Galvanic isolation between the grid and energy storage unit is typically required for bidirectional power distribution systems. Due to the recent advancement in wide-bandgap semiconductor devices, it has become feasible to achieve the galvanic isolation using bidirectional isolated DC/DC converters instead of line-frequency transformers. A survey of the latest generation SiC MOSFET is performed. The devices were compared against each other based on their key parameters. It was determined that under the given specifications, the most suitable devices are X3M0016120K 1.2 kV 16 mohm and C3M0010090K 900 V 10 mohm SiC MOSFETs from Wolfspeed. Two of the most commonly utilized bidirectional isolated DC/DC converter topologies, dual active bridge and CLLC resonant converter are introduced. The operating principle of these converter topologies are explained. A comparative analysis between the two converter topologies, focusing on total device loss, has been performed. It was found that the CLLC converter has lower total device loss compared to the dual active bridge converter under the given specifications. Loss analysis for the isolation transformer in the CLLC resonant converter was also performed at different switching frequencies. It was determined that the total converter loss was lowest at a switching frequency of 250 kHz A prototype for the CLLC resonant converter switching at 250 kHz was then designed and built. Bidirectional power delivery for the converter was verified for power levels up to 25 kW. The converter waveforms and efficiency data were captured at different power levels. Under forward mode operation, a peak efficiency of 98.3% at 15 kW was recorded, along with a full load efficiency value of 98.1% at 25 kW. Under reverse mode operation, a peak efficiency of 98.8% was measured at 17.8 kW. The full load efficiency at 25 kW under reverse mode operation is 98.5%. / Master of Science / Electrical isolation between the grid and energy storage unit is typically required for bidirectional power distribution systems. Traditionally, this isolation is achieved via line-frequency transformers, which tend to be bulky and heavy. This imposes a limit on the overall system power density, which is a crucial performance metric for bidirectional power distribution systems. Alternatively, the required electrical isolation can be implemented through bidirectional power converters. As a result, the overall system power density can be drastically improved. However, the losses incurred by the semiconductor devices in such converters could significantly reduce the overall system efficiency, which is another important performance metric. Due to the recent advancement in semiconductor devices, it has become feasible to design the required bidirectional power converters with high efficiency and high power density. A survey of the latest generation semiconductor devices is performed. A 25 kW converter prototype was designed and built using the selected semiconductor devices. Experimental testing was conducted for the converter prototype and efficiency values exceeding 98% were captured across the entire load range. The converter prototype has a power density of 78 W/in³.

Solar energy as one type of the renewable energy becomes more and more popular which has led to increase the photovoltaic (PV) installations recently. One of the PV installations is the power conditioning system which is to convert the maximum available power output of the PV modules to the utility grid. Single-phase microinverters are commonly used to integrate the power to utility grid in modular power conditioning system. In the two-stage microinverter, each PV module is connected with a power converter which can transfer higher output power due to the tracking maximum power point (MPP) capability. However, it also has the disadvantages of lower power conversion efficiency due to the increased number of power electronics converters. The primary objective of this thesis is to develop a high-efficiency microconverter to increase the output power capability of the modular power conditioning systems. A topology with hybrid modes of operation are proposed to achieve wide-input regulation while achieving high efficiency. Two operating modes are introduced in details. Under high-input conditions, the converter acts like a buck converter, whereas the converter behaves as a boost converter under low-input conditions. The converter operates as the series resonant converter with normal-input voltage to achieve the highest efficiency. With this topology, the converter can achieve zero-voltage switching (ZVS) and/or zero-current switching (ZCS) of the primary side MOSFETs, ZCS and/or ZVS of the secondary side MOSFETs and ZCS of output diodes under all operational conditions. The experimental results based on a 300 W prototype are given with 98.1% of peak power stage efficiency and 97.6% of weighted California Energy Commission (CEC) efficiency including all auxiliary and control power under the normal-input voltage condition. / Master of Science

Battery energy storage system (BESS) is promising to be implemented in residential applications for supporting PV integration, load shifting, and backup power purposes. For this application, 48V second-life battery draws more and more attentions for their cost-effectiveness, safe voltage level, reliability, and potential large market. This thesis proposes the comprehensive control and design of an isolated battery-driven grid interface (IBDGI) with the dual-active-bridge (DAB) converter for residential applications with 48V battery pack. The three-phase DAB converter is a promising candidate as the front-end DC/DC converter in the two-stage IBDGI due to its high efficiency, high power density, and low capacitance requirement. An effective design strategy for the three-phase DAB converter is proposed based on the zero-voltage-switching (ZVS) zone and back-ow power to achieve high efficiency for a wide operating voltage range and different load conditions. Based on the power loss model, an easily-implemented variable switching frequency operating method is proposed to further increase the efficiency at light load conditions. The dead-time effect is observed in the three-phase DAB converter. To avoid the dead-time effect and better understand the phenomena, a comprehensive analysis is proposed. All the cases of the dead-time effect in the three-phase DAB converter are analyzed in terms of the buck, boost, and matching states. The expressions of the transmission power, constraint conditions, and key time of the dead-time effect are derived for each state. The operation waveforms of the dead-time effect are also presented. The hybrid capacitor bank composed by the LC resonant lter with electrolytic capacitor and lm capacitor is utilized for the DC bus of the IBGDI. The electrolytic capacitors work as passive decoupling purpose while the lm capacitor is responsible for high switching harmonic ltering. Moreover, a current sharing method between the hybrid capacitor bank is proposed to extend the electrolytic capacitor's life. The LCL single-phase inverter is applied for the downstream of the IBDGI. A step-by-step design procedure of the LCL lter with passive damping is proposed for the 120V/240V dual grid-tied and standalone modes. The PR controllers are also designed for the LCL inverter for standalone and grid-tied modes. At the system level, a novel second harmonic current (SHC) reduction strategy is proposed for the IBDGI with the three-phase DAB converter by adding a load current feedforward (LCFF) path to the DAB voltage closed-loop controller. This method will suppress the SHC without modi cations of the original controller's bandwidth, which make it easy to be implemented. The small-signal model of the three-phase DAB converter is provided and veri ed by the step response. The parameter sensitivity analysis for the LCFF method is proposed to show that the SHC is well suppressed within ±20% parameter error. The proposed converter and control methods are veri ed by simulation and experimental results. / Thesis / Doctor of Philosophy (PhD)

General-purpose step-down converter is essential in electronic system for processing energy from high-voltage rail to low-voltage circuits. The applications can be found at the auxiliary supplies in automobile, industrial and communication systems. Buck converter is a common circuit topology to fulfill step-down conversion, especially in low-power application since it is well-studied and straightforward. However, it suffers from low duty cycle under high step-down condition, and typically operates in continuous conduction mode (CCM) that generates large switching loss. On the other hand, as an extension of the buck converter, tapped-inductor (TI) buck converter has larger duty cycle while maintaining the structural simplicity. Therefore, the main objective of this thesis is to explore the potential of TI buck converter as a wide conversion range, high power density and high efficiency topology for low power application. To achieve high efficiency at switching frequency of MHz-level, synchronous conduction mode (SCM) is applied for turn-on losses elimination. The operation principle and power stage design of SCM TI buck is first introduced. The design of high switching frequency coupled inductor is emphasized since its size plays a critical role in power density. Loss breakdown is also provided to perform a comprehensive topological study. Secondly, detailed zero-voltage-switching (ZVS) condition of SCM TI buck is derived so that the converter does not experience redundant circulating energy. The experimental results of 15-W SCM TI buck converter prototypes are provided with 90.7% of peak power stage efficiency. The size of coupled inductor is down to 116 mm3. To enhance light-load efficiency, a variable frequency control scheme based on derived ZVS conditions is implemented with the switching frequency ranging from 2 MHz to 2.9 MHz. / Master of Science / General-purpose step-down converter is essential in electronic system for processing energy from high-voltage rail to low-voltage circuits. The applications can be found at the auxiliary supplies in automobile, industrial and communication systems. Typically, the ultimate goals of general-purpose step-down converter are versatility, high efficiency and compact size. Recently, tapped-inductor (TI) buck converter is studied since it could overcome the drawback of commonly used buck converter under high step-down conversion. Therefore, the potential of TI buck converter as a general-purpose step-down converter candidate is explored in this thesis, including control method, hardware design, etc. The thesis verifies that TI buck converter could have compact size while remaining efficient and adaptable.

The demand for increased renewable energy production has led to increased photovoltaic (PV) installations worldwide. As this demand continues to grow, it is important that the costs of PV installations decrease while the power output capability increases. One of the components in PV installations that has lots of room for improvement is the power conditioning system. The power conditioning system is responsible for converting the power output of PV modules into power useable by the utility grid while insuring the PV array is outputting the maximum available power. Modular power conditioning systems, where each PV module has its own power converter, have been proven to yield higher output power due to their superior maximum power point tracking capabilities. However, this comes with the disadvantages of higher costs and lower power conversion efficiencies due to the increased number of power electronics converters. The primary objective of this dissertation is to develop a high-efficiency, low cost microconverter in an effort to increase the output power capability and decrease the cost of modular power conditioning systems. First, existing isolated dc-dc converter topologies are explored and a new topology is proposed based on the highly-efficient series resonant converter operating near the series resonant frequency. Two different hybrid modes of operation are introduced in order to add wide input-voltage regulation capability to the series resonant converter while achieving high efficiency through low circulating currents, zero-current switching (ZCS) of the output diodes, zero-voltage switching (ZVS) and/or ZCS of the primary side active switches, and direct power transfer from the source to the load for the majority of the switching cycle. Each operating mode is analyzed in detail using state-plane trajectory plots. A systematic design approach that is unique to the newly proposed converter is presented along with a detailed loss analysis and loss model. A 300-W microconverter prototype is designed to experimentally validate the analysis and loss model. The converter featured a 97.7% weighted California Energy Commission (CEC) efficiency with a nominal input voltage of 30 V. This is higher than any other reported CEC efficiency for PV microconverters in literature to date. Each operating mode of the proposed converter can be controlled using simple fixed-frequency pulse-width modulation (PWM) based techniques, which makes implementation of control straightforward. Simplified models of each operating mode are derived as well as control-to-input voltage transfer functions. A smooth transition method is then introduced using a two-carrier PWM modulator, which allows the converter to transition between operating modes quickly and smoothly. The performance of the voltage controllers and transition method were verified experimentally. To ensure the proposed converter is compatible with different types of modular power conditioning system architectures, system-level interaction issues associated with different modular applications are explored. The first issue is soft start, which is necessary when the converter is beginning operation with a large capacitive load. A novel soft start method is introduced that allows the converter to start up safely and quickly, even with a short-circuited output. Maximum power point tracking and double line frequency ripple rejection are also explored, both of which are very important to ensuring the PV module is outputting the maximum amount of available power. Lastly, this work deals with efficiency optimization of the proposed converter. It is possible to use magnetic integration so that the resonant inductor can be incorporated into the isolation transformer by way of the transformer leakage inductance in order to reduce parts count and associated costs. This chapter, however, analyzes the disadvantages to this technique, which are increased proximity effect losses resulting in higher conduction losses. A new prototype is designed and tested that utilizes an external resonant inductor and the CEC efficiency was increased from 97.7% to 98.0% with a marginal 1.8% total cost increase. Additionally, a variable frequency efficiency optimization algorithm is proposed which increases the system efficiency under the high-line and low-line input voltage conditions. This algorithm is used for efficiency optimization only and not control, so the previously presented simple fixed-frequency modeling and control techniques can still be utilized. / Ph. D.

In recent years, smart gird has received much attention, and many researchers have devoted to home area network (HAN) to improve the efficiency of grid. This thesis proposes a bidirectional isolated DC-DC converter (Dual Active Bridge, DAB) with phase shift control to implement the concept of HAN. Li+Fe battery is used as storage device of the converter, and a bidirectional communication between Laboratory Virtual Instrumentation Engineering Workbench (LabVIEW) and Digital Signal Processor (DSP) is realized by CAN-bus. By setting the current command given by LabVIEW system, the converter is able to charge the Li+Fe battery with constant current and constant voltage method as well as discharge the Li+Fe battery with constant current. Operation principle of the converter and the associated loss are presented.

Made available in DSpace on 2016-12-12T20:27:37Z (GMT). No. of bitstreams: 1 Joao Cabral.pdf: 4385545 bytes, checksum: c09296f90add051ed37bd87320b15421 (MD5) Previous issue date: 2013-03-06 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This Master Thesis proposes a high gain non-isolated DC-DC converter applied on the processing of PV energy. The proposed converter is a boost converter with quadratic characteristic and with central point at its output. Basic operations and electric characteristics of a PV cell are described, including the procedures to determine its parameters. A model for numeric simulation is presented. A review of the high gain non-isolated DC-DC converters topologies is presented. Shows the converter´s transfer functions and the control strategy adopted as well as the design of control circuits. The control system is consisting of three loops, an internal loop of input current control, an external loop of total output voltage control and an additional loop of voltage unbalance control. The simulation and experimental results are shown to validate the analysis developed and demonstrate the performance of the control system adopted. / Esta Dissertação de Mestrado propõe um conversor CC-CC não isolado de elevado ganho para aplicação no processamento de energia solar fotovoltaica. O conversor proposto é um boost com característica quadrática e com ponto médio na sua saída. Descrevem-se o funcionamento básico e as características elétricas de uma célula fotovoltaica, incluindo-se o procedimento da determinação dos parâmetros e da modelagem dos módulos fotovoltaicos, apresentando-se um modelo para simulação numérica. Apresenta-se uma revisão de topologias de conversores CC-CC não isolados com elevado ganho estáticos. Apresentam-se as funções de transferência do conversor e a estratégia de controle adotada bem como o projeto dos circuitos de controle. O sistema de controle composto por três malhas de controle, uma malha interna de corrente de entrada, uma malha externa de tensão total e uma malha adicional de equalização de tensão. Resultados de simulação e experimentais são apresentados para validar as análises desenvolvidas e demonstrar o desempenho do sistema de controle adotado.

För sammankoppling av multi-terminala HVDC-system med punkt-till-punkt kopplingar ärDC-DC-omvandlaren den enda möjliga sammankopplingen. Därför genomgår problemenmed spänningsmatchning och likspänningsströmbegränsning i högspännings DC-systemomfattande forskning samt ligger i fokus för denna avhandling. Först analyseras toppmodernatopologier för högspännings DC-DC-omvandlare som används för samtrafik av flera terminalaHVDC-system. De analyserade topologierna jämförs sedan baserat på dess olika funktioner.Topologin för en konventionell icke-isolerad DC-DC-omvandlare analyseras när det gäller design,kostnad, storlek, förlust och effektstyrningskapacitet. Först skapas en matematisk modell ochsedan utförs en numerisk analys för olika arbetsområden. Därefter görs en jämförelse av entvåfas-icke-isolerad DC-omvandlare baserad på energilagring, maximal likströmsöverföring ochtotala förluster. Simulering utförs av en tvåfas och en trefas icke-isolerad DC-omvandlare iPSCAD med olika typer av styrenheter. Dessutom tas en isolerad omvandlartopologi och analyserasi detalj från matematisk modellering till validering med hjälp av simuleringsresultat.Olika typer av felanalyser för både isolerad och icke-isolerad omvandlartopologi görs. Slutligenutförs även analyser av DC-felet i olika möjliga anslutningar av omvandlaren i Multi-TerminalGrid, dvs Monopole, Bipole med både symmetriska och asymmetriska konfigurationer. / For interconnection of multi-terminal HVDC systems involving point-to-point links, aDC-DC converter is the only possible way to interconnect. Therefore, the issues of voltagematching and DC fault current limiting in high voltage DC systems are undergoing extensiveresearch and are the focus of this thesis. Starting with analyzing the state of the art highvoltage DC-DC converter topologies for interconnection of multi-terminal HVDC systems andbenchmarking each converter topology based on different functionalities. A basic non-isolatedDC-DC converter topology is analyzed in terms of design, cost, sizing, losses and power controlcapability. First, starting with the mathematical modeling and then the numerical analysis isdone for different operating regions. Next, it is compared with the two-phase non-isolated DCconverter based on energy storage, maximum DC power transfer, and total losses. Simulation oftwo-phase and three-phase non-isolated DC converter is done in PSCAD incorporating differenttypes of controllers. Then, an isolated converter topology is taken and analyzed in detail startingfrom mathematical modeling to validation using simulation results. Different types of faultsanalysis for both isolated and non-isolated converter topology is done. Finally, analyzing the DCfault in different possible connection of the converter in the multi-terminal grid, i.e. monopole,bipole in both symmetric and asymmetric configurations.