The emergence of silicon-carbide (SiC) devices has been a 'game changer' in the field of power electronics. With desirable material properties such as low-loss characteristics, high blocking voltage, and high junction temperature operation, they are expected to drastically increase the power density of power electronics systems. Recent state-of-the-art designs show the power density over 17 ; however, certain factors limit the power density to increase beyond this limit. In this dissertation, three key factors are selected to increase the power density of SiC-based grid-connected three-phase converters. Throughout this dissertation, the techniques and strategies to increase the power density of SiC three-phase converters were investigated. Firstly, a magnetic integration method was introduced for the coupled inductors in the interleaved three-phase converters. Due to limited current-capacity compared to the silicon insulated-gate bipolar transistors (Si-IGBTs), discrete SiC devices or SiC modules, operate in parallel to handle a large current. When three-phase inverters are paralleled, interleaving can be used, and coupled inductors are employed to limit the circulating current. In Chapter 2, the conventional integration method was extended to integrate three coupled inductors into two; one for differential-mode circulating current and the other for common-mode circulating current. By comparing with prior research work, a 20% reduction in size and weight is demonstrated. From Chapter 3 to Chapter 5, a full-SiC uninterruptible power supply (UPS) was investigated. With the high switching frequency and fast switching dynamics of SiC devices, strategies on electromagnetic inference become more important, compared to Si-IGBT based inverters. Chapter 3 focuses on a common-mode equivalent circuit model for a topology and pulse width modulation (PWM) scheme selection, to set a noise mitigation strategy in the design phase. A three terminal common-mode electromagnetic interference (EMI) model is proposed, which predicts the impact of the dc-dc stage and a large battery-rack on the output CM noise. Based on the model, severe deterioration of noise by the dc-dc stage and battery-rack can be predicted. Special attention was paid on the selection of the dc-dc stage's topology and the PWM scheme to minimize the impact. With the mitigation strategy, a maximum 16 dB reduction on CM EMI can be achieved for a wide frequency range. In Chapter 4, an active PWM scheme for a full-SiC three-level back-to-back converter was proposed. The PWM scheme targets the size reduction of two key components: dc-link capacitors and a common-mode EMI filter. The increase in switching frequency calls for a large common-mode EMI filter, and dc-link capacitors in the three-level topology may take a considerable portion in the total volume. To reduce the common-mode noise emission, different combinations of the voltage vectors are investigated to generate center-aligned single pulse common-mode voltage. By such an alignment of common-mode voltage with different vector combinations, noise cancellation between the rectifier and the inverter can be maximally utilized, while the balancing of neutral point voltage can be achieved by the transition between the combinations. Also, to reduce the size of the dc-link capacitor for the three-level back-to-back converter, a compensation algorithm for neutral point voltage unbalance was developed for both differential-mode voltage and the common-mode voltage of the ac-ac stage. The experimental results show a 4 dB reduction on CM EMI, which leads to a 30% reduction on the required CM inductance value. When a 10% variation of neutral point voltage can be handled, the dc-link capacitance can be reduced by 56%. In Chapter 5, a 20 kW full-SiC UPS prototype was built to demonstrate a possible size-reduction with the proposed PWM scheme, as well as a selection of topologies and PWM schemes based on the model. The power density and efficiency are compared with the state-of-the-art Si-IGBT based UPSs. Chapter 6 seeks to improve power density by a change in a modulation method. Triangular conduction mode (TCM) operation of the three-level full-SiC inverter was investigated. The switching loss of SiC devices is reported to be concentrated on the turn-on instant. With zero-voltage turn-on of all switches, the switching frequency of a three-level three-phase SiC inverter can be drastically increased, compared to the hard-switching operation. This contributes to the size-reduction of the filter inductors and EMI filters. Based on the design to achieve a 99% peak efficiency, a comparison was made with a full-SiC three-level inverter, operating in continuous conduction mode (CCM), to verify the benefit of the soft switching scheme on the power density. A design procedure for an LCL filter of paralleled TCM inverters was developed. With 3.5 times high switching frequency, the total weight of the filter stage of the TCM inverter can be reduced by 15%, compared to that of the CCM inverter. Throughout this dissertation, techniques for size reduction of key components are introduced, including coupled inductors in parallel inverters, an EMI filter, dc-link capacitors, and the main boost inductor. From Chapter 2 to 5, the physical size or required value of these key components could be reduced by 20% to 56% by different schemes such as magnetic integration, EMI mitigation strategy through modeling, and an active PWM scheme. An optimization result for a full-SiC UPS showed a 40% decrease in the total volume, compared to the state-of-the-art Si-IGBT solution. Soft-switching modulation for SiC-based three-phase inverters can bring a significant increase in the switching frequency and has the potential to enhance power-density notably. A three-level three-phase full-SiC 40 kW PV inverter with TCM operation contributed to a 15% reduction on the filter weight. / Doctor of Philosophy / The power density of a power electronics system is regarded as an indicator of technological advances. The higher the power density of the power supply, the more power it can generate with the given volume and weight. The size requirement on power electronics has been driven towards tighter limits, as the dependency on electric energy increases with the electrification of transportation and the emergence of grid-connected renewable energy sources. However, the efficiency of a power electronics system is an essential factor and is regarded as a trade-off with the power density. The size of power electronics systems is largely impacted by its magnetic components for filtering, as well as its cooling system, such as a heatsink. Once the switching frequency of power semiconductors is increased to lower the burden on filtering, more loss is generated from filters and semiconductors, thus enlarging the size of the cooling system. Therefore, considering the efficiency has to be maintained at a reasonable value, the power density of Si-based converters appears to be saturated. With the emergence of wide-bandgap devices such as silicon carbide (SiC) or gallium nitride (GaN), the switching frequency of power devices can be significantly increased. This is a result of superior material properties, compared to Si-based power semiconductors. For grid-connected applications, SiC devices are adopted, due to the limitations of voltage ratings in GaN devices. Before commercial SiC devices were available, the power density of SiC- based three-phase inverters was expected to go over 20 𝑘𝑊 𝑑𝑚3 ⁄ . However, the state-of-the art designs shows the power density around 3 ~ 4 𝑘𝑊 𝑑𝑚3 ⁄ , and at most 17 𝑘𝑊 𝑑𝑚3 ⁄ . The SiC devices could increase the power density, but they have not reached the level expected. The adoption of SiC devices with faster switching was not a panacea for power density improvement. This dissertation starts with an analysis of the factors that prevent power density improvement of SiC-based, grid-connected, three-phase inverters. Three factors were identified: a limited increase in the switching frequency, large high-frequency noise generation to be filtered, and smaller but still significant magnetic components. Using a generic design procedure for three-phase inverters, each chapter seeks to frame a strategy and develop techniques to enhance the power density. For smaller magnetic components, a magnetic integration scheme is proposed for paralleled ac-dc converters. To reduce the size of the noise filter, an accurate modeling approach was taken to predict the noise phenomena during the design phase. Also, a modulation scheme to minimize the noise generation of the ac-ac stage is proposed. The validity of the proposed technique was verified by a full-SiC three-phase uninterruptible power supply with optimized hardware design. Lastly, the benefit of soft-switching modulation, which leads to a significant increase in switching frequency, was analyzed. The hardware optimization procedure was developed and compared to hard-switched three-phase inverters.

A single-layer neural network-based voltage compensation technique which generates minimum-distortion sinusoidal output voltages from a three-phase PWM inverter used for uninterruptible power supplies (UPS) is described. The proposed compensation technique is implemented in a microprocessor-based controller constructed in the stationary d-q frame where the controller sampling rate is twice the inverter switching frequency. The structure of a feed-forward artificial neural network connects network inputs and outputs through multiple linear or nonlinear neuron models, and processes these input/output data associations in a parallel distributed manner. Network inputs in the form of UPS load voltage commands and load current feedback are propagated forward in the network each controller sampling period generating the inverter output voltage commands, the network outputs, which are converted to three phase inverter switching Signals using the space vector PWM waveform generation process. / Ph. D.

The goal of this dissertation is to evaluate the effectiveness of model predictive control (MPC) for a magnetically suspended flywheel energy storage uninterruptible power supply (FlyUPS). The reason this research topic was selected was to determine if an advanced control technique such as MPC could perform better than a classical control approach such as decentralised Proportional-plus-Differential (PD) control. Based on a literature study of the FlyUPS system and the MPC strategies available, two MPC strategies were used to design two possible MPC controllers were designed for the FlyUPS, namely a classical MPC algorithm that incorporates optimisation techniques and the MPC algorithm used in the MATLAB® MPC toolbox™. In order to take the restrictions of the system into consideration, the model used to derive the controllers was reduced to an order of ten according to the Hankel singular value decomposition of the model. Simulation results indicated that the first controller based on a classical MPC algorithm and optimisation techniques was not verified as a viable control strategy to be implemented on the physical FlyUPS system due to difficulties obtaining the desired response. The second controller derived using the MATLAB® MPC toolbox™ was verified to be a viable control strategy for the FlyUPS by delivering good performance in simulation. The verified MPC controller was then implemented on the FlyUPS. This implementation was then analysed in order to validate that the controller operates as expected through a comparison of the simulation and implementation results. Further analysis was then done by comparing the performance of MPC with decentralised PD control in order to determine the advantages and limitations of using MPC on the FlyUPS. The advantages indicated by the evaluation include the simplicity of the design of the controller that follows directly from the specifications of the system and the dynamics of the system, and the good performance of the controller within the parameters of the controller design. The limitations identified during this evaluation include the high computational load that requires a relatively long execution time, and the inability of the MPC controller to adapt to unmodelled system dynamics. Based on this evaluation MPC can be seen as a viable control strategy for the FlyUPS, however more research is needed to optimise the MPC approach to yield significant advantages over other control techniques such as decentralised PD control. / Thesis (MIng (Computer and Electronic Engineering))--North-West University, Potchefstroom Campus, 2013

The goal of this dissertation is to evaluate the effectiveness of model predictive control (MPC) for a magnetically suspended flywheel energy storage uninterruptible power supply (FlyUPS). The reason this research topic was selected was to determine if an advanced control technique such as MPC could perform better than a classical control approach such as decentralised Proportional-plus-Differential (PD) control. Based on a literature study of the FlyUPS system and the MPC strategies available, two MPC strategies were used to design two possible MPC controllers were designed for the FlyUPS, namely a classical MPC algorithm that incorporates optimisation techniques and the MPC algorithm used in the MATLAB® MPC toolbox™. In order to take the restrictions of the system into consideration, the model used to derive the controllers was reduced to an order of ten according to the Hankel singular value decomposition of the model. Simulation results indicated that the first controller based on a classical MPC algorithm and optimisation techniques was not verified as a viable control strategy to be implemented on the physical FlyUPS system due to difficulties obtaining the desired response. The second controller derived using the MATLAB® MPC toolbox™ was verified to be a viable control strategy for the FlyUPS by delivering good performance in simulation. The verified MPC controller was then implemented on the FlyUPS. This implementation was then analysed in order to validate that the controller operates as expected through a comparison of the simulation and implementation results. Further analysis was then done by comparing the performance of MPC with decentralised PD control in order to determine the advantages and limitations of using MPC on the FlyUPS. The advantages indicated by the evaluation include the simplicity of the design of the controller that follows directly from the specifications of the system and the dynamics of the system, and the good performance of the controller within the parameters of the controller design. The limitations identified during this evaluation include the high computational load that requires a relatively long execution time, and the inability of the MPC controller to adapt to unmodelled system dynamics. Based on this evaluation MPC can be seen as a viable control strategy for the FlyUPS, however more research is needed to optimise the MPC approach to yield significant advantages over other control techniques such as decentralised PD control. / Thesis (MIng (Computer and Electronic Engineering))--North-West University, Potchefstroom Campus, 2013

Thesis (M.S.) -- Mississippi State University. Department of Electrical and Computer Engineering. / Title from title screen. Includes bibliographical references.

Thesis (M.S.) -- Mississippi State University. Department of Electrical and Computer Engineering. / Title from title screen. Includes bibliographical references.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This master s thesis proposes a three-phase transformerless double-conversion UPS with digital control system and reduced overall cost. The topology is composed of an input stage, a battery bank, an inverter, an auxiliary circuit and transfer switches. The input stage assumes the three-phase rectifier function with LCL filter in normal operation mode and is responsible for discharge the battery bank in backup mode. This configuration reduces the system cost since an available circuit is used to discharge the battery bank. In conventional double-conversion UPS topologies the input stage remains inactive during the backup mode and an additional circuit is employed for this purpose. The auxiliary circuit is formed by an inductor and a switching leg and is used as a battery charger during normal operation mode and is additionally used to balance the bus capacitors voltages in backup mode. In this configuration, the auxiliary circuit can be designed for only a fraction of the UPS rated power, also providing lower total system cost. Transfer switches change the configuration of the input stage and the auxiliary circuit according to the UPS operating mode. The detailed operation of the topology, modelling, digital control structure, and the results obtained by simulation are showed. Finally a 20 kVA prototype is implemented and experimental results are acquired for the validation of the employed design methodology as well as the proposed circuit functionality. / Esta dissertação de mestrado propõem uma topologia de fonte ininterrupta de energia (UPS - Uninterruptible Power Supplies) trifásica de dupla conversão sem transformador com custo reduzido e sistema de controle digital. A UPS proposta é composta por um estágio de entrada, um banco de baterias, um inversor, um circuito auxiliar e chaves de transferência. O estágio de entrada assume a função de retificador trifásico com filtro LCL em modo normal de operação da UPS e é responsável pela descarga do banco de baterias em modo bateria de operação. Esta configuração reduz o custo do sistema, pois permite que seja aproveitado um circuito disponível e dimensionado para a potência nominal da UPS para a descarga do banco de baterias. Em topologias convencionais de UPSs de dupla conversão o estágio de entrada permanece ocioso durante este modo de operação e um circuito adicional é empregado para este fim. O circuito auxiliar é formado por um indutor e um braço de interruptores e é utilizado como carregador de baterias em modo normal de operação e adicionalmente é utilizado para realizar o equilíbrio das tensões dos capacitores de barramento em modo bateria. Com esta configuração, o circuito auxiliar pode ser dimensionado para apenas uma fração da potência nominal da UPS, propiciando também a redução do custo total do sistema. As chaves de transferência alteram as configurações do estágio de entrada e do circuito auxiliar de acordo com o modo de operação da UPS. É apresentada a operação detalhada da topologia, a modelagem, a estrutura de controle digital utilizada e os resultados obtidos via simulação. Por fim, um protótipo de 20 kVA é implementado e resultados experimentais são adquiridos para a validação da metodologia de projeto empregada bem como da funcionalidade do circuito proposto.

This thesis deals with lead-acid battery, its parameters and temperature behaviour of this electro-chemical source of power. In the theory section, types of batteries are presented, followed by their primary parameters and electro-chemical reactions, which take part in battery cells. This section also contains information about charging stages, detailed description of each battery part, used materials, construction of the battery and degradation processes caused by battery operation. The practictal section of this thesis offers a measurement scheme for automated measuring workstation. Furthermore, the practical section presents results of measurements on parameters of chosen lead-acid battery types, it also contains long-term deep battery cycling and measurements under elevated temperatures. Detailed comparison of measurements results and datasheet values is included. Conclusion sums up parameters of chosen lead-acid battery types and their feasibility for further usage in power backuping.

The aim of this thesis is a description of a proposal to ensure safety of DC in power infrastructure and its impact on economics of an enterprise. The first part is focused on the analysis of the whole situation. The work also contains other possibilities of solution with respect to theoretical resources. The attention is also drawn to a particular procedure to minimize the financial loss of the company caused by a power failure and to a plan for providing maximum protection against this power failure.

Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / Grid-connected photovoltaic systems with energy storage have features of conventional gridconnected inverters and uninterruptible power supplies. In these systems, all elements, which include photovoltaic generator, batteries, loads and electric grid, are connected to the same equipment named multifunctional photovoltaic converter. As it is a recent technology, there are many research opportunities in this theme, from classifications and standards to specific problems. This study investigates this field in a comprehensive manner, proposing classifications, technical recommendations and a power management strategy. Initially, classifications are proposed considering system configuration and continuity of supply to the loads. In the following, technical recommendations related to grid and load compatibility, as well as safety technical recommendations, are proposed. The last proposal consists on a power management strategy which defines the operation mode of the static converters and system elements as a function of the energy scarcity. Experimental results of the power management are presented in order to validate the proposals. In this way, this thesis contributes to the development of multifunctional photovoltaic converters technology so that the distributed generation becomes more independent of solar availability and can better collaborate with the electric system. / Sistemas fotovoltaicos conectados à rede com armazenamento de energia são aqueles que conjugam características dos inversores conectados à rede convencionais e de fontes ininterruptas de energia. Nesses sistemas, todos os seus elementos, que incluem o gerador fotovoltaico, as baterias, as cargas e a rede elétrica, são conectados a um mesmo equipamento chamado de conversor fotovoltaico multifuncional. Por se tratar de uma tecnologia recente, existem muitas oportunidades de pesquisa inédita no tema, desde premissas e classificações básicas até a solução de problemas mais específicos. Este trabalho investiga o tema de maneira abrangente, propondo classificações, recomendações técnicas e uma estratégia para gerenciamento interno de potência dos conversores fotovoltaicos multifuncionais. Inicialmente, classificações são propostas considerando a configuração do sistema e a continuidade de fornecimento de energia às cargas. A seguir, são propostas recomendações técnicas relacionadas à compatibilidade com a rede e com as cargas, além de recomendações técnicas de segurança, baseando-se em normas existentes. A última proposta consiste em uma estratégia de gerenciamento interno de potência que define os modos de operação dos conversores estáticos e dos elementos do sistema em função da escassez de energia. Resultados experimentais do método proposto para o gerenciamento interno de potências são apresentados com o objetivo de validar as propostas desenvolvidas. Dessa forma, o presente trabalho contribui com o desenvolvimento da tecnologia de conversores fotovoltaicos multifuncionais, para que a geração distribuída de energia elétrica se torne mais independente da disponibilidade solar e possa melhor colaborar com o sistema elétrico.