In order to meet the increasing demand for sustainable transportation, trains need to run with tighter schedules, more departures and more trains in depot. Multiple trains in depot has been linked to low frequency instability at many locations around the world. The instability originates in the interaction between the infrastructure and trains, specifically the line converter module (LCM). The phenomenon has been studied during the last two decades. Today there are a number of methods used to analyse low frequency stability, although these methods are usually slow and cumbersome. In this project a modelling approach is proposed for a linear small-signal DQ-frame admittance model of the LCM. The model describes the relationship of the low frequency oscillations between the line voltage and line current. Similar linear models have been derived previously, although the design of the specific train modelled is in part significantly different from the design of the trains modelled previously. The main difference is in the DQ-transformation for the phase locked loop (PLL), which in this thesis is implemented with a DFT-based method, and the implementation of the current controller (CC) which in this thesis is implemented in the stationary reference frame. The LCM and infrastructure (modelled as an impedance) is then analysed as a feedback loop using dominant poles analysis, regularly used in control theory. The result from the proposed model was evaluated using methods that are common in the industry. Furthermore, as the LCM is a complex system and the impact of different components and control parameters is not fully understood, the proposed model is used to examine how changing different parameters affect the overall stability of the system. The correlation between the results from the proposed model and the methods used for evaluation implies that the modelling approach is accurate. The two main benefits of the proposed model is that the method is much faster than most other methods for low frequency analysis, the proposed model is useful for testing different parameter configurations and changes in the design of the LCM. / För att möta den ökande efterfrågan på hållbar transport, behöver tåg idag köras med stramare tidtabeller, fler avgångar och fler tåg i depå. Flera tåg i depå har kopplats till lågfrekvent instabilitet på många platser runt om i världen. Instabiliteten har sitt ursprung i interaktionen mellan infrastrukturen och tågen, särskilt nätströmriktaren (LCM). Fenomenet har studerats under de senaste två decennierna. Idag finns det ett antal metoder som används för att analysera lågfrekvent stabilitet, dock är dessa metoder vanligtvis är långsamma och besvärliga. Detta projekt beskriver en modelleringsmetod lågfrekvenssvängningar mellan linjespänningen och linjeströmmen. Liknande linjära modeller har härletts tidigare, skiljer sig utformningen av det specifika tåget modellerad i denna avhandling från de tåg som modellerats tidigare i vissa design val. Den största skillnaden ligger i DQ-transformationen för den synkroniseringsregulatorn (PLL), som i denna avhandling är implementerad med en DFT-baserad metod, och implementeringen av strömregulatorn (CC) som i denna uppsats implementeras i den stationära referensramen. LCM tillsammans med infrastruktur (modellerad som en impedans) analyseras sedan som en stängd återkoppling där det stängda systemets poler härleds. Resultatet från den föreslagna modellen utvärderades med metoder som är vanliga i branschen. Dessutom, eftersom LCM är ett komplext system och effekten av olika komponenter och styrparametrar inte är helt förstod, används modellen för att undersöka hur förändringar i olika parametrar påverkar hela systemets stabilitet. Korrelationen mellan resultaten från det föreslagna modell och de metoder som används för utvärdering av lågfrekvent stabilitet idag innebär att modelleringsmetoden är lovande. De metoden är mycket snabbare än de flesta andra metoder för lågfrekvent analys och att den föreslagna modellen är användbar för att testa olika parameterkonfigurationer och förändringar i utformningen av LCM.

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<p>    </p> <p>Cerebrovascular diseases, such as stroke, constitute the most common life-threatening neurological disease in the United States. To support normal brain function, maintaining adequate brain perfusion (i.e., cerebral blood flow (CBF)) is important. Therefore, it is crucial to assess the brain perfusion so that early intervention in cerebrovascular diseases can be applied if abnormal perfusion is observed. The goal of my study is to develop metrics to measure the brain perfusion through modeling brain physiology using resting-state and task-based blood-oxygenation-level- dependent (BOLD) functional MRI (fMRI). My first and second chapters focused on deriving the blood arrival time using the resting-state BOLD signal. In the first chapters, we extracted the systemic low-frequency oscillations (sLFOs) in the fMRI signal from the internal carotid arteries (ICA) and the superior sagittal sinus (SSS). Consistent and robust results were obtained across 400 scans showing the ICA signals leading the SSS signals by about 5 seconds. This delay time could be considered as an effective perfusion biomarker that is associate with the cerebral circulation time (CCT). To further explore sLFOs in assessing dynamic blood flow changes during the scan, in my second chapter, a “carpet plot” (a 2-dimensional plot time vs. voxel) of scaled fMRI signal intensity was reconstructed and paired with a developed slope-detection algorithm. Tilted vertical edges across which a sudden signal intensity change took place were successfully detected by the algorithm and the averaged propagation time derived from the carpet plot matches the cerebral circulation time. Given that CO₂ is a vasodilator, controlling of inhaled CO₂ is able to modulate the BOLD signal, therefore, as a follow-up study, we focused on investigating the feasibility of using a CO₂ modulated sLFO signal as a “natural” bolus to track CBF with the tool developed from the second chapter. Meaningful transit times were derived from the CO₂-MRI carpet plots. Not only the timing, the BOLD signal deformation (the waveform change) under CO₂ challenge also reveals very useful perfusion information, representing how the brain react to stimulus. Therefore, my fourth chapter focused on characterizing the brain reaction to the CO₂ stimulus to better measure the brain health using BOLD fMRI. Overall, these studies deepen our understanding of fMRI signal and the derived perfusion parameters can potentially be used to assess some cerebrovascular diseases, such as stroke, ischemic brain damage, and steno-occlusive arterial disease in addition to functional activations. </p>

The electric power supply industry has evolved into one of the largest industries. Even though secure and reliable operation of the electric power system is fundamental to economy, social security and quality of modern life, the complicated power grid is now facing severe challenges to meet the high-level secure and reliable operation requirements. New technologies will play a major role in helping today's electric power industry to meet the above challenges. This dissertation has focused on some key technologies among them, including the emerging technologies of energy storage, controlled power electronics and wide area measurement technologies. Those technologies offer an opportunity to develop the appropriate objectives for power system control. The use of power electronics based devices with energy storage system integrated into them, such as FACTS/ESS, can provide valuable added benefits to improve stability, power quality, and reliability of power systems. The study in this dissertation has provided several guidelines for the implementation of FACTS/ESS in bulk power systems. The interest of this study lies in a wide range of FACTS/ESS technology applications in bulk power system to solve some special problems that were not solved well without the application of FACTS/ESS. The special problems we select to solve by using FACTS/ESS technology in this study include power quality problem solution by active power compensation, electrical arc furnace (EAF) induced problems solution, inter-area mode low frequency oscillation suppression, coordination of under frequency load shedding (UFLS) and under frequency governor control (UFGC), wide area voltage control. From this study, the author of this dissertation reveals the unique role that FACTS/ESS technology can play in the bulk power system stability control and power quality enhancement in power system. In this dissertation, almost all the studies are based on the real system problems, which means that the study results are special valuable to certain utilities that have those problems. The study in this dissertation can assist power industry choose the right FACTS/ESS technology for their intended functions, which will improve the survivability, minimize blackouts, and reduce interruption costs through the use of energy storage systems. / Ph. D.

Research has shown that the extent of students' vocabulary in the language of learning and teaching, as an important component of overall language proficiency, plays a crucial role in reading and academic success, whether students are studying through their mother tongue or not. This study compares the vocabulary size of Grade 7 English second language immersion students with that of their English mother tongue classmates, focusing primarily on receptive vocabulary. Two aspects of immersion that South African children may experience are identified: length and quality. It is assumed that the longer the immersion, and the richer the immersion environment, the more positive the effect on vocabulary size will be. Overall results suggest that length has a slightly stronger effect on receptive vocabulary size than quality, though both are generally positive, and that most immersion students are beginning to develop a basic receptive vocabulary size comparable with that of their English mother tongue peers. / Linguistics / (M.A. (Linguistics))

Kadmium telurid je velmi důležitý materiál jak základního, tak i aplikovaného výzkumu. Je to dáno zejména jeho výhodnými elektronickými, optickými a strukturními vlastnostmi, které ho předurčují pro náročné technické aplikace. Dnes se hlavně používá pro jeho vysoké rozlišení k detekci a X-záření. Hlavní výhodou detektorů na bázi CdTe je, že nepotřebují chlazení a mohou spolehlivě fungovat i při pokojové teplotě. To způsobuje efektivnější interakce fotonů než je tomu u Si nebo jiných polovodičových materiálů. Obsahem této práce byla analýza a interpretace výsledků získaných studiem šumových a transportních charakteristik CdTe vzorků. Měření ukázaly že odpor homogenní části CdTe krystalů mírně klesá při připojení elektrického pole na vzorku. Při změně teploty navíc dochází k odlišné reakci u CdTe typu p a n. Právě těmto efektům je v práci věnována pozornost. Pomocí šumové spektroskopie bylo zjištěno, že při nízkých frekvencích je u vzorků dominantní šum typu 1/f, zatímco při vyšších frekvencích je sledován generačně-rekombinační šum a tepelný šum. Všechny měřené vzorky vykazovaly mnohem vyšší hodnotu šumu na nízkých frekvencích než udává Hoogeova rovnice. Byly nalezeny a popsány zdroje nadbytečného šumu.

Les HEMT à base de nitrure de gallium sont des composants très prometteurs en termes de performances en puissance et de fréquence de travail. L'enjeu est donc de développer des technologies performantes et fiables, afin d'intégrer ces transistors aux systèmes hyperfréquences, notamment dans le domaine des télécommunications, et en milieu durci. Les travaux ont été focalisés sur l'étude de la localisation des sources de bruit en excès aux basses fréquences, et de leur évolution suite aux phases de tests de vieillissement accéléré. Les caractérisations électriques ont été réalisées sur des structures fabriquées sur quatre plaques, dont trois sont basées sur une hétérostructure AlGaN/GaN, et la quatrième sur l'hétérostructure AlInN/AlN/GaN. Les résultats obtenus ont permis de valider une méthode de modélisation des sources de bruit en 1/f, localisées dans les zones d'accès aux contacts ohmiques et dans le canal. Des tests de vieillissement accéléré sous contraintes électriques ont permis de détecter des dégradations des performances statiques et du niveau de bruit en excès. Les effets combinés de piégeage et des effets thermiques expliquent ces dégradations, la température s'en étant révélée un facteur d'accélération. / The HEMT based on GaN materials are very promising, speaking of performance in power and frequency. The challenge is to develop efficient and reliable GaN based technologies, to intagrate these transistors to power microwave circuits, especially in the telecommunications field and on harsh environment. The work was focused on the study of the location of low frequency noise sources, and their evolution after accelerated life tests. The electrical characterizations were performed on structures made on four different wafers, three based on the AlGaN/GaN heterostructure, and the fourth based on the AlInN/AlN/GaN heterostructure. Thanks to the achieved results, a method for modeling 1/f noise sources, located in the channel and in the ohmic contacts access areas, has been validated. Life tests under electrical stress have been performed to detect DC and excess noise degradation. These degradations are explained by combined effects of trapping and thermal phenomena, with the temperature as an acceleration factor of degradation.

Ein tieffrequenter lauter Suppressorton verlagert das Corti-Organ periodisch in Richtung Scala vestibuli und Scala tympani. Simultan registrierte DPOAE (Distorsionsprodukte otoakustischer Emissionen) werden in beiden Richtungen unterschiedlich supprimiert. Bei Vorliegen eines endolymphatischen Hydrops (EH) ist die Beweglichkeit der Basilarmembran eingeschränkt, so daß keine oder eine nur geringe Modulation des DPOAE-Pegels entstehen kann. In dieser Arbeit wird die diagnostische Aussagekraft von Messungen tieftonmodulierter DPOAE bei zwei Patientenkollektiven mit Verdacht auf EH im Vergleich mit einem Normalkollektiv (n = 22) geprüft: bei Patienten mit M. Menière (n = 23) und Patienten mit Ohrdruckgefühl ohne Schwindelsymptomatik (n = 8). Bei den M. Menière-Patienten sind die ipsilateralen Modulationstiefen (MD) im Median hochsignifikant geringer als die des Normalkollektivs. Dies wird als Hinweis auf einen EH gewertet. Bei Primärtonpegeln mit L2 = 20 dB SL ergibt sich für den Grenzwert der MD mit 6 dB die Sensitivität von 64% und die Spezifität von 90%. Die kontralateralen MD der M. Menière-Patienten sind im Median signifikant geringer als die des Normalkollektivs. Für Primärtonpegel mit L2 = 20 dB SL liegen 33% der Werte unter dem Grenzwert, von diesen Ohren ist die Hälfte symptomfrei. Auch im kontralateralen Ohr kann ein – eventuell asymptomatischer – Hydrops vorliegen. Die Ergebnisse der Patienten mit Ohrdruckgefühl ohne Schwindel zeigen im Median hochsignifikant geringere MD als die der Normalhörenden und unterscheiden sich nicht signifikant von den Werten der ipsilateralen Ohren der M. Menière-Patienten. Das Ohrdruckgefühl kann auf einen cochleären Hydrops hinweisen und die weitere Entwicklung eines EH ankündigen. Bei den Verlaufsuntersuchungen zeigen die MD beider Patientenkollektive, deren Symptomatik sich während dieses Zeitraums änderte, Variabilitäten: mit Zunahme der Heftigkeit der spezifischen Symptome nimmt die MD ab und umgekehrt. Die Tieftonmodulation scheint den Zustand der Cochlea widerspiegeln zu können. Im Vergleich mit den in der Klinik gängigen Verfahren zur Hydropsdiagnostik wie der ECochG oder dem Glyceroltest erweist sich das in dieser Arbeit angewandte objektive Verfahren als vorteilhaft: es ist schnell durchführbar, nicht belastend und nicht invasiv. Es eignet sich als Screeningverfahren und kann zur Verlaufskontrolle eingesetzt werden. / The cochlear partition is moved periodically towards scala vestibuli and scala tympani by a low-frequency high-level suppressor tone. Simultaneously recorded DPOAEs (distortion product otoacoustic emissions) are suppressed differently in both directions. This modulation of the DPOAE level may be reduced or even missing if the displacement of the basilar membrane is inhibited by endolymphatic hydrops (EH). In this thesis the diagnostic significance of the low-frequency modulation of DPOAEs is tested in two patient groups with suspected EH: patients with Menière’s disease (n = 23) and patients with aural fullness without vertigo (n = 8), compared to a control group of normal hearing adults (n = 22). In the patients with Menière’s disease the ipsilateral modulation depths (median) are high significantly lower than in the control group. This can be interpreted as a sign of EH. At primary tone levels with L2 = 20 dB SL and a limiting value of 6 dB modulation depth (MD), a sensitivity of 64% and a specificity of 90% are found. The contralateral MDs of the patients with Menière’s disease (median) are significantly lower than in the control group. At primary tone levels with L2 = 20 dB SL, 33% of the MDs are below the limiting value, half of these ears are symptom-free. So in the contralateral ear a possibly asymptomatic hydrops may be present as well. The patients with aural fullness without vertigo show high significantly lower MDs (median) than the normal hearing adults and no significant difference to the ipsilateral MDs of the patients with Menière’s disease. Aural fullness can be a sign of cochlear hydrops and may indicate the further development of EH. During the course of disease the MDs of both patient groups vary when symptoms change: with increasing intensity of the specific symptoms the MD is reduced and vice versa. Low-frequency modulation seems to reflect the state of the cochlea. Compared to the generally used clinical tests for the assessment of EH like ECochG or the glycerol test, the objective method presented in this thesis is advantageous: it is fast, not straining and non-invasive. It is suitable for screening and can be used to monitor the course of disease.