359 research outputs found
Primary mediastinal B-cell lymphoma
No full textPrimary mediastinal B-cell lymphoma (PMBCL) is a sub-type of the heterogeneous diffuse large B-cell lymphoma category, and comprises approximately 5% of all non-Hodgkin's lymphomas (NHL). It was first recognized as a distinct clinico-pathologic entity 20 years ago, and recent work has further characterized specific molecular features. Gene expression profiling has suggested a partial overlap with nodular sclerosing Hodgkin lymphoma (HL), with which it shares some clinical features. The optimal management remains a matter of debate. There is uncertainty as to whether weekly alternating chemotherapy regimens may be more effective than CHOP, whether consolidation radiotherapy (RT) to the mediastinum is always required, whether PET scanning can be used to determine this, and whether the use of rituximab as part of initial therapy will change the answers to these questions. The International Extranodal Lymphoma Study Group (IELSG) 26 clinicopathologic study of PMBCL, which has recently opened, represents a first attempt to gather data prospectively on some of these issues. Copyright (c) 2007 John Wiley & Sons, Lt
VSC transmission system using flying capacitor multilevel converters and hybrid PWM control
No full textA high-voltage direct current (HVDC) transmission system based on three-level flying capacitor (FC) multilevel converters with hybrid pulse-width modulation (PWM) is presented in this paper. Selective harmonic elimination PWM (SHE-PWM) is used during normal operating conditions and is switched to phase-shifted sinusoidal PWM (PS-SPWM) during an asymmetric network fault. The generation of the switching patterns under SHE-PWM control for each power device is described taking into account the natural balancing of the FC voltages. A new and simple control method for balancing the FC voltages when using SHE-PWM is proposed which is based on the small change of the firing angle according to the polarity of the load current. The FC voltage ripple under SHE-PWM control is estimated and compared to that under PS-SPWM. A method to implement the proposed hybrid PWM with capacitor voltage balancing is also provided. Simulation studies on a 300-MW/plusmn150 kV voltage-source converter transmission system are presented to confirm the satisfactory performance of the proposed system under active and reactive power variations and single-phase fault conditions
A VSC transmission system using flying capacitor multilevel converters and selective harmonic elimination pWM control
No full textA transmission system based on voltage-source three-level flying capacitor (FC) multilevel converters with selective harmonic elimination pulse-width modulation (SHE-PWM) control is presented in this paper. The generation of the switching patterns for each power device is described taking into account the natural balancing of the FC voltages. A new and simple control method for balancing the FC voltages of the FC converter when using SHE-PWM is proposed which is based on the small change of the firing angle according to the load current polarity. Implementation of the SHE-PWM with capacitor voltage balancing is provided. Simulation studies for a 300MW/+/- 150kV transmission system are presented to confirm the satisfactory performance of the proposed system
New Stationary Frame Control Scheme for Three Phase PWM Rectifiers Under Unbalanced Voltage Dips Conditions
No full textA new stationary frame control scheme for three-phase pulsewidth-modulation (PWM) rectifiers operating under unbalanced voltage dips conditions is proposed in this paper. The proposed control scheme regulates the instantaneous active power at the converter poles to minimize the harmonics of the input currents and the output voltage ripple. This paper's novelty is the development of a new current-reference generator implemented directly in stationary reference frame. This allows using proportional sinusoidal signal integrator (P-SSI) controllers for simultaneous compensation of both positive and negative current sequence components. No phase-locked loop (PLL) strategies and coordinate transformations are needed for the proposed current-reference generator. Experimental results are presented for a 20-kV A alternative current (ac)/direct current (dc) converter prototype to demonstrate the effectiveness of the proposed control scheme. A comparison with two other existing control techniques is also performed. Fast dynamic performance with small dc-link voltage ripple and input sinusoidal currents are obtained with this control scheme, even under severe voltage dips operating condition
Repetitive current control of two-level and interleaved three-phase PWM utility connected converters
No full textThis thesis is mainly concerned with investigations into digital repetitive current controlof two-level and interleaved utility connected PWM converters. The research ismotivated by the relatively poor performance of classical (PI) controllers when theutility voltage harmonic distortion is high. This is due to the low gain, and poordisturbance rejection of the PI controller at the utility harmonic frequencies. Repetitivefeedback controllers have the ability to track or reject periodic disturbances, such asutility harmonics, as they naturally have high gains at the utility voltage harmonicfrequencies, assuming that these frequencies do not change.Repetitive controllers (RC) are known for being sensitive to variations in systemparameters and disturbance frequency, which in practice renders them either ineffectiveor unstable. Another challenge arises from the memory requirements of RC in case ofthe absence of even harmonics, which can make its practical implementation difficultand expensive. In addition, another problem that has not been investigated extensivelyin the literature is that the effectiveness of RC is severely limited by the limitedbandwidth of the plant (the utility connected converter and its filter). Theoreticalanalysis and simulation results presented in this thesis show that RC could noteffectively reject disturbances at frequencies above the closed loop system bandwidth.The design of the converter's output filter bandwidth and the values of its componentsneed to be selected carefully, to enable RC to be used effectively.The research in this thesis focuses on investigating the practical implementation andperformance limits of two types of repetitive controllers (conventional and oddharmonics),used for current control of two-level utility connected converter with LCLoutput filter. The odd-harmonic repetitive controller halves the memory requirementand offers higher gains only at odd harmonic frequencies of interest. The overall controlscheme consists of the traditional classical tracking controller with a dual loop feedbacksystem and RC. The results indicate that the repetitive controller improves the steadystate error and the total harmonic distortion of the output current, provided that theplant's bandwidth is sufficiently large.Finally, a repetitive controller for an interleaved utility connected converter has beendesigned and investigated in this study. The interleaved converter system has higherbandwidth than the two-level converter, which improves the effectiveness of RC. Itprovides good disturbance rejection compared to classical controllers which results inlow output current THD. The RC was demonstrated to be robust despite uncertainty inutility impedance, while achieving a fast almost zero error convergence. The proposedRC has been experimentally implemented using a DSP and the results indicate that thequality of output current complies with international standards on harmonic limits andmatches simulation results obtained from the Matlab/Simulink model of the system
Modelagem e controle de retificadores PWM trifásicos conectados a geradores síncronos a ímãs permanentes em sistemas de conversão de energia eólica
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia ElétricaNo século XXI, mudanças são esperadas nas formas em que a produção de energia ocorre. Com o desenvolvimento de novas tecnologias, o custo da eletricidade a partir da geração eólica caiu mais de 80% durante os últimos anos. A geração de energia através de sistemas de conversão de energia eólica representará uma parcela considerável da energia elétrica gerada. Nesse contexto, turbinas eólicas equipadas com geradores síncronos a ímãs permanentes apresentam vantagens importantes sobre outros tipos de geradores. No entanto, esses geradores precisam ser conectados a uma rede de distribuição através de conversores estáticos de capacidade nominal. Estes conversores permitem que o sistema de conversão de energia eólica seja utilizado em seu ponto ideal de operação a todo instante. Além disto, os conversores estáticos modernos garantem a melhoria da qualidade da energia gerada e que esta seja transmitida à rede com alto rendimento de conversão e de maneira confiável e flexível. Este trabalho propõe uma nova estratégia de controle de corrente de alto desempenho e elevado fator de potência. A estratégia proposta baseia-se na técnica de autocontrole de corrente para retificadores PWM trifásicos, incrementando-a no que diz respeito à compensação de reativos no gerador. Utiliza-se um estágio retificador PWM trifásico robusto e com baixas perdas de condução. Este retificador é analisado e um esquema apropriado de modulação vetorial é proposto. O sistema de conversão de energia eólica é estudado por simulações numéricas e a técnica de controle de corrente proposta é verificada também de forma experimental em um protótipo de laboratório de 6,5 kVA
Transient voltage distribution along LV motor windings fed with PWM converters. Insulation ageing analysis
No full textConversor CC-CC trifásico ZVS PWM de três níveis
No full textTese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia Elétrica.Esta tese propõe uma topologia de conversor CC-CC trifásico de três níveis, modulado por largura de pulso e com comutação suave. A análise estática dos sete modos de condução descontínua e dos seis modos de condução contínua é apresentada no decorrer do documento. A análise dinâmica do conversor voltada para a operação em condução contínua também é apresentada, fornecendo a função de transferência que relaciona a tensão de saída com a razão cíclica. A comutação dos doze interruptores comandados é investigada em detalhes, enfatizando-se a condução contínua, onde há a possibilidade de obtenção de comutação suave do tipo ZVS em todos os doze interruptores do conversor. Para a comprovação das análises apresentadas, realiza-se a simulação numérica e a experimentação através de um protótipo de 10 kW, 800 V de tensão de entrada, 200 V de tensão de saída e 40 kHz do conversor proposto.This thesis proposes a three-level three-phase pulse-width-modulated soft-switched DC-DC converter. The static analysis of seven discontinuous conduction modes and six continuous conduction modes is carried out. The dynamic analysis is also performed, providing the output voltage to duty cycle transfer function. The commutation of all converter switches is detailed, emphasizing the continuous conduction, where there is the possibility of achieving zero-voltage-switching for all the twelve converter switches. In order to validate the presented analyses, the computer aided simulation and experimental results from a prototype rated at 10 kW, 800 V input voltage, 200 V output voltage, and 40 kHz are performed
Iron Losses Prediction with PWM Supply Using Low and High Frequency Measurements: Analysis and Results Comparison
No full textIn this paper, two different methods for iron loss prediction are analyzed. The first method is based on the classical separation of loss contributions (hysteresis, eddy-current, and excess losses). The model requires loss contribution separation using iron loss measurements with sinusoidal supply. In this paper, this method will be called the ldquolow-frequency method.rdquo The second method, named the ldquohigh-frequency method,rdquo is based on the assumption that, under pulsewidth modulation supply, the higher order flux density harmonics do not influence the magnetic work conditions. These magnetic conditions depend only on the amplitude of the fundamental harmonic of the flux density. In this paper, both the proposed methodologies and the related measurements are described in detail, and the obtained results are compared with the experimental ones. The experimental results show that both methods allow getting excellent results. The high-frequency method is better than the lower one but requires a more complex test bench. Depending on the accuracy required by the user, the more handy method can be chosen, with the guarantee that the estimation errors will be lower than 5
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