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Power transformers are one of the most important components in the electrical energy network. Extending transformer life is very economically valuable due to power outage. Therefore the development of instruments to monitor the transformer condition is of great interest. Detection of partial discharges (PDs) in power transformers is an effective diagnostic because it may reveal and quantify an important aging factor and provide information on the condition of the transformer. However, partial discharge diagnostics are still not effectively used for online monitoring of transformers because of the complexity of PD measurements and difficulties of discriminating of PDs and other noise sources. This thesis presents a further study of detection and location of partial discharges in power transformers based on previous work conducted at the Center for Photonics Technology (CPT) at Virginia Tech. The detection and positioning system consists of multiple extrinsic Fabry-Parot interferometric (EFPI) fiber acoustic sensors which can survive the harsh environment of oil-filled transformers. This thesis work is focused on optimal arrangement of multiple sensors to monitor and locate PD activities in a power transformer. This includes the following aspects. First, the sensor design requirements are discussed in order to successfully detect and accurately position the PD sources. In the following sections, Finite Element Method (FEM) is used to model the EFPI sensor fabricated at CPT. Experiments were conducted to measure the angular dependence of the frequency response of the sensor. It is shown that within the range of ±45º incident angles, the sensitivity varies by 3-5dB. Finally, the thesis demonstrates a PD positioning experiment in a 500 gallon water tank (R à H = 74" à 30" cylinder) using a hyperbolic positioning algorithm and time difference of arrival (TDOA). Finally we demonstrated that 100% of the positioning data is bounded by a 22.7à 4.1à 5.3 mm₃ cube, with a sensing range of 810 mm using the leading edge method with FIR filtering. / Master of Science

A partial discharge (PD) is the dissipation of energy caused by the buildup of localized electric field intensity. In high voltage devices such as transformers, this buildup of charge and its release can be symptomatic of problems associated with aging, such as floating components and insulation breakdown. This is why PD detection is used in power systems to monitor the state of health of high voltage transformers. If such problems are not detected and repaired, the strength and frequency of PDs increases and eventually leads to the catastrophic failure of the transformer, which can cause external equipment damage, fires and loss of revenue due to an unscheduled outage. Reliable online PD detection is a critical need for power companies to improve personnel safety and decrease the potential for loss of service. The PD phenomenon is manifested in a variety of physically observable signals including electric and acoustic pulses and is currently detected using a host of exterior measurement techniques. These techniques include electrical lead tapping and piezoelectric transducer (PZT) based acoustic detection. Many modern systems use a combination of these techniques because electrical detection is an older and proven technology and acoustic detection allows for the source to be located when several sensors are mounted to the exterior of the tank. However, if an acoustic sensor could be placed inside the tank, not only would acoustic detection be easier due to the increased signal amplitude and elimination of multipath interference, but positioning could also be performed with more accuracy in a shorter time. This thesis presents a fiber optic acoustic sensing system design that can be used to detect and locate PD sources within a high voltage transformer. The system is based on an optical acoustic (OA) sensor that is capable of surviving the harsh environment of the transformer interior while not compromising the transformer's functionality, which allows for online detection and positioning. This thesis presents the theoretical functionality and experimental validation of a band-limited OA sensor with a usable range of 100-300 kHz, which is consistent with the frequency content of an acoustic pulse caused by a PD event. It also presents a positioning system using the time difference of arrival (TDOA) of the acoustic pulse with respect to four sensors that is capable of reporting the three-dimensional position of a PD to within ±5cm on any axis. / Master of Science

This thesis focuses on the development of a novel polymer based housing for power thyristor devices typically used in long distance high voltage direct current (HVDC) transmission. Power thyristor devices used in HVDC power conversion stations are typically packaged in a hermetically sealed ceramic housing and have demonstrated an excellent history of reliability and performance. However, to avoid increasing the number of thyristors in future higher powered HVDC schemes thyristors having higher power ratings at 8.5 kV and sizes at 125 mm and 150 mm diameters are sought for implementation to achieve higher transmission ratings of, for example, 4000 A at +/- 800 kV. The main disadvantages of such large ceramic-based packages are higher processing cost and weight whilst robustness is also a concern. To overcome these issues, replacing the current ceramic housing with a polymeric material has been investigated in this project. The advantages it is anticipated such packages will provide include lower cost, less weight, robustness, recyclability, etc. However, some challenges it will also offer are: non-hermeticity i.e. polymers are moisture and gas permeable, potentially more complex manufacturing routes, and different electrical, mechanical and thermal properties compared to ceramic materials. The work presented in this thesis was part of a larger project where these challenges have been addressed by developing and testing a prototype polymeric thyristor housing. The prototype is aimed at demonstrating that polymer packages can deliver performance and reliability comparable to, if not better than, current ceramic packages. In this thesis, it is the package development and reliability related studies that are discussed. Because the housings will experience severe electrical stresses and various thermal excursions during their service life, the electrical and thermo-mechanical behaviour of the polymer housing was studied using finite element analysis to gain an understanding of the effects of various design variables and materials properties on performance and the tradeoffs between performance and manufacturability. From these modelling studies, design guidelines have been established for the future development of polymer housings. On the other hand, to identify the physics-of-failure of the prototype that was manufactured as part of the project, accelerated life tests were performed to study its reliability. The knowledge gained from the polymer prototype development was then applied to the design of a larger 125 mm diameter housing using the Taguchi method of experimental design.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: From a systems operation and design perspective, it is important to understand the behaviour of HVDC system insulation when presented with high voltage transients, such as induced by lightning and switching operations. Therefore, this thesis investigates the design, operation and performance of a circuit arrangement that can be used in high voltage laboratories to generate impulse voltage waveforms superimposed on a dc bias voltage. The circuit arrangement consists of an impulse generator and a dc source that supplies continuous dc voltage to stress the test object, which can be any type of insulator, i.e. composite, porcelain, glass, gap arrangements, etc. The composite waveform obtained from the test arrangement is used experimentally to investigate the impulse flashover of insulators. For modelling and analysis purposes, the test circuit was transformed to a Laplace equivalent in order to derive the applicable nodal voltage equations. After substitution of circuit parameter values, the voltage equations are then transformed to time domain equations that predict the time-domain behaviour of the circuit. To validate this mathematical approach, the voltage waveforms obtained with this mathematical model is compared with the waveforms measured under laboratory conditions and also with waveforms simulated with HSPICE software. These comparisons are performed using graphical representations. Good correlation was obtained and the results are presented in this thesis. The final stage of this thesis discusses the application of the designed test arrangement for flashover and withstands tests on a silicon rubber insulator. The determination of the flashover values is done by using the existing statistical methods. The insulator was tested under dry conditions and also under polluted wet conditions for both positive and negative impulses compared to the DC bias voltage polarity. The results show that the dc bias voltage does not affect the total voltage flashover of the insulator significantly. It was also observed that wetting affects the flashover for negative impulse more severely, while the influence of wetting is minimal with positive impulse voltages / AFRIKAANSE OPSOMMING: Vanuit ‘n stelselbedryf en ontwerp perspektief, is dit is belangrik om die gedrag van HSGS stelsels te verstaan wanneer dit onderwerp word aan hoogspanning oorgangsverskynsels soos veroorsaak deur weerlig en skakeloperasies. Daarom ondersoek hierdie tesis die ontwerp, werking en werkverrigting van ‘n stroombaanopstelling wat gebruik kan word in hoogspanningslaboratoriums om impulsspannings gesuperponeer op gelykspanning voorspannings op te wek. Die stroombaan bestaan uit ’n impulsgenerator en ‘n gs-bron wat die langdurige gs-spanning voorsien aan die toetsvoorwerp, wat enige tipe isolator kan wees. bv. porselein, glas, gapings, ens. Die saamgestelde golfvorm wat met die toetsopstelling verkry word, is eksperimenteel gebruik om die impulsoorvonking van isolators te ondersoek. Vir die doel van modellering and analise, is die stroombaan na ‘n Laplace ekwivalent getransformeer om die toepaslike knooppunt spanningsvergelykings af te lei. Na substitusie van die stroombaan parameter waardes, word die spanningsvergelykings getransformeer na die tydgebied vergelykings wat die tydgebied gedrag van die stroombaan voorspel. Om die wiskundige benadering te toets, word die spanningsgolfvorms wat met die wiskundige model voorspel word, vergelyk met golfvorms wat onder laboratorium toestande gemeet is en ook met golfvorms wat met HSPICE programmatuur gesimuleer is. Hierdie vergelykings word gedoen met behulp van grafiese voorstellings. Goeie korrelasie is verkry en die resultate word in die tesis gegee. Die finale stadium van hierdie tesis bespreek die toepassing van die ontwerpte toetsopstelling vir oorvonk- en weerstaantoetse op ‘n silikonrubber isolator. Die bepaling van die oorvonkwaardes word gedoen deur bestaande statistiese metodes te gebruik. Die isolator is onder droë en nat besoedelde toestande gedoen, vir beide positiewe sowel as negatiewe impulse met verwysing na die GS voorspan spanning. Die resultate toon dat die gsvoorspanning nie die oorvonkspanning van die isolator beïnvloed nie. Dit is ook waargeneem dat die benatting die oorvonking neer beïnvloed met ‘n negatiewe impuls terwyl die invloed minimaal is met positiewe impulsspannings.

La filière viti-vinicole se doit de trouver des solutions au cours des années à venir afin de réduire son impact carbone de 20% et de proposer rapidement des alternatives au dioxyde de soufre. Ainsi, ce travail de thèse sur les champs électriques pulsés propose aux professionnels du vin une éco-innovation qui pourrait aider à répondre à ces deux problématiques. L’éco-innovation est de plus en plus considérée comme la clé de la compétitivité future dans le cadre du développement durable. Toutefois, avant d’être intégrée dans la production du vin, l’éco-innovation doit démontrer sa performance et son efficacité sans nuire à la qualité du produit et à la sécurité des consommateurs et doit être approuvée par les instances gouvernementales (OIV, UE...). A ce titre, la thèse a reçu un soutien financier conjoint du CIVB et de l’ADEME. Le principe des Champs Electriques Pulsés (CEP) est d’appliquer à un produit des impulsions électriques de quelques kilovolts durant un temps très court (quelques microsecondes) et répétées n fois. Les cellules contenues dans le produit traité (raisins, moût ou vin) voient leur potentiel transmembranaire augmenter jusqu’à l’induction de pores dans les membranes. L’irréversibilité des pores aboutit à l’extraction des composés cellulaires mais aussi à la mort des cellules. Ainsi, cette technologie, suivant le moment de son application en vinification et les paramètres opératoires, permet l’extraction des composés d’intérêts organoleptiques tels que les polyphénols mais aussi l’inactivation des microorganismes. Les CEP prennent en compte trois aspects importants pour la durabilité d’une technologie: l'environnement, l'économie et la qualité. En effet, cette technique a une série d'avantages pour le producteur de vins: elle est propre, rapide, peu chère, efficace, industrialisable et automatisable. Comparée à d’autres traitements tels que la pasteurisation, la filtration stérilisante, la thermovinification, la macération à chaud ou à froid, la cryoextraction ou encore la flash-détente, la consommation d'énergie est faible (quelques dizaines de kWh/tonne). La technologie des CEP est respectueuse de l’environnement puisqu’elle nécessite peu d’énergie et aucun intrant chimique. Elle est rapide et efficace car la durée du traitement n’est que de quelques dizaines à centaines de millisecondes. Enfin, les CEP sont une technologie non thermique et donc ne dégradent pas les molécules thermosensibles telles que les arômes. Les objectifs de ce travail de recherche ont été de comprendre l’effet des CEP sur les cellules, de déterminer les paramètres de traitement des CEP à employer pour réaliser l’extraction de composés d’intérêt des raisins et pour inactiver des microorganismes afin de stopper la fermentation alcoolique des vins liquoreux et de stabiliser microbiologiquement les vins rouges avant leur mise en bouteille en conservant la qualité du produit traité. / The wine industry needs to find solutions in the coming years to reduce its carbon footprint by 20% and quickly propose alternatives to sulphur dioxide usage. This work on pulsed electric fields offers wine professionals an eco-innovation that could help to address these two issues. Eco-innovation is increasingly seen as the key to future competitiveness in the context of sustainable development. However, before being integrated in wine production, eco-innovation must demonstrate its performance and efficiency without compromising product quality and consumers safety so that it could be approved by government authorities (OIV, EU ...). As such, this thesis has received a joint financial support from the CIVB and the ADEME. The principle of Pulsed Electric Field (PEF) is to apply to a product; electrical pulses of a few kilovolts in a very short period time (a few microseconds) and then repeated n times. During treatment (grapes, grape must or wine) cells transmembrane potential increases till the induction of pores in the membranes. The irreversibility of the pores leads not only to the extraction of cellular components but also cell death. Thus, depending on the time of this technology application in winemaking and operating parameters, allows the extraction of sensory interests compounds such as polyphenols or the inactivation of microorganisms. PEF takes into account three important aspects to the sustainability of a technology: environment, economy and quality. This technique has a number of advantages for wine producers: clean, fast, inexpensive, efficient, industrializable and automated. Compared to other treatments such as pasteurization, sterile filtration, thermovinification, hot or cold maceration, cryoextraction or flash-release, its energy consumption is low (a few tens of kWh / tonne). PEF technology is environmentally friendly, as it requires little energy and no chemical inputs. It is fast and efficient because the processing time is only a few tens to hundreds of milliseconds. Finally, this technique is a non-thermal and therefore does not degrade the heat-sensitive molecules such as flavours. The objectives of this research work was to understand the effect of PEF on the cells, to determine the processing parameters during the extraction of compounds of interest grapes and to inactivate microorganisms in order to stop alcoholic fermentation of sweet wines and stabilize microbiologically red wines before bottling maintaining the quality of the processed product.

Cette étude a pour objet les décharges haute-tension nanoseconde dans l'air à des pressions supérieures ou égale à la pression atmosphérique, en géométrie pointe-plan, et leur application au déclenchement de combustion en mélange air/propane. Ces décharges fortement hors-équilibres sont susceptibles de former une concentration significative d'espèces réactives et nous analysons leur capacité à allumer un mélange combustible. Le générateur conçu est capable de fournir une impulsion de tension carrée de 40 à 80 kV avec un front de montée raide de 3 ns. A la pression atmosphérique, nous observons un type de décharge peu commun dans les précédentes études expérimentales de décharges couronnes : la décharge diffuse. On retrouve une décharge de type filamentaire en augmentant la distance pointe-plan ou en augmentant la pression, toutes choses égales par ailleurs. Nous mettons en avant par imagerie CCD rapide deux phases de développement de ces décharges, également analysées à l'aide de simulations COMSOL. Pour une pression de 1 bar, l'application de ces décharges à un mélange combustible air/propane provoque un allumage à la pointe, avec une énergie minimale de décharge de 8±2 mJ. L’inflammation est obtenue pour une impulsion de tension unique, et la richesse minimum pour l’obtenir est 0,7. La question de la contribution de l'apport radicalaire en comparaison de l'apport thermique à l'apparition du noyau de flamme se pose. L'analyse paramétrique basée sur l'effet de l'atome d'oxygène sur les délais d'inflammation montre qu'il est nécessaire de convertir entre 0,5 et 0,8% d'oxygène moléculaire pour pouvoir allumer à délai équivalent et avec 100 K de moins par rapport à une auto-inflammation. Par ailleurs, la température du gaz à 1 mm de la pointe a été mesurée par spectroscopie Raman spontanée, en collaboration avec le laboratoire CORIA (Rouen) : cette température reste proche de l’ambiante pour une énergie de 30 mJ et une concentration de propane de 1,7 %. Ainsi les radicaux jouent probablement un rôle non négligeable dans le déclenchement de combustion par décharge nanoseconde mono-impulsionnelle. / The present study deals with high voltage nanosecond discharges in air at equal or higher pressure than the atmospheric pressure in a point-to-plane geometry, and how they apply to the combustion triggering in air/propane mixtures. These highly non-equilibrium discharges can produce a significant concentration of reactive species and we analyse their ability to ignite combustion. The generator can produce a pulse voltage of 40 to 80 kV with a steep rise front of 3 ns. We observe an uncommon type of discharge at atmospheric pressure: the diffuse discharge. The common filamentary discharge occurs when we increase the point-to-plane distance or the pressure, all else being equal. With fast CCD imaging, we are able to identify two phases in the discharge evolution, which are also analysed with COMSOL simulations. At pressure of 1 bar, discharges can ignite an air/propane mixture at the point, with a minimal discharge energy depositing of 8±2 mJ. Combustion occurs with a single voltage pulse, the minimal equivalence ratio being 0,7. This brings up the question of the contribution of the radical part as compared to the thermal part as the kernel of the flame. The parametric analysis based on the effect of the oxygen atom on the ignition periods show that it is necessary to convert between 0,5 and 0,8% of molecular oxygen in order to ignite with a similar time period and temperature reduced by 100K, as compared to auto-inflammation. Moreover the temperature of gas at 1 mm from the point has been measured by Raman Spectroscopy, in collaboration with the CORIA laboratory (Rouen): this temperature remains close to the ambient one for deposited energy of 30 mJ and a propane concentration of 1,7%. Thus radicals may play quite an important part in the triggering of combustion by nanosecond mono-pulse discharges.

The Port of Gävle is one of the most important harbours in Sweden as far as size and freight capacity is concerned. Marine traffic is increasing greatly, thus environmental pollution as well as noise and vibrations are of major concern in port cities. Shore to ship power supply systems might be a feasible solution to curtail emissions because the Auxiliary Engines are instead shut down while the ship stays alongside the quay. The literature review shows they are reliable and very appealing in all respects, thereby contributing to sustainable development. Taking into account the kind of vessels that call at the Port of Gävle, a High Voltage Shore Connection is recommendable, in compliance with the International Standards. An own technical survey is developed to gather all the information, as well as personal interviews to collect first-hand data. Technical issues such as the synchronisation procedure and the ground system with regard to safety are briefly discussed. Due to the lack of data, calculations consist of average values: peak and average demand, and fuel consumption during a typical call. Considering updated energy prices for both electricity and fuel, results show that an on‑shore power supply system make energy costs decrease by 71% at berth in comparison with burning marine fuel, which is saved by around 4 tonnes per call. Additionally, up to 5126 tonnes of CO2 are avoided per year, among other pollutants. Shore‑side power has proven to be profitable and appealing to the Port of Gävle; however, vessels need to be retrofitted, which implies relatively high investments. Collaboration agreements and shipping companies’ willingness to undergo changes are key issues that still need to be solved.