Dissertations / Theses on the topic 'Air Breakdown'

Compréhension et maîtrise des phénomènes intervenant sous haute tension à l'interface entre un gaz et un isolant solide.Dans le cadre du remplacement du SF6 dans les appareils moyenne tension, une étude de l’isolation électrique mixte (gaz (air) / solide isolant) a été réalisée. L’objectif étant de déterminer l’influence de la nature du solide sur la tenue au claquage. Pour cela plusieurs matériaux ont été sélectionnés tel que l’époxy / silice (matériau de référence chez Schneider Electric), le PA6T/66.GF50 (nouveau matériau pour cette application) ainsi que plusieurs autre matériaux permettant une meilleure compréhension du phénomène (PTFE, PC, PP, …). Les caractérisations diélectriques ont permis de mesurer des déclins de potentiel, des courants volumiques ainsi que des permittivités en fonction de la nature des matériaux. A partir de ces données, des mesures de claquage ont été effectuées. Deux configurations de champ électrique ont été testées (parallèle et perpendiculaire au solide). La géométrie « pointe-plan » a été sélectionnée, permettant l’observation de décharges partielles avant le claquage et donc une étude de la propagation de la décharge et non de l’initiation de celle-ci. Ces mesures ont permis de déterminer l’influence du solide sur la tension de claquage, avec une influence indirecte de la nature du matériau et sa teneur en eau. Dans notre configuration, la présence d’un isolant solide dégrade la tenue diélectrique. La permittivité est le paramètre influençant la tension de claquage, avec une réduction de celle-ci en présence de matériaux à forte permittivité (comme l’alumine). Aucun lien n’a pu être mis en évidence entre les mesures de déclin de potentiel et les tensions de claquage. Pour caractériser le développement de la décharge électrique le long de l’isolant solide, des visualisations et des mesures de courant ont été réalisés. Les visualisations ont permis d’observer deux types de décharges menant avant le claquage. Dans un premier temps le développement d’une décharge en surface « streamer de surface », puis celui d’une décharge dans le gaz « streamer de volume ». Ces observations ont permis de conclure que le streamer menant au claquage se développe majoritairement dans l’air en configuration de champ parallèle. Les mesures de courant ont apporté des informations sur l’initiation de la décharge, en montrant une réduction de la tension et du temps d’initiation de la décharge en présence d’un solide. Des résultats semblables ont été obtenus pour une large gamme de matériaux. Cependant certains matériaux de faible permittivité (PP, PTFE) se distinguent avec des tensions et des temps d’initiation retardés et aléatoires, semblables à ceux obtenus dans l’air sans solide. Il a été montré que l’initiation et la propagation n’influencent pas la tension de claquage en géométrie pointe-plan. Celle-ci est déterminée par la transition au claquage.Mots-clés : appareillages moyenne tension, tension de claquage, caractérisation diélectriques, streamers, courant transitoires, visualisations
Comprehension and control of phenomena occurring under high voltage at the interface between a gas and a solid insulator.In the context of SF6 replacement in medium-voltage apparatus, a study of mixed electrical insulation (gas (air) / insulating solid) was carried out. The aim is to determine the influence of the nature of the solid on breakdown voltage. For this purpose several materials have been selected such as epoxy / silica (reference material of Schneider Electric), PA6T / 66.GF50 (new material for this application) and several other materials allowing a better understanding of the phenomenon (PTFE, PC, PP, ...). The dielectric characterizations allowed us to measure potential decay, currents and permittivities according to the nature of the materials. From this data, breakdown measurements have been carried out. Two configurations of electric field were tested (parallel and perpendicular to the solid). The "point-to-plane" geometry was selected, allowing the observation of partial discharges before breakdown and therefore a study of the propagation of the discharge and not of the initiation. These measurements show the influence of the solid on the breakdown voltage, with an indirect influence of the nature of the material and its water content. In our configuration, the presence of a solid insulator lowers the dielectric strength. The relative permittivity is the main parameter influencing the breakdown voltage, with a reduction of it in the presence of materials with high permittivity (alumina). It was not possible to establish a link between surface potential decay measurements and breakdown voltages. In order to characterize the development of the electric discharge along the solid insulation, visualizations and transient current measurements were achieved. The visualizations allowed the observation two types of discharges before the breakdown. First, the development of a surface discharge "surface streamer", followed by a discharge in the gas "volume streamer". These observations lead us concluding that streamers leading to breakdown develop predominantly in air above the surface in the parallel field configuration. Current measurements provide information on the initiation of the discharge. There exists a reduction of voltage and initiation time of the discharge in the presence of a solid. Similar results have been obtained for a wide range of materials. However, several low permittivity materials (PTFE, PP) show a different behaviour, with higher and scattered time delay and initiation voltage, similar to those obtain in air without solid. It has been shown that initiation and propagation do not influence the breakdown voltage in poi nt-plane geometry. The transition to breakdown mainly determines the breakdown voltage.Key words: medium voltage apparatus, breakdown voltage, dielectric characterization, streamers, transient currents, visualizations

Channeling an extended electrical breakdown of air by a laser beam is a long-standing challenge in applied laser science. Virtually all previously reported experiments on discharge channeling by femtosecond laser beams relied on the application of a single laser pulse and have been conducted with discharge gaps of less than one meter, in which case the direct ohmic heating of the laser-generated plasma by the applied DC electric field is the dominant channeling mechanism. We report a laboratory-scale demonstration of a channeling approach that makes use of concatenated plasma filaments produced by a sequence of multiple ultrashort laser pulses. Direct ohmic heating of the guiding channel is eliminated through the introduction of large temporal delays between the individual laser pulses in the pulse sequence. We propose an extension of this scheme to channeling kilometer-scale discharges, including natural lightning. Our proposed approach alleviates the fundamental range limitations inherent to the single-pulse schemes reported previously. It can channel discharges propagating in either direction and along curved paths. Published by AIP Publishing.

Thesis (Ph. D.) -- University of Maryland, College Park, 2007.
Thesis research directed by: Dept. of Physics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

The laser beam interaction with matter and the plasma generation have been studied for many years. In some applications what is really important is to understand the composition and the temporal evolution of the species in the interested medium. In this thesis, time resolved optical spectroscopy was employed to understand the evolution of the plasma which is produced by interaction of Infrared (1.064 µ
m) laser beam with air. In this thesis, a new technique is suggested to analyze the time evolution of laser induced breakdown spectroscopy. The suggested method and the instrumentation of the setup are tested with a single gas (He). After the tests, we analyzed time sequence spectra of Laser Induced Air Breakdown. The suggested method is based on triggering the laser and the spectrometer at different time and applying the spectrometer trigger time by adding the time delay (&
#916
t) between them by using the pulse generator. The results show that the decay rates are slowing down microseconds after the excitation of the plasma. The results of the time-resolved measurements of the line spectra show that different component of the air has different decay rate, and lifetime. The lifetime of helium is 20 µ
s, and the decay start 5 µ
s after the initiation of plasma. Air has 12 µ
s lifetime, and the decay start 3 µ
s after the initiation of the plasma. Also, the decay rate and the lifetime depend on the state. We also calculate Doppler velocity for different component and different emission states. Doppler velocities show that the component which has great mass has small velocity, the component which has small mass has high velocity.

Dans cette thèse, un modèle de la dynamique du plasma après un claquage microonde dans l'air à pression atmosphérique a été développé. Ce modèle a permis d'expliquer pour la première fois la formation et la dynamique de structures filamentaires auto-organisées lors du claquage microonde. Le claquage microonde dans l'air à pression atmosphérique a été récemment observé au MIT dans des expériences mettant en oeuvre une source microonde de puissance et des caméras rapides. Les mesures montrent que, lors du claquage, un ensemble structuré de filaments de plasma se forme et se dirige vers la source à une vitesse de plusieurs km/s. Les mécanismes de formation et de propagation de ces structures auto-organisées de plasma ne sont pas bien compris et l'objectif de cette thèse a été de mettre en évidence et de modéliser les phénomènes physiques de base qui en sont responsables. Dans le but de décrire la dynamique du plasma après claquage, les équations de Maxwell ont été couplées à un modèle simple de plasma et résolues numériquement. Le modèle de plasma suppose la quasineutralité et décrit l'évolution de la densité de plasma sous l'effet de la diffusion, de l'ionisation, de l'attachement et de la recombinaison électron-ion. L'ionisation et l'attachement sont supposés dépendre du champ électrique effectif local. La vitesse moyenne électronique est déduite d'une équation de transport de quantité de mouvement simplifiée. La diffusion des particules chargées est ambipolaire au sein du plasma mais devient libre dans le front où la densité chute à zéro. Une expression heuristique de la transition entre diffusion ambipolaire dans le corps du plasma et diffusion libre sur les bords a été établie et validée à l'aide d'un modèle mono-dimensionnel de type dérive-diffusion-Poisson que nous avons développé et dans lequel on ne suppose pas la quasineutralité du plasma. Le modèle plasma-Maxwell quasineutre a ensuite été utilisé pour étudier la dynamique du plasma après claquage dans les conditions des expériences du MIT. Les résultats numériques montrent la formation de structures filamentaires auto-organisées de plasma en excellent accord qualitatif avec les observations expérimentales. Ces structures auto-organisées sont liées aux structures du champ électrique diffracté par le plasma. De nouveaux filaments se forment de façon continue dans le front du plasma par des phénomènes de diffusion-ionisation. Le modèle montre que la formation d'un réseau de filaments de plasma auto-organisé est dû à l'apparition des maxima de champ électrique de l'onde stationnaire formée dans le front du plasma. Dans la dernière partie de la thèse, la formation d'un filament de plasma isolé (ou streamer microonde) au maximum de champ formé à l'intersection de deux faisceaux microondes est analysée à l'aide du modèle. Le streamer microonde s'allonge parallèlement à la direction du champ en raison du renforcement du champ à ses pôles (phénomène de polarisation). L'intensité du champ aux extrémités du filament est modulée dans le temps en raison de phénomènes de résonance pour des longueurs de filaments voisines de multiples de la demi longueur d'onde.

Für die Menschen zählen Gewitter mit Donner und Blitz zu den ältesten bekannten Phänomenen der Natur. Die Tatsache, dass Blitze dennoch bis heute nicht abschließend erforscht sind, liegt in dem äußerst komplexen, schnellen und stochastischen Verhalten der Blitzentladung begründet. Der Autor befasste sich in Rahmen dieser Arbeit u.a. mit der Entwicklung eines modernen Messsystems, durch das die Erfassung von sowohl elektrischen als auch optischen Parametern der im Nanosekundenbereich ablaufenden Prozesse der Entladungen möglich wurde. Es handelt sich um technisch anspruchsvolle Messungen von hohen Stoßspannungen und schwachen Vorentladungsströmen in einem großen Frequenzbereich. Außerdem gelang es durch eine geschickte Synchronisation, die Entladungsprozesse präzise und erstmalig in einer Fotosequenz aufzunehmen. Auf Grundlage der Messergebnisse konnte der Entladungsprozess in mehrere Phasen der Vorentladung unterteilt und beschrieben werden. Vor allem beweisen die Messergebnisse die Existenz einer kanalartigen Vorentladung, die nicht zwingend zu einem Durchschlag führt. Dieser Mechanismus, der bei langsam ansteigenden Spannungen als stabile Leaderentladung benannt und mit Thermoionisation gekennzeichnet ist, war bei Blitzstoßspannungen bisher umstritten
The breakdown behavior of long air gaps by Lightning Voltages has been investigated and the results of a new measurement system are reported. A series of up to eight photos (made by a high-speed camera) have recorded the expansion of discharge activities during the breakdown process. The pre-discharge current and voltage have been measured as well. The modern measurement techniques have made possible the synchronization of the photos with the current and voltage measurements in nanosecond-ranges. The combination of the measured current, voltage, and photos can precisely explain the breakdown process. The experiments have been carried out with the lightning impulses up to 2.4 MV and the air gaps up to 4 m. Despite of such high voltages, the experimental system has been verified to be appropriate for measuring the pre-discharge current in mA-ranges

The RAF has been undertaking Land Quality Assessments across the retained estate. As part of this study the author has been interested in the environmental hazard posed by disposal to land by burial and or burning of the Royal Air Force's WWII (World War 2) Chemical Weapons (CW) stockpile. The investigation describes a Hazard Rating strategy to address the potential contamination. The development of a novel method of screening sites using dogs is also described. The study charts the development of chemical weapons for use by the RAF and looked at the production and distribution of these weapons from factories to stations. Mustard gas was the major chemical weapon agent adopted by the United Kingdom government during this period. The 65 Ib Light Casing (LC) bomb was deemed to be one of the most potentially successful weapons in the CW arsenal. As of 1 April 1945 the total number of 65 Ib LC bombs manufactured for use in Northwest Europe was 389,937 of which 388,800 were available for use by the RAF and 1,137 were transferred to the USAAF. The study describes the problems of leakage especially with the Mk 1 variant and its associated Mk 1 wooden crate. It is highly likely that leaking weapons were buried on RAF sites and that by the end of the war a considerable quantity of mustard gas agent has potentially leaked into the ground. Mustard gas is likely to remain on stations. American weapons, distribution and potential storage sites have also been discussed.

Cette thèse est consacrée à l’étude de la décharge positive dans les grands intervalles d’air. L’objectif initial est de se doter d’un modèle capable de déterminer la tension U50 ainsi que le facteur d’intervalle k, deux facteurs importants entrant dans le dimensionnement des ouvrages hautes tensions, dans des géométries complexes. Pour ce faire un bilan des modèles de la décharge disponibles dans la littérature a été fait. Notre choix s’est porté sur le modèle de type circuit développé par le groupe du professeur Beroual et appliqué aussi bien à la décharge positive qu’à la décharge négative. Ce modèle est basé sur un schéma électrique équivalent, ses paramètres variant avec le temps en fonction des caractéristiques du canal et de la géométrie de la décharge. La propagation du leader est basée sur un critère lié au calcul du champ à sa tête et où le caractère aléatoire du trajet de la décharge est pris en compte. Comme la plupart des modèles présents dans la littérature, ce modèle ne s’intéresse qu’à l’intervalle de type pointe-plan. Une partie de ce travail a consisté à étendre le domaine d’applicabilité de ce modèle aux intervalles de géométrie complexes. Une étude détaillée de la géométrie pointe-pointe nous a permis de valider notre modèle dans sa capacité à simuler un exemple d’intervalle complexe. De plus, des essais expérimentaux nous ont permis de comparer la valeur de la U50 déterminée à l’aide de notre modèle et celle issue des expériences pour cette géométrie pointe-pointe. Les résultats obtenus sont en bon accord avec une marge d’erreur inférieure à 5%. Au vu du bon accord résultant de la confrontation modèle - expérience de laboratoire et de la grande similarité, grande étincelle - décharge atmosphérique, un modèle de foudre positif a été élaboré. Ce modèle nous a permis d’utiliser pour la première fois le modèle circuit pour modéliser le processus d’attachement de la foudre. Les résultats obtenues permettent de valider la faisabilité de trouver, aux méthodes standards de protection contre la foudre, une alternative numérique basé sur une simulation de la foudre. Une estimation du champ rayonné par le canal du leader a été effectuée. Ce qui ouvre une voie à l’étude de l’interaction de ces champs rayonnés avec les systèmes d’ingénierie
This thesis is devoted to the study of the positive discharge in large air gaps. The initial goal is to develop a model able to determine the voltage U50 and the k-factor, two important factors involved in the design of high voltage structures in complex geometries. To do this, a review of the discharge models available in the literature was done. We chose the circuit model developed by Professor Beroual’s group and applied to the positive discharge and also to the negative discharge. This model is based on an equivalent circuit diagram, his parameters varying with time according to the leader channel characteristics and the geometry of the discharge. The spread of the leader is based on a criterion related to the calculation of the field at its head and where the randomness of the discharge path is taken into account. As most of models found in the literature, this model applies only to the point-plane type interval. Part of this work was to extend the domain of applicability of this model to complex geometries. A detailed study of the rod-rod gap allowed us to validate our model in its ability to simulate an example of complex geometry. Moreover, experimental tests allowed us to compare the value of the U50 voltage determined using our model and the one from experiments for to this rod-rod geometry. The results are in good agreement with a margin of error of less than 5%. Given the good agreement resulting from the confrontation model - laboratory experience and the great similarity, large spark - atmospheric discharge, a positive lightning model was developed. This model allowed us to use for the first time the circuit model to model the lightning attachment process. The results obtained allow to validate the feasibility to find, to the standard methods of protection against lightning, digital alternative based on a simulation of lightning. An estimate of the field radiated by the leader channel was performed. This opens a way for the study of the interaction of these fields radiated with engineering systems

L'objectif de ce travail a ete d'etudier les phenomenes physiques mis en jeu lors des mouvements de granules conducteurs spheriques ou cylindriques, de taille millimetrique, dans des champs electriques continus ou intermittents, entre electrodes distantes de quelques centimetres, dans l'air atmospherique. L'etude a ete effectuee en champ harmonique puis en presence des decharges de type couronne positives ou negatives. Un logiciel fonde sur la methode des elements de frontiere a ete utilise pour calculer la charge acquise par un granule, la force agissant sur celui-ci et le renforcement du champ electrique qu'il produit, lorsqu'il est en contact avec ou a proximite d'une electrode plane. On a evalue les conditions d'apparition d'une decharge electrique entre un granule charge et une electrode de polarite opposee, et analyse les circonstances dans lesquelles la presence d'un petit objet conducteur provoque le claquage de l'air ionise. En utilisant plusieurs dispositifs experimentaux (electrodes pointe-plan, fil et cylindre coaxiaux, matrice de pointes plan) on a determine le champ de decollement des granules de formes, de dimensions et de masses volumiques variees. Les experiences ont permis egalement de mesurer les tensions de claquage de l'air entre electrodes de configurations diverses, en presence d'un granule conducteur fixe ou mobile. Un modele mathematique a ete propose afin de decrire par calcul numerique les mouvements des granules et les claquages engendres par ceux-ci. L'influence des divers parametres electriques (niveau de la haute tension, densite de la charge d'espace) ou mecaniques (coefficient de rebond des granules sur une electrode) a ete etudiee systematiquement. La meilleure comprehension des phenomenes physiques acquise lors de ce travail a contribue a realiser des ameliorations a des installations industrielles telles que les separateurs electrostatiques

Für die Menschen zählen Gewitter mit Donner und Blitz zu den ältesten bekannten Phänomenen der Natur. Die Tatsache, dass Blitze dennoch bis heute nicht abschließend erforscht sind, liegt in dem äußerst komplexen, schnellen und stochastischen Verhalten der Blitzentladung begründet. Der Autor befasste sich in Rahmen dieser Arbeit u.a. mit der Entwicklung eines modernen Messsystems, durch das die Erfassung von sowohl elektrischen als auch optischen Parametern der im Nanosekundenbereich ablaufenden Prozesse der Entladungen möglich wurde. Es handelt sich um technisch anspruchsvolle Messungen von hohen Stoßspannungen und schwachen Vorentladungsströmen in einem großen Frequenzbereich. Außerdem gelang es durch eine geschickte Synchronisation, die Entladungsprozesse präzise und erstmalig in einer Fotosequenz aufzunehmen. Auf Grundlage der Messergebnisse konnte der Entladungsprozess in mehrere Phasen der Vorentladung unterteilt und beschrieben werden. Vor allem beweisen die Messergebnisse die Existenz einer kanalartigen Vorentladung, die nicht zwingend zu einem Durchschlag führt. Dieser Mechanismus, der bei langsam ansteigenden Spannungen als stabile Leaderentladung benannt und mit Thermoionisation gekennzeichnet ist, war bei Blitzstoßspannungen bisher umstritten.
The breakdown behavior of long air gaps by Lightning Voltages has been investigated and the results of a new measurement system are reported. A series of up to eight photos (made by a high-speed camera) have recorded the expansion of discharge activities during the breakdown process. The pre-discharge current and voltage have been measured as well. The modern measurement techniques have made possible the synchronization of the photos with the current and voltage measurements in nanosecond-ranges. The combination of the measured current, voltage, and photos can precisely explain the breakdown process. The experiments have been carried out with the lightning impulses up to 2.4 MV and the air gaps up to 4 m. Despite of such high voltages, the experimental system has been verified to be appropriate for measuring the pre-discharge current in mA-ranges.

Study One: Special Educational Needs and Disability Tribunals (SENDIST) provide independent adjudication of parental appeals against Local Authority (LA) decisions. The Parent Partnership Service (PPS) and Disagreement Resolution Services (DRS) are both arranged to reduce disagreements and, specifically, to prevent tribunals. Study One aimed to explore parental experiences of Local Disagreement Resolution Services (LDRSs) including the PPS and DRS. A secondary aim of Study One was to identify barriers to and facilitators of disagreement resolution from a parental perspective. Methods: Study One utilised semi-structured interviews as a means of exploring seven parents' experiences and constructs. Interview transcripts were analysed using thematic analysis in order to specify key themes relating to the resolution of disagreements about SEN. Results: Parents reported a sense of embattlement with the Local Authority that appeared to act as a barrier to the resolution of disagreements. Parents also identified a number of facilitators of disagreement resolution including: Feeling 'listened to'; Having access to a 'legitimate decision-maker'; and becoming better informed. A number of barriers to disagreement resolution were also reported, including but not limited to: a perception that no one is listening to them; a perception that LA staff lack independence; a perception that the LA cannot be trusted to deliver SEN provision. Study Two:Special Educational Needs and Disability Tribunals (SENDIST) provide independent adjudication of parental appeals against Local Authority (LA) decisions. The Parent Partnership Service (PPS) and Disagreement Resolution Services (DRS) are both arranged to reduce disagreements and, specifically, to prevent tribunals. Study Two aimed to explore experiences of professionals working within Local Disagreement Resolution Services (LDRSs) including the PPS and DRS. A secondary aim of Study Two was to identify barriers to and facilitators of disagreement resolution from a professional perspective. A final aim of Study Two was to synthesise the perceptions reported by professionals in Study Two with those reported by parents in Study One. Methods: Study Two utilised semi-structured interviews as a means of exploring six LA-employed professionals' experiences and constructs. Interview transcripts were analysed using thematic analysis in order to specify key themes relating to the resolution of disagreements about SEN. Findings emerging from Study Two were integrated with findings from Study One using the themes generated through thematic analysis. Results: Professionals reported a number of parental factors that were perceived to act as a barrier to disagreement resolution. These included but were not limited to: weak understanding of SEN systems and a lack of confidence to engage in mediation processes. Professionals did not identify any parental factors perceived to be conducive to disagreement resolution. A smaller number of facilitators of disagreement resolution were reported, including: early intervention; and face-to-face meetings. Synthesis of findings from Studies One and Two resulted in the creation of clusters of themes that can inform future policy and practice. Findings from Studies One and Two indicate that disagreement resolution is best supported where Local Authorities can promote: collaboration, information-sharing, and reassurance for parents.

The diploma thesis deals with the use of Laser-Induced Breakdown Spectroscopy (LIBS) for depth profiling and 3D mapping of the zinc-coated steel used in the automotive industry. Before creating depth maps and depth profiles, optimization of the experiment was performed. It was shown that the LIBS technique is suited for making depth profiles and depth maps. The theoretical part deals with the description of the LIBS instrumentation, characteristics of plasma, and assembling of scientific papers which reflect the up to date knowledge about depth profiling and mapping with the use of the LIBS technique. The experimental part describes the optimization of the experiment. Gate delay, the depth and the diameter of craters using the profilometer, the position of the focal plane relative to the sample surface, and selection of spectral lines with the smallest residual signal and small scattering of data were optimized. Depth profiles of zinc, iron, chromium and manganese with the depth map of zinc and iron were made, and also the depth resolution for both elements was calculated.

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, in fulfilment of the requirements for the degree of Master of Sclenice in Engineering February 20th, 1998
All available model used to determine the 50% breakdown voltage for rod-type ami conductor-type gaps subject to switching is impulse wave forms has been applied to a tower-conductor window gap configurution. The results for rod-plane, conductor-piane and tower-conductor window gaps have all been compared and correspond well with practical data. III order to app(v the model, a charge simulation technique has been adopted ill conjunction with the 'Coulomb 3D' charge simulation package. Additional tests have been performed where parameters of'the charge simulation method am! the electrode geometl:p hare been adjusted and consequent conclusions made. Recommendations for further application of tile model have been suggested.
MT2017

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering, 2016
The validity of the atmospheric correction method presented in the IEC 60060-1 (2010) standard is analysed and evaluated by means of theoretical and laboratory work. In order to understand the problem, the evolution of the atmospheric correction methods, from as early as 1914, has been presented. A procedure (Calva prediction method) for predicting the direct current (DC) breakdown voltage for an air gap at any altitude was discovered and was also analysed along with the IEC 60060-1 (2010). A critique of some of the atmospheric correction methods commonly used standards was also done. Experiments were carried out at altitudes of 1 740 m (Wits University), 130 m (UKZN HVDC centre) and at less than 2 m above sea level (Scottburgh beach, Clansthal). More tests were conducted using a pressure vessel where high altitude relative air density was simulated. All tests were conducted on rod-plane air gaps using a 15 mm diameter at tip rod. Test results from Scottburgh beach were used as the standard breakdown voltages of the air gaps tested since the environmental conditions were the closest to the conventional standard conditions (stp). The test results obtained were compared with predictions using the Calva method in order to validate the method. The test results were also corrected according to IEC 60060-1 (2010) and compared to the standard breakdown voltages obtained at Scottburgh beach. It was shown that the IEC 60060-1 (2010) is quite suitable for atmospheric correction for data obtained at low altitudes (about 130 m). When applied to high altitude (1 740 m) data, the correction method is accurate and suitable for very small air gaps less than 0.1 m. As the air gap length increased, the corrected results began to deviate from the expected standard voltage. The same trend was shown with the corrected results from the pressure chamber tests. The prediction method by Calva was accurate when compared to the experimental data from the high altitude and low altitude test results. When compared to the data from the pressure chamber, the prediction method had a linear error factor which was di erent for each gap length. It was concluded that the IEC 60060-1 (2010) is not only unsuitable for atmospheric correction for data at relative air densities below 0.8, but also that the correction method is prone to an increase in error as the air gap length increases when the relative air density is higher than 0.8. The Calva prediction method was found to be suitable to use after additional factors are added when applied to high altitude conditions.
GR2016

This dissertation investigates the role that is played by high temperatures of air gaps on the breakdown voltage levels under DC positive and negative polarity applied voltages. Due to past experience of AC transmission lines tripping as a result of sugar-cane fires that occur under these lines during cultivation seasons, this study was initiated to investigate this effect under DC applied voltages. Results were obtained from laboratory work conducted and these were closely analysed to understand the behaviour of air gaps under these conditions. A 17mm2 square-cut brass rodrod electrode configuration was used to carry out these tests at the various air gap temperatures. These were induced by a gas burner for both the positive and negative polarities at 200C – 3000C for the 10 mm – 150 mm air gap range and 200C – 1500C for the 200 mm – 500 mm air gap range. Later particles were introduced into the air gap to determine the subsequent behaviour. These were introduced vertically from the top into an air gap via a vibrating micro sieve mechanism to regulate the consistency of the introduction of these particles in the air gap. A reduction of 55% and 50% was observed on the breakdown voltage under positive and negative polarity applied voltages respectively from ambient conditions to 3000C. Additionally the breakdown behaviour of both negative and positive DC was found to be linear which is similar to the AC case. However, air gaps subjected to positive DC applied voltages were found to portray an inferior dielectric strength as opposed to the equivalent negative DC polarity. The study found that the effect of particles in the air gap is practically negligible and that for practical purposes, only the temperature effect plays a role due to the reduced air density at high temperatures. Empirical models for both the positive and negative DC polarities have been proposed by the study that incorporate the effect of the temperature in the air gap to enable the determination or prediction of the breakdown voltage level at various temperatures. These models may be utilised for DC transmission line design for servitudes in areas that are known to be prone to fires.
Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2012.

Vortex breakdown is an important phenomenon observed in swirling flows involving the development of a stagnation point on the axis of the vortex followed by a region of recirculation when the swirl increases beyond a particular level. It has been studied extensively over past 50 years and various theories have been proposed to explain its various aspects. However, a single model explaining all the aspects together is yet to emerge. Numerical simulations of breakdown have been performed using a variety of grid-based as well as vortex methods. Vortex methods are a Lagrangian alternative to grid-based methods wherein the motion of the vorticity is determined by the local fluid velocity convection, with models for viscous effects when considered. The fluid velocity is obtained from the vorticity field. Only the rotational regions of the flow need to be considered leading to significant economy of computational effort for simulations of vorticity dominated flows, such as vortex breakdown. The inviscid vortex filament method has been used to simulate several aspects of the vortex breakdown phenomenon. The vortex filament method however, cannot easily simulate viscous effects. To simulate the viscous effects the viscous vortex particle method needs to be used. This work was intended to be a first step towards this end by initially evaluating the effectiveness of the inviscid version of the vortex particle method in simulating the breakdown phenomenon. The inviscid vortex particle method was found to satisfactorily simulate most qualitative aspects involved in the formation of vortex breakdown such as the retardation of axial velocity along centerline, radial swelling of the vortex core, formation of stagnation points, creation of azimuthal vorticity gradient from axial vorticity gradient and the turning of vortex lines along with the formation of a bubble-like structure with recirculating flow within. The effect of a wall placed adjacent to the vortex core was simulated by using image vortices. The wall was not found to influence the location of breakdown. However, the initiation of the spiral mode was found to occur earlier when a wall was present. For a quantitative assessment, a simulation of the experimental results of Faler and Leibovich (1978) was attempted. The simulation managed to predict the location of the breakdown and the extent of the bubble. The shape and height of the bubble obtained however were not in accord with the experimental observations. A single vortical cell was obtained in the interior of the bubble.

碩士
國立雲林科技大學
電子工程系
107
A high electron mobility transistor (HEMT) is composed of one or more III-V heterostructures. It has excellent performance with high electron density and mobility. This study focuses on optimizing the structure of an AlGaN/GaN HEMT to improve the breakdown voltage. Sentaurus TCAD was used to simulate the characteristics of AlGaN/GaN HEMTs. This thesis firstly introduces the physical models and boundary conditions used in the TCAD. Next, parameter setting in each step of the device simulation is explained. The study started with a depletion-mode double-channel AlGaN/GaN HEMT structure. A air-bridge field plate was added on the structure, which reduced the electric field at the gate. Because the peak electric field was still present, four approaches were applied separately: 1. a recessed air-bridge field plate; 2. an add-on slant field plate under the recessed air-bridge field plate, 3. a slant field plate at the gate only, and 4. slant field plates under both the gate and air-bridge field plate. The study found that the third approach was the most effective in reducing the peak electric field. It resulted in a breakdown voltage of 200 V, a significant improvement in contrast to the 19 V of the initial structure.

A model for predicting the e ect of a Floating Conductor (FC) on the breakdown voltage is presented and critically analysed in conjunction with results from laborat- ory work. Firstly, the scenario of live-line maintenance on HVDC lines is modelled by a FC within a rod-plane gap. The use of a rod-plane gap is to produce a non- uniform eld distribution, representing the non-uniform eld that would surround the conductor of a transmission line. This test object is then further simpli ed into simple gap geometries such that the breakdown voltage of the two gaps separated by the FC can be numerically calculated and the breakdown voltage of the entire test object predicted. The DC breakdown voltage of the rod-plane gap is then presented as a function of the position of the FC in the gap. The simulations are performed for both positive and negative DC voltages. The laboratory work uses a similar test object, with the U50 breakdown voltage recorded as a function of the position of the FC within the rod-plane gap. The simulation model is then critically analysed by comparing the laboratory and simulation results. It is concluded that although the model predicts the change in the breakdown strength from the starting to the ending points, the developed model has omitted detail that leads to discrepancies in the predicted results when compared to the laboratory results. This omitted detail includes the e ect of corona discharge from the FC, additional parameters a ecting the potential of the FC, the oversimpli cation of the gap geometry for numerical calculation, and the lack of a dynamic model for the changing electrostatic scenario as an applied voltage increases towards the breakdown voltage. The results also show that the position of a FC a ects the breakdown strength of a non-uniform gap. This e ect is due to the polarity dependent nature of breakdown in a non-uniform gap and in particular, the di erence in the breakdown strength of a quasi-symmetrical (rod-rod) and asymmetrical (rod-plane) non-uniform gap. The e ects of charged particles and their interaction with the oating conductor in the gap is also deemed to have a signi cant e ect on the breakdown strength, with their behaviour also in uenced by the sustained electric eld that is a result of HVDC stress.

In South Africa, live work is routinely performed on high voltage apparatus at various voltages up to 400 kV ac. With regard to HVDC, there are unqualified parameters relating to the development of the safe live work standards, which are currently based on extrapolation of ac and transient voltage test data. Furthermore, it is generally accepted that the mechanism for air breakdown under dc is different when compared between ac and dc voltage. The air breakdown mechanism under HVDC conditions and the corresponding live work related parameters need to be researched further before live work may be performed. Experimentation using floating objects, has in the past, been used to study parameters related to live working calculations. This research report presents the results of HVDC air breakdown tests using a 300 mm diameter floating metallic sphere with a 30 mm protrusion, in a point-to-plane configuration with a total gap length ranging between 0.75 m and 1.4 m. Both positive and negative polarity cases were tested. The results indicated that the position of the floating sphere does not significantly affect the flashover-voltage magnitude. There is, however, a definite reduction in the strength of the air gap, between 27 % and 29 %, for the cases tested. Further, the static charge on the floating object did not influence the breakdown voltage. There is also linearity in the air breakdown voltages of simple point-to-plane air gaps. Humidity and temperature also contribute to variations in the breakdown voltage. Two international publications have been published based on the research presented in this report. These are listed in Appendix D and E, respectively.

Ο εξοπλισμός και οι αγωγοί που μεταφέρουν την υψηλή τάση ακολουθούν συγκεκριμένες τιμές ασφάλειας και ελέγχου. Με τον όρο υπερτάσεις εννοούμε τις τάσεις που εμφανίζονται στα ηλεκτρικά δίκτυα και που συνήθως έχουν εύρος μεγαλύτερο από εκείνο της κανονικής τάσης λειτουργίας. Οι υπερτάσεις έχουν κατά κανόνα ανώμαλη μορφή και είναι ικανές να προκαλέσουν διηλεκτρικές καταπονήσεις μεγάλου μεγέθους στις μονώσεις ενός συστήματος. Κατά τον σχεδιασμό της μόνωσης ενός συστήματος υπάρχουν δύο εξαιρετικά σημαντικές παράμετροι που πρέπει να ληφθούν υπ’όψην από τους κατασκευαστές: 1) Ο καθορισμός της μορφής των τάσεων που καταπονούν την μόνωση 2) Ο καθορισμός της αντοχής της μόνωσης όταν υφίσταται καταπόνηση από συγκεκριμένες μορφές τάσεων. Σκοπός της παρούσας εργασίας είναι η ανάλυση της καταπόνησης που υφίσταται διάκενο ακίδα-πλάκα συνολικού μήκους 6cm στο οποίο παρεμβλήθηκε μετακινούμενο διάφραγμα. Τα συμπεράσματα που εξήχθησαν αφορούν την μελέτη της τάσης διάσπασης και τον τρόπο με τον οποίο αυτή μεταβάλλεται σε σχέση με την γεωμετρία του διακένου αλλά και τις ατμοσφαιρικές συνθηκες. Τα συμπεράσματα που προκύπτουν από την ανάλυση της καταπόνησης που υφίσταται διάκενο ακίδας-πλάκας είναι ότι καθώς απομακρύνεται το διάφραγμα από την ακίδα η τάση διάσπασης μειώνεται. Αξίζει επίσης να σημειωθεί ότι όσο πλησιάζει το διάφραγμα την ακίδα μειώνεται η τυχαιότητα του φαινομένου της διάσπασης πράγμα το οποιό σημαίνει πως ο σχεδιασμός ενός συστήματος μόνωσης δύναται να γίνει πιο ακριβής. Τέλος παρατηρείται ότι ανάμεσα σε σετ μετρήσεων με ίδια απόσταση διαφράγματος από την ακίδα υπάρχει μια απόκλιση στις τιμές της U50%. Αυτή η απόκλιση οφείλεται σε διάφορους παράγοντες. Ένας εξ’αυτών είναι η μικρή μεταβολή των ατμοσφαιρικών συνθηκών κατά την χρονική απόσταση που παρεμβλήθηκε ανάμεσα στα σετ μετρήσεων. Έναν δεύτερο παράγοντα αποτελούν οι πιθανές υπερπηδήσεις που συνέβησαν κατά την διάρκεια των επιβολών τάσης στο διάκενο.
The equipment and conductors that "carry" high voltage are taking specific values of security and control. Using the term, hyper-voltage, the researchers mean those voltages that appear in electricity networks and they usually have a width value greater than the normal operating voltage. Most of the times, hyper-voltages are typically have irregular form and they are capable of causing large dielectric stresses in the insulation of a system. When designing an insulation system there are two very important parameters that have to be taken under consideration by the manufacturers: 1) The determination of a voltage form that stresses the insulation equipment 2) The determination of the insulation resistance when there is strain from specific voltage forms. The purpose of this paper is to analyze the voltage stress that is happening in a 6cm gap of pin-plate which was inserted by removable diaphragm. The conclusions drawn are concerning the study of breakdown voltage and how it changes with respect to the geometry of the gap and the atmospheric conditions. One conclusion that arises from the stress analysis of the pin-plate gap is that when the diaphragm is moving away from the pin the breakdown voltage is decreasing. It is also worth noting that as the diaphragm is moving towards the pin, the breakdown phenomenon is becoming less random. This means that the design of an insulation system can be made more accurate. Another conclusion is related to the discrepancy in the values of U50% between different sets of measurements. The distance between the diaphragm and the spike is influencing the above observation. This discrepancy is due to several factors. The first factor is related to the small change of weather conditions during the different sets of measurements. The second factor is related to the possible jumps that occurred during the voltage imposition in the gap.

Τα τελευταία χρόνια, η φασματοσκοπία πλάσματος επαγόμενο από λέιζερ (LIBS) έχει προσελκύσει μεγάλο ερευνητικό ενδιαφέρον καθώς αποτελεί μία πειραματικά απλή και αποτελεσματική τεχνική, η οποία παρέχει τη δυνατότητα λήψης μετρήσεων για απευθείας ποιοτική και ποσοτική στοιχειακή ανάλυση. Η τεχνική LIBS στηρίζεται στη δημιουργία σπινθήρα/πλάσματος μέσω ισχυρά εστιασμένης δέσμης λέιζερ στην επιφάνεια ή στο εσωτερικό του δείγματος, στην ακόλουθη διέγερση και ατομοποίηση των στοιχείων του στόχου και στην τελική καταγραφή και φασματοσκοπική ανάλυση της εκπεμπόμενης ακτινοβολίας του πλάσματος. Λόγω των πολλών πλεονεκτημάτων που συγκεντρώνει η τεχνική, το LIBS έχει προταθεί για πληθώρα πρακτικών, τεχνικών και τεχνολογικών εφαρμογών σε ένα ευρύ φάσμα ερευνητικών πεδίων. Από την άλλη μεριά, στον τομέα της καύσης, η ποσότητα καυσίμου σε ένα εύφλεκτο μίγμα είναι αντικείμενο μείζονος σημασίας καθώς επηρεάζει σημαντικά την απόδοση των χημικών διεργασιών και την παραγωγή και εκπομπή ρύπων. Επομένως, δημιουργείται η ανάγκη ανάπτυξης μίας γρήγορης και μη παρεμβατικής διαγνωστικής τεχνικής για τη μέτρηση της περιεκτικότητας του καυσίμου τοπικά στη φλόγα με καλή τόσο χωρική όσο και χρονική ανάλυση. Στα πλαίσια της παρούσας διδακτορικής διατριβής, η τεχνική LIBS η οποία συγκεντρώνει όλα αυτά τα πλεονεκτήματα χρησιμοποιήθηκε για αυτό το σκοπό. Κατά τη διάρκεια των πειραμάτων, χρησιμοποιήθηκαν πηγές λέιζερ διάρκειας παλμών ns και fs, ενώ τα συστήματα καύσης που μελετήθηκαν ήταν φλόγες υδρογονανθράκων-αέρα, στρωτής και τυρβώδους ροής, απλής και συνθετότερης γεωμετρίας. Από τα LIBS φάσματα φλογών διαφορετικής σύστασης, προέκυψε λοιπόν ότι υπάρχει μία ισχυρή εξάρτηση μεταξύ των εντάσεων διαφόρων φασματικών γραμμών με το λόγο ισοδυναμίας. Επομένως, μέσω της συσχέτισης αυτής μπορεί να επιτευχθεί με μεγάλη ακρίβεια τόσο η μέτρηση της περιεκτικότητα σε καύσιμο φλογών άγνωστης σύστασης όπως επίσης και η μέτρηση της κατανομής του καυσίμου τοπικά μέσα σε όλη την έκταση της φλόγας παρέχοντας σημαντικές πληροφορίες για την δομή της. Τέλος, εφαρμόστηκε μία παραπλήσια διαγνωστική τεχνική, κατά την οποία η διηλεκτρική κατάρρευση του μέσου ήταν αποτέλεσμα ενός ηλεκτρικού σπινθήρα: electrical Spark Induced Breakdown Spectroscopy (SIBS) όπου και πραγματοποιήθηκε η συγκριτική μελέτη της ακτινοβολίας του πλάσματος επαγόμενο μέσω οπτικής και ηλεκτρικής διέγερσης.
Laser induced breakdown spectroscopy (LIBS) has attracted a lot of scientific interest during the last two decades as it is generally considered to be an experimentally simple and efficient laser-based technique which can perform real-time, qualitative and quantitative elemental analysis. The basic idea of LIBS is the creation of spark/plasma through tight focusing of a laser beam on the surface or into a sample, the subsequent excitation and atomization of the species of the sample at the location where the spark is formed and the final detection and spectroscopic analysis of the emitted radiation from the decaying plasma. Seeing the numerous advantages holding the technique, LIBS has been proposed for many practical, technical and technological applications in various scientific areas. On the other hand, in the field of combustion, the proportion of fuel in a combustible mixture is of great importance as it strongly affects the efficiency of the chemical processes and the production of soot emissions. Therefore, there is a continuously increasing need for the development of a rapid and non-perturbative diagnostic technique for the determination of the fuel content locally in the flame structure with good spatial and temporal resolution. Ιn the present dissertation, LIBS technique which offers such advantages has been applied for combustion diagnostics purposes. During the experiments, laser systems with pulse duration in the scale of ns and fs have been applied as excitation sources, while the combustible mixtures under investigation were hydrocarbon-air flames, of laminar and turbulent flow with simple and more complicated structures. From the LIBS spectra in flames of different compositions, it was exhibited that there is a strong dependence of the intensities of various spectral lines on the equivalence ratio, which demonstrates that the precise determination of the amount of fuel can be performed. Also based on this correlation, the determination of the equivalence ratio locally everywhere within the flame can be achieved giving useful information about its structure. Finally, a similar diagnostic technique has been employed. The dielectric breakdown is held using a spark generator and the technique is called electrical Spark Induced Breakdown Spectroscopy (SIBS). The emitted light of the two plasmas induced by optical and electrical excitation was collected and a comparative study was performed.