Temporal and wavelet characteristics of initial breakdown pulses are meticulously studied especially during the earliest moment of lightning events. Any possible features during the earliest moment that may exist which lead to either negative cloud-to-ground (CG), positive cloud-to-ground, cloud or isolated breakdown flashes in Sweden are investigated. Moreover, the occurrence of narrow bipolar pulses (NBPs) as part of a CG event that has been recorded from tropical thunderstorms are also included in the investigation. Electric field signatures selected from a collection of waveforms recorded using fast electric field broadband antenna system installed in Uppsala, Sweden and Skudai, South Malaysia are then carefully analyzed in order to observe any similarities or/and differences of their features. Temporal analysis reveals that there are significant distinctions within the first 1 ms among different types of lightning flashes. It is found that a negative CG flash tends to radiate pulses more frequently than other flashes and a cloud flash tends to radiate shorter pulses than other flashes but less frequently when compared to negative CG and isolated breakdown flashes. Perhaps, the ionization process during the earliest moment of negative CG flashes is more rapid than other discharges. Using a wavelet transformation, it can be suggested that the first electric field pulse of both negative CG and cloud flashes experiences a more rapid and extensive ionization process compared to positive CG and isolated breakdown flashes. Further temporal analysis on NBPs found to occur as part of CG flashes show the disparity of the normalized electric field amplitude between the NBPs prior to and after the first return stroke. This indicates that the NBPs intensities were influenced by the return stroke events and they occurred in the same thundercloud. The similarity between the temporal characteristics of NBPs as part of CG flashes and isolated NBPs suggests that their breakdown mechanisms might be similar.

Η παρούσα διπλωματική εργασία πραγματεύεται την καταμέτρηση της κεραυνικής πυκνότητας στην ευρύτερη περιοχή Πατρών. Η εργασία αυτή εκπονήθηκε από το Εργαστήριο Υψηλών Τάσεων του Τμήματος Ηλεκτρολόγων Μηχανικών και Τεχνολογίας Υπολογιστών της Πολυτεχνικής Σχολής του Πανεπιστημίου Πατρών. Σκοπός είναι η εκτίμηση των αριθμών των εκκενώσεων μεταξύ νέφους και γης ανά τετραγωνικό χιλιόμετρο και ανά έτος που έπληξαν την ευρύτερη περιοχή της Πάτρας χρησιμοποιώντας κατάλληλα συστήματα μετρητών. Αρχικά επεξηγείται το φαινόμενο του κεραυνού και τα φυσικά του χαρακτηριστικά. Αναλύονται επίσης οι λόγοι για τους οποίους είναι χρήσιμο να γνωρίζουμε τον αριθμό των κεραυνών που έπληξαν μία περιοχή. Στη συνέχεια ορίζεται ο όρος της κεραυνικής πυκνότητας και ποιά είναι η πρακτική του αξία. Παραθέτονται διάφοροι τρόποι με τους οποίους μπορούμε να την υπολογίσουμε θεωρητικά και να την συσχετίσουμε με τον αριθμό των ημερών καταιγίδας μιας περιοχής ανάλογα με τα τοπογραφικά της δεδομένα. Το επόμενο βήμα είναι η μελέτη των συστημάτων με τα οποία μπορεί να εκτιμηθεί πειραματικά ο αριθμός των κεραυνών που έπεσαν εντός κάποιας εμβέλειας. Αναφέρονται επίσης σύγχρονα συστήματα με τα οποία μπορούμε να κάνουμε απευθείας εκτίμηση της κεραυνικής πυκνότητας καθώς και να αντλήσουμε πληροφορία για την φύση των εκκενώσεων που πλήττουν μία περιοχή. Τέλος με χρήση ενός παγκόσμιας αποδοχής μετρητή κεραυνών (CIGRE 10 kHz) αναλύεται έρευνα η οποία εκπονήθηκε για την εκτίμηση της τιμής της κεραυνικής πυκνότητας στην ευρύτερη περιοχή Πατρών. Σχολιάζονται τα αποτελέσματα που προέκυψαν μετά από καταμέτρηση χρονικής περιόδου ενάμιση χρόνου, συγκρίνονται με παλαιότερες έρευνες που είχαν εκπονηθεί υπό την αιγίδα του Εργαστηρίου Υψηλών Τάσεων και γίνεται εξαγωγή συμπερασμάτων. Τα αποτελέσματα της έρευνας έδειξαν ότι η κεραυνική πυκνότητα στην ευρύτερη περιοχή Πατρών κυμαίνεται περί τους 7,9 κεραυνούς ανά τετραγωνικό χιλιόμετρο ανά έτος. Η τιμή αυτή δεν συμφωνεί με παλαιότερες έρευνες που είχαν δείξει ότι η κεραυνική πυκνότητα στην ευρύτερη περιοχή Πατρών κινείται μεταξύ 3 - 5 κεραυνών ανά τετραγωνικό χιλιόμετρο ανά έτος. Για να εκτιμηθεί αν η τιμή που υπολογίστηκε είναι ορθή, αλλά και για να εκτιμηθεί εκ νέου η κεραυνική πυκνότητα στην ευρύτερη περιοχή Πατρών, πρέπει να συνεχιστούν οι καταγραφές για αρκετά χρόνια ακόμα, έτσι ώστε να υπολογιστεί μία πιο σφαιρική εικόνα, καθώς δεν μπορούμε να βασιστούμε στα δεδομένα μόνο μιας μικρής περιόδου για την εξαγωγή γενικών συμπερασμάτων. / The following project deals with measuring the lightning flash density in the broader area of Patras. This project was carried out by the High Voltage Laboratory of the Department of Electrical and Computer Engineering in the University of Patras. This paper aims to estimate the number of cloud to ground lightning flash charges per square kilometer and per year hitting the broader area of Patras with the use of appropriate measuring systems. At the start an attempt to define and explain the nature of lightning is made. Then follows an analysis of the practical reasons why it helps to know the number of lightning strikes in an area. Next comes a definition of the term lightning density and its practical implications. A variety of methods, with which we can calculate it theoretically and associate it with the number of thunder days in an area in relation to the topographic data of the area, are cited. The next step is to study the systems with which the number of lightning charges striking within a specific range can be assessed. In addition, modern systems with which we can directly estimate lightning density as well as derive information about the nature of the charges an area receives. Finally, with the use of a worldwide accepted lightning flash counter (CIGRE 10 kHz), the research conducted to estimate the value of lightning flash density in the broader area of Patras is analyzed. The findings which resulted after measurements for a time span of eighteen months are presented and compared to earlier researches conducted under the auspices of High Voltage Laboratory and in the end some conclusions are presented. The findings of the research indicate that the lightning flash density in the broader area of the city comes up to 7,9 charges per km2 per year. This value is not in accordance with earlier researches showing that the lightning density in the broader area of Patras ranges from 3 to 5 charges per km2 per year. In order to assess the validity of the findings, as well as to reassess the lightning density in the broader area of Patras, more measurements must be recorded for more years to come so that a better, more rounded image can be presented before we safely come to conclusions.

Lightning causes important transient disturbances on transmission and distribution systems, with consequent damages to equipment, outages, and general decrease of the power quality. The assessment of the lightning dielectric strength of power equipment is generally based on tests performed using the standard lightning impulse voltage (1.2 / 50 µs waveshape), although the characteristics of the lightning overvoltages depend on many parameters and may vary widely. The behavior of insulators when subject to non-standard impulses depends both on the voltage amplitude and waveshape, and therefore a reliable model is required to produce the corresponding volt-time curves. Although there is no method universally accepted for this purpose, one of the most used is the Disruptive Effect (DE) model, which is based on the integration method concept. The application of this model involves the estimation of some parameters for which different procedures have been proposed in the literature, as for instance the procedures by Darveniza and Vlastos, by Hileman, by Chowdhuri et al., and by Ancajima et al. Tests of representative lightning overvoltages were performed to obtain the critical flashover overvoltages (CFO) and the volt-time curves of typical porcelain pin-type insulators considering three standard medium-voltage distribution classes (15 kV, 24 kV, and 36 kV) and five impulse voltage waveshapes, of both polarities. These tests provided data for the analyses of the insulators\' behavior and the results obtained using the different procedures for estimating the DE parameters. It is shown that in some cases insulator flashover is not predicted. A new method is then developed and proposed for evaluating the dielectric behavior of MV insulators. The method is validated using the typical insulators of the three voltage classes and the five lightning impulse voltages considered, of positive and negative polarities. The calculated volt-time curves showed in general a good agreement with the measured results for all the cases studied. The mean difference between the measured and calculated times to breakdown, for all the cases considered, was about 1.3 s; while the maximum difference was 4.0 s. The application of the proposed method to evaluate the occurrence of insulator flashovers in the shield wire line (SWL) system implemented in the State of Rondônia due to nearby lightning strikes supports previous conclusions that indicate that lightning has a significant impact on the SWL system performance in regions with high ground flash density. / Descargas atmosféricas produzem distúrbios transitórios significativos em sistemas de transmissão e distribuição, com consequentes danos em equipamentos, interrupções e redução geral na qualidade de energia elétrica. A avaliação da suportabilidade dielétrica frente às descargas atmosféricas do equipamento de potência é geralmente baseada em ensaios realizados usando o impulso atmosférico de tensão normalizada (forma de onda 1.2 / 50 µs), contudo as características das sobretensões atmosféricas dependem de muitos parâmetros e podem variar amplamente. O comportamento dos isoladores quando sujeitos a impulsos não normalizados depende tanto da magnitude como da forma de onda da tensão, e então um modelo confiável é necessário para se obter as curvas tensão-tempo correspondentes. Embora não haja um método universalmente aceito para essa finalidade, um dos mais utilizados é o modelo de Efeito Disruptivo (DE), que é baseado no conceito do método de integração. A aplicação desse modelo envolve a estimativa de alguns parâmetros para os quais diferentes procedimentos têm sido propostos na literatura, como por exemplo, os procedimentos de Darveniza e Vlastos, de Hileman, de Chowdhuri et al. e de Ancajima et al. Ensaios de sobretensões atmosféricas representativas foram feitos para obter as tensões de descarga disruptiva (CFO) e as curvas de tensão-tempo de isoladores de porcelana típicos, tipo pino, considerando três classes de tensão de distribuição de média tensão (15 kV, 24 kV e 36 kV) e cinco formas de onda de impulso de tensão, de ambas as polaridades. Estes ensaios proporcionaram dados para as análises do comportamento dos isoladores e os resultados obtidos usando os diferentes procedimentos para estimar os parâmetros necessários para a aplicação do modelo do Efeito Disruptivo. É mostrado que em alguns casos a disrupção no isolador não é prevista por tais procedimentos. Um novo método é, então, desenvolvido e proposto para avaliar o comportamento dielétrico dos isoladores de média tensão. O método é validado usando os isoladores típicos das três classes de tensão e as cinco formas de tensões de impulso atmosféricos consideradas, de polaridades positiva e negativa. As curvas tensão-tempo calculadas mostraram, em geral, boa concordância com os resultados medidos para todos os casos estudados. A diferença média entre os tempos de disrupção medidos e calculados, para todos os casos considerados, foi da ordem de 1,3 s; enquanto a máxima diferença foi de 4,0 s. A aplicação do método proposto para avaliar a ocorrência de disrupções em isoladores do Sistema de Cabo Para-raios Energizados (PRE), implementado no estado de Rondônia, devido a descargas atmosféricas indiretas, apoia as conclusões previamente obtidas que indicam que as descargas atmosféricas têm impacto significativo sobre o desempenho do sistema PRE em regiões com alta densidade de descargas para solo.

Lightning causes important transient disturbances on transmission and distribution systems, with consequent damages to equipment, outages, and general decrease of the power quality. The assessment of the lightning dielectric strength of power equipment is generally based on tests performed using the standard lightning impulse voltage (1.2 / 50 µs waveshape), although the characteristics of the lightning overvoltages depend on many parameters and may vary widely. The behavior of insulators when subject to non-standard impulses depends both on the voltage amplitude and waveshape, and therefore a reliable model is required to produce the corresponding volt-time curves. Although there is no method universally accepted for this purpose, one of the most used is the Disruptive Effect (DE) model, which is based on the integration method concept. The application of this model involves the estimation of some parameters for which different procedures have been proposed in the literature, as for instance the procedures by Darveniza and Vlastos, by Hileman, by Chowdhuri et al., and by Ancajima et al. Tests of representative lightning overvoltages were performed to obtain the critical flashover overvoltages (CFO) and the volt-time curves of typical porcelain pin-type insulators considering three standard medium-voltage distribution classes (15 kV, 24 kV, and 36 kV) and five impulse voltage waveshapes, of both polarities. These tests provided data for the analyses of the insulators\' behavior and the results obtained using the different procedures for estimating the DE parameters. It is shown that in some cases insulator flashover is not predicted. A new method is then developed and proposed for evaluating the dielectric behavior of MV insulators. The method is validated using the typical insulators of the three voltage classes and the five lightning impulse voltages considered, of positive and negative polarities. The calculated volt-time curves showed in general a good agreement with the measured results for all the cases studied. The mean difference between the measured and calculated times to breakdown, for all the cases considered, was about 1.3 s; while the maximum difference was 4.0 s. The application of the proposed method to evaluate the occurrence of insulator flashovers in the shield wire line (SWL) system implemented in the State of Rondônia due to nearby lightning strikes supports previous conclusions that indicate that lightning has a significant impact on the SWL system performance in regions with high ground flash density. / Descargas atmosféricas produzem distúrbios transitórios significativos em sistemas de transmissão e distribuição, com consequentes danos em equipamentos, interrupções e redução geral na qualidade de energia elétrica. A avaliação da suportabilidade dielétrica frente às descargas atmosféricas do equipamento de potência é geralmente baseada em ensaios realizados usando o impulso atmosférico de tensão normalizada (forma de onda 1.2 / 50 µs), contudo as características das sobretensões atmosféricas dependem de muitos parâmetros e podem variar amplamente. O comportamento dos isoladores quando sujeitos a impulsos não normalizados depende tanto da magnitude como da forma de onda da tensão, e então um modelo confiável é necessário para se obter as curvas tensão-tempo correspondentes. Embora não haja um método universalmente aceito para essa finalidade, um dos mais utilizados é o modelo de Efeito Disruptivo (DE), que é baseado no conceito do método de integração. A aplicação desse modelo envolve a estimativa de alguns parâmetros para os quais diferentes procedimentos têm sido propostos na literatura, como por exemplo, os procedimentos de Darveniza e Vlastos, de Hileman, de Chowdhuri et al. e de Ancajima et al. Ensaios de sobretensões atmosféricas representativas foram feitos para obter as tensões de descarga disruptiva (CFO) e as curvas de tensão-tempo de isoladores de porcelana típicos, tipo pino, considerando três classes de tensão de distribuição de média tensão (15 kV, 24 kV e 36 kV) e cinco formas de onda de impulso de tensão, de ambas as polaridades. Estes ensaios proporcionaram dados para as análises do comportamento dos isoladores e os resultados obtidos usando os diferentes procedimentos para estimar os parâmetros necessários para a aplicação do modelo do Efeito Disruptivo. É mostrado que em alguns casos a disrupção no isolador não é prevista por tais procedimentos. Um novo método é, então, desenvolvido e proposto para avaliar o comportamento dielétrico dos isoladores de média tensão. O método é validado usando os isoladores típicos das três classes de tensão e as cinco formas de tensões de impulso atmosféricos consideradas, de polaridades positiva e negativa. As curvas tensão-tempo calculadas mostraram, em geral, boa concordância com os resultados medidos para todos os casos estudados. A diferença média entre os tempos de disrupção medidos e calculados, para todos os casos considerados, foi da ordem de 1,3 s; enquanto a máxima diferença foi de 4,0 s. A aplicação do método proposto para avaliar a ocorrência de disrupções em isoladores do Sistema de Cabo Para-raios Energizados (PRE), implementado no estado de Rondônia, devido a descargas atmosféricas indiretas, apoia as conclusões previamente obtidas que indicam que as descargas atmosféricas têm impacto significativo sobre o desempenho do sistema PRE em regiões com alta densidade de descargas para solo.