Oxide thin films were grown on 4H-SiC at low pressure and reduced temperatures using a remote plasma afterglow thermal oxidation method, achieving significantly faster growth rates than standard atmospheric furnace processes. The resulting SiO2/SiC structures were characterized by a non-contact corona-voltage metrology technique in order to extract capacitance-voltage information, to facilitate further analysis of the afterglow oxidation growth mechanism, and to determine the electrical behavior of defects. In addition, mass spectrometry experiments revealed the concentration of nitric oxide species in the afterglow reactor gas exhaust produced by the cracking of N2O molecules in the microwave plasma discharge. Oxidations were performed on n- and p-doped epitaxial 4H-SiC wafers at growth temperatures between 700°C and 1100°C. The afterglow oxidation process was determined to be primarily in the parabolic growth regime, and thus rate-limited by diffusion processes. Analysis of the parabolic growth rate temperature dependence revealed a break in activation energy between 0.46 eV and 1.51 eV at lower and higher temperature ranges, indicating a change in the dominating oxidation mechanism. In the proposed transport-limited mechanism, afterglow oxidation was suggested to be rate-limited by parallel diffusion of atomic oxygen radicals and excited singlet oxygen molecules to the SiO2/SiC interface. An alternative stress-relief mechanism suggested that viscous flow of SiO2 could relieve compressive stress in the oxide above 960°C. In this case, growth would be stress-limited at low temperatures and diffusion-limited at higher temperatures. Regardless of the exact mechanism or temperature range, the data developed in this work suggest that afterglow oxidation rates of 4H-SiC are faster than atmospheric growth rates mainly because significant quantities of atomic and excited oxygen are generated in the microwave discharge independent of temperature. Using flatband voltages and accumulation capacitance values extracted from C-V measurements, worst-case charge densities associated with the oxide-semiconductor interfacial region were estimated. The charged defects were found to exist in the 1012/cm2 range regardless of growth temperature or oxide thickness. The charged defects were attributed to interface traps which capture majority carriers while the SiC is electrically stressed into accumulation during measurement. It was suggested that the traps failed to emit their charges within the time of measurement, even when the semiconductor was swept into depletion, and thus caused a shift in the observed flatband voltage. Mass spectrometry analysis showed that no thermal cracking of gas species occurs in the furnace at the detection level of the measurement, but rather significant quantities of nitric oxide are produced by the cracking of N2O molecules in the microwave plasma discharge independent of furnace temperature.

The main objective of this research was to study DC conductor corona loss (CL) and audible noise (AN) in the context of local climatic conditions, through corona cage measurements, and do a comparative analysis of the measured data with results available in literature and EPRI TLW software simulation results. The ultimate aim was to assess the applicability of the software to our local conditions and hence determine, if necessary, appropriate correction factors for application in HYDC transmission line designs. For this study, short term measurements of corona AN and currents were carried out in an indoor meshed cylindrical corona cage, under DC and AC voltages. The cage was later converted into a short test line and some of the measurements repeated. The DC supply was obtained from a two stage ±500 kV Walton-Cockroft generator. The AC voltages were obtained from a 2x 100 kV, 50 Hz, AC test transformer set. The tests were performed using single solid and stranded aluminum conductors with three different diameters (1.6 cm, 2.8 cm, and 3.5 cm). All the measurements were carried out at low altitude. A CoroCAM I camera was used to determine the corona inception gradients and to observe the corona activity at different surface gradients and under different voltages and polarities. AN measurements at different conductor surface gradients were done using the Rohde&Schwarz and the Bruel&Kjaer sound level meters. To obtain the frequency spectra, a Bruel&Kjaer octave-band filter set attached to the sound level meter was used. The measured data was corrected for both height and length effects, and then compared with simulations from the EPRI-TLW software through curve fitting. A digital micro-ammeter connected to the centre of the cage through a 560 .Q measuring resistor was used to measure the corona current. Current pulses were viewed using a digital storage oscilloscope. To verify the corona current results obtained from the cage measurements, current measurements were also done for a point-plane spark gap. The corona current data was, later on, used to evaluate the total corona power loss for DC. The results obtained from test line measurements were used to compare the CL and AN for different configurations. The effect of the space charge under DC voltages was assessed through current measurements. The measurements were done with the cage covered with an aluminum foil to trap the charge and then repeated with the cage uncovered. On the test line, the space charge effect was investigated using a high power fan blowing along the conductor, to simulate the wind factor. The results of this study have shown the characteristics of corona discharges under different system voltages. The results also give an understanding of how factors such as conductor surface conditions and size, polarity and system voltage affect CL and AN. Both CL and AN were found to increase with conductor size for the same conductor surface gradient and to be higher for stranded conductors. Positive polarity DC and AC noise levels were higher than the negative polarity levels. CL under positive polarity DC was lower than the negative polarity loss. The effects of space of space charge were noted to some extend. The comparison of test line results and cage results showed that CL depends more on the gap size and the shorter the gap the higher the loss. Hence CL results were not compared with the software simulations. The comparison between the corrected AN results and the software simulations showed a very good agreement. The comparison was done for the 3.5 cm and the 2.8 cm diameter conductors under both positive and negative polarities. The trends compared through curve fitting were quite similar and the trend line equations were of the same order of magnitude. The magnitudes of the corrected noise levels were higher than the CRIEPI and BPA predictions but closer to the EPRl prediction. Generally there is a very good and encouraging agreement between the available literature, simulation results and the results obtained from the laboratory measurements. It is proposed, as part of further studies, to extend this work to high altitude regions and use bundled conductors as well. Consideration of different and larger test configurations will provide an understanding of the effects of geometry on corona discharges. Space charge analysis will also assist in determining the effect of space charge on different configurations. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2007.

En aviation civile, les communications entre pilotes et contrôleurs aériens sont d'une importance primordiale. Un brouillage a été observé sur des stations sol fonctionnant dans la gamme VHF (118- 134 MHz), qui a perturbé les communications air-sol. Ce phénomène est dû à des décharges corona qui se produisent sur le pylône lors d'un fort champ électrostatique ambiant. Le but de cette thèse est de développer un modèle pour prédire le bruit dû aux décharges corona pour une géométrie et un champ ambiant connus. Premièrement, nous localisons les lieux où les décharges se produisent le plus probablement via des simulations électrostatiques. Puis, nous proposons et validons un modèle de dipôle élémentaire pour représenter le rayonnement des décharges. Enfin, nous évaluons les niveaux d'interférence introduits par les décharges corona aux ports des antennes au moyen de simulations dans le domaine fréquentiel. Les résultats de simulation sont comparés avec des mesures.

Mémoire (Sc.A.)--Université du Québec à Chicoutimi, 1986. / Document électronique également accessible en format PDF. CaQCU

Mémoire (M.Sc.A.)-- Université du Québec à Chicoutimi, 1991. / Bibliogr.: f. 86-91. Document électronique également accessible en format PDF. CaQCU

Mémoire (M.Eng.)--Université du Québec à Chicoutimi, 2000. / Document électronique également accessible en format PDF. CaQCU

Η παρούσα εργασία αναφέρεται στη μελέτη της ηλεκτρικής εκκένωσης τύπου κορώνας και ιδιαίτερα στα ηλεκτρικά και υδροδυναμικά (ηλεκτρικός άνεμος) φαινόμενα που τη συνοδεύουν. Μια εκκένωση κορώνας είναι το φαινόμενο που δημιουργείται σε ένα αέριο διάκενο μεταξύ δύο ηλεκτροδίων διαφορετικής ακτίνας καμπυλότητας, όταν αυτά βρίσκονται υπό την επίδραση διαφοράς δυναμικού μερικών kV. Η εκκένωση κορώνας συνοδεύεται από την εμφάνιση ηλεκτρικών, οπτικών, ακουστικών, χημικών, αεροδυναμικών (ηλεκτρικός άνεμος) και ραδιοηλεκτρικών φαινομένων. Ο ηλεκτρικός άνεμος είναι ένα φαινόμενο το οποίο δημιουργείται από τη μετάδοση της ορμής των ιόντων στα ουδέτερα μόρια του αερίου οπότε παρατηρείται μία κίνηση του αερίου από το ηλεκτρόδιο με μικρή ακτίνα καμπυλότητας, προς το άλλο ηλεκτρόδιο. Το κύριο αντικείμενο της εργασίας αυτής είναι η μελέτη του φαινομένου της θετικής κορώνας, μεταξύ μιας μη συμμετρικής διάταξης ηλεκτροδίων, στον ατμοσφαιρικό αέρα. Στην άνοδο τοποθετήθηκαν μια και δύο ακίδες της ίδιας ακτίνας καμπυλότητας ενώ χρησιμοποιήθηκαν τρεις διαφορετικές κάθοδοι. Οι πειραματικές μετρήσεις είχαν ως στόχο τον προσδιορισμό της χαρακτηριστικής του μέσου ρεύματος της εκκένωσης συναρτήσει της τάσης διακένου, των παλμών του ρεύματος για διάφορες τιμές της τάσης , τον προσδιορισμό της κατανομής και της χαρακτηριστικής της ταχύτητας του ηλεκτρικού ανέμου καθώς και της χαρακτηριστικής της θερμοκρασίας του ηλεκτρικού ανέμου. Πρέπει να αναφέρουμε ότι οι μετρήσεις πραγματοποιήθηκαν για συνεχή και παλμική τάση λειτουργίας. / In this thesis, the effect of the positive corona discharge was studied. Especially, electric and hydrodynamic phenomena (corona wind) were studied. By the term “corona discharge”, one generally refers to the ensemble of phenomena which occur in a gaseous medium in the vicinity of conductors of small radius of curvature, subjected to intense, but not disruptive, electric fields. The main part of this thesis is experimental procedures that were held between a non-symmetrical set-up of electrodes in atmospheric air in order to, initially define the V-I characteristics and study the current impulses for different voltage values. Furthermore, we studied the velocity distribution and velocity characteristics of corona wind. In the last part of this work the temperature characteristics are present. It is important to mention that the experimental measurements were carried out on continuous and pulsed voltage.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Power utilities are expected to keep the cost of electricity as low as possible. They are also expected to be environmentally friendly and, amongst other things, not to produce unacceptable audible noise. When the electric field on a conductor is high enough corona is produced and this is accompanied by audible noise. Air pressure, which is directly related to altitude, has an effect on the voltage at which corona will start. It is more difficult to ionise the air at sea level (high air pressure) than at high altitude (low air pressure). Altitude does not only affect the corona inception voltage, but also the intensity of the audible noise. A thorough scan of literature revealed that there is very little evidence of prior research work on the effect of air density on corona under fair weather (dry) conditions. In South Africa, transmission lines are built at altitudes higher than 1800 m above sea level. The cost of a 400 kV line is in the region of R2M per km. It is important to predict the noise levels under a proposed line accurately, before it is energised. This research indicated that the altitude correction for conductor corona audible noise, under dry conditions, might be steeper than the general accepted correction of 1 dB/300m. This correction, however, appears to be valid for heavy rain conditions. Under heavy rain conditions the corona is mainly determined by the water droplets, whereas under dry conditions the condition of the conductor plays the biggest role. The air density therefore has a bigger effect on the corona performance under dry conditions. The implication of a steeper altitude correction for dry conditions is that too low noise levels will be predicted for a higher altitude, which could lead to complaints. On the other hand, predictions for lower altitudes will be too conservative. / AFRIKAANSE OPSOMMING: Daar word van kragvoorsieners verwag om die koste van elektrisiteit so laag as moontlik te hou. Hulle is verder onder druk om omgewingsvriendelik te wees en om onder andere nie onaanvaarbare hoorbare geraas te veroorsaak nie. Wanneer die elektriese veld op ‘n geleier hoog genoeg is, kan korona ontstaan wat dan hoorbare geraas veroorsaak. Die lugdruk, en daarom die hoogte bo seevlak, beïnvloed die spanning waarby ‘n geleier in korona sal gaan. Dit is moeiliker om die lug te ioniseer by seevlak (hoë lugdruk) as hoog bo seevlak (lae lugdruk). Die hoogte bo seevlak beïnvloed daarom nie net die spanning waarby korona sal begin nie maar ook die intensiteit van die hoorbare geraas. Dit wil voorkom of die effek van hoogte bo seevlak, op hoorbare geraas, a.g.v. geleier korona tot op datum baie skraps nagevors is. Baie min kon in die literatuur gevind word op die effek onder droë toestande. In Suid-Afrika is dit nodig om transmissielyne op hoogtes van 1800 m en hoër te bou. So ‘n lyn (400 kV) kos in die omgewing van R2M per km. Dit is daarom van uiterste belang om die geraasvlakke wat ‘n beplande lyn sal veroorsaak, akkuraat te bepaal, voordat so ‘n lyn aangeskakel word. Hierdie navorsing het gewys dat die effek van hoogte bo seespieël op hoorbare korona geraas onder droë toestande groter kan wees as wat algemene aanvaar word. Die helling van die korreksiefaktor vir hoogte bo seevlak blyk steiler as 1 dB/300 m te wees vir droë toestande. Die implikasie hiervan is dat geraas voorspellings vir hoër hoogtes bo seespieël te laag sal wees en die vir lae hoogtes te konserwatief kan wees. Die navorsing stem egter saam met die korreksiefaktor van 1 dB/300 m onder swaar reën toestande. Dit wil voorkom of die invloed van die waterdruppels op geleier korona groter is as lugdruk. Onder droë toestande speel die toestand van die geleier 'n groter rol en is die effek van lugdruk groter.

abstract: Composite insulators on overhead lines are frequently subjected to corona discharges due to increased electric field intensities under various conditions. These discharges can cause localized heating on the surface and affect the hydrophobicity of the insulator. A study has been undertaken to quantify and evaluate the thermal degradation that composite insulation is subjected to from corona discharges. This has been conducted primarily at the power frequency (60 Hz) and at the low frequency range (37 kHz). Point to plane corona discharge experiments have been performed in the laboratory at both the frequencies and varying levels of thermal degradation has been observed. The amplitude and the frequency of current spikes have been recorded at different voltage levels. A temperature model based on the amplitude and the frequency of current data has been formulated to calculate the maximum temperature attained due to these discharges. Visual thermal degradation has been found to set in at a low frequency range while there is no visual degradation observed at power frequency even when exposed to discharges for relatively much longer periods of time. However, microscopic experiments have been conducted which revealed degradation on the surface at 60 Hz. It has also been found that temperatures in excess of 300 Celsius have been obtained at 37 kHz. This corroborates the thermo gravimetric analysis data that proves thermal degradation in silicone rubber samples at temperatures greater than 300 Celsius. Using the above model, the maximum temperature rise can be evaluated due to discharges occurring on high voltage insulation. This model has also been used to calculate the temperature rise on medium voltage distribution equipment such as composite bushings and stand-off plugs. The samples were subjected to standard partial discharge tests and the corresponding discharge magnitudes have been recorded. The samples passed the tests and the corresponding temperatures plotted have been found to be within thermal limits of the respective insulation used on the samples. The experimental results concur with the theoretical model. A knowledge of the maximum temperatures attained due to these discharges can help in design of insulation with better thermal properties. / Dissertation/Thesis / M.S. Electrical Engineering 2010

The problem of how the Sun's corona is heated is of central importance to solar physics research. In this thesis we model three main areas. The first, annihilation, is a feature of non-ideal MHD and focusses on how magnetic field of opposite polarity meets at a null point and annihilates, after having been advected with plasma toward a stagnation point in the plasma flow. Generally, the null point of the field and the stagnation point of the flow are coincident at the origin, but in chapter 2 a simple extension is considered where an asymmetry in the boundary conditions of the field moves the null point away from the origin. Chapter 3 presents a model of reconnective annihilation in three dimensions. It represents flux being advected through the fan plane of a 3D null, and diffusing through a thin diffusion region before being annihilated at the spine line, and uses the method of matched asymptotic expansions to find the solution for small values of the resistivity. The second area of the thesis covers null collapse. This is when the magnetic field in close proximity to a null point is disturbed, causing the field to fold up on itself and collapse. This is a feature of ideal MHD, and causes a strong current to build up, allowing non-ideal effects to become important. When using linearised equations for the collapse problem, we are in fact looking at a linear instability. If this instability initiates a collapse, this is only a valid model until non-linear effects become important. By talking about collapse in chapters 4 and 5 (as it is talked about in the literature), we mean that the linear instability initiates collapse, which in principle, non-linear effects could later stop. Chapter 4 introduces a two-dimensional model for collapse, using the ideal, compressible, linearised MHD equations. It is a general solution in which all spatially linear nulls and their supporting plasma flows and pressure gradients can be checked for susceptibility to collapse under open boundary conditions. Chapter 5 uses the model introduced in chapter 4 to investigate the collapse of three-dimensional, potential nulls (again, spatially linear) for all possible supporting plasma flows and pressure gradients. Using this model, all nulls under consideration are found to collapse and produce large currents, except for a group of 2D O-type nulls supported by highly super-Alfvenic plasma flows. The third area of this thesis involves numerically simulating a model of heating by coronal tectonics (Priest et al, 2002). A simple magnetic field is created and the boundary is driven, also in a simple manner. Current sheets which scale with grid resolution are seen to build up on the quasi-separatrix layers, and there is some evidence of magnetic reconnection.