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In the design of most of the modern systems, lightning threat needs to be considered at the design phase itself. This demands a suitable model and owing to associated complexity, only simplified modeling have been attempted. As a consequence, it does not provide adequate insight into the phenomena. Considering these, a more realistic time-¬ domain electromagnetic model for the return stroke current evolution has been developed by incorporating the following underlying physical processes: (i) excitation formed by the electric field due to charge distribution along the channel, cloud and that induced on ground, (ii) the transient enhancement of series conductance at the bridging regime, which initiates the return stroke,( iii) the non¬linear variation of channel conductance along with (iv) the associated dynamic Electromagnetic Fields(EMFs) that supports the current evolution. The intended modeling begins from the instant of bridging and the pre-¬return stroke charge distribution along the channel is calculated using Charge Simulation Method(CSM). For the calculation of dynamic EMFs, the thin wire Time Domain Electric Field Integral Equation(TD¬EFIE) is employed The transient enhancement of conductance at the bridging/streamer region is emulated using Toepler’s spark law while that along the matured section of the channel is described by first order arc model. The macroscopic physical model developed depicts most of the salient features of current evolution and resulting remote electromagnetic fields in a self¬ consistent manner. The work is not limited by the simplifications adopted for the channel geometry. The strength of the model was exploited for investigating a couple of practically important questions, one of which had divided opinion in the literature. Firstly, analysis showed that the "secondary" current waves generated by successive reflection within struck TGO and that fed by branches do not get reflected at the main wave front. It is shown that the dynamic spatial resistance profile of the channel at the main wave front is primarily responsible for this behavior. Secondly, it is shown that the abrupt change in radii at TGO ¬channel junction is mainly responsible for reflection at the junction. In summary, a novel time-¬domain macroscopic physical model for the first return stroke of a downward cloud¬-to-¬ ground lightning has been successfully developed, which is capable of providing much deeper insight in to the complex phenomena.

Lightning is a natural phenomenon involving transient high current discharge in the atmosphere. Cloud-to-ground lightning, wherein the discharge occurs between the cloud and the ground is quite hazardous to systems on the ground. Apart from threat to life, the devastating effects of lightning can be mainly of thermal, mechanical and electromagnetic origin. Many a times, thermal and electromagnetic effects are of main concern. A direct hit, wherein the system under consideration becomes a part of the lightning path, could be quite catastrophic to many vulnerable systems like oil rigs, chemical factories, missile/satellite launch pads. From the safety and operational point of view, lightning is of serious concern for electrical systems including transmission lines and substations, nuclear power stations, telecommunication station and data banks. Lightning cannot be avoided, however, by employing a suitable Lightning Protection System (LPS), adequate protection against a direct hit can be provided to ground based systems. A typical lightning protection system involves: 1) Air termination network, which is responsible for stroke interception, 2) Down conductor system, which provides to the stroke current a minimal impedance path to the ground and 3) Earth termination network, for safe dissipation of current into the ground. Similarly, for the indirect effects, which are basically of electromagnetic origin, suitable protection can be designed. The key factors in a protective action involve interception of the dangerous strokes, minimization of the consequential potential rise on down conductors, as well as, at earth termination and keeping the field in the protective volume within an acceptable level. The last aspect can be generally categorized into secondary level protection. For critical systems, the lightning protection system is generally isolated from it. In such designs, potential rise on LPS governs the physical isolation required between the protected and protection system. For a given level of bypass strokes, cost of the LPS increases with the amount of physical separation employed. All most all of the earlier works have concentrated on lightning surge response of power transmission line towers. Apart from their relatively moderate heights, the intention was to arrive at a model, which can be incorporated in circuit simulation software like EMTP. Consequently, they envisage or approximate the mode of propagation to be TEM. In reality, for down conductors of height greater than say 30 m, only TM mode prevails during the initial critical time period. Hence the earlier models cannot be extended to general lightning protection schemes and for down conductor of larger lengths. Only limited literature seems to be available on the characteristics of general down conductor configurations. The problem in hand is very important and some serious research efforts are very much essential. In view of the above, the present work aims to evaluate the rise in potential as well as current injected into the soil at the base for: (i) practical range of down conductor configurations involving single down conductor (with height exceeding 30 m) and (ii) pertinent values of stroke current parameters. The protection schemes considered are isolated vertical down conductor, isolated tower (both square and triangular cross-section) and, tower with insulated lightning mast carrying ground wires. The parameters under consideration are: (i) height and cross section for the down conductor, (ii) clearance between the down conductor and the protected system, (iii) channel geometry, wherein only inclination is to be considered, (iv) velocity of current along the channel and (v) wave shape and rise time for the stroke current. For the evaluation of lightning surge response of transmission line towers, many theoretical and experimental approaches are found in the literature. However, works considering the TM mode of current propagation is relatively limited. In that both experimental and theoretical approaches have been adopted. Theoretical approach invariably adopted numerical field computation in frequency domain using Numerical Electromagnetic code (NEC-2). Fourier Transform techniques are employed to extract the time domain quantities. This approach is very economical, free from experimental errors and least time consuming. Hence it is selected for the present work. However, there are certain limitations in this approach. In NEC simulation, there is a restriction on the size and the arrangement of individual elements. Therefore, although fairly complex tower structures can be simulated, some simplification in the geometry is unavoidable. Such an approximation has been reported to cause insignificant error. NEC is not accurate for calculations in low frequency regime. But in the present work, the initial time regime is of concern wherein the high frequency components dominate. Therefore the above said limitation is not of any serious concern. In order to validate the approach, potential rise is computed for 120 m tall cylindrical down conductor and tower. Results are compared favorably with earlier works, which are based on potential lead wire method. A careful re-look into the ’potential rise’ on the down conductors reveal several things. The electric field in the region between the protection system and protected system is the root cause for the breakdown/flashover. For a given geometry, the integral of the electric field along the shortest path between the two systems must be representing the overall stress on the air gap. Further, for the later time periods, this integral coincides with the well-known quasi-static potential. All the available data and models for breakdown of long air gaps are basically in terms of this quasi-static potential. In view of this, the above path integral is defined as ’equivalent potential rise’ (which will be hereafter termed as ’potential rise’), and taken as the index for surge response. Further, observation of the computed spatio-temporal radial electric field around the down conductor reveals some additional features, which are not common in the quasi-static regime. Electric field reverses its polarity in space, which is due to the opposite current flowing in the lightning channel. Therefore, ’potential rise’, which is taken as the representative for the dielectric stress on the air, should not be evaluated for larger distances. Considering this and noting that the protected system generally lies well within a distance of 50% of the H, height of the down conductor, potential rise is evaluated by integrating electric field within this distance (12.5%H, 25%H, 50%H). Three heights (100%H, 75%H, 50%H) are considered for the evaluation of the potential. The influences of various down conductor and lightning channel parameters are analyzed. Finally vertical channel with full velocity for current propagation is arrived for the investigations. Also, the influence of neighboring conducting objects is briefly studied. It is argued that it needs to be ignored for the general study. Analysis is carried out for a range of down conductor configurations of heights ranging from 45 m to 120 m. Cylindrical down conductor is selected for the detailed study on the overall characteristics and its dependency on pertinent parameters. The characteristics of potential rise are found to be significantly different from that given by the commonly employed uniform transmission line model. In the regime of very fast front currents, down conductor of comparable heights have comparable potential rise. For the larger time to crest, behavior tends more to wards that for quasi-static regime. The dependency of the potential rise on radius of the down conductor seems to be logarithmic in nature. Surge response of isolated towers of both square and triangular cross sections is studied for heights ranging from 45 m to 120 m. The overall characteristics are found to be similar to cylindrical down conductor. Dispersive propagation is found to exist on towers. As a result, the base currents are slightly lower and potential rise exhibits less oscillations. Data curves on potential rise at three different heights and for three different spatial extents are generated for the range of down conductor heights with rise time of the stroke current as the variable. Several interesting observations have been made. Next the investigation is taken up for the insulated mast scheme. The parameters of the study are taken as the number of ground wires, grounding location of ground wires and length of the insulation cylinder. Potential across the insulation, tower base currents, and ground wire end currents are deduced. The basic characteristics of the potential rise are shown to be quite similar to that for the transmission line. For fast front currents the temporal variation is bipolar with a smooth decay. In other words, oscillations are sustained for considerably longer duration. Voltage stress across the insulation surface for one ground wire design is found to be higher by 1.4 - 2.4 times than that for isolated tower. The highest amplification of the ground end current, which occurs for fast front currents, is about 1.8 times. Potential difference across the insulation for two-ground wire design is higher by a factor of 1.3 - 1.85 than that for isolated tower. For the design with four ground wires, potential across the insulation is comparable with that for the tower. However, the mechanical strength of the insulating support should also be considered in the selection of number of ground wires. There exists, especially for fast front strokes, significant induction to the supporting tower. The height of the insulation seems to possess no appreciable influence on the potential rise and base currents. Several issues need to be considered before selecting this design. The contribution made by the present work can be summarized as follows. It basically deals with lightning surge response of isolated down conductors of height in the range 45 - 120 m. The configurations considered are, cylindrical down conductor, tower with both square and triangular cross section and insulated mast scheme. It makes a careful study on the ’potential rise’ on down conductors and a suitable definition for the same is proposed. Basic characteristics of potential rise and ground end currents are studied for the above-mentioned designs. Their salient features are enumerated. For the towers, design data curves are provided for relevant range of stroke current rise time. The issues that need to be considered in the insulated mast scheme are discussed along with the data on potential rise and base currents. The findings of this work are believed to be very useful for the design of lightning protection scheme involving isolated down conductor. Further the results are useful in analyzing the consequential lightning generated threat of being close to tall towers.

The work presented in this thesis aims at investigating the attachment of lightning flashes to wind turbines. Modern wind turbines are highly exposed to lightning strikes, due to the increase of their height and the rotation of the blades. Upward lightning is the dominant mechanism of lightning strikes to them. Therefore, this study evaluates the initiation of the initial upward leader discharge and the process of lightning attachment of dart leaders taking place prior to the first return stroke in upward flashes. This work extends the self-consistent leader inception and propagation model (SLIM) to evaluate the lightning attachment of dart and dart-stepped leaders to grounded objects. SLIM was originally proposed to evaluate the lightning attachment of stepped leaders. Unlike the well-studied lightning attachment of stepped leaders, upward connecting leaders initiated in response to dart and dart-stepped leaders develop under a significantly faster change of the ambient electric field. Additionally, these connecting leaders could develop in warm air pre-conditioned by the previous strokes in the same flash. An analytical expression to evaluate the charge required to thermalize the connecting leader per unit length is also developed in the extended model. This model is validated through the analysis of three attachment events recorded in rocket-triggered lightning experiments. Good agreement between the predicted properties of the upward leaders and the measurements has been found. The model is utilized to evaluate the different conditions where connecting leaders can develop prior to the return strokes in upward lightning. The extended model of SLIM is also applied to study the interception of lightning dart leaders by upward connecting leaders initiated from wind turbines. The evaluation considers the influence of the return stroke peak current, the blade rotation and wind on the attachment of lightning dart leaders to wind turbines. The probability of lightning strikes to the receptors along the blade and on the nacelle is calculated for upward lightning flashes. It is shown that the lightning attachment of dart leaders is a mechanism that can explain the lightning damages to the inboard region of the blades (more than 10 meters from the tip) and the nacelle of wind turbines. Furthermore, the critical stabilization electric field required to initiate upward lightning from wind turbines is evaluated for both ‘self-initiated’ and ‘other-triggered’ upward flashes. The calculation shows that the stabilization electric field of an operating wind turbine periodically changes due to the rotation of its blades.  The initiation of upward lightning is greatly facilitated by the electric field change produced by nearby lightning events. However, the rate of rise of the electric field only has a weak impact on the stabilization electric field. The evaluation of the stabilization electric field provides essential information needed for the estimation of the incidence of upward lightning to wind turbines. / <p>QC 20161201</p>

Thesis (Ph.D.)--Mississippi State University. Department of Electrical and Computer Engineering. / Title from title screen. Includes bibliographical references.

Στα πλαίσια της παρούσας διπλωματικής εργασίας γίνεται αναφορά και μελέτη των επιπτώσεων κεραυνικών πληγμάτων σε διατάξεις χαμηλής τάσης και των τρόπων με τους οποίους ο άνθρωπος μπορεί να προστατέψει αυτές τις κατασκευές. Μετά την παρουσίαση των βασικών παραμέτρων του κεραυνού και των συνεπειών των πληγμάτων στις κατασκευές, μελετήθηκε και σχεδιάστηκε ένα σύστημα αντικεραυνικής προστασίας για ένα μοντέλο (υπάρχουσα κτηριακή κατασκευή στην Αχαΐα), το οποίο εν συνεχεία υποβλήθηκε σε κεραυνικό ρεύμα, μέσω προγράμματος εξομοίωσης, ώστε να παρατηρηθεί η συμπεριφορά του και να διαπιστωθούν τα επίπεδα ασφάλειας του. Σκοπός είναι η αποφυγή ζημιών λόγω κεραυνικών πληγμάτων. Η μελέτη έγινε σύμφωνα με τους διεθνείς κανονισμούς και πρότυπα ασφαλείας. Κάνοντας χρήση του λογισμικού προσομοίωσης EMTP – ATP , μελετήθηκε η μεταβατική συμπεριφορά δύο διαφορετικών συστημάτων γείωσης, ώστε να κριθεί αν είναι αποτελεσματικά ή όχι. Στο Κεφάλαιο 1 περιγράφεται το φυσικό φαινόμενο του κεραυνού, τα είδη του, ο μηχανισμός των ατμοσφαιρικών εκκενώσεων και τα φυσικά χαρακτηριστικά του. Στο Κεφάλαιο 2 αναφέρονται οι άσχημες συνέπειες που προκαλούν τα κεραυνικά πλήγματα σε κατασκευές, τη φύση και τον άνθρωπο. Στο Κεφάλαιο 3 παρουσιάζονται οι μέθοδοι προστασίας κατασκευών από κεραυνούς. Αναλύεται και περιγράφεται πλήρως η μελέτη και η διαδικασία επιλογής του συστήματος αντικεραυνικής προστασίας. Εν συνεχεία, παρουσιάζεται αναλυτικά η σχεδίαση του εξωτερικού συστήματος αντικεραυνικής προστασίας μιας κατασκευής τηρώντας τους κανονισμούς. Στο Κεφάλαιο 4 παρουσιάζεται η υπό μελέτη κτηριακή κατασκευή και αναλυτικά τα επιμέρους τμήματα του συστήματος αντικεραυνικής προστασίας που σχεδιάσαμε. Στο Κεφάλαιο 5 αναφέρονται διάφοροι τρόποι μοντελοποίησης του συστήματος και παρουσιάζεται ο επικρατέστερος, που χρησιμοποιήθηκε και στην παρούσα εργασία. Στη συνέχεια υπολογίζονται τα στοιχεία του κυκλώματος γείωσης, καθώς και του συστήματος συλλογής και των αγωγών καθόδου. Μετά τον υπολογισμό των παραμέτρων αυτών σχεδιάζεται το [5] ισοδύναμο κύκλωμα της εγκατάστασης και γίνεται η εισαγωγή τους στο πρόγραμμα εξομοίωσης ATP. Στο Κεφάλαιο 6 γίνεται η επιβολή του κεραυνικού ρεύματος στο κύκλωμα και παρατηρείται η συμπεριφορά του δυναμικού στα σημεία εκχύσεως καθώς και σε γειτονικούς κόμβους. Έπειτα παρουσιάζονται τα αποτελέσματα υπό μορφή γραφημάτων και συγκεντρωτικών πινάκων. Στο τέλος, παρατίθενται παρατηρήσεις και συμπεράσματα. / In this thesis reference and study is made concerning the impact of lightning strikes in low voltage systems and the ways in which humans can protect such structures. After the presentation of the basic parameters of lightning and its consequences on structures, a lightning protection system model was studied and designed (for an actual house in Achaia). After been subjected to lightning current through emulation, observation of its behavior is made, and conclusion to the safety levels. Its purpose is to prevent damage due to lightning strikes. The emulation was made according to the international regulations and standards. Using the simulation software EMTP - ATP, we studied the transient behavior of two different grounding systems, to determine whether they are effective or not. Chapter 1 describes the natural phenomena of lightning, the categories, the mechanism of atmospheric discharges and natural features. Chapter 2 deals with the bad consequences caused by lightning strikes to structures, nature and man. Chapter 3 presents the methods of construction of lightning protection. Fully analyzed and described the study and selection process of the lightning protection system. It then gives a detailed design of the external lightning protection system of a structure in compliance with the regulations. Chapter 4 presents the study brick and mortar construction and detail the individual parts of the lightning protection system that we designed. Chapter 5 suggests several ways of modeling the system and the dominant, the one that we preferred in this thesis, is presented. Then there is the calculation of the grounding circuit elements, the collection system and conductors. After the calculation of these parameters, an equivalent circuit of the installation is designed and they are imported into the simulation software ATP. In Chapter 6 we impose the lightning current in the circuit and observe the dynamic behavior of the discharge points and adjacent nodes. Then follows the presentation of the results in forms of graphs and concentrated tables. In the end, remarks and conclusions are made.

Thesis (M.S.)--Florida State University, 2004. / Advisor: Dr. Henry E. Fuelberg, Florida State University, College of Arts and Sciences, Dept. of Meteorology. Title and description from dissertation home page (viewed Sept. 24, 2004). Includes bibliographical references.

Thesis (M.S.)--Florida State University, 2004. / Advisor: Dr. Henry E. Fuelberg, Florida State University, College of Arts and Sciences, Dept. of Meteorology. Title and description from dissertation home page (viewed Sept. 24, 2004). Includes bibliographical references.

In this dissertation, experimental data was used to investigate the influence of a nearby mast structure on the striking distance to a lightning rod. The results of this research helped identify and understand the impact of different factors such as stroke polarity, lightning rod height, striking distance to the ground, lateral distance from the lightning stroke to an adjacent mast, and height of the adjacent mast on the striking distance of a lightning rod. Moreover, a system identification methodology was employed for the development and validation of striking distance models from experimental work performed at the Mississippi State University High Voltage Laboratory. Striking distance models were obtained to represent the striking distance to the ground, striking distance to an isolated lightning rod, and striking distance to a lightning rod in the presence of an adjacent mast. In the case of the striking distance to the ground the system identification approach was used for the extraction of the parameters of the black-box model proposed. From the results the relationship between the striking distance to ground and the leader voltage were obtained for both polarities of the lightning stroke. The system identification approach was then expanded to obtain the models for the striking distance to the lightning rod. The system identification approach was used to evaluate different mathematical models based on the ones found in the literature. The models were trained on experimental data, their quality evaluated, and the best model was selected for both positive and negative polarity. Furthermore, the model for negative polarity was evaluated against data from real lightning conditions in order to corroborate the model extrapolation capability. Building on the success obtained on the cases of the striking distance to the ground and to an isolated lightning rod the focus was turned to expanding the models to incorporate the influence of an adjacent mast. Models for positive and negative polarity were obtained and the quality of the equations was evaluated.

The Maxwell current density (J(s)), electric field (E), and positive and negative air conductivities were measured simultaneously under several thunderstorms at the NASA Kennedy Space Center (KSC), Florida, during the summer of 1981. The components of J(m) include displacement current as well as field-dependent (J(s)) and convection (J(c)) currents. The measurements under active storms show that: (a) J(m) is usually dominated by displacement currents when E is close to zero, (b) J(m) is steady with time in the intervals between lightning discharges, (c) J(m) is usually not altered significantly by lightning, and (d) the average values of J(m) change slowly over time scales that are comparable to those required for storm development. Field-mill data have been used to derive estimates of the time-average J(m), ‘J(m), under a number of storms at KSC in the years 1976-1978 and 1981. Maps of ‘J(m) are consistent with the locations of radar echoes and lightning charges, and the patterns of ‘J(m) develop and change shape slowly with time. Maximum values of ‘J(m) for large storms are typically on the order of 10 to 15 na/m², and those for small storms are 2 to 4 na/m². Since J(m) is a solenoidal vector, area-integrals of the ‘J(m) maps (‘I) on the ground provided at least a lower limit to the total storm current aloft. Maximum values of ‘I for small convective storms are on the order of 0.1 to 0.5 A, and the maximum values for large storms are at least 3 to 5 times larger. Attempts to infer the location, magnitude, and geometry of the current sources aloft from the field-derived estimates of ‘J(m) have been hampered by a 10-20% variance in the values of ‘J(m). These errors prevent a unique characterization of the current sources aloft unless other data can be included in the analysis. Polar conductivities have been found to be highly variable in a thunderstorm environment, but the total conductivity often remains comparable to that found in fair weather. Conductivities derived from Maxwell current estimates of Jₑ vs. E are about an order of magnitude larger than the direct measurements; therefore, the Jₑ vs. E method of estimating σ may not be valued.