a set of factors associated to the lightning event and to the line characteristics. Due to its frequency, the negative downward stroke is the event of major interest as far as lightning overvoltage is involved. A. The source of lightning effects on transmission and distribution lines The primary source of lightning overvoltage consists in the return stroke current. The parameters of such current (mainly the peak value I P, the front time t F and waveform) define the features of the overvoltage wave stressing the line insulators. The median peak current of first negative stroke may vary from 3 ka [3,4] to 45 ka [5], while for subsequent strokes it can vary from 12 ka to 16 ka respectively. According to such

2 2 references, the median front time of current is around 4 to 6 µs and.8 to.9 µs respectively for first and subsequent strokes. B. The most influent factors on the occurrence of line outages Concerning the phenomenon, there are basically two main factors that govern the line performance: o the local cloud-to-ground flash density N g that determines the frequency that lightning strikes the line; o the characteristic of lightning striking current, since this current is the source of the overvoltage responsible for lightning effects, either direct or induced one. III. RECENT ADVANCES ON LIGHTNING DETECTION, ON MEASUREMENT AND SIMULATION A. Introduction In the last decade, some technological advances have contributed to improve the assessment of the lightning performance of lines and to develop practices to ensure the performance improvement. B. Dissemination of Lightning Detection Systems Presently Lightning Location Systems (LLS) are installed all over the world, covering large areas. The LLS uses the detection of remote fields radiated from lightning currents to estimate the individual stroke position and time of occurrence [1,6]. Though the results of such systems are affected by environmental conditions that influence the electromagnetic field propagation, the technological advances of such systems have improved the quality and reliability of their results [7]. In the perspective of lightning performance, the main parameter obtained from LLS is the local N g, derived from the historical accumulated data they provide. Presently, N g can be reliably determined for small areas, around 1 km 2. Thus, it is possible to determine the expectation of strikes at each span along the line based on historical data, if the line is georeferred. Fig.2, taken from [8], depicts this kind of application for a real 23 kv line, where the historical rate of lightning strikes along a 1-km-wide strip (5 m at each line side) was determined. The system also provides rough estimates of the peak current for each detected stroke. In spite of the advanced technology of such systems, it is important to understand clearly their features and limitations. The author considers the error in peak current estimates still significant and prefers not to use such estimates in lightning performance evaluations. In this respect, a relevant contribution [9] presented a summary of directly-measured peak currents versus LLS estimates based on measurement of triggered lightning or of very fast pulses in upward lightning currents. Recently, the authors' research group developed the original evaluation of peak-current estimates by lightning detection system by comparison with natural first- and subsequent-stroke currents. The comparison considered the records of the Brazilian LLS and currents measured during two years in Morro do Cachimbo Station [1]. The results confirm the expectation of large errors, corresponding to underestimations in the mean peak currents around 4% and 7% for first and subsequent strokes respectively [1]. C. Instrumentation for measurement of Lightning Currents The knowledge about lightning currents is extremely relevant since they are the main source of lightning effects and some aspects of the physical processes involved in the lightning discharge formation can be inferred from the patterns of such currents. Traditionally, the parameters of currents are obtained from measurements performed at instrumented towers, where a threshold of current is usually adopted to trigger the device responsible for recording the current wave. In most cases the record of the current preceding the threshold is lost and the wavefront has very poor resolution. Recent technologies have allowed improving substantially the quality of the records of lightning currents. In this respect, the measuring system installed in Morro do Cachimbo Station is an excellent reference. Presently, the transducers installed at the tower mast are able to measure currents within a range from 2 A to 2 ka, feeding an 8-channel data acquisition board with a sampling rate of 6 MS/s. The system stores 1-s continuous records for any current exceeding 6 A flowing along the tower, with 33-ns time resolution and 3-ms pretrigger period. Optionally, it can be set to store.5-s-long records with a 17-ns resolution and 15-ms pre-trigger period). Fig. 3, taken from [11], illustrates the possibility to explore different windows of time and current-amplitude to reveal details of measured currents of great interest for lightning protection. This record corresponds to the first stroke of a seven-stroke flash measured in October 7, 29. Note that that the window of time and of current amplitude can decreased to depict details of the impulsive waveform of the first-stroke current, including the beginning of the maximum time derivative around the half-peak or to see details of the unipolar pulses of current developed in response to the steps of the stepped leader approaching the ground during the period the lightning channel is still under formation. Fig. 2. Illustration of LLS application to determine the expected lightning strike rate for each span along the line. Adapted from [8].

3 Voltage (kv) (a) (c) (b) (d) Fig.4. Representative lightning current waves of: (a) first stroke and (b) subsequent lightning stroke measured in MCS; (c) first stroke and (d) subsequent lightning stroke measured in MSS. Adapted from [17]. Fig. 3. Measured lightning-current record shown in different scales of time and amplitude. Adapted from [11]. Determining typical waveforms of real lightning currents is very important to achieve representative overvoltage waveforms required for a correct evaluation of the flash occurrence across insulators. As denoted in Fig. 3(d) typical waveforms of first-stroke currents include an initial concavity followed by an abrupt rise around the half-peak and several subsidiary peaks, being the second peak usually the highest one, as discussed in [12]. The waveform of most subsequent-stroke currents presents a single peak and a relatively smooth shape. With such an approach the current waves of Fig. 4 were determined to simulate realistic waveforms of lightning overvoltages developed in response to direct or nearby strikes. Details on such curves and their analytical representation are available in [12, 13]. It is worth noticing the significant differences of both waveforms in relation to the traditional median waveform representation by Berger [3,4], notably for the first-stroke current. The waveforms of Fig. 4 represents much better real currents of first- and subsequent-stroke currents [17], considering the median parameters of the currents measured at Morro do Cachimbo Station MCS (T d3 front-times of 4.8 ms and.7 ms) [15] or corresponding parameters measured by Berger in Mont San Salvatore - MSS. D. Use of advanced electromagnetic models to simulate lightning effects Another fundamental tool to determine practices to ensure appropriate lightning performance of lines consists in electromagnetic models. They are required to simulate accurately lightning overvoltages developed across insulator strings from a consistent representation of the relevant part of the electrical system stressed by the lightning currents. To develop such an accurate result, the model must be able to represent all the main influent parameters in simulations, such as the tower and geometrical configuration of conductors (shield wires and phase cables), the arrangement of grounding electrodes and the line spans. The international literature presents some reliable models of this type, such as those described in [14-16]. The author developed the Hybrid Electromagnetic Model HEM [14] for such kind of evaluation, which has been applied worldwide to solve lightning related-problems. Figure 5(a) represents the overvoltage calculated across the upper insulator string of a real 23-kV transmission line due to the impression of the first-stroke current represented in Fig. 4(a), simulating a lightning strike to the top of the guyed-tower, as indicated in Fig. 5(b) (a) r = 2.m, e r = 2 r(w), e(w) Fig. 5. Simulated overvoltage across upper insulator string under the assumption of constant and frequency-dependent soil parameters, assuming a 5 m long counterpoise buried in a 2 m soil (a), considering a direct strike to the guyed tower (b). Adapted from [31]. (b)

4 Voltage (kv/ka) 4 IV. GENERAL CONSIDERATIONS ON THE LIGHTNING PROTECTION OF LINES It is not feasible to protect distribution lines against effects of direct lightning strikes, since the related overvoltages are extremely higher than the line insulation withstand. Therefore, the lightning performance of distribution lines is mainly governed by the line response to overvoltages induced by nearby strikes. In this respect, in spite of being relevant to characterize the local flash density and lightning current parameters and to have computational tools available to determine the stress imposed by lightning effects, the protective measures of distribution lines rest basically on the use of surge arresters conjugated with appropriate grounding practices. Surge arresters are very effective to mitigate the prevailing lightning induced effects that are typically associated with currents lower than 1 ka flowing along the distribution system. Furthermore, though surge arresters are not able to avoid insulation failures due to direct strikes, they can usually avoid damages to the distribution network. In this context and considering the huge extension of the subject "lightning protection of lines", the author decided to focus this document on the lightning performance of transmission lines. Nevertheless, in order to supply the readership with details on lightning protection of distribution lines, references [17-24] covering this issue are suggested. V. LIGHTNING PROTECTION OF TRANSMISSION LINES A. General considerations Lightning is a frequent cause of transmission line outages. Three mechanisms are responsible for developing lightning overvoltages that might cause electrical discharges across insulator strings, leading to line faults: the flashover, associated to strikes to phase conductors; the backflashover, associated to strikes to the tower or to shield wires at the tower vicinities; and the midspan flashover, as detailed in [2, 25]. Backflashover largely prevails in transmission lines below 5 kv, installed in regions of moderate and high soil resistivity (natural Brazilian conditions) [25]. Therefore, this mechanism, which is mainly governed by the value of tower-footing grounding impedance, is the main focus of techniques intended to improve the lightning performance of lines. B. Traditional techniques for improving the lightning performance of transmission lines Conventional practices to decrease the frequency of backflashover events comprise basically two actions: (i) the reduction of tower footing resistance and (ii) the installation of surge arresters to prevent flashovers. Most power utilities concentrate their efforts in the first practice due to the costs associated to the installation and maintenance of line surge arresters. It is worth mentioning that reducing grounding resistance is an indirect way to achieve the actual goal, which is to decrease the tower-footing grounding impedance. As explained in [26, 27] the low-frequency grounding resistance can be quite different from the impulse grounding impedance, which is the parameter typically used to characterize the response of electrodes subjected to lightning currents. Nevertheless, the measurement of such impedance is not feasible in field conditions. Since, under certain circumstances, both parameters are related, the measured low-frequency resistance is used to verify the grounding quality instead of the impedance, as discussed in [26]. C. Influent parameters Since backflashover is the main lightning stressing mechanism of transmission lines, it is important to assess the relevance of tower-footing grounding impedance as a factor of major influence on the line performance. The relevant effect of the grounding impedance to decrease the amplitude of lightning overvoltage is illustrated in Figure 6, taken from [25]. This shows simulated overvoltage waves across the upper insulator string of a guyed tower of a 23-kV line, due to a strike to the tower top, approaching the incident lightning current by a triangular 2/5 ms wave. It is worth mentioning that reducing grounding resistance Rg is an indirect way to decrease the tower-footing grounding impedance, as explained in [26]. The overvoltage waves were calculated varying Rg from 8 to 1, considering different values of soil resistivity for a 5-m long counterpoise (as shown in Fig. 5(b)) R g = 8 - r = 64.m R g = 4 - r = 32.m R g = 3 - r = 24.m R g = 2 - r = 16.m R g = 1 - r = 8.m Fig. 6. Overvoltage across upper insulator string of a 23 kv line for different grounding-resistances (2/5 µs triangular current wave). Adapted from [25]. Once a tower or the shielding wires close to the tower are stricken by a lightning event, the balance between the amplitude of the overvoltage experienced across insulator strings and the insulation withstand determine whether a backflashover occurs or not. In the present stage of technology, the evaluation of the flashover condition is done by means of the integration method, also called Disruptive Effect (DE) method [28], that takes the amplitude and waveform of the developed overvoltage into account. This leads to another fundamental influent factor concerning the lightning performance of line: the lightning current wave. The use of realistic current waves is essential to simulate accurately the developed overvoltage wave across insulators, which is integrated by the DE method to determine whether a flashover will occur (or not). Thus, the lightning current waveform, the peak and the front-time parameters have major impact on the resulting overvoltage wave and are all relevant influent parameters. The peak current is almost proportional to the overvoltage amplitude and the current waveform influences strongly the overvoltage waveform, mainly at the wavefront.

5 Voltage (kv/ka) 5 Furthermore, it is important to use reliable statistic distributions of lightning current parameters (at least of peak currents) to determine the expected percentage of faults for each tested condition. Also other line parameters have significant influence on the developed overvoltage wave and, therefore, affect the probability of backflashover occurrence, notably the insulation withstand (strongly influenced by the voltage level of the line), the distance between adjacent towers, the number of shielding wires and the geometrical disposal of the line conductors. D. Recent findings including Non-conventional techniques The large number of influent parameters stresses the need for advanced computational models, such as the HEM model, to calculate accurately the lightning overvoltage experienced by insulators, considering real conditions of line, of lightning current representation, along with consistent expectations of stroke frequencies, in order to assess the backflashover occurrence from the application of the DE method. Basically two main returns might be obtained from exploring the application of the mentioned approach. First, it is possible to obtain the expectations of outages for the given conditions of line. A very recent work [3] applies such an approach to determine the expected outage rate of a 69 kv line, considering different distributions of tower-footing grounding resistance. For the specific conditions described in [3], the application of the approach reveal a new and unexpected finding: subsequent strokes may play an important role in the lightning performance of the transmission line when R LF is limited to 1, as indicated in Table I, taken from [3]. TABLE I CALCULATED OUTAGE-RATE OF 69-KV LINES (NG = 4 FLASH/KM 2 /YEAR) Hypotheses for the distribution of R LF Outages/1km/year 2 (%) 1 (%) 5 (%) below 3 (%) Subs. First strokes strokes Total However, the most relevant potential of this approach concerns the possibility to determine the conditions required for a given line to ensure an aimed lightning performance. For example, for a given line and specific soil condition at the tower location, it is possible to determine the arrangement and dimension of electrodes to achieve a very low defined probability of backflashover. In this respect a recent work that explores this approach deserves to be mentioned because of its original results [31]. If the decrease of soil resistivity for increasing frequency and real permittivity values of soil are taken into account, as described in[32,33], the improvement of the response of the line to lightning strikes is very significant. This makes it possible to determine feasible arrangements of electrodes (of low cost) to ensure appropriate lightning performance of line, releasing the need for applying surge arrester in most conditions [31]. The lower overvoltage curve in Fig. 5 (below the curve obtained with the assumption of constant soil resistivity and permittivity 2 m and e r =2) denotes the significant reduction of the amplitude of the lightning overvoltage developed across the upper insulator string of a 138-kV line due to this effect. It is worth mentioning that this effect is more relevant with increasing soil resistivity. Another relevant recent contribution explores the possibility to achieve additional reductions of lightning overvoltages by means of non-conventional techniques, such as installing the so called underbuilt wires. Fig. 7, taken from [25] shows the overvoltage experienced across upper insulator string of the guyed tower in Fig. 5(b) of a 23 kv line due to the impression of the first-stroke current represented in Fig. 4 on the tower top. A 2- grounding-resistance was assumed for the tower-footing. The curves indicate the significant reduction of the overvoltage amplitude due to the installation of 1 and 2 underbuit wires (4 m below the lower phase). This practice is more effective than reducing the grounding impedance to its half value. The underbuilt wires are required to be installed only in the spans adjacent to the tower whose performance is intended to be improved and the practice effectiveness is raised with increasing soil resistivity. Phase conductor Underbuilt wire d d Shield wire Phase conductors Underbuilt wire original configuration +1 underbuilt wire +2 underbuilt wires Fig. 6. Overvoltage developed across upper insulator string due to the strike of a realistic current wave to the tower top with 1 and 2 underbuilt wires. Adapted from [25] In cases where it is not possible to apply elaborate calculating methods, the traditional simplified practice consisting of adopting a threshold value of tower-footing grounding resistance R LF (low frequency resistance) [29] can be improved for application. In these cases, it is suggested to observe, at least, limits of R LF that depends on the voltage level of line. In [2], it is suggested to limit the maximum value of R LF to 8, 25, 35, 39 and 5, respectively for lines of 69, 138, 23, 345 and 5 kv. However, it is worth mentioning that this threshold resistance (R Max ) is the limit of acceptable values for towers in critical conditions of soil. A lower value of such resistance should be pursued all along the line. An average value around.5.r Max is considered a good aim. Furthermore, it is very important to note that the value of this threshold resistance have to be achieved for electrodes shorter than the effective length, as explained in [26].

6 6 Finally, the mentioned techniques can be used to minimize the cases that require the application of surge arresters. Thus, their application would become limited to very critical conditions, reducing the costs and maintenance. In summary, presently it is possible to use the aforementioned advances to determine efficient practices and recommendations to improve the lightning performance of transmission lines. VI. REFERENCES [1] S. Visacro, Lightning: an Engineering Approach, (book in Portuguese), Ed. ArtLiber, São Paulo, Brazil, pp , 25. [2] S. Visacro, Direct strokes to transmission lines: Considerations on the mechanisms of overvoltage formation and their influence on the lightning performance of lines, J. Lightning Res., vol.1, pp. 6-68, 27. [3] K. Berger, R.B. Anderson, H. Kroninger, Parameters of lightning flashes, Electra, no.8, pp , [4] R. B. Anderson and A. J. Eriksson, Lightning parameters for engineering application, Electra, vol.69, pp , 198 [5] S. Visacro, M. A. O. Schroeder, A. Soares J., L. C. L. Cherchiglia and V. J. Sousa, Statistical analysis of lightning current parameters: measurements at Morro do Cachimbo station, J. Geophys. Res., vol. 19, No. D115, pp. 1-11, 24. [6] C. R. Mesquita, Lightning Detection Systems, MSC. Thesis - Sup. S. Visacro, Electrical Eng. Program, Federal University of Minas Gerais, Belo Horizonte, Brazil, 21 [7] Cummins, K.L, Murphy, M.J, Bardo, E.A., Hiscox, W.L., Pyle, R.B., Pifer, A.E., A combined TOA/MDF Technology Upgrade of the U.S. National Lightning Detection Network. Journal of Geophysical Research, Vol. 13, N D8, Pages [8] Visacro S., Dias R. N. and Mesquita C. R., "Novel approach for determining spots of critical lightning performance along transmission lines", IEEE Trans. Power Delivery, Vol. 2, pp , Apr. 25. [9] Diendorfer, G., Cummins, K.L., Rakov, V.A., Hussein, A.M., Heidler, F. Mair, M., Nag, A., Picher, H., Schulz, W., Jerauld, J. Janischewskyj, W., LLS-estimated versus directly measured currents based on data from tower-initiated and rocket-triggered lightning, in Proc. 29th International Conference on Lightning Protection, Uppsala, 28. [1] Mesquita, C. R. ; Dias, Rosilene N. ; Visacro S., Comparison of peak currents estimated by lightning location system and ground truth references obtained in Morro do Cachimbo Station. Atmospheric Research, v. I, p. ATMOS 2479, 211. [11] Visacro, S. ; Vale, M. H. M. ; Correa, G. M. ; Teixeira, A. M.. The early phase of lightning currents measured in a short tower associated with direct and nearby lightning strikes. Journal of Geophysical Research, v. 15, p. D1614, 21. [12] S. Visacro, A representative curve for lightning current waveshape of first negative stroke, Geophys. Res. Lett., vol. 31, L7112, Apr. 24. [13] A. De Conti and S. Visacro, Analytical representation of single- and double-peaked lightning current waveforms, IEEE Trans. Electromagn. Compat., vol.49, No.2, pp , May 27. [14] S. Visacro and A. Soares Jr., "HEM: a model for simulation of lightning-related engineering problems," IEEE Trans. Power Del., vol.2, no. 2, pp , Apr. 25. [15] G. J. Burke and A. J. Poggio, Numerical electromagnetic code - Method of Moments, Part I: Program description, theory, Naval Electronics System Command (ELEX 341). Washington, DC, Tech. Doc. 116, [16] M. Tsumura, Y. Baba, N. Nagaoka, and A. Ametani, "FDTD simulation of a horizontal grounding electrode and modeling of its equivalent circuit," IEEE Trans. Electromagn. Compat., vol. 48, no. 4, pp , Nov. 26. [17] Silveira, F. H. ; Visacro, S. ; Herrera, J. ; Torres, H., Evaluation of Lightning-Induced Voltages Over a Lossy Ground by the Hybrid Electromagnetic Model. IEEE Transactions on Electromagnetic Compatibility, doi: 1.119/TEMC , v. 51, p , 29. [18] Silveira, F. H. ; Conti, A. R. ; Visacro, S., Lightning Overvoltage Due to First Strokes Considering a Realistic Current Representation. IEEE Transactions on Electromagnetic Compatibility, doi: 1.119/TEMC , v. 52, p , 21. [19] Conti, A. R.; Visacro, S.. Evaluation of lightning surges transferred from medium voltage to low voltage distribution lines. IEE Proceedings. Generation, Transmission & Distribution, England, v. 152, n. 3, p , 25. [2] Conti, A. R. ; Perez, E. ; Soto, E. ; Silveira, F. H. ; Visacro, S. ; Torres, H., Calculation of lightning-induced voltages on overhead distribution lines including insulation breakdown. IEEE Transactions on Power Delivery, doi: 1.119/TPWRD , v. 25, p , 21. [21] Conti, A., Silveira, F. H., Visacro, S., Lightning overvoltages on complex low-voltage distribution, Electric Power Systems Research (211), doi 1.116/j.epsr [22] Silveira, F. H.; Visacro, S.. The Influence of Attachment Height on Lightning-Induced Voltages. IEEE Transactions on Electromagnetic Compatibility, v. 5, p. 1-5, 28. [23] Silveira, F. H.; Conti, A. R. ; Visacro, S., Voltages induced in singlephase overhead lines by first and subsequent negative lightning strokes: influence of the periodically grounded neutral conductor and the ground resistivity. IEEE Transactions on Electromagnetic Compatibility, doi: 1.119/TEMC , v. 53, p , 211. [24] Silveira, F. H.; Visacro, S., On the Lightning-Induced Voltage Amplitude: First Versus Subsequent Negative Strokes. IEEE Transactions on Electromagnetic Compatibility, doi: 1.119/TEMC , v. 51, p , 29. [25] Visacro, S. ; Silveira, F. H.; Conti, A. R.. The use of underbuilt wires to improve the lightning performance of transmission lines. IEEE Transactions on Power Delivery, v. 27, p , 212. [26] S. Visacro, "A comprehensive approach to the grounding response to lightning currents," IEEE Trans. Power Del., vol. 22, no. 1, pp , Jan. 27. [27] S. Visacro and G. Rosado, Response of grounding electrodes to impulsive currents: An experimental evaluation, IEEE Trans. Electromagn. Compat., v. 51, p , Feb. 29. [28] M. Darveniza and A. E. Vlastos, The generalized integration method for predicting impulse volt-time characteristics for non-standard wave shapes A theoretical basis, IEEE Trans. Elect. Insul., vol. 23, no. 3, pp , Jun [29] S. Visacro, Grounding and Earthing: Basic Concepts, Measurements and Instrumentation, Grounding Strategies (book in Portuguese), 2nd ed., São Paulo, Brazil: ArtLiber Edit., pp , 22. [3] Silveira, F. H.; Visacro, S.; Conti, A. R.; Mesquita, C. R.. Backflashovers of Transmission Lines Due to Subsequent Lightning Strokes. IEEE Transactions on Electromagnetic Compatibility, Vol. 54, n. 2, pp , DOI 1.119/TEMC , 212. [31] S. Visacro, F. H. Silveira, S. Xavier, H. B. Ferreira, "Frequency dependence of soil parameters: the influence on the lightning performance of transmission lines", submitted to the 31st International Conference on Lightning Protection, Vienna, 212. [32] Visacro, Silverio; Alipio, Rafael ; Murta Vale, Maria Helena ; Pereira, Clever. The Response of Grounding Electrodes to Lightning Currents: The Effect of Frequency-Dependent Soil Resistivity and Permittivity. IEEE Transactions on Electromagnetic Compatibility v. 53, no. 2, p , 211. [33] Visacro, Silverio; Alipio, Rafael; Frequency Dependence of Soil Parameters: Experimental Results, Predicting Formula and Influence on the Lightning Response of Grounding Electrodes, IEEE Transactions on Power Delivery, v. 27, issue 2, p , 212 [34] A. Soares J., M.A.O. Schroeder, and S. Visacro, "Transient voltages in transmission lines caused by direct lightning strikes", IEEE Trans. Power Del., vol. 2, no.2, pp , Apr. 25.

7 7 [35] S. Visacro, E. P. Antunes, V. G. Machado, V. T. Guedes, Improving the lightning performance of 69 KV to 23 KV transmission lines, in Proc. Int. Conf. on Grounding and Earthing and 2 nd Int. Conf. on Lightning Physics and Effects (GROUND 26 and 2nd LPE), Maceió, Brazil, pp , 26. [36] Visacro, Silverio ; Mesquita, Claudia R. ; De Conti, Alberto ; Silveira, Fernando H.. Updated statistics of lightning currents measured at Morro do Cachimbo Station. Atmospheric Research, v. 1, p. 1-9, 211. VII. BIOGRAPHY Silvério Visacro (M ) received the B.Sc. and M.Sc. degrees in electrical engineering from the Federal University of Minas Gerais (UFMG), Belo Horizonte, in 198 and 1984, respectively, and the Ph.D. degree from the University of Rio de Janeiro, Rio de Janeiro, Brazil, in Since 198, he has been with UFMG, working in Applied Electromagnetics. Currently, he is Full Professor of the Electrical Engineering Department and the Head of the Lightning Research Center (LRC). He has developed many investigation projects of a theoretical and applied nature and is responsible for a successful relationship between his university and industry in his fields of activity. He has authored or coauthored more than 3 scientific papers published in reviewed journals and presented at international conferences. He is the author of two books on lightning and grounding, respectively. He is an Associate Editor of the Journal of Lightning Research and a regular reviewer of the IEEE Trans. EMC and of IEEE Trans. Power Delivery. His current research interests include electromagnetic modeling, grounding and lightning physics, as well as protection. Prof. Visacro is an effective member of the American Geophysical Union (AGU) and International Council on Large Electric Systems (CIGRE), where he convened the Working Group Response of Grounding Electrodes to Lightning Currents.