Power Quality in Electrified Transportation Systems

"Power Quality in Electrified Transportation Systems" has covered interesting horizontal topics over diversified transportation technologies, ranging from railways to electric vehicles and ships. Although the attention is chiefly focused on typical railway issues such as harmonics, resonances and reactive power flow compensation, the integration of electric vehicles plays a significant role. The book is completed by some additional significant contributions, focusing on the interpretation of Power Quality phenomena propagation in railways using the fundamentals of electromagnetic theory and on electric ships in the light of the latest standardization efforts

Control of modular multilevel converters in high voltage direct current power systems

This thesis focuses on a comprehensive analysis of Modular Multilevel Converters (MMC) in High Voltage Direct Current (HVDC) applications from the viewpoint of presenting new mathematical dynamic models and designing novel control strategies. In the first step, two new mathematical dynamic models using differential flatness theory (DFT) and circulating currents components are introduced. Moreover, detailed step-by-step analysis-based relationships are achieved for accurate control of MMCs in both inverter and rectifier operating modes. After presenting these new mathematical equations-based descriptions of MMCs, suitable control techniques are designed in the next step. Because of the nonlinearity features of MMCs, two nonlinear control strategies based on direct Lyapunov method (DLM) and passivity theory-based controller combined with sliding mode surface are designed by the use of circulating currents componentsbased dynamic model to provide a stable operation of MMCs in HVDC applications under various operating conditions. The negative effects of the input disturbance, model errors and system uncertainties are suppressed by defining a Lyapunov control function to reach the integralproportional terms of the flat output errors that should be finally added to the initial inputs. Simulation results in MATLAB/SIMULINK environment verify the positive effects of the proposed dynamic models and control strategies in all operating conditions of the MMCs in inverter mode, rectifier mode and HVDC applications.Esta tese visa proceder a uma análise abrangente de conversores multinível modulares (MMC) para transmissão a alta tensão em corrente contínua (HVDC), almejando apresentar novos modelos matemáticos em sistemas dinâmicos e projetar novas estratégias de controlo. Na primeira etapa são introduzidos dois novos modelos matemáticos dinâmicos que usam differential flatness theory e as componentes de correntes circulantes. Ainda, é estabelecida uma modelação matemática para o controlo preciso dos MMCs, operando em modo inversor ou modo retificador. Depois de apresentar as novas equações matemáticas, as técnicas de controlo mais adequadas são delineadas. Devido às características não lineares dos MMCs, são projetadas duas estratégias de controlo não-lineares baseadas no método direto de Lyapunov e no controlo do tipo passivity theory-based combinado com controlo por modo de deslizamento através do uso de modelos dinâmicos baseados em correntes circulantes para fornecer uma operação estável aos MMCs em aplicações de HVDC sob várias condições de operação. Os efeitos negativos das perturbações de entrada, erros de modelação e incertezas do sistema são suprimidos através da definição da função de controlo de Lyapunov para alcançar os termos de integraçãoproporcionalidade dos erros de saída para que possam finalmente ser adicionados às entradas iniciais. Os resultados da simulação computacional realizados em ambiente MATLAB/SIMULINK verificam os efeitos positivos dos modelos dinâmicos propostos e das novas estratégias de controlo em todas as condições de operação dos MMCs no modo inversor, retificador e em aplicações HVDC

Comparative Analysis of Multilevel Converters for Medium-Voltage Applications

The electric energy demand has been steadily growing during the last century, and all forecasts indicate that it will keep growing in the following years. Within this frame, and due to all the problems that this demand increase generate in the environment, it is necessary improving the current techniques of electric energy conversion and transmission in order to increase the whole system efficiency. On the other hand, it is also necessary increasing the renewable energy resources exploitation through more efficient generation systems. According to these lines, the power electronics systems that have been installed in the last decades allowed to obtain better efficiency from the renewable natural resources like the wind or the solar power. These systems have also notably improved the quality of the power supplied, reducing the losses through what are known as power quality applications. Power converters are currently essential in any power electronics system. Within them, the multilevel converters specially suppose a breakthrough compared with the classical two level converters, as they allow obtaining voltage and current signals with lower harmonic content, what means fewer losses in high power medium voltage applications. In this Thesis a comparative study of some multilevel converter topologies normally used in high power medium voltage applications is done. The objective is analyzing in detail each topology and comparing it with the rest following different criteria, with the aim to know the advantages and drawbacks of each one and to realize which one is more suitable for each application

Operation and control of cascaded H-bridge converter for STATCOM application

In the last decade, particular attention has been paid to the use of Modular Multilevel Converters (MMC) for grid applications. In particular, for STATCOM applications the phase leg of the converter is constituted by a number of single-phase full-bridge converters connected in cascade (here named Cascaded H-Bridge, CHB, converter). This multilevel converter topology is today considered the industrial standard for STATCOM applications and has replaced other converter topologies, mainly due to its small footprint, high achievable voltage levels (allowing transformer-less operation), modularity and reduced losses. However, there are still areas of research that need to be investigated in order to improve the performance and the operational range of this converter topology for grid-applications. The aim of this thesis is to explore control and modulation schemes for the CHB-STATCOM, both under balanced and unbalanced conditions of the grid, highlighting the advantages but also the challenges and possible pitfalls that this kind of topology presents for this specific application. The first part of the thesis is dedicated to the two main modulation techniques for the CHBSTATCOM: the Phase-Shifted PulseWidthModulation (PS-PWM) and the Level-Shifted PWM (LS-PWM) with cells sorting. In particular, the focus is on the impact of the adopted modulation on the active power distribution on the individual cells of the converter. When using PS-PWM, it is shown that non-ideal cancellation of the switching harmonics leads to a non-uniform active power distribution among the cells and thereby to the need for an additional control loop for individual DC-link voltage balancing. Theoretical analysis proves that a proper selection of the frequency modulation ratio leads to a more even power distribution over time, which in turns alleviates the role of the individual balancing control. Both PS-PWM and cells sorting schemes fail in cell voltage balancing when the converter is not exchanging reactive power with the grid (converter in zero-current mode). To overcome this problem, two methods for individual DC-link voltage balancing at zero-current mode are proposed and verified. Then, the thesis focuses on the operation of the CHB-STATCOM under unbalanced conditions. It is shown analytically that regardless of the configuration utilized for the CHB-STATCOM (star or in delta configuration), a singularity exists when trying to guarantee balancing in the DC-link capacitor voltages. In particular, it is shown that the star configuration is sensitive to the level of unbalance in the current exchanged with the grid, with a singularity in the solution when positive- and negative-sequence currents have the same magnitude. Similar results are found for the delta configuration where, in a pure duality with the star configuration, the system is found to be sensitive to the level of unbalance in the applied voltage. The presence of these singularities represents an important limit of this topology for STATCOM applications

Design and Control of Power Converters for High Power-Quality Interface with Utility and Aviation Grids

Power electronics as a subject integrating power devices, electric and electronic circuits, control, and thermal and mechanic design, requires not only knowledge and engineering insight for each subarea, but also understanding of interface issues when incorporating these different areas into high performance converter design.Addressing these fundamental questions, the dissertation studies design and control issues in three types of power converters applied in low-frequency high-power transmission, medium-frequency converter emulated grid, and high-frequency high-density aviation grid, respectively, with the focus on discovering, understanding, and mitigating interface issues to improve power quality and converter performance, and to reduce the noise emission.For hybrid ac/dc power transmission,• Analyze the interface transformer saturation issue between ac and dc power flow under line unbalances.• Proposed both passive transformer design and active hybrid-line-impedance-conditioner to suppress this issue.For transmission line emulator,• Propose general transmission line emulation schemes with extension capability.• Analyze and actively suppress the effects of sensing/sampling bias and PWM ripple on emulation considering interfaced grid impedance.• Analyze the stability issue caused by interaction of the emulator and its interfaced impedance. A criterion that determines the stability and impedance boundary of the emulator is proposed.For aircraft battery charger,• Investigate architectures for dual-input and dual-output battery charger, and a three-level integrated topology using GaN devices is proposed to achieve high density.• Identify and analyze the mechanisms and impacts of high switching frequency, di/dt, dv/dt on sensing and power quality control; mitigate solutions are proposed.• Model and compensate the distortion due to charging transition of device junction capacitances in three-level converters.• Find the previously overlooked device junction capacitance of the nonactive devices in three-level converters, and analyze the impacts on switching loss, device stress, and current distortion. A loss calculation method is proposed using the data from the conventional double pulse tester.• Establish fundamental knowledge on performance degradation of EMI filters. The impacts and mechanisms of both inductive and capacitive coupling on different filter structures are understood. Characterization methodology including measuring, modeling, and prediction of filter insertion loss is proposed. Mitigation solutions are proposed to reduce inter-component coupling and self-parasitics

Development of a multilevel converter topology for transformer-less connection of renewable energy systems

The global need to reduce dependence on fossil fuels for electricity production has become an ongoing research theme in the last decade. Clean energy sources (such as wind energy and solar energy) have considerable potential to reduce reliance on fossil fuels and mitigate climate change. However, wind energy is going to become more mainstream due to technological advancement and geographical availability. Therefore, various technologies exist to maximize the inherent advantages of using wind energy conversion systems (WECSs) to generate electrical power. One important technology is the power electronics interface that enables the transfer and effective control of electrical power from the renewable energy source to the grid through the filter and isolation transformer. However, the transformer is bulky, generates losses, and is also very costly. Therefore, the term "transformer-less connection" refers to eliminating a step-up transformer from the WECS, while the power conversion stage performs the conventional functions of a transformer. Existing power converter configurations for transformer-less connection of a WECS are either based on the generator-converter configuration or three-stage power converter configuration. These configurations consist of conventional multilevel converter topologies and two-stage power conversion between the generator-side converter topology and the high-order filter connected to the collection point of the wind power plant (WPP). Thus, the complexity and cost of these existing configurations are significant at higher voltage and power ratings. Therefore, a single-stage multilevel converter topology is proposed to simplify the power conversion stage of a transformer-less WECS. Furthermore, the primary design challenges – such as multiple clamping devices, multiple dc-link capacitors, and series-connected power semiconductor devices – have been mitigated by the proposed converter topology. The proposed converter topology, known as the "tapped inductor quasi-Z-source nested neutral-point-clamped (NNPC) converter," has been analyzed, and designed, and a prototype of the topology developed for experimental verification. A field-programmable gate array (FPGA)-based modulation technique and voltage balancing control technique for maintaining the clamping capacitor voltages was developed. Hence, the proposed converter topology presents a single-stage power conversion configuration. Efficiency analysis of the proposed converter topology has been studied and compared to the intermediate and grid-side converter topology of a three-stage power converter configuration. A direct current (DC) component minimization technique to minimize the dc component generated by the proposed converter topology was investigated, developed, and verified experimentally. The proposed dc component minimization technique consists of a sensing and measurement circuitry with a digital notch filter. This thesis presents a detailed and comprehensive overview of the existing power converter configurations developed for transformer-less WECS applications. Based on the developed 2 comparative benchmark factor (CBF), the merits and demerits of each power converter configuration in terms of the component counts and grid compliance have been presented. In terms of cost comparison, the three-stage power converter configuration is more cost-effective than the generatorconverter configuration. Furthermore, the cost-benefit analysis of deploying a transformer-less WECSs in a WPP is evaluated and compared with conventional WECS in a WPP based on power converter configurations and collection system. Overall, the total cost of the collection system of WPP with transformer-less WECSs is about 23% less than the total cost of WPP with conventional WECs. The derivation and theoretical analysis of the proposed five-level tapped inductor quasi-Z-source NNPC converter topology have been presented, emphasizing its operating principles, steady-state analysis, and deriving equations to calculate its inductance and capacitance values. Furthermore, the FPGA implementation of the proposed converter topology was verified experimentally with a developed prototype of the topology. The efficiency of the proposed converter topology has been evaluated by varying the switching frequency and loads. Furthermore, the proposed converter topology is more efficient than the five-level DC-DC converter with a five-level diode-clamped converter (DCC) topology under the three-stage power converter configuration. Also, the cost analysis of the proposed converter topology and the conventional converter topology shows that it is more economical to deploy the proposed converter topology at the grid side of a transformer-less WECS