This dissertation is devoted to a unified and comprehensive study of zero-current-transition (ZCT) soft-switching techniques for high-power three-phase PWM inverter applications. Major efforts in this study are as follows: 1) Conception of one new ZCT scheme and one new ZCT topology; 2) Systematic comparison of a family of ZCT inverters; 3) Design, implementation and experimental evaluation of two 55-kW prototype inverters for electric vehicle (EV) motor drives that are developed based on the proposed ZCT concepts; and 4) Investigation of the ZCT concepts in megawatts high-frequency power conversions. The proposed ZCT techniques are also applicable to three-phase power-factor-correction (PFC) rectifiers. In order to minimize switching losses, this work first proposes a new control scheme for an existing three-phase ZCT inverter circuit that uses six auxiliary switches. The proposed scheme, called the six-switch ZV/ZCT, enables all main switches, diodes and auxiliary switches to be turned off under zero-current conditions, and in the meantime provides an opportunity to achieve zero-voltage turn-on for the main switches. Meanwhile, it requires no modification to normal PWM algorithms. Compared with existing ZCT schemes, the diode reverse-recovery current is reduced significantly, the switching turn-on loss is reduced by 50%, the resonant capacitor voltage stress is reduced by 30%, and the current and thermal stresses in the auxiliary switches are evenly distributed. However, a big drawback of the six-switch ZV/ZCT topology, as well as of other types of soft-switching topologies using six auxiliary switches, is the high cost and large space associated with the auxiliary switches. To overcome this drawback, this work further proposes a new three-phase ZCT inverter topology that uses only three auxiliary switches-- the three-switch ZCT. The significance of the proposed three-switch ZCT topology is that, among three-phase soft-switching inverters developed so far, this is the only one that uses fewer than six auxiliary switches and still has the following three features: 1) soft commutation for all main switches, diodes and auxiliary switches in all operation modes; 2) no modification to normal PWM algorithms; and 3) in practical implementations, no need for extra resonant current sensing, saturable cores, or snubbers to protect the auxiliary switches. The proposed six-switch ZV/ZCT and three-switch ZCT inverters, together with existing ZCT inverters, constitute a family of three-phase ZCT inverters. To explore the fundamental properties of these inverters, a systematic comparative study is conducted. A simplified equivalent circuit is developed to unify common traits of ZCT commutations. With the visual aid of state planes, the evolution of the family of ZCT inverters is examined, and their differences and connections are identified. Behaviors of individual inverters, including switching conditions, circulating energy, and device/component stresses, are compared. Based on the proposed six-switch ZV/ZCT and three-switch ZCT techniques, two 55-kW prototype inverters for EV traction motor drives have been built and tested to the full-power level with a closed-loop controlled induction motor dynamometer. The desired ZCT soft-switching features are realized together with motor drive functions. A research effort is carried out to develop a systematic and practical design methodology for the ZCT inverters, and an experimental evaluation of the ZCT techniques in the EV motor drive application is conducted. The design approach integrates system optimization with characterizations of the main IGBT device under the ZCT conditions, selection, testing and characterization of the auxiliary devices, design and selection of the resonant inductors and capacitors, inverter loss modeling and numerical analysis, system-level operation aspects, and layout and parasitic considerations. Different design aspects between these two ZCT inverters are compared and elaborated. The complexity of the 55-kW prototype implementations is compared as well. Efficiencies are measured and compared under a group of torque/speed points for typical EV drive cycles. Megawatts high-frequency power conversion is another potential application of the ZCT techniques. The integrated gate commutated thyristor (IGCT) device is tested and characterized under the proposed six-switch ZV/ZCT condition, and the test shows promising results in reducing switching losses and stresses. Improvements in the IGCT switching frequency and simplification of the cooling requirements under ZCT operations are discussed. In addition, a generalized ZCT cell concept is developed based on the proposed three-switch ZCT topology. This concept leads to the discovery of a family of simplified multilevel soft-switching inverters that reduce the number of auxiliary switches by half, and still maintain desirable features. / Ph. D.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This thesis contributes to the analysis, project and implementation of discrete-time controllers based on the Internal Model Principle, aiming to improve the performance of three-leg three-phase PWM inverters with insulating output transformer usually found in the output stage of medium power double-conversion UPS. Initially, an investigation about issues concerning to the discrete-time models of three-phase inverters with space vector modulation and the LC filter have been carried out. This investigation shows the impact of different switching sequences, sampling instants and filter cut-off frequency on the harmonic spectrum of sampled variables used for feedback. This is a concern when the switching frequency is low to limit the switching losses as in medium and high power UPS. In this way, this thesis proposes sampling methods that make possible to reduce the low order harmonics on the interest variables, which is relevant when state feedback is used. These methods allow to obtain discrete-time average linear models useful for the controller design. In addition, the saturation of the insulating transformer is addressed in details. The dc component arising from the digital implementation and from the circuit measures non idealities, and then amplified by an inadequate choice of the controller, may lead the transformer to saturate. In order to solve this problem, and in accordance with the Internal Model Principle, internal models adequate to the plant under consideration as well as discrete-time voltage controllers in stationary and synchronous frames which are not prone to amplify the dc component, are proposed. Another goal of this thesis is the improvement of the UPS output voltage transient response due to linear and non linear load steps. This is obtained considering the sampling methods before mentioned, combined with internal models with reduced number of poles and low sampling rate. It is demonstrated that it is possible to improve significantly the output voltage transient responses, as well as to satisfy the rigorous classification of the standard IEC62040-3 for UPS without degrading the steady state performance. In addition, these control structures have enough stability margins, as proved in each case, and they results in simple and attractive solutions to be implemented in 8 or 16 bits fixed-point arithmetic microcontrollers and DSP with reduced memory space. The feasibility of the proposed solutions is verified with experimental results demonstrating both transient and steady-state performances. Finally, a comparative analysis of the proposed control structures, over the light of the standard IEC62040-3, is presented. / A presente tese de doutorado contribui para a análise, projeto e implementação de controladores discretos baseados no Princípio do Modelo Interno, com o intuito de aprimorar o desempenho de inversores trifásicos PWM a três braços com transformador isolador usualmente utilizados no estágio de saída de UPS de dupla conversão de média potência. Inicialmente se realiza uma investigação sobre os aspectos relacionados à obtenção do modelo discreto de inversores trifásicos com modulação space vector e do filtro de saída. Essa investigação mostra o impacto de diferentes seqüências de comutação, instantes de amostragem e freqüência de corte do filtro no conteúdo harmônico das variáveis amostradas usadas para realimentação. Isso é uma preocupação quando a freqüência de comutação é baixa para limitar as perdas em UPS de média e alta potência. Nesse sentido essa tese propõe métodos de amostragem que possibilitam reduzir as harmônicas de baixa ordem nas variáveis de interesse o que adquire importância quando se realiza a realimentação dos estados da planta. Esses métodos permitem a obtenção de modelos médios lineares úteis para o projeto do controlador. Além disso, a saturação do transformador de isolação é analisada em detalhe. A componente contínua oriunda da implementação digital e das não idealidades dos circuitos de medição, e amplificada por uma escolha inadequada do controlador, pode levar o transformador à saturação. Com o intuito de solucionar esse problema, e de acordo com a teoria do Princípio do Modelo Interno, são propostos modelos internos adequados à planta em questão bem como controladores discretos de tensão em eixos estacionários e síncronos, que não tem tendência a amplificar a componente contínua. Outro dos objetivos dessa tese é o aprimoramento da resposta transitória das tensões de saída da UPS na presença de degraus de carga linear e não linear. Isso é obtido considerando-se os métodos de amostragem mencionados acima, combinado com modelos internos com número de pólos e taxa de amostragem reduzidos. Demonstra-se que é possível aprimorar significativamente a resposta transitória das tensões de saída bem como satisfazer a exigente classificação da norma IEC62040-3 para UPS, sem degradar o desempenho de regime permanente. Além disso, essas estruturas de controle possuem suficiente margem de estabilidade, como provado em cada caso, e estas resultam em soluções simples e atrativas para serem implementadas em microcontroladores e DSP de aritmética de ponto fixo, com palavras de 8 e 16 bits e capacidade reduzida de memória. A viabilidade prática das propostas realizadas é verificada com resultados experimentais em regime permanente e transitório. Finalmente, apresenta-se uma análise comparativa das estruturas de controle propostas, sob o enfoque da norma IEC62040-3.