Thesis (Ph. D.)--Ohio State University, 2005. / Available online via OhioLINK's ETD Center; full text release delayed at author's request until 2006 Nov 28

O número crescente de cargas baseadas em conversores eletrônicos criou a preocupação com a distorção harmônica em sistemas de energia elétrica. Qualidade de energia é um assunto importante tanto para as companhias distribuidoras e geradoras de energia elétrica como para os consumidores. A utilização de equipamentos para a eliminação de harmônicos tornou-se necessária e são diversas as soluções possíveis. Em sistemas trifásicos com quatro fios, os filtros ativos de potência e os sistemas de energia ininterrupta são equipamentos empregados para a melhoria da qualidade de energia elétrica. Tais equipamentos apresentam em sua estrutura um conversor trifásico com quatro fios podendo ser do tipo fonte de tensão ou fonte de corrente. Este estudo propõe uma estratégia para a minimização da ondulação das correntes de linha e de neutro em um conversor do tipo fonte de tensão trifásico com quatro fios. A estratégia consiste em injetar uma tensão de seqüência zero nas referências dos blocos PWM\'s que não altere o valor médio da corrente, mas somente a sua ondulação. Propõe-se uma metodologia que possibilita a escolha de um valor ótimo para a tensão de seqüência zero. São também apresentados a região dos vetores sintetizáveis, a análise da tensão de saída e o modelamento do conversor. Propõe-se uma estratégia de controle de corrente com reguladores PI para um sistema trifásico com quatro fios usando conversor com quatro fios. O comportamento do conversor é verificado por simulações numéricas e resultados experimentais, confirmando o desempenho da estratégia de PWM proposta. / The growing number of power electronics-based loads has created concern about harmonic distortion in electric systems. Power quality is an important issue both for utilities and consumers. The use of harmonic filtering equipment has become necessary and many solutions have been studied. In three phase systems, active power filters and uninterruptible power supply are used for improving power quality. These equipments present in their topologies a three-phase four-wire voltage source converter or three-phase four-wire current source converter. This study proposes a strategy for the minimization of the line and neutral ripple current of a three-phase four-wire Voltage Source Converter. The strategy consists of injecting an instantaneous zero sequence to the references of a triangular carrier PWM that does not affects the local average of the converter output voltages, but only the ripple currents. A methodology to evaluate the instantaneous optimal value of the injected zero sequence voltage is presented. The locus of the synthesizable voltage vectors and the modeling of the converter are also shown. It presents a strategy of current control for three-phase four-wire systems using three-phase four-wire converter. The behavior of the converter is verified by numerical simulation and experimental results, confirming the performance of the proposed PWM strategy.

This dissertation is dedicated to a comprehensive study of static synchronous compensator (STATCOM) systems utilizing cascaded-multilevel converters (CMCs). Among flexible AC transmission system (FACTS) controllers, the STATCOM has shown feasibility in terms of cost-effectiveness in a wide range of problem-solving abilities from transmission to distribution levels. Referring to the literature reviews, the CMC with separated DC capacitors is clearly the most feasible topology for use as a power converter in the STATCOM applications. The controls for the CMC-based STATCOM were, however, very complicated. The intricate control design was begun without well-defined system transfer functions. The control compensators were, therefore, randomly selected. The stability of the system was achieved by trial and error processes, which were time-consuming and ineffective. To be able to operate in a high-voltage application, a large number of DC capacitors are utilized in a CMC-based STATCOM. All DC capacitor voltages must be balanced in order to avoid over-voltages on any particular link. Not only do these uneven DC voltages introduce voltage stress on the semiconductor switches, but they also lower the quality of the synthesized output waveforms of the converter. Previous researches into DC capacitor voltage-balancing techniques were very straightforward, in that individual voltage compensators were added into the main control loop. However, the compensator design for these individual loops is very problematic because of the complexity of the voltage-loop transfer functions. Basically, the trial and error technique again provides the simplest way to achieve acceptable compensators. Moreover, the greater number of voltage levels, the more complex the control design, and the main controller must perform all of the feedback control procedures. As a result, this approach potentially reduces the reliability of the controller. The goal of this dissertation is to achieve high-performance, reliable, flexible, cost-effective power stages and controllers for the CMC-based STATCOM. Major contributions are addressed as follows: 1) optimized design for the CMC-based STATCOM power stages and passive components, 2) accurate models of the CMC for reactive power compensations in both ABC and DQ0 coordinates, 3) an effective decoupling power control technique, 4) DC-link balancing strategies; and 5) improvements in the CMC topology. To enhance the modularity and output voltage of the CMC, the high-switching-frequency, high-power H-bridge building block (HBBB) and the optimized design for its power stage and snubber circuits are first proposed. The high-switching-frequency feature is achieved by utilizing the Virginia Tech-patented emitter turn-off (ETO) thyristor. Three high-power HBBB prototypes were implemented, and their performance was experimentally verified. To simplify the control system design, well-defined models of the CMC in both ABC and DQ0 coordinates are proposed. The proposed models are for the CMC with any number of voltage levels. The key system transfer functions are achieved and used in the control design processes. To achieve independent power control capability, the control technique, called the decoupling power control, is proposed. By applying this control technique, real and reactive power components can be controlled separately. In order to balance the DC capacitor voltages, a new, effective pulse width modulation (PWM) technique, which is suitable for any number of H-bridge converters, is proposed. The proposed cascaded PWM algorithm can be practically realized into the field programmable gate arrays (FPGA), and its complexity is not affected by the number of voltage levels. In addition, the complexity of the main controller, which is essentially based on the digital signal processor (DSP), is no longer a function of the number of the output voltage levels. The basic structure of the cascaded PWM is modular, which, in general, enhances the modularity of the CMC power stages. With the combination of the decoupling power control and the cascaded PWM, a CMC with any number of voltage levels can be simply modeled as a three-level cascaded converter, which is the simplest topology to deal with. This significantly simplifies and optimizes the control design process. To verify the accuracy of the proposed models and the performance of the control system for the CMC-based STATCOM, a low-power, seven-level cascaded-based STATCOM hardware prototype is implemented. The key control procedures are performed by a main controller, which consists of a DSP and an FPGA. The simulation and experimental results indicate the superior performance of the proposed control system, as well as the precision of the proposed models. / Ph. D.

This thesis is dedicated to a comprehensive study of static series synchronous compensator (SSSC) systems utilizing cascaded-multilevel converters (CMCs). Among flexible AC transmission system (FACTS) controllers, the SSSC has shown feasibility in terms of cost-effectiveness in a wide range of problem-solving abilities from transmission to distribution levels. Referring to the literature reviews, the CMC with separated DC capacitors is clearly the most feasible topology for use as a power converter in the SSSC applications. The control for the CMC-Based SSSC is complicated. The design of the complicated control strategy was begun with well-defined system transfer functions. The stability of the system was achieved by trial and error processes, which were time-consuming and ineffective. The goal of this thesis is to achieve a reliable controller design for the CMC-based SSSC. Major contributions are addressed as follows: 1) accurate models of the CMC for reactive power compensations in both ABC and DQ0 coordinates, and 2) an effective decoupling power control technique. To simplify the control system design, well-defined models of the CMC-Based SSSC in both ABC and DQ0 coordinates are proposed. The proposed models are for the CMC-Based SSSC focus on only three voltage levels but can be expanded for any number of voltage levels. The key system transfer functions are derived and used in the controller design process. To achieve independent power control capability, the control technique, called the decoupling power control used in the design for the CMC-Based STATCOM is applied. This control technique allows both the real and reactive power components to be independently controlled. With the combination of the decoupling power control and the cascaded PWM, a CMC with any number of voltage levels can be simply modeled as a three-level cascaded converter, which is the simplest topology to deal with. This thesis focuses on the detailed design process needed for a CMC-Based SSSC. / Master of Science

Distributed power system (DPS) is widely adopted in Power supplies for the telecom, computer and network applications. Constant on-time current mode control and V2 control are widely used as point-of-load (POL) converters and voltage regulators (VR) in DPS systems. Series resonant converters (SRC) are widely used in aerospace systems and LLC resonant converters are widely used as Front-end converters in DPS systems. The technological innovations bring increasing demand for optimizing the dynamic performance of the switching regulators in these applications. There has been a strong desire to develop simple and accurate equivalent circuit models to facilitate the design of these converters. Constant on-time current-mode control has been widely used in POL and VRM converters. For multi-phase application, external ramp is required to improve jittering performance using pulse distribution method. Chapter II analyzes the effect of external ramp on small-signal model of constant on-time current mode control. It is found that external ramp brings additional dynamics by introducing a moving pole and a static zero. Next, a three-terminal switch model is proposed based on non-ideal current source concept, where the non-idealness of the current source is presented by a Re2-Le2 branch. Based on the proposed model, design guidelines are proposed based on either worst case design strategy or auto-tuning strategy. V2 control has advantages of simple implementation and fast transient response and is widely used in industry for POL and VR applications. However, the capacitor voltage sideband effect, which casues the instability problem when ceramic capacitors are employed, also needs to be taken into consideration in modeling. Chapter III proposed a unified equivalent circuit model of V2 control, the model is built based on non-ideal voltage source concept. The model represents capacitor voltage sideband effect with a Re2-Le2 branch, which forms the double pole by resonating with power stage output capacitor. The equivalent circuit model is a complete model and can be used to examine all the transfer functions. Bsed on the unified equivalent circuit model, design guidelines for VR applications and general POL applications are provided in Chapter IV, for both constant on-time V2 control and constant frequency V2 control. For resonant converters, the small-sginal modelling is very challenging as some of the state variables do not have dc components but contain strong switching frequency component and therefore the average concept breaks down. For SRC, the equivalent circuit model proposed by E. Yang in [E26] based on the results by the extended describing function concept is the most successful model. However, the order of the equivalent circuit model is too high and the transfer functions are still derived based on numerical solution instead of analytical solutions. Chapter V proposes a methodology to simplify the fifth-order equivalent circuit of SRC to a third-order equivalent circuit. The proposed equivalent circuit model can be used to explain the beat frequency dynamics: when switching frequency is far away from resonant frequency, beat frequency will occur; when the two frequencies are close, beat frequency will disappear and another double pole which is determined by equivalent inductor and output capacitor will be formed. For the first time, analytical solutions are provided for all the transfer functions which are very helpful for feedback design. LLC resonant converters are widely adopted as front-end converter in distributed power system for the telecom, computer and network applications [F2]. Besides, LLC resonant converters are also very popular in other applications, such as LCD, LED and plasma display in TV and flat panels [F3]-[F6]; iron implanter arc power supply[F7]; solar array simulator in photovoltaic application[F8]; fuel cell applications[F9],and so on. For LLC, no simple equivalent circuit model is available and no analytical expressions of transfer functions are presented. Chapter VI proposes an equivalent circuit model for LLC resonant converter. When Fs ≥ Fo, Lm is clamped by the output voltage and LLC behaves very similar as SRC. As a result, the dynamic behavior is similar as SRC: when switching frequency is larger than resonant frequency, the beat frequency double pole show up and the circuit is third-order; when switching frequency is close to resonant frequency, beat frequency double pole disappear and a new double pole formed by equivalent inductor Le and equivalent output capacitor Cf show up. The circuit reduces to second order. When Fs<Fo, Lm participates in resonance during some time periods and the circuit is essentially a multiresonant structure. An approximated model is proposed where the equivalent resonant inductor is modified to include the effect of Lm. As a result, the double pole will move to a little lower frequency. For the first time, analytical solutions are provided for all the transfer functions which are very helpful for feedback design. In conclusion, the works shown in this dissertation focus on small-signal equivalent circuit modeling for Buck converters with advanced control schemes and also resonant converters. The models are simple and accurate up to very high frequency range (1/2 fsw). / Ph. D.

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Automotive catalytic converters are increasingly used to reduce emissions from internal combustion engines to comply with emission regulations. Maldistributed flow across the catalyst affects its warm up, light off time, ageing, and conversion efficiency. This thesis concerns flow distribution in automotive catalytic converters and methods to improve CFD predictions. Previous studies showed that modelling the monolith flow resistance using the Hagen- Poiseuille’s formulation under predicted flow maldistribution. The predictions were improved by incorporating an additional pressure loss term V2 2 1  , where V is transverse velocity just upstream of a monolith channel, for oblique entry of the flow into the monolith known as the entrance effect. Further improvement was obtained by incorporating the critical angle of attack method. However, there was no experimental evidence to support these oblique entry loss formulations. There also remained the possibility that under prediction of flow maldistribution might be due to the failure to predict flow in the diffuser accurately. A one-dimensional oblique angle flow rig was designed and built to measure the effect of oblique entry flow losses in monoliths. Experiments were performed at different angles of attack (α), using different lengths of substrate and a methodology was developed to obtain the oblique flow entrance losses. The results showed that the pressure loss attributed to the entrance effect increased with the angle of attack. The entrance effect was also found to be dependent on channel Reynolds number and substrate length. The theoretical assumption of V2 2 1  predicts accurately at low Reynolds number but looses its validity at high Reynolds number. From the experimental studies, an improved correlation for the entrance effect has been derived as a function of major controlling variables, i.e., angle of attack, length of the substrates and Reynolds number. A two-dimensional rig was designed to measure the flow field using PIV in a 2-D diffuser placed upstream of two different length substrates. The results showed that the flow in a wide angle diffuser consisted of a central core, free shear layer and recirculation regions. The near-field region was found similar to that of a plane jet. The flow field was found to be independent of Reynolds number. Increasing the substrate length resulted in a flattening of the axial profiles close to the substrate face. A CFD study was undertaken to predict maldistributed flow at the exit of the substrate for an axisymmetric catalyst model by incorporating the measured entrance effect correlation. A fixed critical angle of attack (αc,F) approach was used whereby the entrance effect is assumed constant for α>αc,F. Incorporating the entrance effect with αc,F= 810 improved the prediction of maldistribution in the flow profiles. A 2-D CFD study was undertaken to predict the flow distribution in the diffuser and downstream of the substrate. A comparison of the CFD predictions in the diffuser using different turbulence models showed that all the turbulence models used in this study over predicted the width of the central core region and the V2F turbulence model gave velocity predictions that compared best with PIV. Incorporating the entrance effect improved the predictions close to the diffuser-substrate interface and downstream of the substrate.

QC 351 A7 no. 38 / A brief description is given of the various types of image tubes presently used in astronomical research and a review is presented of the past applications of image tubes to direct astronomical photography. A detailed laboratory evaluation of the Carnegie image tube is summarized and photographs at the telescope are presented to confirm and extend the results obtained in the laboratory. Iris photometry of stellar images can be carried out on Carnegie tube photographs with about the same accuracy as is obtained by normal photographic techniques. Compared to unaided plates the image tube typically requires about 1/15 the exposure time to record stellar images of a specified threshold magnitude. When exposures are made to near the sky limit, however, the Carnegie tube cannot record stars as faint as can be recorded with an unaided plate. When exposed at a given focal length telescope, the limiting magnitude of an image tube record is about 1 magnitude brighter than that of an unaided photograph. Primarily two characteristics of the Carnegie tube, an over-all mottled sensitivity pattern and a light- induced background, are found to be responsible for the loss in limiting magnitude of a Carnegie tube record. The mottle pattern is characterized by an rms variation in sensitivity of ±1.3 percent. It modulates the photographic record of the night-sky radiation and seriously affects the signal -to -noise ratio of the threshold images. The additional background produced by the light- induced background of the image tube generally amounts to 25 percent of the night-sky radiation on a sky-limited photograph. In order to record the same sky-limited magnitude on a Carnegie tube plate and an unaided plate, the image tube record must be exposed at a longer focal length telescope. The exposure time required by the image tube is then about 1/2 to 1/3 that of the unaided plate. Because of the higher scale of the image tube photograph in such a case, however, the effective gain provided by the image tube over the unaided plate is generally somewhat larger than the relative exposure time. The photography of extended objects is found to be particularly affected by the nonuniformities of the image tube. Besides reducing the over-all signal-to-noise ratio of the image tube record, the generalmottle pattern and additional discrete patches and ripples in sensitivity of the image tube tend to mimic low contrast features of galaxies and nebulae. The rather subjective effects of the nonuniformities can be significantly reduced by using telescopes with moderately long focal lengths, so that the seeing image is then large in comparison to the nonuniformities. The photography of astronomical sources through narrowband interference filters has been found to be a particularly promising application of the Carnegie image tube. Preliminary tests reported in the present study include the photography of supernova remnants, planetary nebulae, galaxies, and reflection nebulae. The basic quality criterion for comparing the image tube to unaided photographic emulsions is argued to be the detective quantum efficiency. Typical values of the gain over unaided emulsions provided by the Carnegie tube are calculated to be in the range 10 to 20. It is emphasized, however, that because of the variety of requirements in specific research areas and because of the several unique characteristics of a given image tube, no single figure of merit may be defined that will predict the usefulness of an image tube in all applications. It is suggested that the resolution of a detector should not generally be combined into the calculation of a single figure of merit but should be considered as a separate quality criterion. Certain problems with the Carnegie tube (and other image tubes as well) potentially limit its usefulness in specific research areas. Besides the problems already mentioned, other problems include low resolution, geometrical distortion, the complexities of analyzing the final record (as compared to an unaided photograph), and the limited field of the image tube. Each of these characteristics can be highly significant or entirely inconsequential in different applications.

M.Phil. (Electrical and Electronic Engineering) / There are two methods that can be used when power is converted from one frequency to another. One way uses a DC link as an intermediate stage and the other uses no intermediate stage. When no DC link exists, the conversion process is referred to as AC-AC conversion, (Cha, Enjeti & Ratanapanachote, 2004:2237). This method essentially chops the AC signal in a specific manner which yields an AC signal with a different frequency directly. Depending on how the AC signal is chopped, it would result in a varied number of wave shapes are constructed. The construction of these various wave shapes results in devices that can accomplish different tasks. One such device is called a unity displacement factor frequency converter (UDFFC). This research attempts to develop a model, to simulate and design a unity displacement factor frequency converter. The primary idea about this device is that the power factor or displacement factor is unity regardless of the load. There are three parts in the development of the UDFFC. The power circuit is an array or matrix of switches that link the three phase supply to the three phase load. The power circuit is interfaced to the control circuit which generates pulses that control the matrix of switches. The control circuit utilizes embedded control software that performs the control logic in a microcontroller. The development of the UDFFC has a potential to solve real power engineering problems, such as eliminating the DC link in a converter. This achieves the conversion of AC directly to AC in the absence of a DC link. The other major advantage is the variation of the output frequency to a desired value. The most important is that the displacement factor can be controlled as well.