Faculty of Engineering & Built Environment, School of Electrical & Info Engineering, Dissertation / The primary objective of this paper is to present results regarding data obtained from Eskom’s Lightning Positioning and Tracking System (LPATS) and is a continuation of the work presented at the two SAUPEC Conferences in Pretoria and Stellenbosch [1, 2]. LPATS provides some useful information regarding the lightning field measurements around the Brixton and Hillbrow Towers, in Johannesburg, for the two seasons of June 2001 to June 2003. The results suggest that there is a significant increase in apparent ground flash density in the vicinity of the towers when compared to the surrounding areas. The observation of mean current values in the order of -20kA suggests that the increased contribution of upward flashes to the total incidence of flashes in tall structures should lead to a decrease in measured current amplitudes.

Σκοπός αυτής της διπλωματικής εργασίας, είναι να παραθέσει τους τρόπους με τους οποίους προστατεύουμε ψηλά κτίρια - κατασκευές από κεραυνικά πλήγματα. Η προστασία των ψηλών κατασκευών είναι εντελώς διαφορετική από την προστασία χαμηλότερων κατασκευών αφού παύουν να ισχύουν οι κανόνες και τα επίπεδα προστασίας για κτίρια μέχρι 60m που ισχύουν στους διεθνείς οργανισμούς και τον Ε.Λ.Ο.Τ. Από τα 60m και πάνω οι συνθήκες είναι πολύ διαφορετικές, γι αυτό το λόγο γίνεται επεξήγηση για όλους τους παράγοντες που επηρεάζουν μια ψηλή κατασκευή όσον αφορά την προστασία της με τη γειωμένη μεταλλική ράβδο του αλεξικέραυνου του Franklin. Επίσης, μέσω της εφαρμοσμένης υπολογιστικής εξομοίωσης γίνεται προσπάθεια να βρεθεί η απόσταση διάσπασης ( stricking distance ) δηλαδή η ακτίνα προστασίας που καλύπτει μια κατασκευή με μέθοδο προστασίας την ακίδα Franklin. Τα αποτελέσματα και τα συμπεράσματα συγκρίνονται με πειραματικές μετρήσεις που έγιναν σε εργαστήριο. Παρακάτω παρατίθενται τα περιεχόμενα του κάθε κεφαλαίου της εν λόγω εργασίας. Στο 2ο κεφάλαιο γίνεται πλήρης ανάλυση για τη φυσική των κεραυνών. Γίνεται κατηγοριοποίηση των φάσεων που εξελίσσονται σε κεραυνό από τη γη μέχρι τα σύννεφα και παρουσιάζεται ο μηχανισμός των ατμοσφαιρικών εκκενώσεων. Ακόμα, γίνεται εξήγηση για τους ανοδικούς συνδετικούς οχετούς και την απόσταση διάσπασης από τα’ αλεξικέραυνα και τους οχετούς καθόδου. Στο 3ο κεφάλαιο αναπτύσσονται οι βασικοί παράμετροι και εξισώσεις που ισχύουν για ψηλά κτίρια όπως η ελάχιστη ακτίνα προστασίας-απόσταση διάσπασης, η ισοδύναμη επιφάνεια, η πιθανότητα της ελάχιστης ακτίνας διάσπασης και ανοδικών leader από τέτοια ψηλά κτίρια, ο επηρεασμός της ακίδας προστασίας και απόστασης διάσπασης από θετικούς κεραυνούς, τη σχέση που έχουν τα ψηλά κτίρια με την απόσταση διάσπασης και την επίδραση των γειτονικών κατασκευών. Ακολούθως, γίνεται περιγραφή της μεθόδου CVM για το χειρισμό ψηλών κατασκευών με επίπεδα και γωνίες προστασίας και πίνακες ρίσκου βασισμένα σε στατιστικές από κεραυνούς. Μετέπειτα, βλέπουμε πως επηρεάζεται η απόσταση διάσπασης από τη γεωμετρία της κατασκευής, από την γεωμετρία της ακίδας προστασίας Franklin, αλλά και από τη βέλτιστη ακτίνα κορυφής της ακίδας προστασίας Franklin. Στο 4ο κεφάλαιο γίνεται προσπάθεια να προσδιορίσουμε τη ζώνη προστασίας με τη χρήση υπολογιστικού μοντέλου. Αρχικά, αναφέρουμε κάποια στοχαστικά μοντέλα διάσπασης διηλεκτρικών. Μετά προχωρούμε στην περιγραφή με λεπτομέρεια των υπολογιστικών εξομοιώσεων που πραγματοποιήσαμε και την τακτική επεξεργασίας τους. Ακολούθως, προσδιορίσαμε τη ζώνη προστασίας των εξομοιώσεων για ύψος ακίδας 80cm και 100cm με γραμμικές εξισώσεις από προσαρμογή των μετρήσεων στις γραφικές παραστάσεις που δείχνουμε και αντίστοιχα στοιχεία για τις δυο ακίδες με αύξηση της τάσης 10%. Στο τέλος γίνεται επεξεργασία των δεδομένων και συγκρίνουμε τις μετρήσεις που βρήκαμε μεταξύ τους αλλά και με άλλους μελετητές. Αναφέρουμε τα αποτελέσματα της διεξαγωγής των υπολογιστικών εξομοιώσεων και τα συμπεράσματα. Στο 5ο κεφάλαιο κάνουμε ανακεφαλαίωση των θεωρητικών στοιχείων που ισχύουν για τις ψηλές κατασκευές και γενική συζήτηση. Επίσης εξάγονται χρήσιμα συμπεράσματα από τις υπολογιστικές εξομοιώσεις που πραγματοποιήθηκαν τόσο για την απόσταση διάσπασης όσο και για τις εξομοιώσεις που πραγματοποιήσαμε / The purpose of this project is to set out the possible ways that protect tall structures from lightning strikes. The protection of the tall structures is a completely different task from the protection of the shorter structures. That is because the rules and the protection levels applied by National Organizations (International Committee) and Ε.Λ.Ο.Τ. that concern structures to 60 meters, cease to exist in the case of taller structures. Concerning the structures that are taller than 60 meters the protection circumstances are very different from those of shorter structures. That is why this thesis explain all the factors that affect a tall structure, as far as its protection with the “Franklin Rod” is concerned. Furthermore, through computer simulation the author attempted to determine the striking distance, which is the protection radius that covers a structure, by utilizing as a method of protection the Franklin Rod. The results and conclusions that arose were compared with experimental measurements that took place in the lab. Below, the content of each chapter of this thesis is described. In the second chapter it is attempted a thorough analysis of the nature of lightning. Then there is a categorization of the phases that evolve to a lightning, from the ground to the clouds. The mechanism of atmospheric evacuation is also presented. Moreover, the upward connection leaders, the striking distance from the lightning rods and the downward leaders are described and explained. In the third chapter, the basic parameters and equations that apply to tall buildings are described. Some of these parameters are the attractive radius, the striking distance, the equivalent exposure area, the weighted average attractive area, the upward leaders from such tall buildings, the influence of the Franklin rod, the striking distance from positive flashes, the relation that the tall structures have with the striking distance and finally the influence of the surrounding structures. In addition, the CVM (Collection Volume Method) is described which deals with tall structures by utilizing protection levels and derating angles and risk analysis tables based on lightning’s statistics. Moreover, we see how the striking distance is affected by the structure geometry, by the geometry of the Franklin rod but also by the optimum tip radius of Franklin rod. In the fourth chapter the author attempted to determine the protection zone by using a computer model. Firstly, some stochastic models of dielectric breakdown are described. Furthermore, a detailed description of the computer simulations that we accomplished and the method of processing them are described. Moreover, the author determined the protection zone of the simulations for rod height: 80cm and 100cm with linear equations, by using measurement fitting to graphs where we show the respective elements for the two rods by raising 10 % of the Volts of the measurement. In the end, the data were processed and a comparison of this thesis’ findings and that of other authors were compared. The author also describes the results of the computer simulations and the conclusions that arose. In the fifth chapter the author revised the theoretical elements that apply to the tall structures and makes reflections on the findings. Moreover, useful conclusions arise from computer simulations that took place as far as the striking distance as well as the simulation is concerned.

Nonlinear time history analysis is increasingly being used in the design of tall steel structures, but member sizes still must be determined by a designer before an analysis can be performed. Often the distribution of story strength is still based on an assumed first mode response as determined from the Equivalent Lateral Force (ELF) procedure. For tall buckling restrained braced frames (BRBFs), two questions remain unanswered: what brace distribution will minimize total brace area, while satisfying story drift and ductility limits, and is the ELF procedure an effective approximation of that distribution? In order to investigate these issues, an optimization algorithm was incorporated into the OpenSees dynamic analysis platform. The resulting program uses a genetic algorithm to determine optimum designs that satisfy prescribed drift/ductility limits during nonlinear time history analyses. The computer program was used to investigate the optimized distribution of brace strength in BRBFs with different heights. The results of the study provide insight into efficient design of tall buildings in high seismic areas and evaluate the effectiveness of the ELF procedure.

No description available.

No description available.

Electromagnetic effects of lightning currents during a direct hit to tall communication towers, other instrumented towers and chimneys can be hazardous to associated cables, as well as, electrical and electronics systems. The standard practice in telecommunication and other related fields is to bond the cable sheath to the tower and ground connection is made before it enters the base station. However, in some specific cases when power, signal and data logging cables are to be supported on the same tower, isolation of power cables is demanded. In a totally different situation, attempts are also made to have a dedicated isolated down conductor. A critical review of the situation demanded a more quantitative answer to the following questions: (i) whether it is possible to electrically isolate a dedicated down conductor, (ii) is it possible to electrically isolate the cables and their terminal equipment both mounted on towers serving as down conductor and if so, what will be the nature of current induced in the cables and (iii) as per the standard practice, if the cable sheaths are connected to the tower/structure, what will be the nature of the current shared by them. Addressing these important issues formed the scope of the present work. For the tall structures considered in this work, for the critical time periods, wave nature of the current dominates. This called for electromagnetic modeling covering Transverse Magnetic (TM) mode of the wave propagation. Owing to the complex geometrical features involved with the problem, both experiments on electromagnetically scaled laboratory models, as well as, theoretical simulation is attempted. An electromagnetically scaled laboratory model is employed for the time domain experimental investigation. This approach, which has been validated earlier, is further scrutinized to ensure its adequacy. In order to achieve generality and noting the fact that the associated parameters are rather difficult to be varied in the experimentation, theoretical investigation is also employed. For this, both NEC-2, as well as, an in-house thin wire time domain code developed for this work is employed. NEC-2 could handle multi-wire multi-radius junctions, while in-house time domain code could handle proximity and non-cylindrical shapes encountered with tower lattice elements. The investigation of induction to isolated cables on simple down conductors and towers is considered first. The induced current is shown to be bipolar oscillatory with the period of oscillation governed by the length of the cable. It is shown that the level of induction for good earth termination is below 5 – 10 % while that with moderate inductance in the earth termination can enhance the induction to higher levels. The level of induction is shown to be not critically dependent on the length of the cable, gap between cable and down conductor/tower. When multiple cables are mounted, they seem to influence each other and individually carry currents of lower amplitude. Also, the effect of shape and proximity of the tower lattice elements on induction is investigated. If the cable is housed inside a metallic tray, the amplitude of induced current is shown to be quite small. Subsequently, the evaluation of electrical stress between the isolated down conductor on tower and simplified representation of the structure is considered. A suitable definition of the electric stress for the wave regime is evolved and then it is shown that, at present, the voltage difference defined by the path integral of electric field across shortest path between the two entities is the best indicator for the stress. The electrical stress in the case of isolated down conductor on tower, as well as, down conductor with isolated cable is shown to reach very dangerous levels. On the other hand, the stress on the isolated cables on towers also serving as down conductors is shown to be relatively moderate. Interestingly, it is shown that the electrical stress and the voltage difference is dependent on the gap and for the critical time period, can be much lower than that calculated as a product of equivalent tower surge impedance and the stroke current, even before the arrival of ground end reflections. Finally, the current shared by cables connected to the down conductor is investigated. For the case of simple cylindrical down conductor with cable connected to it at the top, it is shown that the amount of current shared by the cable is not dependent on its length and the relative radii (cross section) have only a weak influence. For the case with down conductor formed by L and + angles, it is shown that the placement of cable at their interior corner can reduce the initial current shared by the cable. In order to model best possible situation with towers, experiments are conducted with cable inside an aluminum pipe. Even in this case, cable current builds up with successive reflections to become comparable with the current through the pipe itself. Subsequent investigation with 1:40 and 1:20 tower models lead to several interesting observations. Cables running along leg/face of the tower whether placed inside or outside the tower, always shares good amount of current. Further, frequent bonding of the sheath to the tower increases the current shared by the cable. Cable when housed in a metallic tray shares less than 50% of the current shared without the tray. Even though a complete quantification is not to be achieved in this work, it has made a good beginning with some significant contribution towards lightning protection issues pertaining to tall towers and structures.