Дисертації: "High-frequency transient"

1

Cargill, Steven M. "Corona screen effectiveness in large machines under high voltage, high frequency transient conditions." Thesis, Robert Gordon University, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.298316.

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2

Zedan, Buthenah. "Characterisation of substation earth grid under high frequency and transient conditions." Thesis, Cardiff University, 2005. http://orca.cf.ac.uk/55986/.

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Earthing systems are designed to perform satisfactorily under normal system voltage conditions as well as power frequency faults. The performance of most earth electrode geometries is now fairly well understood under these conditions. However, the response of earthing systems under high frequency and transient conditions is yet to be fully clarified, and there are several aspects of earthing systems that require further investigations. In this thesis, both modelling and experimental studies were carried out using high frequency and impulse current injection. Generic earth electrodes as well as the full earthing grid of an operating substation were investigated. The studies carried out in this work have confirmed some of the previous findings published in the open literature, and have clarified some aspects of conduction in earthing systems. The literature review on injuries due to lightning currents has highlighted the importance of good earthing systems. A comprehensive parametric simulation study was conducted on vertical rods, horizontal electrodes as well as earth grids under variable frequency and impulse currents. The effects of geometry and soil characteristics were also studied. It was demonstrated that significant inductive effects appear at high frequency, and the size of the earthing systems was found to reach an "effective dimension" beyond which negligible performance benefit is obtained. For horizontal electrodes the concept of effective length is investigated and for grids the effective area was used instead. The Simulation techniques developed for these simple electrodes were applied to an operating transmission substation, and similar trends were seen under high frequency and impulse current conditions. The safety voltages were calculated but no conclusion could be drawn as there are no recommended safety guidelines for safety at high frequency and impulse current. These Generic studies have led to a new proposal for earthing systems so that the short fall of poor performance due to inductive effects and "effective dimensions" are minimized. It was shown that this proposal is a major improvement on the existing enhancement techniques currently used in practice. Parallel to the simulation programme, an experimental set up was used to study the performance of laboratory earth electrode models under fast impulse current. It was found that highly non-linear conduction phenomena take place in such configurations. These complex conduction processes were explained by thermal effects and soil ionisation.

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3

Mousa, Salah. "Experimental investigation of enhanced earth electrode systems under high frequency and transient conditions." Thesis, Cardiff University, 2014. http://orca.cf.ac.uk/60855/.

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This thesis is primarily concerned with experimental tests and computer simulations to determine the high frequency and transient performance of earth electrode systems. The work has involved an extensive review of published literature, theoretical and analytical investigations of earth electrode systems. The experimental investigations on earth electrodes were carried out an outdoor site prior to electrode testing the test site soil resistivity were undertaken. Specifically, the soil resistivity was obtained at the location of the vertical test rod. In order to obtain better understanding of the soil stratification, fourteen profiles of soil resistivity were measured at the field site. From the measurements 2D soil models were constructed to visualise both horizontal and vertical resistivity variation. High frequency and impulse characteristics of vertical test rods up to 6m length and horizontal electrodes up to 88m buried in a non-uniform soil outdoor test site were tested. DC, AC and impulse test results show that increasing the length of electrode reduces the earthing resistance but not impedance. It was shown that, the earth resistance/impedance is constant over a low frequency range, while higher or lower impedance values are observed in the high-frequency range due to inductive or capacitive effects, depending on the length of earth electrode. Improved high- frequency and transient response of earth rods was determined experimentally by connecting horizontal electrode enhancements in star or cross formation at the top of vertical rods. Using these additional enhancements, a reduction in both resistance and impedance has been demonstrated. The addition of horizontal enhancements to the vertical rod can reduce the earth potential rise (EPR) by approximately 70% and 48% for 1.2m and 6m rods respectively. Voltage and current distributions of earth electrode systems under low/high frequency and impulse conditions, for different lengths of vertical rods with horizontal electrode enhancements and along a horizontal electrode with and without insulated conductor, were investigated experimentally and verified by computer simulation. In the case of the of the rods with an added ‘4-cross’ horizontal conductor enhancement, it was shown that the rods carry the majority of the current at low frequency, but this proportion decreases significantly as frequency increases The field test results show that current distribution in earth conductor systems is significantly different under high-frequency and impulse energisation compared with power frequency conditions. Close agreement was obtained between the measured and computed current and voltage High voltage tests in the ground around the vertical electrodes were investigated experimentally. It was observed that when a sufficiently high current magnitude is injected through vertical electrodes, a significant reduction in the impulse resistance by increase in current with a sudden fall of voltage is observed which is called soil ionisation. Such phenomenon does not occur when the vertical electrodes with horizontal enhancements is tested, where the current through all earth electrodes is small.

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4

Deng, Haifei. "Modeling and Design of a Monolithic High Frequency Synchronous Buck with Fast Transient Response." Diss., Virginia Tech, 2005. http://hdl.handle.net/10919/26226.

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With the electronic equipments becoming more and more complicated, the requirements for the power management are more and more strict. Efficient performance, high functionality, small profile, fast transient and low cost are the most wanted features for modern power management ICs, especially for mobile power. In order to reduce profile, the number of external components should be as small as possible, which means that compensator, ramp compensation, current sensor, driver and even power devices should be all implemented on a single chip, i.e. monolithic integration. Comparing with discrete switching DC-DC converter, monolithic integration brings a number of benefits and new design challenges. Besides monolithic integration, high switching frequency is another trend for power management ICs due to its higher bandwidth and the ability to further reduce external passive component size. Comparing with low frequency counterparts, high frequency switching converter design is more difficult in terms of the stability modeling, high switching loss and difficult current sensing etc. The objective of this dissertation is to study the design issues for monolithic integration of high frequency switching DC-DC converter. For this purpose, a high frequency, wide input range monolithic buck converter ASIC with fast transient response is designed based on advanced trench BCD technology. Stability is the fundamental requirement in designing switching converter ASIC. Achieving this requires an accurate loop gain design, especially for monolithically integrated high frequency switching converter since compensator is fixed on silicon and loop delay is comparable with switching cycle. Since DC-DC switching converters are time-varying system, traditional small signal analysis in SPICE cannot be directly used to simulate the loop gain of this kind of system. A periodic small signal analysis based method is proposed to analyze and simulate DC-DC switching converter inside a SPICE like simulator without the need for averaging. This general method is suitable for any switching regulators. The results are accurate comparing with average modeling and experiment results even at high frequency part. A general procedure to design loop gain is proposed. Several novel design concepts are proposed for monolithic integration of high frequency switching DC-DC converter; a novel control scheme-Cotangent Control (Ctg control) is proposed for fast transient response; In order to realize on-chip implementation of the compensator, especially for low frequency zero, active feedback compensator is developed and a general design procedure is proposed. Adaptive compensation concept is proposed to stabilize the whole system for a wide application range. Multi-stage driver and multi-section device concepts are investigated for high efficiency and low noise power stage design. And finally, a new noise insensitive lossless RC sensor is proposed for high speed current sensing. At the end of this dissertation, the test results of the fabricated chip are presented to verify the correctness of these design concepts.
Ph. D.

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5

Zhelyazkov, Aleksandar. "Development of a Damage Indicator Based on Detection of High-Frequency Transients Monitored in Bridge Piers During Earthquake Ground Shaking." Doctoral thesis, Università degli studi di Trento, 2020. http://hdl.handle.net/11572/272732.

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Real-time structural health monitoring is a well established tool for post-earthquake damage estimation. A key component in the monitoring campaign is the approach used for processing the data from the structural health monitoring system. There is a large body of literature on signal processing approaches aimed at identifying ground-motion induced damage in civil engineering structures. This dissertation expands on a specific subgroup of processing approaches dealing with the identification of damage induced high-frequency transients in the monitoring data. The underlying intuition guiding the current research can be formulated in the following hypothesis - the time difference between the occurrence of a high-frequency transient and the closest deformation extremum forward in time is proportional to the degree of damage. A mathematical deduction is provided in support of the above hypothesis followed by a set of shaking table tests. For the purposes of this research two shaking table tests of reinforced concrete bridge piers were performed. Data from a shaking table test performed by another research group was also analyzed. The cases in which the proposed procedure could find a practical application are examined along with the present limitations.

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6

Le, Guennec Yves. "Transient dynamics of beam trusses under impulse loads." Thesis, Châtenay-Malabry, Ecole centrale de Paris, 2013. http://www.theses.fr/2013ECAP0016/document.

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Ce travail de recherche est dédié à la simulation de la réponse transitoire des assemblages de poutres soumis à des chocs. De tels chargements entraînent la propagation d’ondes haute fréquence dans l’ensemble de la structure. L’énergie qu’elles transportent peut être dommageable pour son fonctionnement ou celui des équipements embarqués. Dans des études précédentes, il a été observé sur des structures expérimentales qu’un régime vibratoire diffusif tend à s’installer pour des temps longs. Le but de cette étude est donc de développer un modèle robuste de la réponse transitoire des assemblages de poutres soumis à des chocs permettant de simuler, entre autres, cet état diffusif. Les champs de déplacement étant très oscillants et la densité modale élevée, la simulation numérique de la réponse transitoire à des chocs peut difficilement être menée par une méthode d’éléments finis classique. Une approche utilisant un estimateur de la densité d’énergie de chaque mode de propagation a donc été mise en œuvre. Elle permet d’accéder à des informations locales sur les états vibratoires, et de contourner certaines limitations intrinsèques aux longueurs d’onde courtes. Après avoir comparé plusieurs modèles de réduction cinématique de poutre à un modèle de Lamb de propagation dans un guide d’ondes circulaire, la cinématique de Timoshenko a été retenue afin de modéliser le comportement mécanique haute fréquence des poutres. En utilisant ce modèle dans le cadre de l’approche énergétique évoquée plus haut, deux groupes de modes de propagation de la densité d’énergie vibratoire dans une poutre ont été isolés : des modes longitudinaux regroupant un mode de compression et des modes de flexion, et des modes transversaux regroupant des modes de cisaillement et un mode de torsion. Il peut être également montré que l’´evolution en temps des densités d’énergie associées obéit à des lois de transport. Pour des assemblages de poutres, les phénomènes de réflexion/transmission aux jonctions ont du être pris en compte. Les opérateurs permettant de les décrire en termes de flux d’´energie ont été obtenus grâce aux équations de continuité des déplacements et des efforts aux jonctions. Quelques caractéristiques typiques d’un régime haute fréquence ont été mises en évidence, tel que le découplage entre les modes de rotation et les modes de translation. En revanche, les champs de densité d’énergie sont quant à eux discontinus aux jonctions. Une méthode d’éléments finis discontinus a donc été développée afin de les simuler numériquement comme solutions d’´equations de transport. Si l’on souhaite atteindre le régime diffusif aux temps longs, le schéma numérique doit être peu dissipatif et peu dispersif. La discrétisation spatiale a été faite avec des fonctions d’approximation de type spectrales, et l’intégration temporelle avec des schémas de Runge-Kutta d’ordre élevé du type ”strong stability preserving”. Les simulations numériques ont donné des résultats concluants car elles permettent d’exhiber le régime de diffusion. Il a été remarqué qu’il existait en fait deux limites diffusives différentes : (i) la diffusion spatiale de l’´energie sur l’ensemble de la structure, et (ii) l’équirépartition des densités d’énergie entre les différents modes de propagation. Enfin, une technique de renversement temporel a été développée. Elle pourra être utile dans de futurs travaux sur le contrôle non destructif des assemblages complexes et de grandes tailles
This research is dedicated to the simulation of the transient response of beam trusses under impulse loads. The latter lead to the propagation of high-frequency waves in such built up structures. In the aerospace industry, that phenomenon may penalize the functioning of the structures or the equipments attached to them on account of the vibrational energy carried by the waves. It is also observed experimentally that high-frequency wave propagation evolves into a diffusive vibrational state at late times. The goal of this study is then to develop a robust model of high-frequency wave propagation within three-dimensional beam trusses in order to be able to recover, for example, this diffusion regime. On account of the small wavelengths and the high modal density, the modelling of high-frequency wave propagation is hardly feasible by classical finite elements or other methods describing the displacement fields directly. Thus, an approach dealing with the evolution of an estimator of the energy density of each propagating mode in a Timoshenko beam has been used. It provides information on the local behavior of the structures while avoiding some limitations related to the small wavelengths of high-frequency waves. After a comparison between some reduced-order beam kinematics and the Lamb model of wave propagation in a circular waveguide, the Timoshenko kinematics has been selected for the mechanical modelling of the beams. It may be shown that the energy densities of the propagating modes in a Timoshenko beam obey transport equations. Two groups of energy modes have been isolated: the longitudinal group that gathers the compressional and the bending energetic modes, and the transverse group that gathers the shear and torsional energetic modes. The reflection/transmission phenomena taking place at the junctions between beams have also been investigated. For this purpose, the power flow reflection/transmission operators have been derived from the continuity of the displacements and efforts at the junctions. Some characteristic features of a high-frequency behavior at beam junctions have been highlighted such as the decoupling between the rotational and translational motions. It is also observed that the energy densities are discontinuous at the junctions on account of the power flow reflection/transmission phenomena. Thus a discontinuous finite element method has been implemented, in order to solve the transport equations they satisfy. The numerical scheme has to be weakly dissipative and dispersive in order to exhibit the aforementioned diffusive regime arising at late times. That is the reason why spectral-like approximation functions for spatial discretization, and strong-stability preserving Runge-Kutta schemes for time integration have been used. Numerical simulations give satisfactory results because they indeed highlight the outbreak of such a diffusion state. The latter is characterized by the following: (i) the spatial spread of the energy over the truss, and (ii) the equipartition of the energy between the different modes. The last part of the thesis has been devoted to the development of a time reversal processing, that could be useful for future works on structural health monitoring of complex, multi-bay trusses

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7

Murnane, Owen D., and John K. Kelly. "The Effects of High-Frequency Hearing Loss on Low-Frequency Components of the Click-Evoked Otoacoustic Emission." Digital Commons @ East Tennessee State University, 2003. https://dc.etsu.edu/etsu-works/1791.

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Click-evoked otoacoustic emission (CEOAE) input/output (I/O) functions were measured in ears with normal hearing and in ears with sensorineural hearing loss above 2000 Hz. The low- to midfrequency CEOAEs obtained from the ears with high-frequency hearing loss were significantly reduced in level compared to the CEOAEs obtained from the ears with normal hearing even though there were no significant group differences in the 250-2000 Hz pure-tone thresholds. The findings are discussed within the context of two hypotheses that explain the low- to midfrequency reduction in transient-evoked otoacoustic emission (TEOAE) magnitude: (1) subclinical damage to the more apical regions of the cochlea not detected by behavioral audiometry, or (2) trauma to the basal region of the cochlea that affects the generation of low-frequency emissions. It is proposed that localized damage at basal cochlear sites affects the generation of low- to midfrequency CEOAE energy.

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8

Smith, Norman Alan. "The steady-state and post-ignition transient luminous behaviour of the tubular fluorescent lamp operating throughout the dimmed mode range using high frequency dimming." Thesis, University of Sheffield, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.387764.

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9

Kam, Shui-Cheong. "Assessing of circuit breaker restrike risks using computer simulation and wavelet analysis." Thesis, Queensland University of Technology, 2012. https://eprints.qut.edu.au/53144/1/Shui-Cheong_Kam_Thesis.pdf.

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A breaker restrike is an abnormal arcing phenomenon, leading to a possible breaker failure. Eventually, this failure leads to interruption of the transmission and distribution of the electricity supply system until the breaker is replaced. Before 2008, there was little evidence in the literature of monitoring techniques based on restrike measurement and interpretation produced during switching of capacitor banks and shunt reactor banks in power systems. In 2008 a non-intrusive radiometric restrike measurement method and a restrike hardware detection algorithm were developed by M.S. Ramli and B. Kasztenny. However, the limitations of the radiometric measurement method are a band limited frequency response as well as limitations in amplitude determination. Current restrike detection methods and algorithms require the use of wide bandwidth current transformers and high voltage dividers. A restrike switch model using Alternative Transient Program (ATP) and Wavelet Transforms which support diagnostics are proposed. Restrike phenomena become a new diagnostic process using measurements, ATP and Wavelet Transforms for online interrupter monitoring. This research project investigates the restrike switch model Parameter „A. dielectric voltage gradient related to a normal and slowed case of the contact opening velocity and the escalation voltages, which can be used as a diagnostic tool for a vacuum circuit-breaker (CB) at service voltages between 11 kV and 63 kV. During current interruption of an inductive load at current quenching or chopping, a transient voltage is developed across the contact gap. The dielectric strength of the gap should rise to a point to withstand this transient voltage. If it does not, the gap will flash over, resulting in a restrike. A straight line is fitted through the voltage points at flashover of the contact gap. This is the point at which the gap voltage has reached a value that exceeds the dielectric strength of the gap. This research shows that a change in opening contact velocity of the vacuum CB produces a corresponding change in the slope of the gap escalation voltage envelope. To investigate the diagnostic process, an ATP restrike switch model was modified with contact opening velocity computation for restrike waveform signature analyses along with experimental investigations. This also enhanced a mathematical CB model with the empirical dielectric model for SF6 (sulphur hexa-fluoride) CBs at service voltages above 63 kV and a generalised dielectric curve model for 12 kV CBs. A CB restrike can be predicted if there is a similar type of restrike waveform signatures for measured and simulated waveforms. The restrike switch model applications are used for: computer simulations as virtual experiments, including predicting breaker restrikes; estimating the interrupter remaining life of SF6 puffer CBs; checking system stresses; assessing point-on-wave (POW) operations; and for a restrike detection algorithm development using Wavelet Transforms. A simulated high frequency nozzle current magnitude was applied to an Equation (derived from the literature) which can calculate the life extension of the interrupter of a SF6 high voltage CB. The restrike waveform signatures for a medium and high voltage CB identify its possible failure mechanism such as delayed opening, degraded dielectric strength and improper contact travel. The simulated and measured restrike waveform signatures are analysed using Matlab software for automatic detection. Experimental investigation of a 12 kV vacuum CB diagnostic was carried out for the parameter determination and a passive antenna calibration was also successfully developed with applications for field implementation. The degradation features were also evaluated with a predictive interpretation technique from the experiments, and the subsequent simulation indicates that the drop in voltage related to the slow opening velocity mechanism measurement to give a degree of contact degradation. A predictive interpretation technique is a computer modeling for assessing switching device performance, which allows one to vary a single parameter at a time; this is often difficult to do experimentally because of the variable contact opening velocity. The significance of this thesis outcome is that it is a non-intrusive method developed using measurements, ATP and Wavelet Transforms to predict and interpret a breaker restrike risk. The measurements on high voltage circuit-breakers can identify degradation that can interrupt the distribution and transmission of an electricity supply system. It is hoped that the techniques for the monitoring of restrike phenomena developed by this research will form part of a diagnostic process that will be valuable for detecting breaker stresses relating to the interrupter lifetime. Suggestions for future research, including a field implementation proposal to validate the restrike switch model for ATP system studies and the hot dielectric strength curve model for SF6 CBs, are given in Appendix A.

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10

GALLESI, FEDERICO. "Investigation, Experimental Development, and Field Application of Non-Invasive Sensors for Partial Discharge Measurement." Doctoral thesis, Università degli studi di Genova, 2022. http://hdl.handle.net/11567/1082296.

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Electrical insulating systems represent one of the weakest elements of the entire electrical equipment. In fact, the lifetime of any electrical device depends on the capability of its insulation to last in time. In general, all organic materials, and in particular all dielectrics, are subjected to ageing. The ageing process is caused by different stresses: thermal, electrical, mechanical and environmental. By focusing on the electrical stress, it is possible to state that it is fundamentally characterized by two phenomena: the presence of an electric field and/or the presence of partial discharges (PDs), a phenomenon in which the interelectrode zone is partially short-circuited causing degradation of the material. It is clear, therefore, how partial discharges are both the symptom and the cause of the deterioration of the insulating materials. Thus, the measurement of this phenomenon leads to obtain an “image” of the status of the insulating system allowing to perform a predictive diagnostic by an expert user or by sophisticate artificial intelligence algorithms. These diagnostic techniques lead to reduce the maintenance costs since they allow to perform a condition-based maintenance that is carried out only when this is necessary, according to the real state of the component. To measure the partial discharge activity there are different methods, but it is possible to classify them into two groups: electrical detection methods and non-electrical detection methods. If the electrical methods typically need invasive procedures to perform the partial discharge measurement, the non-electrical ones, on the contrary, can be implemented on the electrical component without any change (non-invasive) in the normal operating of the equipment. The focus of this research activity is to study, develop and optimize prototypes, test them in laboratory and in real applications of different non-invasive sensors in order to measure partial discharges. Two kind of sensors are analysed: Transient Earth Voltage Sensors and High Frequency Current Transformers ones. Transient Earth Voltage Sensors are based on the homonymous operating principle, and they can be used when the electrical apparatus is placed inside a metal box like switchgears or gas insulated systems. Starting from a first investigation on the effect of the geometrical dimensions of different sensors prototype in order to maximize the signal output, a first preliminary on-field test campaign is performed on an industrial air insulated medium voltage switchgear. Then, the entire partial discharge measurement system configuration, based on a definitive version of TEV sensor, is optimized and tested on the industrial test set-up, by comparing the obtained PD pattern with those collected from an IEC 60270 standard compliant measurement system. The results suggest how the non-invasive PD measurement system provides information comparable to those collected by the standard-compliant one. The High Frequency Current Transformer Sensors are studied and optimized during an experimental laboratory activity where different magnetic core material and different sensor configurations are tested. The results allow to implement the HFCT sensor on an optimized PD measurement system introduced to perform a research investigation on the phenomenon of electrical treeing on HVDC cable insulation, carried out during an exchange program at the RWTH Aachen University in Germany. The aim of this investigation is to understand the behaviour of the electrical treeing with different voltage frequencies and amplitudes during long-term tests since some preliminary tests give unexpected results. In fact, it is generally accepted that increasing the frequency leads to an accelerated ageing process, mainly due to partial discharge activity, and therefore, to a reduced time to breakdown. The collected results show that increasing the applied voltage frequency, the time to breakdown increases since the electrical treeing shape changes. During this analysis, PD measurement is carried out in order to monitor the electrical treeing evolution during the degradation process by adopting the optimized HFCT-based measurement system.

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11

Imdad, Kashif. "High frequency modeling of power transformers under transients." Doctoral thesis, Universitat Politècnica de Catalunya, 2017. http://hdl.handle.net/10803/457774.

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This thesis presents the results related to high frequency modeling of power transformers. First, a 25kVA distribution transformer under lightning surges is tested in the laboratory and its high frequency model is proposed. The transfer function method is used to estimate its parameters. In the second part, an advanced high frequency model of a distribution transformer is introduced. In this research, the dual resonant frequency distribution transformer model introduced by Sabiha and the single resonant frequency distribution transformer model under lightning proposed by Piantini at unloaded conditions are investigated and a modified model is proposed that is capable to work on both single and dual resonant frequencies. The simulated results of the model are validated with the results of Sabiha and Piantini that have been taken as reference. Simulations have shown that the results of the modified model such as, secondary effective transfer voltages, transferred impedances and transformer loading agree well with the previous models in both time and frequency domains. The obtained objectives of this research are: * Methodology for determining the parameters of a power transformer. * High frequency modeling of a transformer in order to simulate its transient behavior under surges. * Modification of high frequency model for single and dual resonance frequency. The originality and methodology of this research are: * High frequency transformer model is derived by means of the transfer function method. In the literature, the transfer function method has been used in many applications such as the determination of the mechanical deformations or insulation failure of interturn windings of transformers. In this thesis, the parameters of the proposed model are estimated using the transfer function method. * Modification of high frequency model for single/dual resonance frequency using the transfer function method. The transfer function can also be used to determine the state of the transformer. The modification in the developed model using the proposed technique has been validated. The high frequency transformer model was presented by Sabiha at two resonance frequency under both condition loading and unloading was used as reference model for modification and further enhancement. A transformer with 25kVA capacity was tested in UPC Terrassa Spain in High voltage lab under the effect of impulse voltage and the digital data was stored via oscilloscope in computer. An algorithm was developed to estimate the transformer parameters by transfer function method using fast Fourier transform analysis. In this scheme the two port network theory concept was taken for black box analysis of transformer. The series of transient¿s frequencies of experimental digital data was noted. The transformer parameters such as Z11, Z12, Z21, and Z22 calculated on all these frequencies in order to generate a narrow band of correct frequency at which the transients was developed experimentally and therefore it has to be developed on that specific frequencies. Earlier the transfer function method was used for the mechanical deformation analysis in the transformer, now similar method of modeling used to estimate the parameters of transformer and propose accurate transformer model for two resonance frequencies only and the parameters estimation was based simply placing RLC elements. The proposed model also tested and validated for accuracy and reliability. In the second phase of research high frequency models of transformer for protection from the transients based on experimental data are presented. Which were tested and validated for unloading and loading for Single and Two resonant frequencies, and modeled using transfer function method. The proposed single model leads to others two models which are verified by two port network theory, unloaded transfer under time domain and frequency domain analysis, transformer loading under different loads and transfer function
Esta tesis presenta los resultados relacionados con el modelado en alta frecuencia de transformadores de potencia. Primero, un transformador de distribución de 25 kVA es ensayado a perturbaciones tipo rayo y se propone un modelo de alta frecuencia. El método de la función de transferencia es utilizado para estimar sus parámetros. En segundo lugar, se introduce un modelo avanzado de alta frecuencia. El modelo resulta de la investigación y modificación de los modelos de resonancia dual introducido por Sabiha y el de resonancia única introducido por Piantini con condiciones sin carga. El modelo propuesto es capaz de representar tanto la resonancia única como dual. El modelo es validado a partir de los resultados de referencia de Sabiha y Piantini. Los resultados de simulación indican que el modelo representa las sobretensiones transferidas al secundario, las impedancias de transferencia y el trabajo en carga del transformados coinciden con los modelos anteriores tanto en el dominio temporal y de la frecuencia. Los objetivos obtenidos en esta tesis son: * Metodología para determinar los parámetros de un transformador de potencia. * Modelo de alta frecuencia para simular el comportamiento transitorio frente a perturbaciones. * Modificación del modelo de alta frecuencia para representar una y dos resonancias. La originalidad y metodología de la tesis son: * El modelo de alta frecuencia del transformador ha sido obtenido a partir de la metodología de la función de transferencia. En la literatura, esta metodología ha sido usada en muchas aplicaciones como en la determinación de deformaciones mecánicas o fallo de aislamiento en espiras de transformadores. En esta tesis, los parámetros del modelo propuesto son obtenidos mediante esta metodología. * Modificación del modelo de alta frecuencia con resonancia única o dual utilizando el método de la función de transferencia. La metodología propuesta puede ser utilizada para determinar el estado del transformador. Sabiha propuso un modelo de alta frecuencia representado dos resonancias en condiciones de carga y de vacío. Este modelo ha sido utilizado como referencia para la modificación y mejora. En la tesis, el transformador de distribución 25 kVA disponible en el Departamento de Ingeniería Eléctrica de la UPC en la ESEIAAT ha sido ensayado a impulsos tipo rayo. Un algoritmo ha sido desarrollado para estimar los parámetros del transformador mediante el método de la función de transferencia utilizando la transformada de Fourier. En este esquema, una red de dos puertos ha sido adoptada para el análisis del transformador. Los parámetros del transformador como la Z11, Z12., Z21 y Z22 son calculadas para representar las frecuencias consideradas. Mediante el método de la función de transferencia los parámetros del transformador son determinados. El modelo propuesto se basa en elementos simples RLC capaces de representar con precisión dos frecuencias de resonancia. En la segunda parte de la tesis, un nuevo modelo es introducido capaz de representar ambos modelos de resonancia única y dual en uno solo. Se analiza el caso de carga y vacío.

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12

Chartier, Caroline. "Caractérisation des propriétés mécaniques du tissu cutané par élastographie impulsionnelle haute fréquence : applications en dermatologie et en cosmétique." Thesis, Tours, 2017. http://www.theses.fr/2017TOUR3304/document.

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L’exploration du tissu cutané est aujourd’hui limitée par le peu de méthodes dites quantitatives permettant de décrire objectivement les propriétés mécaniques du tissu cutané. L’élastographie permet une exploration locale d’un milieu et offre la possibilité pour certaines méthodes d’estimer quantitativement le module d’élasticité (module d’Young). Nous avons mis au point une technique d’élastographie ultrasonore impulsionnelle haute fréquence 1D (HF-TE) et haute résolution permettant une description micrométrique des propriétés mécaniques du tissu cutané pour des applications en cosmétique et en dermatologie
Nowadays, exploration of cutaneous tissue is limited by the few number of available approaches, known as quantitative methods, allowing an objective description of the mechanical properties of skin tissue. Elastography allows a local exploration of a medium and offers the possibility for some strategies to quantitatively estimate the modulus of elasticity (Young’s modulus). A 1-D high-frequency ultrasonic transient elastography method (HF-TE) allowing a micrometric description of the mechanical properties of skin tissue has been designed for cosmetic and dermatological applications. An experimental system of high-frequency transient elastography has been developed : software, hardware and measurement methodology. The HF-TE technique has been validated using simulation and measurements in monolayer and bilayer calibrated phantoms developed in the laboratory. The Young’s modulus values measured in monolayer media were then compared with those measured by two others dynamic techniques

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13

Probert, Sarah Ann. "Simulation, analysis, assessment and diagnosis of high frequency power system transients." Thesis, Brunel University, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.249793.

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14

Babany, Michel David. "Decision support for disruption management on high frequency transit lines." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/99549.

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Thesis: S.M., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2015.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 155-159).
Incidents (due to equipment failures, passenger emergencies, infrastructure problems, human errors, etc.) routinely occur in metro systems. Such incidents can cause significant disruptions in service (from slowdown to full closure of the line), with serious impacts on passengers, especially in the core of high frequency lines operating near capacity. Disruption consists of two distinct phases. The incident phase is the period from the start of the incident to the moment when its cause has been resolved. The second phase of the disruption is the recovery, which starts at the end of the incident and lasts until normal service is restored. Dealing efficiently with disruptions is crucial and agencies use real-time control strategies to mitigate those impacts and improve performance. This thesis proposes an approach for supporting controllers decision-making in the recovery phase of disruption management. While the method is applied to the Piccadilly Line on the London Underground, It is applicable to other high frequency transit rail lines. After reviewing the main challenges controllers face during incident management and the main strategies they use, the thesis formulates the recovery phase problem as an optimization problem that integrates timetable revision and crew rescheduling (train reformation problem, TRP). The approach focuses on modeling common control strategies such as short-turning and train renumbering. It explicitly incorporates the scarcity of resources and associated constraints, especially with respect to crews. The method consists of two phases: the generation of a large number of candidate journeys; and the selection of the journeys (recovery timetable) that optimize some measure of performance, involving the effectiveness of the recovery and the passenger service. The model is first applied to an incident that happened on January 2014 on the Piccadilly Line. The actual controllers response is compared with the output of the train reformation problem, and a sensitivity analysis of the model parameters is performed. The results suggest that using more complex reformations and less short-turns may lead to better passenger service during the recovery phase. The train reformation problem is then applied to a hypothetical incident. The results support current practices that canceling trains during the incident phase enables a shorter and more efficient recovery.
by Michel David Babany.
S.M.

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15

Sánchez-Martínez, Gabriel Eduardo. "Real-time operations planning and control of high-frequency transit." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/99550.

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Thesis: Ph. D. in Transportation, Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2015.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 135-140).
High-frequency transit systems are essential for the socioeconomic and environmental well-being of large and dense cities. The planning and control of their operations are important determinants of service quality. Transit operators are increasingly adopting data collection devices that enable real-time monitoring of vehicle locations and demand, but existing models and current practice limit the utility of this information. This research develops new concepts, frameworks, and models for real-time optimization of operations, utilizing both historical and real-time information originating from connected data collection devices, including automated vehicle location, automated fare collection, and automatic passenger counting systems. Previous control strategies either do not forecast system states or rely on forecasts based on running times and demand assumed to be static. This research develops an optimization model for holding-based control that incorporates dynamics, producing a holding policy that accounts not only for the current state of the system, but also for expected changes in running times and demand, due to both exogenous and endogenous dynamics. This information advantage can lead to improved performance when a transit service faces typical changes in running times and demand over time, as well as potentially disruptive events such as signal failures, disabled rolling stock, and demand surges. Anticipatory control policies allow the transit service to react before disruptions develop. It is shown that information about dynamics is particularly valuable when it leads to better predictions of capacity being reached. Although headway and optimization-based control strategies generally outperform schedule-adherence strategies, high-frequency operations are mostly planned with schedules, in part because operators must observe resource constraints (neglected by most control strategies) while planning and delivering service. This research develops a schedule-free paradigm for high-frequency transit operations, in which trip sequences and departure times are optimized in real-time, employing stop-skipping strategies and utilizing real-time information to maximize service quality while satisfying operator resource constraints. Following a discussion of possible methodological approaches, a simple methodology is applied to operate a simulated transit service without schedules. Results demonstrate the feasibility of the new paradigm and suggest possible methodology improvements.
by Gabriel Eduardo Sánchez-Martínez.
Ph. D. in Transportation

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16

Brauer, Patrik. "High-Frequency Voltage Distribution Modelling of a Slotless PMSM from a Machine Design Perspective." Thesis, KTH, Elkraftteknik, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-224174.

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The introduction of inverters utilizing wide band-gap semiconductors allow for higher switching frequency and improved machine drive energy efficiency. However, inverter switching results in fast voltage surges which cause overvoltage at the stator terminals and uneven voltage distribution in the stator winding. Therefore, it is important to understand how next generation machine drives, with higher switching frequency, affect the voltage distribution. For this purpose, a lumped-parameter model capable of simulating winding interturn voltages for the wide frequency range of 0-10 MHz is developed for a slotless PMSM. The model includes both capacitive and inductive couplings, extracted from 2D finite element simulations, as well as analytically estimated resistive winding losses. The developed model of a single phase-winding is used to investigate how machine design aspects such as insulation materials and winding conductor distribution affects both voltage distribution and winding impedance spectrum. Validation measurements demonstrate that the model is accurate for the wide frequency range. The sensitivity analysis suggests that the winding conductor distribution affect both impedance spectrum and voltage distribution. For the slotless machine, capacitance between the winding and the stator is several times smaller than capacitance between turns. Therefore, the high-frequency effects are dominated by the capacitance between turns. Insulation materials that affect this coupling does therefore have an impact on the impedance spectrum but does not have any significant impact on the voltage distribution.
Nästa generations inverterare för styrning av elektriska maskiner, baserade på bredbandgaps komponenter, tillåter högre switchfrekvenser vilket skapar en energieffektivare drivlina. Nackdelen är att snabba spänningsflanker från den höga switchfrekvensen skapar överspänning på stators anslutningar och en ojämn spänningsfördelning i statorlindningen. Det är därför betydelsefullt att förstå hur dessa nya drivlinor påverkar lindningens spänningsfördelning. I denna rapport används en modell kapabel att simulera lindningens spänningsfördelning i det breda frekvensspektrumet 0-10 MHZ. Modellen är framtagen för en faslindning av en PMSM, utan statoröppning, som inkluderar både kapacitiva och induktiva kopplingar samt analytiskt beräknade lindningsförluster. Modellen används för att undersöka spänningsfördelningen i lindningen samt inverkan från designparametrar som isolationsmaterial och lindningsdistribution. Känslighetsanalysen visar att lindingsdistributionen har en signifikant påverkan på både impedansspektrumet och spänningsfördelningen. För den studerade maskintypen är det kapacitansen mellan varv som är dominerande för högfrekventa fenomen. Isolationsmaterial som påverkar denna koppling har en påverkan på impedansspektrumet men är liten för spänningsfördelningen.

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17

Coor, Gordon T. (Gordon Thomas). "Analysis of the short-turning strategy on high-frequency transit lines." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/42587.

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18

Maltzan, David (David W. ). "Using real-time data to improve reliability on high-frequency transit services." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/99541.

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Thesis: S.M. in Transportation, Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2015.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 152-153).
In recent years, automatically-collected data from many transit agencies have been made available to the public in real time. This has dramatically improved the experience of riding transit, by allowing passengers to use detailed information on the current state of service to make more informed travel decisions. The "open data" movement has allowed independent mobile-phone app developers to create a variety of useful tools to improve the passenger experience. However, agencies' use of real-time data for operational purposes has lagged behind customer-facing app development. This research examines the use of real-time data for the application of operational control strategies on transit services. Two high-frequency bus routes of the Massachusetts Bay Transportation Authority are used as a case study. It begins with the development of an application to download, interpret, and present data on bus service and recommended control actions in a graphical user interface. This application is then used to conduct an experiment with a terminal-based holding strategy on MBTA Route 1. The results of this experiment drive further investigation into the causes of deviations from scheduled or assigned departure times at terminals. To supplement the experimental data, a simulation model of MBTA Routes 1 and 28 is developed. This simulation is used to test additional control strategies, as well as the effect of reducing unexplained operator deviations from assigned departure times. The research finds that real-time data can be used to create significant operational improvements. In particular, holding strategies at terminals, along with reducing unexplained operator deviations from assigned terminal departure times, have a strong effect. Several specific recommendations are made for a number of strategies that the MBTA can use to improve the precision of terminal departure times on bus services. This research also finds that holding at midpoints and short-turning can provide some additional benefit, but the costs and benefits to passengers of these strategies are more complicated and should be investigated with further research and implemented using optimization schemes rather than the heuristic rules used here.
by David Maltzan.
S.M. in Transportation

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19

Chartier, Sébastien [Verfasser]. "SiGe HBT ICs with high operational to transit frequency ratio: design and design re-use / Sébastien Chartier." Ulm : Universität Ulm. Fakultät für Ingenieurwissenschaften und Informatik, 2011. http://d-nb.info/1016718497/34.

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20

Zhelyazkov, Aleksandar. "Development of a Damage Indicator Based on Detection of High-Frequency Transients Monitored in Bridge Piers During Earthquake Ground Shaking." Doctoral thesis, Università degli studi di Trento, 2020. http://hdl.handle.net/11572/272732.

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Real-time structural health monitoring is a well established tool for post-earthquake damage estimation. A key component in the monitoring campaign is the approach used for processing the data from the structural health monitoring system. There is a large body of literature on signal processing approaches aimed at identifying ground-motion induced damage in civil engineering structures. This dissertation expands on a specific subgroup of processing approaches dealing with the identification of damage induced high-frequency transients in the monitoring data. The underlying intuition guiding the current research can be formulated in the following hypothesis - the time difference between the occurrence of a high-frequency transient and the closest deformation extremum forward in time is proportional to the degree of damage. A mathematical deduction is provided in support of the above hypothesis followed by a set of shaking table tests. For the purposes of this research two shaking table tests of reinforced concrete bridge piers were performed. Data from a shaking table test performed by another research group was also analyzed. The cases in which the proposed procedure could find a practical application are examined along with the present limitations.

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21

Ramli, Mohd Shamir. "Investigation of circuit breaker switching transients for shunt reactors and shunt capacitors." Thesis, Queensland University of Technology, 2008. https://eprints.qut.edu.au/17822/1/Mohd_Shamir_Ramli_Thesis.pdf.

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Switching of shunt reactors and capacitor banks is known to cause a very high rate of rise of transient recovery voltage across the circuit breaker contacts. With improvements in circuit breaker technology, modern SF6 puffer circuits have been designed with less interrupter per pole than previous generations of SF6 circuit breakers. This has caused modern circuit breakers to operate with higher voltage stress in the dielectric recovery region after current interruption. Catastrophic failures of modern SF6 circuit breakers have been reported during shunt reactor and capacitor bank de-energisation. In those cases, evidence of cumulative re-strikes has been found to be the main cause of interrupter failure. Monitoring of voltage waveforms during switching would provide information about the magnitude and frequency of small re-ignitions and re-strikes. However, measuring waveforms at a moderately high frequency require plant outages to connect equipment. In recent years, there have been increasing interests in using RF measurements in condition monitoring of switchgear. The RF measurement technique used for measuring circuit breaker inter-pole switching time during capacitor bank closing is of particular interest. In this thesis, research has been carried out to investigate switching transients produced during circuit breaker switching capacitor banks and shunt reactors using a non-intrusive measurement technique. The proposed technique measures the high frequency and low frequency voltage waveforms during switching operations without the need of an outage. The principles of this measurement technique are discussed and field measurements were carried out at shunt rector and capacitor bank installation in two 275 kV air insulated substations. Results of the measurements are presented and discussed in this thesis. The proposed technique shows that it is relatively easy to monitor circuit breaker switching transients and useful information on switching instances can be extracted from the measured waveforms. Further research works are discussed to realise the full potential of the measuring technique.

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22

Ramli, Mohd Shamir. "Investigation of circuit breaker switching transients for shunt reactors and shunt capacitors." Queensland University of Technology, 2008. http://eprints.qut.edu.au/17822/.

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Switching of shunt reactors and capacitor banks is known to cause a very high rate of rise of transient recovery voltage across the circuit breaker contacts. With improvements in circuit breaker technology, modern SF6 puffer circuits have been designed with less interrupter per pole than previous generations of SF6 circuit breakers. This has caused modern circuit breakers to operate with higher voltage stress in the dielectric recovery region after current interruption. Catastrophic failures of modern SF6 circuit breakers have been reported during shunt reactor and capacitor bank de-energisation. In those cases, evidence of cumulative re-strikes has been found to be the main cause of interrupter failure. Monitoring of voltage waveforms during switching would provide information about the magnitude and frequency of small re-ignitions and re-strikes. However, measuring waveforms at a moderately high frequency require plant outages to connect equipment. In recent years, there have been increasing interests in using RF measurements in condition monitoring of switchgear. The RF measurement technique used for measuring circuit breaker inter-pole switching time during capacitor bank closing is of particular interest. In this thesis, research has been carried out to investigate switching transients produced during circuit breaker switching capacitor banks and shunt reactors using a non-intrusive measurement technique. The proposed technique measures the high frequency and low frequency voltage waveforms during switching operations without the need of an outage. The principles of this measurement technique are discussed and field measurements were carried out at shunt rector and capacitor bank installation in two 275 kV air insulated substations. Results of the measurements are presented and discussed in this thesis. The proposed technique shows that it is relatively easy to monitor circuit breaker switching transients and useful information on switching instances can be extracted from the measured waveforms. Further research works are discussed to realise the full potential of the measuring technique.

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23

Jwad, Dhrgham Mousa. "Transients fault analysis based on the wavelet transform for fault identification and protection on cycloconverter based high voltage low frequency transmission system." Thesis, University of Leicester, 2017. http://hdl.handle.net/2381/40863.

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This thesis presents a study on HVLF AC transmission systems for a long distance Offshore Wind Farm (OWF) grid connection. A particular scheme highlights the use of a high voltage cycloconverter as a frequency changer at the sending end of the transmission system, in which the voltage is stepped up at 60 Hz before inputting to the cycloconverter. This eliminates the need for a high voltage low frequency transformer on the offshore platform, and also it allows the use of standard 50/60 Hz generating equipment in the wind turbines. A modelling study has been undertaken to validate the operation of the system, including with the presence of transmission line faults. The study shows the effects of the pre-fault harmonics generated by the cycloconverter on the fault generated transients. Thus, the need to develop a new frequency based fault protection system for HVLF transmission is addressed. New transmission line fault detection and location algorithms for the HVLF system have been developed. Firstly, the frequency range of the post-fault generated transients were identified using Fast Fourier Transform (FFT) analysis. It was shown that these transient components, ranging from DC to high frequency, are distributed throughout the spectrum of the three-phase current signals. However, the FFT analysis provides the frequency information of these transients but without time information. In this thesis, the Wavelet Packets Transform (WPT) is introduced for the fault identification. The fault generated transients were detected by monitoring wavelet coefficients over a time window. The performance of the protection system under all possible fault scenarios of the HVLF transmission line are investigated. Finally, practical considerations, such as the impact of the fault inception angle and the switching and control of the cycloconverter on the fault detection and location algorithm’s accuracy were also investigated.

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24

Peñaloza, Ana Karen Apolo. "Localização de faltas em sistemas de distribuição de energia elétrica : uma abordagem baseada na análise de transitórios de alta frequência." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2017. http://hdl.handle.net/10183/165231.

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Os Sistemas de Distribuição de Energia Elétrica (SDEE) estão constantemente expostos à ocorrência de faltas, o que torna de primordial importância para as concessionárias que estas sejam localizadas com rapidez e precisão. Com isso, a degradação da confiabilidade do sistema e as perdas econômicas são minimizadas. Com este intuito, diversas pesquisas vêm sendo conduzidas nas últimas décadas tendo em vista o desenvolvimento de métodos computacionais para Localização de Faltas (LF) em SDEE. Embora as técnicas disponíveis atualmente sejam capazes de estimar a distância das faltas com relativa exatidão, algumas características intrínsecas aos SDEE ainda impõem limitações à LF em alimentadores radiais. Em geral, o aspecto econômico requer o uso de dados provenientes de um único terminal de medição. Ainda, a quantidade significativa de ramais laterais tipicamente presentes nos SDEE faz com que o problema das múltiplas estimativas da LF seja considerado como a principal limitação à efetividade das técnicas existentes na atualidade. Neste contexto, este trabalho apresenta uma metodologia baseada na análise dos transitórios de alta frequência gerados pelas faltas, a qual é capaz de fornecer uma estimativa única para a LF em SDEE ramificados a partir de medições somente no terminal local. O ramal em falta e a distância da falta em relação ao terminal de medição são determinados através da comparação entre as frequências características identificadas no espectro do transitório, e as frequências teóricas calculadas para os possíveis caminhos de propagação. Uma das principais contribuições deste trabalho consiste na formulação de um modelo detalhado das linhas de distribuição desequilibradas. A formulação proposta é baseada na modificação nas equações de Carson, considerando a dependência da frequência e a inclusão dos efeitos pelicular e da corrente de retorno pela terra para altas frequências. Como resultado, esta abordagem permite uma maior exatidão na determinação dos parâmetros modais que descrevem a propagação de transitórios em SDEE desequilibrados, eliminando as aproximações adotadas pela maioria das técnicas propostas na literatura atual. A técnica proposta foi avaliada considerando simulações de faltas nos alimentadores IEEE 13 e 34 barras através do ATP/EMTP. Os resultados apresentados incluem diversos cenários de faltas, bem como a comparação com um método de LF para SDEE considerado estado da arte atualmente.
Electric power distribution systems (EPDS) are continuously exposed to faults, therefore fast and accurate fault location is of paramount importance for utilities. Thus, degradation of system reliability and economic losses are minimized. In this sense, several studies have been conducted in the last decades aiming to the development of computational methods for Fault Localization (FL) in EPDS. Although the currently available techniques are able to estimate fault distance with relative accuracy, some intrinsic characteristics of EPDS still impose limitations to FL in radial feeders. In general, the economic aspect requires use of data from one-terminal measurements. Also, typical EPDS have a large number of branches, which makes the problem of multiple FL estimates the main limitation to the effectiveness of the existing techniques. In this context, this work presents a methodology based on the analysis of the high frequency transient generated by faults, which is able to provide a unique FL estimate in branched EPDS by using only one-terminal measurements. The faulted branch and the fault distance from the measurement terminal are determined by correlating the characteristic frequencies identified in the transient spectrum and theoretical frequencies calculated for the possible propagation paths. One of the main contributions of this work is the formulation of a detailed model of unbalanced distribution lines. The proposed formulation is based on the modification of Carson’s equations, considering frequency dependence and inclusion of skin effects and the ground current return at high frequencies. As a result, this approach allows a greater accuracy in determining the modal parameters that describe the transients’ propagation in unbalanced EPDS, thus eliminating the approximations adopted by most of the techniques proposed in the current literature. The proposed technique was evaluated considering fault simulations in the IEEE 13 and 34 nodes feeders through the ATP/EMTP. Results presented include several fault scenarios as well as the comparison with a FL method for SDEE currently considered as the state of the art.

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25

Fabian, Joshua Javier. "Improving high-frequency transit reliability : a case study of the MBTA Green Line through simulation and field experiments of real-time control strategies." Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/111426.

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Thesis: S.M. in Transportation, Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 175-179).
Service reliability is a major concern for public transportation agencies. Transit services experience natural variability in scheduled service, due to factors such as traffic congestion, irregular demand, multi-route and branching corridors, and operator behavior. This variability leads to irregular headways, resulting in longer passenger waits and decreased effective capacity as gaps in service form. Real-time control strategies allow controllers to intervene at terminals and en route to regulate headways and improve performance. This research tests the effectiveness of holding control strategies on the Massachusetts Bay Transportation Authority (MBTA) Green Line in Boston, a complex, four-branch light rail line. A simulation model is developed to estimate and compare the benefits of different schedule-based and headway-based holding strategies. Dispatching trains at terminals to target headways is found to minimize wait time, and the addition of en route holding improves service further, albeit slightly. The simulation results inform the design of a field experiment, in which headway-based dispatching is implemented at a Green Line branch terminal. Terminal personnel are provided with tablet computers showing departure times optimized by an even-headway policy. When optimized departure times are adhered to, peak-hour headway variability is reduced by 40%. The average wait is shortened by 15% (30 seconds), and the 90th percentile wait is shortened by 21% (90 seconds). Compliance with the recommended departure times in the experiment was hampered by various human factors and station features. During the experiment, only 49% of trips left within 45 seconds of the departure times recommended by the algorithm. These results show that adopting headway-based dispatching at terminals promises significant benefits to service and passengers if operational changes are accompanied by improved supervision practices. This research fully supports the idea that transit agencies, such as the MBTA, should allocate supervisory resources for high-frequency services to prioritize terminal headway control versus en route and schedule-based strategies.
by Joshua Javier Fabian.
S.M. in Transportation

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26

Ramirez-garcia, Eloy. "Analyse expérimentale et modélisation du bruit haute fréquence des transistors bipolaires à hétérojonctions SiGe et InGaAs/InP pour les applications très hautes fréquences." Thesis, Paris 11, 2011. http://www.theses.fr/2011PA112082/document.

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Le développement des technologies de communication et de l’information nécessite des composants semi-conducteurs ultrarapides et à faible niveau de bruit. Les transistors bipolaires à hétérojonction (TBH) sont des dispositifs qui visent des applications à hautes fréquences et qui peuvent satisfaire ces conditions. L’objet de cette thèse est l’étude expérimentale et la modélisation du bruit haute fréquence des TBH Si/SiGe:C (technologie STMicroelectronics) et InP/InGaAs (III-V Lab Alcatel-Thales).Accompagné d’un état de l’art des performances dynamiques des différentes technologies de TBH, le chapitre I rappelle brièvement le fonctionnement et la caractérisation des TBH en régime statique et dynamique. La première partie du chapitre II donne la description des deux types de TBH, avec l’analyse des performances dynamiques et statiques en fonction des variations technologiques de ceux-ci (composition de la base du TBH SiGe:C, réduction des dimensions latérales du TBH InGaAs). Avec l’aide d’une modélisation hydrodynamique, la seconde partie montre l’avantage d’une composition en germanium de 15-25% dans la base du TBH SiGe pour atteindre les meilleurs performances dynamiques. Le chapitre III synthétise des analyses statiques et dynamiques réalisées à basse température permettant de déterminer le poids relatif des temps de transit et des temps de charge dans la limitation des performances des TBH. L’analyse expérimentale et la modélisation analytique du bruit haute fréquence des deux types de TBH sont présentées en chapitre IV. La modélisation permet de mettre en évidence l’influence de la défocalisation du courant, de l’auto-échauffement, de la nature de l’hétérojonction base-émetteur sur le bruit haute fréquence. Une estimation des performances en bruit à basse température des deux types de TBH est obtenues avec les modèles électriques
In order to fulfil the roadmap for the development of telecommunication and information technologies (TIC), low noise level and very fast semiconductor devices are required. Heterojunction bipolar transistor has demonstrated excellent high frequency performances and becomes a candidate to address TIC roadmap. This work deals with experimental analysis and high frequency noise modelling of Si/SiGe:C HBT (STMicroelectronics tech.) and InP/InGaAs HBT (III-V Lab Alcatel-Thales).Chapter I introduces the basic concepts of HBTs operation and the characterization at high-frequency. This chapter summarizes the high frequency performances of many state-of-the-art HBT technologies. The first part of chapter II describes the two HBT sets, with paying attention on the impact of the base composition (SiGe:C) or the lateral reduction of the device (InGaAs) on static and dynamic performances. Based on TCAD modelling, the second part shows that a 15-25% germanium composition profile in the base is able to reach highest dynamic performances. Chapter III summarizes the static and dynamic results at low temperature, giving a separation of the intrinsic transit times and charging times involved into the performance limitation. Chapter IV presents noise measurements and the derivation of high frequency noise analytical models. These models highlight the impact of the current crowding and the self-heating effects, and the influence of the base-emitter heterojunction on the high frequency noise. According to these models the high frequency noise performances are estimated at low temperature for both HBT technologies

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27

Muševič, Sašo. "Non-stationary sinusoidal analysis." Doctoral thesis, Universitat Pompeu Fabra, 2013. http://hdl.handle.net/10803/123809.

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Muchos tipos de señales que encontramos a diario pertenecen a la categoría de sinusoides no estacionarias. Una gran parte de esas señales son sonidos que presentan una gran variedad de características: acústicos/electrónicos, sonidos instrumentales harmónicos/impulsivos, habla/canto, y la mezcla de todos ellos que podemos encontrar en la música. Durante décadas la comunidad científica ha estudiado y analizado ese tipo de señales. El motivo principal es la gran utilidad de los avances científicos en una gran variedad de áreas, desde aplicaciones médicas, financiera y ópticas, a procesado de radares o sonar, y también a análisis de sistemas. La estimación precisa de los parámetros de sinusoides no estacionarias es una de las tareas más comunes en procesado digital de señales, y por lo tanto un elemento fundamental e indispensable para una gran variedad de aplicaciones. Las transformaciones de tiempo y frecuencia clásicas son solamente apropiadas para señales con variación lenta de amplitud y frecuencia. Esta suposición no suele cumplirse en la práctica, lo que conlleva una degradación de calidad y la aparición de artefactos. Además, la resolución temporal y frecuencial no se puede incrementar arbitrariamente debido al conocido principio de incertidumbre de Heisenberg. \\ El principal objetivo de esta tesis es revisar y mejorar los métodos existentes para el análisis de sinusoides no estacionarias, y también proponer nuevas estrategias y aproximaciones. Esta disertación contribuye sustancialmente a los análisis sinusoidales existentes: a) realiza una evaluación crítica del estado del arte y describe con gran detalle los métodos de análisis existentes, b) aporta mejoras sustanciales a algunos de los métodos existentes más prometedores, c) propone varias aproximaciones nuevas para el análisis de los modelos sinusoidales existentes i d) propone un modelo sinusoidal muy general y flexible con un algoritmo de análisis directo y rápido.
Many types of everyday signals fall into the non-stationary sinusoids category. A large family of such signals represent audio, including acoustic/electronic, pitched/transient instrument sounds, human speech/singing voice, and a mixture of all: music. Analysis of such signals has been in the focus of the research community for decades. The main reason for such intense focus is the wide applicability of the research achievements to medical, financial and optical applications, as well as radar/sonar signal processing and system analysis. Accurate estimation of sinusoidal parameters is one of the most common digital signal processing tasks and thus represents an indispensable building block of a wide variety of applications. Classic time-frequency transformations are appropriate only for signals with slowly varying amplitude and frequency content - an assumption often violated in practice. In such cases, reduced readability and the presence of artefacts represent a significant problem. Time and frequency resolu

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Wickert, Humberto Margel. "Modelagem e simulação computacional de transformadores de potência para estudo de transitórios em altas frequências." Universidade Federal de Santa Maria, 2016. http://repositorio.ufsm.br/handle/1/8600.

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This paper presents a proposed the modeling of power transformers for high-frequency electromagnetic transients. The application of this model allows the evaluation of the windings behavior against the voltage surges imposed by the electric power system. Besides the model, the required calculations to determine the concentrated and distributed parameters which define the representative circuit are detailed. The calculations were performed for a 25 MVA power transformer, using the data obtained from a technical audit between both client and manufacturer, known as design review. The representation of the equipment for the simulation is realized through a descriptive language, known as netlist. The impossibility of measuring the voltage along the power transformer windings subjected to voltage surges has motivated the development of the proposed model and computer simulation. Aiming to evaluate the potentialities of the developed model, the simulations for the power transformer were performed focusing on the determination of the lightning impulse transient voltage distribution along the windings and fully evaluating the obtained results. Alterations of the studied transformer s project were also addressed, considering the windings traditionally used by the manufacturers. A qualitative comparison between the responses of both power transformers is also presented.
Este trabalho apresenta uma proposta de modelagem de um transformador de potência para transitórios eletromagnéticos em altas frequências. O emprego deste modelo possibilita avaliar o comportamento dos enrolamentos frente aos surtos de tensão resultantes de transitórios no sistema elétrico de potência. Além do modelo, são detalhados os cálculos necessários para a determinação dos parâmetros distribuídos e concentrados que definirão o circuito representativo do modelo. Os cálculos foram realizados para um transformador de potência de 25 MVA, a partir dos dados obtidos de uma auditoria técnica entre cliente e fabricante, também conhecida como design review. A representação do modelo do equipamento para a simulação é realizada através de uma linguagem descritiva, netlist. A impossibilidade de medições de tensão ao longo dos enrolamentos do transformador, submetidos a surtos de tensão, motivaram o desenvolvimento do modelo proposto e a sua validação via simulação computacional. Com o objetivo de avaliar as potencialidades do modelo desenvolvido realizaram-se simulações enfatizando a determinação da distribuição das tensões transitórias de impulso atmosférico ao longo dos enrolamentos do transformador, sendo estes resultados avaliados detalhadamente. Também foram abordadas alterações no projeto do transformador estudado, considerando enrolamentos tradicionalmente utilizados pelos fabricantes. É apresentada uma comparação qualitativa entre a resposta do transformador projetado e os estudos de modificação de projeto.

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Camargo, Matheus de Castro. "Metodologia para modelagem de transformadores de potencial indutivo e capacitivos para estudos de transitórios em altas frequências." Universidade Federal de Santa Maria, 2015. http://repositorio.ufsm.br/handle/1/8577.

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This dissertation proposes a modeling methodology for Inductive and Capacitive Voltage Transformers (IVT and CVT) for high-frequency electrical transients analysis, up to 3 MHz. The model construction starts from the VT s sweep frequency test, carried out by a commercial Sweep Frequency Response Analyzer (SFRA) with special connections, in order to obtain its short-circuit Admittance Matrix. The SFRA is designed for another purpose, to detect displacement of windings or fault in the magnetic core of power transformers based on voltage transfer analysis. Therefore, due to this fact, the measured data needs to be corrected and recalculated, through and external routine, to adequately represent the proper and mutual admittance values of the Matrix. After its correction, the Admittance Matrix serves as input data to the Matrix Fitting method, a frequency-response approximation tool by means of modified rational functions, which is part of the Vector Fitting (VF) routine, freely available for non-commercial purposes in MATLAB®. The results are an approximated rational function, in state-space or pole-residue model, and a RLC equivalent network proper to be used in electromagnetic transients analysis software, such as the Alternative Transients Program (ATP). The models validation is accomplished based on the comparison between the transient responses of the created model, through simulation in ATP/EMTP, and the IVTs and CVTs laboratory tests when both are submitted to a step function excitation.
Esta dissertação propõe uma metodologia para a modelagem de Transformadores de Potencial Indutivos (TPIs) e Capacitivos (TPCs) para estudos de transitórios eletromagnéticos em altas frequências, na ordem de até 3 MHz. A obtenção dos modelos inicia através do ensaio de varredura em frequência dos TPs utilizando um Sweep Frequency Response Analyzer (SFRA) comercial por meio de conexões especiais de modo a obter as Matrizes de Admitâncias de curto-circuito. Os fabricantes do SFRA projetaram este equipamento com outro objetivo, o da realização de estudos de transferência de tensão, para detectar possíveis deformações mecânicas ou falhas internas em transformadores de potência. Desse modo, os dados obtidos dessa medição necessitam ser corrigidos e recalculados, através de uma rotina externa, a fim de representar adequadamente os valores das admitâncias próprias e mútuas desta Matriz. Uma vez corrigida, essa Matriz de Admitâncias servirá como entrada para inicializar o método do Ajuste Matricial (Matrix Fitting), que é uma ferramenta fundamental para aproximação da resposta em frequência por funções racionais modificadas que integra a rotina de Ajuste Vetorial, disponível livremente para fins não comerciais no MATLAB®. Como resultado, são obtidas sua função racional aproximada, em modelos de espaço de estados ou de polo-resíduo, e uma rede RLC equivalente complexa, propícia para o uso direto em programas de análises de transitórios eletromagnéticos, como o Alternative Transients Program (ATP). A validade dos modelos criados foi verificada por meio da comparação das respostas transitórias dos equivalentes gerados, através de simulações no EMTP/ATP, com os ensaios feitos em laboratório e em campo em TPIs e TPCs quando submetidos a uma excitação com uma função degrau.

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Chimklai, Suthep. "High-frequency transformer model for switching transient studies." Thesis, 1995. http://hdl.handle.net/2429/8775.

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The objective of this thesis is to develop a simplified high-frequency model for three-phase, two- and three-winding transformers. The model is an extension of the classical 60 Hz model which includes two important factors prevailing in transformers under transient conditions: stray capacitances which cause transformers to resonate and frequency dependent characteristics of the leakage flux and winding resistances due to skin effects. The model is not aimed to represent internal details of the transformer and only lumped circuit parameters are used in order to simulate terminal behaviours of the transformer. However, it is different from other terminal models in that it is not just an impedance or admittance black box derived from measured transfer functions. Only the meaningful parameters which correspond to the physical components in the real transformer are included in the model. The short-circuit impedances T-form of the classical model is retained which makes it possible to separate the frequency-dependent series branch form the constantvalued capacitances. In addition, it enables the model to be built at the coil level which is independent of winding connections. The model stray capacitances are placed at the corresponding coils terminals. If they link two coils they will be split into two halves with one half connected at the upper ends and the other half at the lower ends. The frequency dependent series branch is divided into sections corresponding to various sections in the transformer coil which can be assumed uniform. An RL equivalent network is used to synthesise the frequency dependent behaviour of each section. The values of R's and L's are calculated from minimum-phase-shift approximations which guarantees numerical stability of the resulting network. With the use of symmetrical components, mathematical complications of fitting mutual impedance functions are avoided and also the number of impedance functions to be fitted by rational functions is reduced. A number of short-circuit tests on the actual power transformers installed in the Thailand 's power system were performed to determine the parameters of the model. The frequency responses calculated from the model are compared with the tests. Also, a timedomain test was conducted and the result was used for comparison with the simulation from the model.

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31

Fattal, Feras. "High frequency model for transient analysis of transformer windings using multiconductor transmission line theory." 2017. http://hdl.handle.net/1993/32172.

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Transients encountered by transformers in power stations during normal operation can have complex oscillatory overvoltages containing a large spectrum of frequency components. These transients can coincide with the natural frequencies of the transformers windings, leading to voltages that can be greater or more severe than the current factory proof tests. This may lead to insulation breakdown and catastrophic failures. Existing lumped parameter RLCG transformer models have been proven to be less accurate for very fast transient overvoltages (VFTO) with frequencies over 1 MHz. A white box model for transient analysis of transformer windings has been developed using Multiconductor Transmission Line (MTL) Theory. This model enables the simulation of natural frequencies of the transformer windings up to frequencies of several MHz, and can be used to compute voltages between turns by representing each turn as a separate transmission line. Both continuous and interleaved disk windings have been modelled and a comparison and validation of the results is presented.
May 2017

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32

Cheng, Yung Ming, and 鄭永銘. "Derivation of A High Frequency Electromagnetic Transient Model from the geometrical Structure of A Transformer." Thesis, 1995. http://ndltd.ncl.edu.tw/handle/26942444525732262856.

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Hsiao, Po-Yu, and 蕭柏宇. "The Effects and Coping Strategies of Transient Voltage and Frequency with High Penetration Wind Power for Taipower System." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/70446356333136269132.

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碩士
國立中正大學
電機工程研究所
101
Taiwan power Company (TPC) develops renewable energy actively in recent years. Moreover, TPC has started planning a high penetration wind power system and building offshore wind farms around the coast of Zhang-bin, Yun-lin and Peng-hu. The large scale of wind farms will inevitably encounter greater disturbances from wind that is randomly characterized. It is a key factor influencing both the quality and stability of a wind farm’s power supply. In this study, after high penetration wind power is integrated, the impacts of system frequency and the dispatch of conventional units will be discussed. Besides, the hybrid system combing wind power with pumped-storage will be planning to reduce the effect of system frequency. Then, the required low voltage ride through (LVRT) capability issued by TPC will be evaluated when many wind turbines are integrated into the grid. A suitable LVRT curve will be formulated in order to avoid the widespread tripping of wind generators after a bus fault occurs near the wind farms.

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Radic, Aleksandar. "Practical Volume-reduction Strategies for Low-power High-frequency Switch Mode Power Supplies." Thesis, 2014. http://hdl.handle.net/1807/44131.

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The miniaturization of dc–dc switch-mode power supplies (SMPS) is of a key importance in volume-sensitive portable devices, such as cell phones, tablet computers, and digital cameras. In these systems, multiple SMPS are required to provide well regulated voltage and power to various electronic components such as the central processing unit (CPU) and random-access memory (RAM). The combined volume, weight, and surface area footprint of these SMPS is usually the largest component. Traditionally, SMPS volume reduction has been achieved through increased switching frequencies; however, for power-sensitive applications this is undesirable due to the increased switching losses. This thesis presents two alternative, power-efficient, SMPS miniaturization methods: one control and one topology based. The presented controller recovers from load transients with virtually minimum possible output voltage deviation, reducing the reactive component size. The controller utilizes a simple algorithm, requiring no knowledge of the converter parameters and virtually no processing power. The simplicity of the control concept enabled the design of an area and power efficient integrated circuit (IC) implementation. The entire IC is implemented in a CMOS 0.18µm process on a 0.26 mm2 silicon area, which is comparable to the state-of-the-art analog solutions. For the experimental system the deviation (output capacitor size) is about four times smaller than that of a fast PID compensator having a 1/10th of the switching frequency bandwidth. The second solution is a complementary converter topology that has a smaller output filter volume, improved dynamic response, and lower switching losses compared to the state-of-the-art solutions. To reduce the volume and switching losses, the input-to-output voltage difference is decreased with a capacitive attenuator that replaces the input filter capacitor and has approximately the same volume. Both the attenuator and the downstream buck converter share the same set of switches, minimizing conduction losses. A single multi-mode digital controller governs operation of both stages, seamlessly regulating the output and input center-tap voltages. Experiments with a 5–1.5-V, 2.5-A, 1-MHz prototype show that, compared to the conventional buck, the merged topology has 43% smaller inductor, 36% smaller output capacitor, up to 30% lower power losses, and a 25% faster transient response.

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35

(9826433), Rodney Peters. "Transient overvoltages in mine power systems." Thesis, 1998. https://figshare.com/articles/thesis/Transient_overvoltages_in_mine_power_systems/20341575.

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This thesis investigates the nature and magnitude of switching transient overvoltages that can be expected in inductively grounded underground coal mines. Computer models of power system equipment have been developed to analyse single-phase fault isolations; motor starts (including prestriking) and motor turn offs. The computer models that were developed have been validated against field recordings. The "Alternative Transients Program" (ATP) software was used to conduct the computer simulation studies.

The underground substations used in Central Queensland coal mines are grounded through a Neutral Earthing Reactor (NER). The primary purpose of a NER is to limit earth fault currents. However, when a single -phase -to -ground fault occurs, energy is stored in the NER. Consequent to the isolation of such a fault, the energy stored in the NER is released. The release of this energy manifests itself as a transient overvoltage. This thesis investigates the effect of the rating of the NER and the value of the electric current at the instant of earth fault isolation on transient overvoltages.

Two alternate earthing schemes were investigated in an attempt to reduce the severity of the transient overvoltages consequent to the isolation of earth faults. The first alternative that was investigated was the addition of a Resistor -Capacitor Snubber in parallel with a NER. Computer simulations indicate that the proper choice of Snubber parameters is very effective in reducing transient overvoltages following the isolation of earth faults. Consequently, a decision was made to manufacture a R -C Snubber and conduct field tests on the same. A summary of the field tests results has been included in the thesis.

The other alternate grounding scheme that was investigated was the substitution of a Neutral Earthing Reactor with a Neutral Earthing Resistor. Computer simulations indicate that transient overvoltages following the isolation of earth faults are virtually eliminated if a Neutral Earthing Resistor is used.

Consequent to this research work, the use of R -C Snubbers has become a recommended practice at various coal mine sites in Australia which use NER grounded, container type substations. The R -C Snubber is the preferred alternative at such installations as it is very compact and also eliminates the heating/reliability problems associated with resistance grounding.

Vacuum contactors are used throughout the Central Queensland mines to frequently turn motors on and off. One of the most significant characteristics of vacuum contactors is their ability to interrupt high frequency currents. The consequence of this is that a considerable number of high frequency prestriking transients can be generated whenever a motor is turned on. These high frequency transients can lead to the premature aging of insulation. This thesis presents a computer model that was developed to simulate prestriking transients using the TACS (Transient Analysis of Control Systems) programming feature of the ATP software. Further investigation of prestriking transients was considered to be beyond the scope of the present research project. However, the unique TACS based prestriking model of a vacuum contactor that has been developed will be a useful tool for conducting further research.

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(6922148), Pei Zhang. "Large Eddy Simulation/Transported Probability Density Function Modeling of Turbulent Combustion: Model Advancement and Applications." Thesis, 2019.

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Studies of turbulent combustion in the past mainly focus on problems with single-regime combustion. In practical combustion systems, however, combustion rarely occurs in a single regime, and different regimes of combustion can be observed in the same system. This creates a significant gap between our existing knowledge of combustion in single regime and the practical need in multi-regime combustion. In this work, we aim to extend the traditional single-regime combustion models to problems involving different regimes of combustion. Among the existing modeling methods, Transported Probability Density Function (PDF) method is attractive for its intrinsic closure of treating detailed chemical kinetics and has been demonstrated to be promising in predicting low-probability but practically important combustion events like local extinction and re-ignition. In this work, we focus on the model assessment and advancement of the Large Eddy Simulation (LES)/ PDF method in predicting turbulent multi-regime combustion.

Two combustion benchmark problems are considered for the model assessment. One is a recently designed turbulent piloted jet flame that features statistically transient processes, the Sydney turbulent pulsed piloted jet flame. A direct comparison of the predicted and measured time series of the axial velocity demonstrates a satisfactory prediction of the flow and turbulence fields of the pulsed jet flame by the employed LES/PDF modeling method. A comparison of the PLIF-OH images and the predicted OH mass fraction contours at a few selected times shows that the method captures the different combustion stages including healthy burning, significant extinction, and the re-establishment of healthy burning, in the statistically transient process. The temporal history of the conditional PDF of OH mass fraction/temperature at around stoichiometric conditions at different axial locations suggests that the method predicts the extinction and re-establishment timings accurately at upstream locations but less accurately at downstream locations with a delay of burning reestablishment. The other test case is a unified series of existing turbulent piloted flames. To facilitate model assessment across different combustion regimes, we develop a model validation framework by unifying several existing pilot stabilized turbulent jet flames in different combustion regimes. The characteristic similarity and difference of the employed piloted flames are examined, including the Sydney piloted flames L, B, and M, the Sandia piloted flames D, E, and F, a series of piloted premixed Bunsen flames, and the Sydney/Sandia inhomogeneous inlet piloted jet flames. Proper parameterization and a regime diagram are introduced to characterize the pilot stabilized flames covering non-premixed, partially premixed, and premixed flames. A preliminary model assessment is carried out to examine the simultaneous model performance of the LES/PDF method for the piloted jet flames across different combustion regimes.

With the assessment work in the above two test cases, it is found that the LES/PDF method can predict the statistically transient combustion and multi-regime combustion reasonably well but some modeling limitations are also identified. Thus, further model advancement is needed for the LES/PDF method. In this work, we focus on two model advancement studies related to the molecular diffusion and sub-filter scale mixing processes in turbulent combustion. The first study is to deal with differential molecular diffusion (DMD) among different species. The importance of theDMD effects on combustion has been found in many applications. However, in most previous combustion models equal molecular diffusivity is assumed. To incorporate the DMD effects accurately, we develop a model called Variance Consistent Mean Shift (VCMS) model. The second model advancement focuses on the sub-filter scale mixing in high-Karlovitz (Ka) number turbulent combustion. We analyze the DNS data of a Sandia high-Ka premixed jet flame to gain insights into the modeling of sub-filter scale mixing. A sub-filter scale mixing time scale is analyzed with respect to the filter size to examine the validity of a power-law scaling model for the mixing time scale.

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Lukic, Zdravko. "Design and Practical Implementation of Advanced Reconfigurable Digital Controllers for Low-power Multi-phase DC-DC Converters." Thesis, 2012. http://hdl.handle.net/1807/33855.

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The main goal of this thesis is to develop practical digital controller architectures for multi-phase dc-dc converters utilized in low power (up to few hundred watts) and cost-sensitive applications. The proposed controllers are suitable for on-chip integration while being capable of providing advanced features, such as dynamic efficiency optimization, inductor current estimation, converter component identification, as well as combined dynamic current sharing and fast transient response. The first part of this thesis addresses challenges related to the practical implementation of digital controllers for low-power multi-phase dc-dc converters. As a possible solution, a multi-use high-frequency digital PWM controller IC that can regulate up to four switching converters (either interleaved or standalone) is presented. Due to its configurability, low current consumption (90.25 μA/MHz per phase), fault-tolerant work, and ability to operate at high switching frequencies (programmable, up to 10 MHz), the IC is suitable to control various dc-dc converters. The applications range from dc-dc converters used in miniature battery-powered electronic devices consuming a fraction of watt to multi-phase dedicated supplies for communication systems, consuming hundreds of watts. A controller for multi-phase converters with unequal current sharing is introduced and an efficiency optimization method based on logarithmic current sharing is proposed in the second part. By forcing converters to operate at their peak efficiencies and dynamically adjusting the number of active converter phases based on the output load current, a significant improvement in efficiency over the full range of operation is obtained (up to 25%). The stability and inductor current transition problems related to this mode of operation are also resolved. At last, two reconfigurable digital controller architectures with multi-parameter estimation are introduced. Both controllers eliminate the need for external analog current/temperature sensing circuits by accurately estimating phase inductor currents and identifying critical phase parameters such as equivalent resistances, inductances and output capacitance. A sensorless non-linear, average current-mode controller is introduced to provide fast transient response (under 5 μs), small voltage deviation and dynamic current sharing with multi-phase converters. To equalize the thermal stress of phase components, a conduction loss-based current sharing scheme is proposed and implemented.

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