No description available.

La thèse porte sur le développement d'une décharge à barrière diélectrique de surface pour la dépollution de l'air. Il s'agit d'une thématique tout à fait d'actualité. En effet l'application des réglementations sur la qualité de l'air nécessite de nouvelles technologies. Parmi elles, la dépollution par plasma froid est un candidat on ne peut plus prometteur. Dans un tel contexte, il est nécessaire de savoir approcher le problème sous différents aspects: Le premier est bien sûr de comprendre la physique fondamentale de ce type de plasma. Cela requiert une démarche expérimentale adaptée aux échelles du phénomène. Ces décharges sont constituées de filaments de petites dimensions (quelques centaines de micromètres de diamètre, quelques centimètres de long) se propageant extrêmement vite, avec une durée de vie très courte (quelques dizaines de nanosecondes). Ces caractéristiques en font des objets difficiles à étudier. Les résultats ont montré des phénomènes d'autodéclenchement des filaments de plasma. Ce comportement tout à fait original est expliqué par l'effet mémoire de la surface diélectrique, qui peut retenir les charges sur des temps très longs (plusieurs minutes). Ces charges peuvent alors être photodésorbées et déclencher le départ de plusieurs streamers en un temps très court (40 ns). L'organisation et la propagation des filaments ont été étudiées par imagerie iCCD avec une résolution temporelle raffinée à la nanoseconde. Le deuxième aspect est d'identifier les qualités du système qui peuvent être exploitées du point de vue procédé: efficacité chimique, coût énergétique, géométrie du dispositif. L'identification de ces points a permis de déposer un brevet sur le système. Par la suite, un travail d'étude de marché a été réalisé grâce à l'encadrement d'étudiants de l'ISTIA (Institut des Sciences et Techniques de l'Ingénieur d'Angers) pour leur stage de M2.

Streamer characteristics, breakdown strengths and gassing behaviour of insulating liquids under electric stresses are taken into account for a reliable design and safe operation of the transformer. Ester liquids which are biodegradable and have high fire point have been widely used in distribution transformers and some power transformers in recent years. It is also interesting to introduce ester liquids into High Voltage Direct Current (HVDC) converter transformers due to the fast development of HVDC transmission lines. Therefore, this thesis aims to investigate the pre-breakdown, breakdown characteristics and gassing behaviour of a synthetic ester liquid under DC and various impulse voltages where a mineral oil is tested as the benchmark. A comprehensive study of streamer characteristics and breakdown strength of the mineral oil and the synthetic ester liquid under both positive and negative DC voltages was carried out in the point-plane electric fields. Characteristics of streamer length, propagation velocity and shape were analysed based on shadowgraph images obtained at a gap distance of 10 mm, using a multi-channel ultra-high speed camera. Streamer inception voltages with the tip radii of 5 µm, 10 µm, 20 µm and 50 µm and breakdown voltages at various gaps of 2 mm, 5 mm, 10 mm, 20 mm and 30 mm were also investigated. The results indicate that there is no obvious streamer propagation (less than about 10% of the gap distance) under negative polarity even when the applied voltage approaches breakdown voltage. At the same applied voltage level, the streamer in the synthetic ester liquid propagates faster and further than that in the mineral oil. As a result, the breakdown voltages of the synthetic ester liquid are lower than those of the mineral oil at all the gap distances investigated under both polarities. Experimental and modelling studies of pre-breakdown and breakdown phenomena in the mineral oil and the synthetic ester liquid under impulse waveforms with different tail-time were carried out in the point-plane electric fields. A compact solid-state switch based impulse generator was used to provide different impulse waveforms from short tail-time to 'step-like' tail-time: 0.8/8 µs, 0.8/14 µs, 0.8/30 µs and 0.8/3200 µs. A point-plane electrode configuration with a small gap distance of 10 mm and a tip radius of 10 µm was used. The results indicate that the shorter tail-time impulse waveform results in a shorter stopping length and higher breakdown voltage; however it does not affect the instantaneous breakdown voltage and time to breakdown. A mathematical model is therefore described to predict the breakdown voltage under different impulse waveforms. In addition, with the similar stopping length, higher energy injected from the short tail-time impulse caused the streamers to have more branches than those under the long tail-time impulse. The characteristics of fault gas generation in the mineral oil and the synthetic ester liquid under various levels of electrical faults were studied. A test platform with functions of automatic spark fault control and data acquisition was developed. The effects of spark numbers (from 20 to 500), gap distance (5 mm and 10 mm) and voltage levels (Vb-99.9% and 1.5Vb-99.9%) on fault gas generation in liquids were studied. The key gases in the mineral oil are H2 and C2H2, while the key gases in the synthetic ester liquid are H2, C2H2 and CO. The amount of fault gas generation increases linearly with the number of sparks. However, the number of sparks does not have an obvious effect on fault gas pattern and gas generation per unit fault energy in µL/J. Spark at a larger gap distance or under a higher applied breakdown voltage generates more fault gases due to higher injected fault energy.

No description available.

The towed streamer controlled source electromagnetic (CSEM) system collects data faster than the conventional static node-based CSEM system. However, the towed streamer CSEM is typically much noisier than the conventional static node-based CSEM. Identifying and quantifying various sources of noise is important for the development of future robust electromagnetic streamer system. This is the problem I address in this thesis. I achieve this in three parts. First, I examine the idea that the towed streamer suffers from noise induced by its motion through the Earth’s magnetic field according to Faraday’s law of induction. I derive expressions for the motionally-induced noise for the cases of a horizontal streamer parallel to the acquisition vessel’s path and a curved streamer caused by a constant cross-current. These expressions demonstrate that the motionally-induced noise is sensitive to the magnitude of the feather angle at the head and at the tail of the streamer, and to the vertical and lateral motion of the streamer. The key finding is that no motionally-induced noise is generated when the streamer is horizontal and moving in a constant magnetic field. By contrast, when the streamer shape is curved because of cross-currents, motionally-induced noise is generated if the velocity of the streamer varies over time. Second, I analyse and compare the noise recorded using the first generation of towed streamer with the noise recorded using a static ocean bottom cable (OBC) CSEM. I find out that within the frequency range of interest, 0.01–1 Hz the towed streamer noise is 20 dB greater (factor of 10) than the noise recorded with the OBC CSEM. I show also that the motion of the telluric cable between the pair of electrodes in the towed streamer is responsible for this difference in amplitude between the two systems. In the frequency ranges, 0.03–0.1 Hz and 0.03–0.2 Hz, the motionally-induced noise is shown to be uncorrelated across all channels. However, within the frequency band 0.1–0.3 Hz, the motionally-induced noise correlation gradually increases and becomes well correlated at about 0.2 Hz. This correlated noise could be caused by ocean swell from surface waves, water flowing around the streamer or cross-currents. Finally, to identify and quantify the contribution of several distinct sources of noise, and to describe the mechanisms generating each source of noise, I co-designed a prototype towed streamer CSEM. I carried out an experiment with the prototype streamer suspended 1 m below the water surface in the controlled environment of the Edinburgh wave tank located in King’s building campus (the University of Edinburgh). I then subjected the streamer to flow running at velocities of 0–1ms−1 along its length and to waves propagating in the same direction, at 45°, and perpendicular relative to the streamer direction.

No description available.

Data from satellites and reanalysis products are analyzed to study the behavior of wave trains in the three major Southern Hemisphere Convergences zones: the South Pacific, the South Atlantic, and the South Indian. Using composites on high rain-rate days, a wave pattern is identified that is characteristic of high rain events. This wave pattern is then compared to the patterns of variability of brightness temperature using empirical orthogonal functions. A linear regression technique is used to examine the behavior of potential vorticity corresponding to the patterns of maximum variance. Planetary-scale waves, propagating in favorable regions, slow and break, dragging streamers of moisture from the tropics into higher latitudes. These streamers, combined with lifting, lead to the enhanced rain seen in the Southern Hemisphere’s convergence zones. It is concluded that the convergence zones are areas of enhanced streamer activity and that a more thorough study of streamers will yield more information on the structure and behavior of the convergence zones.

Σκοπός της παρούσας διπλωματικής εργασίας είναι η πειραματική μελέτη των κυμάτων ιονισμού που παράγονται από ηλεκτρικές εκκενώσεις σε ανομοιογενές ηλεκτρικό πεδίο. Συγκεκριμένα, εξετάζουμε τη δημιουργία και διάδοση των streamer στις ηλεκτρικές εκκενώσεις του αζώτου και του ατμοσφαιρικού αέρα. Οι παράμετροι που καθορίζουν την εξέλιξη της εκκένωσης είναι η κατανομή του πεδίου, η πίεση και η φύση του αερίου το οποίο μελετάμε. Στο κεφάλαιο 1 γίνεται μια εισαγωγή στους φυσικούς μηχανισμούς των ηλεκτρικών εκκενώσεων καθώς και στις χαρακτηριστικές V-I αυτών και θα παρουσιαστούν συνοπτικά τα «φωτεινά-οπτικά» χαρακτηριστικά τους. Βάση για όλες τις θεωρίες που πραγματεύονται την παραγωγή πλάσματος συνιστά η θεωρία Townswed. Σύμφωνα με αυτή, τα αέρια κατά την εφαρμογή μιας κρίσιμης τιμής τάσης παύουν να είναι μονωτές, υφίστανται ηλεκτρική διάσπαση και μετατρέπονται σε αγωγούς. Προτείνεται έτσι μια πολύ καλή εξήγηση των μηχανισμών των ηλεκτρικών εκκενώσεων που αφορά όμως, μόνο τα ομογενή πεδία. Η πλήρης ερμηνεία τους επιτυγχάνεται με τη θεωρία των κυμάτων ιονισμού (streamer) που λαμβάνει υπόψην τα μη αμελητέα φορτία χώρου στα οποία εξάλλου και αποδίδει τη μεγάλη ταχύτητα διάδοσης του φαινομένου. Συμπληρωματικά, παρουσιάζεται συνοπτικά η εκκένωση αίγλης που συνιστά κατηγορία φαινομένων που αναπτύσσονται όταν το ρεύμα που διαρρέει το διάκενο γίνεται σημαντικό (Ι>10 μΑ), δηλαδή όταν το κύριο χαρακτηριστικό της εκκένωσης είναι η έντονη παραμόρφωση του ηλεκτρικού πεδίου που προκαλείται από το φορτίο χώρου που συσσωρεύεται στο διάκενο. Στο κεφάλαιο 2 περιγράφεται πλήρως το εργαστήριο, η πειραματική διάταξη που χρησιμοποιήθηκε, τα όργανα των μετρήσεων καθώς και τα αέρια που εξετάζουμε. Κατόπιν, αναφέρονται συνοπτικά τα στάδια που πειράματος καθώς οι δυσκολίες που συναντήθηκαν. Επιπλέον, διερευνάται η ευστάθεια της ηλεκτρικής εκκένωσης που εξαρτάται κυρίως από την επιλογή της αντίστασης φορτίου . Στο κεφάλαιο 3, παρουσιάζονται τα πειραματικά αποτελέσματα για τις ηλεκτρικές εκκενώσεις του αζώτου και του ατμοσφαιρικού αέρα για πιέσεις 1-100mbar. Μελετάται η συμπεριφορά των εκκενώσεων για μεταβολές του διακένου, της ανόδου και της καθόδου όπως αυτά αναδεικνύονται από τις χαρακτηριστικές καμπύλες V-I αλλά και τα αντίστοιχα παλμογραφήματα. Συμπληρωματικά, σχολιάζονται τα ηλεκτρικά και τα οπτικά χαρακτηριστικά τους και επιχειρείται σύγκριση μεταξύ των δύο αερίων. Καθοριστική για την εξέλιξη της εκκένωσης επιβεβαιώνεται ότι είναι η φύση, εν προκειμένω η ηλεκτραρνητικότητα ή μη, του κάθε αερίου καθώς και το υλικό της καθόδου και η πίεση, φυσικά. Στο κεφάλαιο 4 γίνεται μια ανακεφαλαίωση των παρατηρήσεων που προέκυψαν από το πείραμα και παρουσιάζονται κάποιες σκέψεις. Γίνεται προσπάθεια να αντιστοιχεί κάθε μια περιοχή της εκκένωσης στην αντίστοιχη που προβλέπεται από τη θεωρία. Στο κεφάλαιο 5, παρουσιάζεται η αρχή λειτουργίας του φωτοπολλαπλασιαστή. Τέλος, παρουσιάζεται η βιβλιογραφία. / The purpose of the present assignment is the experimental study of the streamers which are produced by electrical discharges at inhomogeneous electrical fields. Specifically, we examine the creation and spreading of the streamers at the electrical discharges of pure nitrogen and atmospheric air. The parameters which define the discharge’s evolution are the field’s allocation, the pressure and the nature of the gas. In chapter 1, an introduction to the physic mechanism of the electrical discharges and the characteristic curves V-I takes place. Their light and optical characteristics are presented, as well. The basis for all the theories that deal with plasma’s production is Townsed’s theory. According to this, gases subsisting a crucial value of voltage stop being insulators, incur an electrical disruption and become conductors. Thus, a very nice explanation of the mechanisms of the electrical discharges is suggested but it refers only to the homogeneous fields. Full interpretation is achieved through the streamers’ theory which takes the space charges into consideration and attributes to them the great spreading’s velocity of the phenomenon. Additionally, the glow discharges are presented. Glow discharges constitute a category of phenomena which evolve when the current in the gap becomes significant. In other words, they evolve when the discharge’s main characteristic is the intense distortion of the electrical field because of the space charges. In chapter 2, the lab, the experimental set-up, the machinery used and the gases are described. Moreover, the experiment’s steps and the difficulties we met are briefly reported. Furthermore, the stability of the discharge that depends on the load resistance is examined. In chapter 3, we present the experimental results concerning the electrical discharges of nitrogen and atmospheric air for pressures 1-100 mbar. The behavior of the discharges is studied while changing the gap, the anode, the cathode and is highlighted at the curves V-I and the oscillograph. In addition, we comment on the electrical and optical characteristics of the discharges and a comparison between the two gases is attempted. The nature of the gas turns out to be significant to the evolution of the discharge (the existence of electronegativity makes a difference). The same thing happens with the cathode’s material and the pressure, too. In chapter 4, we sum up the remarks on the experiment and present some thoughts and questionings. An effort to match the discharge’s regions to the counterpart ones predicted by the theory also takes place. Finally, in chapter 5, the principles of the photomultiplier are explained. Last but not least, the bibliography that has been used is presented.

Electric breakdown in power transformers is preceded by pre-breakdown events such as streamers. The understanding of these phenomena is important in order to optimize liquid insulation systems. Earlier works have derived a model that describes streamers in transformer oil and utilized a finite element method to produce numerical solutions. This research investigates the consequences of changing the numerical method to a finite volume-based solver implemented in OpenFOAM. Using a standardized needle-sphere geometry, a number of oil and voltage combinations were simulated, and the results are for the most part similar to those produced by the previous method. In cases with differing results the change is attributed to the more stable numerical performance of the OpenFOAM solver. A proof of concept for the extension of the simulation from a two-dimensional axial symmetry to three dimensions is also presented. / Elektriska genomslag i högspänningstransformatorer föregås av bildandet av elektriskt ledande kanaler som kallas streamers. En god förståelse av detta fenomen är viktigt vid konstruktionen av oljebaserad elektrisk isolation. Tidigare forskning i ämnet har tagit fram en modell för fortplantningen av streamers. Denna modell har sedan lösts numeriskt av ett beräkningsverktyg baserat på finita elementmetoden. I denna uppsats undersöks konsekvenserna av att byta metod till finita volymsmetoden genom att implementera en lösare i OpenFOAM. En standardiserad nål-sfär-geometri har ställts upp och ett flertal kombinationer av oljor och spänningar har simulerats. De flesta resultaten visar god överensstämmande med tidigare forskning medan resultat som avviker har tillskrivits de goda numeriska egenskaperna hos OpenFOAM-lösaren. En ny typ av simulering har även genomförts där simulationen utökas från en tvådimensionell axisymmetrisk geometri til tre dimensioner.

Cette thèse porte sur la simulation tridimensionnelle (3D) des décharges couronnes à l'aide du calcul haute performance. Lorsqu'on applique une impulsion de haute tension entre une pointe et un plan, les lignes de champ électrique fortement resserrées autour de la pointe induisent la propagation simultanée de plusieurs streamers et la formation d'une décharge couronne de structure arborescente. Dans ces conditions, seule une simulation électro-hydrodynamique 3D est apte à reproduire cette structure et fournir les ordres de grandeur de l'énergie déposée et de la concentration des espèces créées durant la phase de décharge. Cependant, cette simulation 3D est très consommatrice en temps et mémoire de calcul et n'est désormais accessible que grâce à l'accroissement permanent de la puissance des ordinateurs dédié au calcul haute performance. Dans le cadre d'une simulation électro-hydrodynamique 3D, une attention particulière doit être prise concernant l'efficacité des solveurs à résoudre les équations elliptiques 3D car leur contribution en termes de temps de calcul peut dépasser 80% du temps global de la simulation. Ainsi, une partie de manuscrit est consacrée aux tests de performances de méthodes de résolution d'équations elliptiques directes ou itératives telle que SOR R&B, BiCGSTAB et MUMPS, en utilisant le calcul massivement parallèle et les librairies MPI. Les calculs sont réalisés sur le supercalculateur EOS du réseau CALMIP, avec un nombre de cœurs de calcul allant jusqu'à 1800, et un nombre de mailles atteignant 8003 (soit plus 1/2 Milliard de mailles). Les tests de performances sont réalisés en statique sur le calcul du potentiel géométrique et en dynamique en propageant une densité de charge d'espace analytique caractéristique des streamers. Pour réaliser une simulation complète 3D de la décharge il faut également intégrer au programme un algorithme capable de résoudre les équations de transport de particule chargée à fort gradients de densité caractéristiques aux streamers. Dans ce manuscrit, l'algorithme MUSCL est testé dans différentes conditions de propagation d'un cube de densité (à vitesse homogène ou non homogène spatialement) afin d'optimiser le transport des densités d'espèces chargées impliquées. Le code 3D, conçu pour résoudre le modèle électro- hydrodynamique complet de la décharge (couplant les équations de transport, de Poisson et de cinétique réactionnelle) est ensuite validé par la confrontation des résultats 3D et 2D dans une condition de simulation présentant une symétrie de révolution autour de l'axe de propagation d'un streamer. Enfin, les premiers résultats des simulations 3D de la phase décharge avec la propagation d'un ou plusieurs streamers asymétriques sont présentés et analysés. Ces simulations permettent de suivre la structure arborescente de la décharge lorsqu'on applique une tension pulsée entre une pointe et un plan. L'initiation de la structure arborescente est étudiée en fonction de la position de spots plasmas et de leur influence sur l'amorçage des streamers. / This work is devoted to the three dimensional (3D) simulation of streamer corona discharges in air at atmospheric pressure using high-performance parallel computing. When a pulsed high-voltage is applied between a tip and a plane in air, the strong electric field lines constricted around the tip induce the simultaneous propagation of several streamers leading to a corona discharge with a tree structure. Only a true 3D electro-hydrodynamics simulation is able to reproduce this branching and to provide the orders of magnitude of the local deposited energy and the concentration of the species created during the discharge phase. However, such a 3D simulation which requires large computational memory and huge time calculation is nowadays accessible only when performed with massively parallel computation. In the field of 3D electro-hydrodynamics simulations, a special attention must be paid to the efficiency of solvers in solving 3D elliptic equations because their contribution can exceed 80% of the global computation time. Therefore, a specific chapter is devoted to test the performance of iterative and direct methods (such as SOR R&B, BiCGSTAB and MUMPS) in solving elliptic equations, using the massively parallel computation and the MPI library. The calculations are performed on the supercomputer EOS of the CALMIP network, with a number of computing cores and meshes increasing up to respectively 1800 and 8003 (i.e. more than 1/2 Billion meshes). The performances are compared for the calculation of the geometric potential and in a dynamic simulation conditions consisting in the propagation of an analytical space charge density characteristic of the streamers. To perform a complete 3D simulation of the streamer discharge, must also involve a robust algorithm able to solve the coupled conservation equations of the charged particle density with very sharp gradients characteristic of the streamers. In this manuscript, the MUSCL algorithm is tested under different propagation conditions of a cubic density (with uniform or non-uniform velocity field). The 3D code, designed to solve the complete electro-hydrodynamics model of the discharge (coupling the conservation equations, the Poisson equation and the chemical kinetics) is validated by comparing the 3D and 2D results in a simulation conditions presenting a rotational symmetry around the propagation axis of a mono-filamentary streamer. Finally, the first results of the 3D simulations of the discharge phase with the propagation of one or several asymmetric streamers are presented and analyzed. These simulations allow to follow the tree structure of a corona discharge when a pulsed voltage is applied between a tip and a plane. The ignition of the tree structure is studied as a function of the initial position of the plasma spots.