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This book presents novel contributions in the development of solid-state-transformer (SST) technology both for medium-voltage (MV) and low-voltage (LV) utility grid interfaces, which can potentially augment the grid modernization process in the evolving power system paradigm. For the MV interface, a single-stage AC-DC SST submodule topology has been proposed, and its modulation and soft-switching possibilities are analysed, experimentally validated and adequately benchmarked. A control scheme with power balance capability among submodules is developed for MV grid-connected single-stage AC-DC SST for smooth operation under inevitable parameter drift scenario, and experimental validation shows excellent performance under drastic load change conditions. A novel machine learning-aided multi-objective design optimization framework for grid-connected SST is developed and experimentally validated, which equips a power electronics design engineer with meagre computational resources to find out the most optimal SST design in a convenient time-frame. This book has also contributed towards the development of dual-active-bridge (DAB)-type and non-DAB-type LV grid-interfaced isolated AC-DC converters by providing solutions to specific topology and modulation-related shortcomings in these two types of topologies. A comprehensive comparison of the DAB and non-DAB-type LVAC-LVDC converters reveals the superiority of DAB-type conversion strategy.
With the development of renewable energies, such as wind energy and solar energy, the dc power system becomes a promising candidate to manage and transfer the re-newable energy source, which stimulates the study of the dc-dc converters in the past decades. Among various dc-dc converters, the dual-active-bridge (DAB) dc-dc con-verter is regarded as one of the most promising candidates for the dc power conver-sion due to merits like isolated, high-efficiency, bidirectional, and ultrafast dynamic characteristics. Except the DAB dc-dc converter, there are some other isolated dc-dc converters such as full bridge dc-dc converter, three-phase DAB dc-dc converter, etc. They normally have similar dynamic characteristics as the DAB dc-dc converter featuring intermediary inductive ac-link (I2ACL) configuration. However, they are rarely investigated in existing literature, especially for better dynamic control performance. To fill such a gap, the dynamic equivalence between the DAB dc-dc converter and other I2ACL isolated dc-dc converters is revealed with the thorough overview of the existing I2ACL topologies in this work. Further, a unified fast-dynamic direct-current control scheme is proposed for significantly improving the dynamic performance of these I2ACL isolated dc-dc converters. With this predetermined analysis, the dynamic control schemes for the DAB-based dc-dc converter systems can be easily extended to other I2ACL converters with the same configurations. The single DAB dc-dc converter has been extensively investigated, but its modular-ized converter systems such as input-parallel output-parallel (IPOP), input-independent output-parallel (IIOP), in-put-parallel output-series (IPOS), and input-series output-parallel (ISOP) configurations have been seldomly covered in the existing research. Particularly, it is emergent to improve the dynamics, e.g. the input-voltage disturbance, the load-condition change and the power sharing disturbance. In this work, the advanced dynamic controls for these modular DAB dc-dc converter systems are proposed, featuring the flexible power sharing control performances with fast-dynamic responses. Moreover, to realize the reliable operation of these DAB-based systems, the hot swap operations are presented. To ensure the desired power sharing performance, the circuit-parameter estimating methods are proposed for these DAB-based converter systems. This work expands scope of the application of the DAB-based converter system in the partial power processing (PPP). Different from the existing literatures focusing on embedding renewable energy source into the strong ac system, this work proposes a PPP converter system, which can realize the independent control of the renewable energy source and the stabilization of the total dc bus. Combining with DAB module, the DAB-based PPP converter system is proposed. Then, as one of the important functions, the stabilization of the total dc bus should be further improved for this DAB-based converter system. In detail, a high-robustness control strategy is proposed to realize the fast-dynamic control, and the operation when one renewable energy source is out of work is also presented. Notably, the renewable energy should feature the current output and the limited output-voltage regulation such as PV, fuel cell and wind turbine with ac-dc conversion. By using the PV as an example, the effectiveness of the novel system is verified with following results: 1). The maximum power point tracking of the PV panels can be realized by using the existing method. 2). By using the proposed high-robustness control scheme, the total dc-link voltage can maintain at its desired value when the irradiance of PV panels, the voltage of the battery and the load condition are changed, and even when the PV panel is heavily shaded.
This thesis proposes and investigates application of a three-phase AC-AC Matrix Converter (MC), as an alternative to the conventional AC-DC-AC converter system, to interface a Micro-Turbine-Generator (MTG) unit as a Distributed Generation (DG) unit to a utility distribution grid. As compared with a conventional AC-DC-AC converter system, lack of storage elements in a MC results in a stronger coupling and interactions between the AC sides of the MC and thus necessitates more stringent control of the MC to prevent/mitigate such effects. This thesis develops a novel dynamic model of the MC to analytically investigate and quantify the interaction phenomenon and design controllers of the MC. This thesis introduces a novel voltage control strategy for the MC to enable operation of a MC-interfaced MTG (MTG-MC) unit in (i) a grid-connected mode, (ii) an autonomous (islanded) mode, and (iii) transition between the two modes. The control strategy also provides an inherent islanding detection method without non-detection zone, and disturbance ride-through capability. The proposed voltage controller is intended for operation of the MTG-MC unit under balanced grid/load conditions. The MC voltage controller is augmented with a negative-sequence current controller to enable the MTG-MC unit also to operate under unbalanced grid/load conditions as a DG unit. The studies reported in this thesis are based on eigen analyses of the overall system linearized dynamic model, in the MATLAB environment, and digital time-domain simulation studies of the system nonlinear model, in the PSCAD/EMTDC environment.
The book presents the analysis and control of numerous DC-DC converters widely used in several applications such as standalone, grid integration, and motor drives-based renewable energy systems. The book provides extensive simulation and practical analysis of recent and advanced DC-DC power converter topologies. This self-contained book contributes to DC-DC converters design, control techniques, and industrial as well as domestic applications of renewable energy systems. This volume will be useful for undergraduate/postgraduate students, energy planners, designers, system analysis, and system governors.
In this thesis we present a two-stage ac/dc grid-connected converter for computer applications. Also known as off-line power supplies, these converters have to meet various demanding specifications such as a wide input voltage range (typically 0-376 V), large voltage step down (typical output voltages range from 12-48 V), harmonic current limits and galvanic isolation. The focus of this work is in the reduction in volume of ac/dc converters while keeping efficiency constant or improving it, which is challenging to achieve while meeting all the specifications. The thesis breaks down the converter in subsystems and explores architectural and topological trade-offs, modeling, component selection and control methods. The performance of each individual subsystem is experimentally verified. The first stage of the converter is a step-down power factor correction (PFC) converter. This stage interacts with the grid and draws the necessary ac power from the line and rectifies it. Following the PFC is a capacitor bank, which is used to both buffer the ac power from the line and to provide hold-up energy to the output. The capacitor selection process is detailed in the thesis. The second stage of the converter provides isolation and regulation to the output. Two different approaches to the second stage converter are presented: using commercially available, "plug and play" converters and developing a custom converter. The full system is evaluated with both solutions and is compared to other state of the art converters. The final prototype achieves an efficiency of 95.33% at full power (250 W) and 230 Vac input, and a power density of 35 W/in3.
With the increasing penetrations of renewable energy resources, the energy storage system (ESS) is becoming necessary to minimize the impact of the variable power generation on the grid operation. Among many different types of storage, the battery energy storage system (BESS), mostly based on the Li-ion battery, is the fastest-growing due to the decreased system cost, and fast real and reactive power dispatch capabilities, which can be used for various applications such as voltage and frequency support, as well as for economic dispatch applications. BESS requires a bidirectional power electronics converter between the dc battery and the ac grid while having the capability to handle a wide range dc battery voltage. For interfacing with a three-phase ac grid, a single-stage configuration is used. The single-stage design can eliminate the ac grid side contactor, as it has a small ac-link capacitor and the inrush current is small. The three-phase single-stage design can also minimize the bulky dc-link capacitor. The proposed design includes three identical single-phase modules. Each module includes an unfolding bridge and a single-stage bidirectional (DAB) or series-resonant dual-activebridge (SR-DAB). This modular topology can also be used for both the single-phase and three-phase grid-connected BESS. The unfolding bridge will rectify the ac voltage to twice the line frequency in ac-dc operation (charging of the battery), or invert the voltage to the ac grid for dc-ac operation (discharging of the battery). In the meantime, DAB can convert ac energy with the absolute value of the sinusoidal voltage to the battery side or converter dc energy to the ac side with a high-frequency transformer, while providing zero voltage switching (ZVS) for the whole ac voltage range. A single-stage DAB topology is proposed in Chapter 2. The power flow for both directions is introduced and the novel combined dual phase shift modulation and variable frequency modulation are explained with advantages over the single phase shift modulation and fixed frequency modulation in terms of the inductor rootmean-square (rms) current. The power factor circuit (PFC) requirement and the ZVS constraints are investigated for the single-stage DAB with dual phase shift and variable frequency modulation. A novel online calculation control algorithm for the single-stage DAB is explained in Chapter 3. The control is proposed to minimize the dc low voltage side maximum turn-off current. A detailed explanation is provided for the control algorithm with the variable frequency and with a fixed frequency range. The extended ZVS ranges are proposed for the control algorithm to guarantee the ZVS over a wide range of the dc battery voltage and loads. A dual loop close loop control is introduced with its capability of dealing with the transit charging/discharging current response. An adaptive deadtime method is utilized to optimize the deadtime loss while working with a varying switching current over line period. A single-stage SR-DAB is proposed to further optimize the turn-off current and (rms) current of the single-stage topology, and is included in Chapter 4. The operation principle of a single-stage SR-DAB is proposed and its numerical expression of the equivalent model is analyzed with its PFC requirement and ZVS constraint investigated. The advantage of the dual phase shift and variable frequency control modulation is explained with its comparison over the single phase shift and fixed frequency modulation. An optimization algorithm is proposed aiming to minimize the system overall loss for the single-stage SR-DAB. A range of comparisons over the switching current and rms current between the single-stage DAB and SR-DAB are made, and the advantage of a single-stage SR-DAB is verified. A comprehensive loss model including a transistor loss such as conduction loss, switching loss, driving loss and deadtime loss, and magnetic loss such as transformer and inductor loss is introduced and well analyzed in Chapter 5. An optimization algorithm aimed to optimize the system loss is introduced based on the comprehensive loss model. With this algorithm, hardware optimizations are conducted and the optimal values of the transformer turns ratio, auxiliary inductor for a single-stage DAB, a resonant inductor and capacitor for a single-stage SR-DAB, the snubber capacitor for the dc low voltage side transistors are determined to ensure optimal performance of the converter. The ac side input capacitance and inductance are also determined to ensure a small switching voltage ripple and guarantee relay-less operation. In Chapter 6, the hardware that is utilized to verify the single-stage DAB and single-stage SR-DAB is explained in detail. Advanced implementation and switching performance of the power stage are presented. The system operation parameters as well as the major components used are included. Experimental results for single-stage DAB and single-stage SR-DAB at 1 kW and 2 kW single phase operation, and three-phase operation are displayed to verify the single-stage concept and present its performance. Thermal image, loss breakdown, and efficiency map/curve are presented. The single-stage DAB and single-stage SR-DAB provide a good solution for three-phase ac to dc battery with bidirectional power flow and requirement of isolation. The operation principle, PFC requirement, and ZVS constraint for both converters is well analyzed in the main content. A loss model is established and a hardware optimization is conducted to ensure the converter is operating at optimal efficiency. Experimental verification is included to verify the capability of the single-stage DAB and single-stage SR-DAB
The use of single-phase line-interfaced power converters in electrical power systems is rapidly growing due to the changing nature and power quality requirements of electrical loads. Most applications require these single-phase line-interfaced power converters to be compact and efficient, and depending on application meet additional performance, cost, and reliability targets. This thesis presents innovative system architectures, circuit topologies, design methodologies, and control strategies for highly compact and efficient single-phase ac-dc and ac-ac line-interfaced power converters. First, a comprehensive design methodology for step-down isolated two-stage ac-dc converters is presented which compares various designs and operating modes and selects the optimal design based on overall volume and efficiency. Additionally, a new control strategy is presented for a compact front-end soft-switched power-factor correction (PFC) stage to ensure compliance with strict electromagnetic interference (EMI) regulations. A 1-kW universal-input to 28 V-output isolated ac-dc prototype converter is built to showcase performance benefits of proposed design and control strategies. This prototype achieves a high-power-density of 84W/in3 and maintains greater than 93% efficiency across a wide output power range. Next, the functionality of the proposed ac-dc converter is further enhanced by incorporating a new droop control strategy for parallel operation of multiple similar ac-dc converter modules. The proposed control strategy uses the input current of the secondary dc-dc stage of two-stage ac-dc converters in conjunction with variable droop resistance to achieve near-perfect parallel operation. A multi-module ac-dc conversion system is built to validate the proposed droop control strategy. The parallel modules achieve a current distribution error of less than 2% near their maximum output power. Multiple ac-ac conversion applications are also addressed in this thesis. For highly cost-sensitive applications, two compact and efficient single-stage ac-ac converters are presented which utilize a comprehensive design methodology centered around minimizing the total cost of components. Moreover, innovative control strategies are presented for both ac-ac converters to enable output voltage regulation under input voltage and output load fluctuations. Both single-stage ac-ac prototype converters, utilizing the proposed design and control strategies, are built and tested. The 600-W 480 Vrms to 264 Vrms prototypes achieve power densities exceeding 40W/in3 while maintaining conversion efficiencies of greater than 96% across majority of the output load. Finally, a much more feature-rich ac-dc-ac converter is also proposed for advanced ac-ac conversion applications, such as data center online uninterruptible power supplies (UPS). The proposed transformer-less two-stage ac-ac converter is based on a new circuit topology which can operate at high switching frequencies (up to several MHz) and utilize 50% lower dc-bus capacitance than conventional split-dc-bus topologies. A 1-k VA 120 Vrms prototype ac-dc-ac converter is built and extensively tested to showcase performance improvements. This prototype achieves high peak conversion efficiency of greater than 95% and high power density of 26.4W/in3 while utilizing long-life but relatively bulky film dc-bus capacitors.
Microgrids are the most innovative area in the electric power industry today. Future microgrids could exist as energy-balanced cells within existing power distribution grids or stand-alone power networks within small communities. A definitive presentation on all aspects of microgrids, this text examines the operation of microgrids – their control concepts and advanced architectures including multi-microgrids. It takes a logical approach to overview the purpose and the technical aspects of microgrids, discussing the social, economic and environmental benefits to power system operation. The book also presents microgrid design and control issues, including protection and explaining how to implement centralized and decentralized control strategies. Key features: original, state-of-the-art research material written by internationally respected contributors unique case studies demonstrating success stories from real-world pilot sites from Europe, the Americas, Japan and China examines market and regulatory settings for microgrids, and provides evaluation results under standard test conditions a look to the future – technical solutions to maximize the value of distributed energy along with the principles and criteria for developing commercial and regulatory frameworks for microgrids Offering broad yet balanced coverage, this volume is an entry point to this very topical area of power delivery for electric power engineers familiar with medium and low voltage distribution systems, utility operators in microgrids, power systems researchers and academics. It is also a useful reference for system planners and operators, manufacturers and network operators, government regulators, and postgraduate power systems students. CONTRIBUTORS Thomas Degner Aris Dimeas Alfred Engler Nuno Gil Asier Gil de Muro Guillermo Jiménez-Estévez George Kariniotakis George Korres André Madureira Meiqin Mao Chris Marnay Jose Miguel Yarza Satoshi Morozumi Alexander Oudalov Frank van Overbeeke Rodrigo Palma Behnke Joao Abel Pecas Lopes Fernanda Resende John Romankiewicz Christine Schwaegerl Nikos Soultanis Liang Tao Antonis Tsikalakis