Power losses in the grid are important, and as the power losses decrease the efficiency increases. Not much research has been done recently on the Newton-Raphson Power Flow (NRPF) method in polar form for systems with High Voltage Direct Current (HVDC) subsystems. The point of departure for this thesis is based on decoupling the NR Power Flow method Power flow problems are solved for many fundamental problems in the operation and planning of the power system. Although many methods are available to solve these problems, this thesis focuses on developing an enhanced HVDC power flow method with improved computational efficiency and convergence stability. A comparison of the results with full Newton-Raphson Power Flow method is presented to evaluate the performance of the proposed method. Simulations were conducted on the 14-bus and 30-bus IEEE systems. Two and three converters are shown to improve the voltage magnitude, active and reactive power profile .The overall results indicate which mode is the best mode compared to others depending on the bus importance.

This thesis comprises modeling and simulation of an AC-DC converter (Battery charger). An AC-DC converter may cause a high frequency distortion in the electrical power network or augment the existing distortion caused by other devices connected to the network. The goal is to design a controller for suppressing this noise at a reasonable level. We hope the thesis can be considered as a step forward to solve the original problem. One needs an accurate model of the AC-DC converter, to design such a controller. This study tries to clarify the effects of theline inductance on the performance of the converter by modeling and simulating the converter during the commutation time. The idea is to model and simulate the converter for two different conditions; first in the Normal condition by neglecting the effect of line impedance, second in the Commutation condition by considering the effect of the line impedance on commutation of the diodes. One can perform a complete simulation of the converter with combining these two models. The thesis deals with AC-DC converters, Hamiltonian-port modeling, simulation and MATLAB programming using the functionality of the S-function and SIMULINK.

Harvesting energy from ultra-low-power vibration energy sources typically employs a rectifier circuit as the first power conditioning stage. The Schottky diode has a 0.15 V - 0.2 V threshold voltage and can not extract energy efficiently at low voltage. Other technologies such as MOSFET bridge or active diode are designed to minimize the voltage drop to reduce the conduction loss. However, these designs require either additional power supplies to operate comparators or have a larger threshold turn-on voltage than Schottky. Therefore, most rectifiers have an unresponsive or significant low-efficiency zone when the input power is low. This dissertation will elaborate on a backward diode based self-powered micro-circuit diode that will operate in the extremely weak or low alternating source applications, where the existing approaches offer poor outcomes. This proposed micro-circuit diode was compared to a Schottky diode in several experiment setup. The micro-circuit based half-wave rectifier circuit harvested 3.1 mV DC at a 239.5 Ohm load when the input magnitude is 50 mV while the Schottky diode was unable to convert this ultra-low AC power. This dissertation also provides the analysis of two alternating sources, the oscillatory electromagnetic generator and the piezoelectric energy harvester, to conduct experiments in a more realistic context. The micro-circuit diode shows excellent advantages in electromagnetic generator experiment, the micro-circuit based half-wave rectifier circuit harvested 5.16 mV DC at a 0.5 kOhm load when the input magnitude is 40 mV. However, due to the large leakage current in negative resistance region, this micro-circuit is unable to show advantages in piezoelectric energy harvester applications.

abstract: This thesis investigates different unidirectional topologies for the on-board charger in an electric vehicle and proposes soft-switching solutions in both the AC/DC and DC/DC stage of the converter with a power rating of 3.3 kW. With an overview on different charger topologies and their applicability with respect to the target specification a soft-switching technique to reduce the switching losses of a single phase boost-type PFC is proposed. This work is followed by a modification to the popular soft-switching topology, the dual active bridge (DAB) converter for application requiring unidirectional power flow. The topology named as the semi-dual active bridge (S-DAB) is obtained by replacing the fully active (four switches) bridge on the load side of a DAB by a semi-active (two switches and two diodes) bridge. The operating principles, waveforms in different intervals and expression for power transfer, which differ significantly from the basic DAB topology, are presented in detail. The zero-voltage switching (ZVS) characteristics and requirements are analyzed in detail and compared to those of DAB. A small-signal model of the new configuration is also derived. The analysis and performance of S-DAB are validated through extensive simulation and experimental results from a hardware prototype. Secondly, a low-loss auxiliary circuit for a power factor correction (PFC) circuit to achieve zero voltage transition is also proposed to improve the efficiency and operating frequency of the converter. The high dynamic energy generated in the switching node during turn-on is diverted by providing a parallel path through an auxiliary inductor and a transistor placed across the main inductor. The paper discusses the operating principles, design, and merits of the proposed scheme with hardware validation on a 3.3 kW/ 500 kHz PFC prototype. Modifications to the proposed zero voltage transition (ZVT) circuit is also investigated by implementing two topological variations. Firstly, an integrated magnetic structure is built combining the main inductor and auxiliary inductor in a single core reducing the total footprint of the circuit board. This improvement also reduces the size of the auxiliary capacitor required in the ZVT operation. The second modification redirects the ZVT energy from the input end to the DC link through additional half-bridge circuit and inductor. The half-bridge operating at constant 50% duty cycle simulates a switching leg of the following DC/DC stage of the converter. A hardware prototype of the above-mentioned PFC and DC/DC stage was developed and the operating principles were verified using the same. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

Orientador: Falcondes Jose Mendes de Seixas / Banca: Guilherme de Azevedo e Melo / Banca: Jurandir de Oliveira Soares / Resumo: Este trabalho propõe um conversor CA-CC, modulado por largura de pulso (PWM), de elevado fator de potência, dotado de um controle variável da razão cíclica. O estágio de potência do conversor pré-regulador do fator de potência (PFP) proposto é composto por múltiplas células entrelaçadas. A integração das técnicas de entrelaçamento (interleaving) à técnica de eliminação da ponte retificadora da entrada (bridgeless) busca diminuir as perdas de energia e distribuir os esforços de corrente nos semicondutores da estrutura conversora de potência. O levantamento bibliográfico foi realizado com intuito de analisar resultados e características de conversores CA-CC baseados em estruturas PFP boost modificadas. Comparações foram realizadas levando em consideração as características de distorção harmônica, fator de potência, complexidade do circuito de controle, eficiência energética, esforços nos semicondutores de potência e emissão de interferência eletromagnética (IEM). O conversor proposto é então equacionado e os valores dos componentes acumuladores de energia determinados são escritos como função das especificações de projeto. Análises gráficas indicam o comportamento da distorção harmônica e do fator de potência da estrutura atuando em diferentes níveis de tensão. Simulações são realizadas como forma de comprovar a eficácia do sistema conversor no que se refere à correção do fator de potência e à distribuição dos esforços nos semicondutores de potência. Os principais resultados estão dispostos em tabelas comparativas, que indicam um fator de potência praticamente unitário. O sistema de regulação da tensão saída, testado via simulação computacional, apresenta resultados que demonstram boa dinâmica na resposta transitória e erro nulo a regime frente às variações de carga e afundamentos na... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: This paper proposes an AC-DC converter, pulse width modulated (PWM) with high power factor and a variable duty cycle control. The power factor preregulator (PFP) converter proposed is composed of multiple bridgeless boost interleaved cells. The integration of these techniques allows a reduction in the energy losses and sharing the stress in power semiconductors. The literature review was performed in order to analyze the results and characteristics of AC-DC converters based on modified PFC boost structures. Comparisons were made taking into account the characteristics of harmonic distortion, power factor, complexity of control circuitry, energy efficiency, current stress in power semiconductors, and emission of electromagnetic interference (EMI). The proposed converter is mathematically equated and the values of the reactive components are written as a function of design specifications. Graphical analysis indicates the behavior of harmonic distortion and power factor of the structure at different voltage levels. Simulations are performed in order to demonstrate compliance of the converter system with respect to the power factor correction and distribution efforts in the power semiconductors. The main results are arranged in comparative tables which indicate power factor very closed to unity. The control system applied to the converter was evaluated through the computer simulations which showed good dynamics at transient response and null steady-state error faced to load variations and sags in the voltage supply. The prototype for the power stage implemented in the laboratory operated according... (Complete abstract click electronic access below) / Mestre

Este trabalho tem como principal objetivo apresentar uma investigação e uma metodologia de projeto, até o presente momento inédita, de uma topologia de um conversor ca-cc trifásico de comutação forçada. Através da análise desenvolvida neste trabalho, determina-se um modelo matemático do conversor ca-cc para os quatro modos que determinam o processo da comutação nesse circuito. Esse modelo resulta em sistemas de equações fundamentais na forma de equações diferenciais, que são resolvidos com a finalidade de determinar o comportamento do circuito do conversor durante o processo de comutação e de obter uma metodologia de projeto. Com o objetivo de validar a investigação e o método de projeto foi implementada simulação computacional, no programa Simmon, dos intervalos de comutação do conversor ca-cc de comutação forçada. Essa estrutura, que durante a sua operação emprega tanto a comutação forçada como a natural, quando comparada com os conversores de comutação natural apresenta várias vantagens, como um alto fator de potência e a eliminação dos harmônicos de baixa ordem na linha ca (utilização da comutação forçada com a técnica de modulação por largura de pulso PWM), como mostra vátios trabalhos publicados, que estão desctitos na parte de referências bibliográficas deste texto. / The main objective of this work is to present an investigation and a design method, which up to now is not available in the literature, of a three-phase force commutated ac-dc convetier. From the analysis developed in this work one obtains the mathematical model of the ac-dc converter for its four modes of commutation, which determine the commutation process in the converter circuit. The fundamental equations, which have been written in the form of differential equations, are solved to determine the behaviour of the converter circuit during commutation, and also to obtain a design method. To validate the analysis and the design procedure, computer simulation of the commutation intervals of the ac-dc converter was implemented using the program Simnon. The three-phase ac-dc converter, which uses both forced and line commutation, when compared with the conventional line commutated converter, presents several advantages as a high power factor and the elimination of lower harmonics of the ac line (utilization of the forced commutation and the pulse width modulation strategy), as it is shown in several published works, which can be found in the references of this work.

The development of voltage source converter (VSC) based high voltage direct current (HVDC) transmission has progressed rapidly worldwide over the past few years. The UK transmission system is going through a radical change in the energy landscape which requires a number of VSC HVDC installations to connect large Round 3 windfarms and for interconnections to other countries. For bulk power long distance transmission, VSC HVDC technology offers flexibility and controllability in power flow, which can benefit and strengthen the conventional AC system. However, the associated uncertainties and potential problems need to be identified and addressed. To carry out this research, integrated mathematical dynamic AC/DC system models are developed in this thesis for small disturbance stability analysis. The fidelity of this research is further increased by developing a dynamic equivalent representative Great Britain (GB) like system, which is presented as a step-by-step procedure with the intention of providing a road map for turning a steady-state load flow model into a dynamic equivalent. This thesis aims at filling some of the gaps in research regarding the integration of VSC HVDC technology into conventional AC systems. The main outcome of this research is a systematic assessment of the effects of VSC controls on the stability of the connected AC system. The analysis is carried out for a number of aspects which mainly orbit around AC/DC system stability issues, as well as the control interactions between VSC HVDC and AC system components. The identified problems and interactions can mainly be summarized into three areas: (1) the effect of VSC HVDC controls on the AC system electromechanical oscillations, (2) the potential control interactions between VSC HVDC and flexible alternating current transmission systems (FACTS) and (3) the active power support capability of VSC HVDC for improving AC system stability. The effect of VSC controls on the AC system dynamics is assessed with a parametric sensitivity analysis to highlight the trade-offs between candidate VSC HVDC outer control schemes. A combination of analysis techniques including relative gain array (RGA) and modal analysis, is then applied to give an assessment of the interactions – within the plant model and the outer controllers – between a static synchronous compensator (STATCOM) and a VSC HVDC link operating in the same AC system. Finally, a specific case study is used to analyse the capability of VSC HVDC for providing active power support to the connected AC system through a proposed frequency droop active power control strategy.

The aim of this thesis is to study the possibilities of controlling high-brightness white LEDs using circuits operating at high voltages. The job itself is a division of existing light sources and mainly focus on semiconductor technology LED. The following describes the voltage of DC / DC and AC / DC. The work contains a description of the three high-voltage inverters are designed for power LEDs. Finally, it created a draft scheme of the PCB. As a control circuit is chosen ATMEGA 162 microcontroller from Atmel and source for LED diode is formed HV9910 and HV9861 circuits.

This thesis aims to provide a recommended power system design for optimal efficiency, reliability, and cost in off-grid applications. The power system examined in this project is a residence in an off-grid community called Quail Springs that generates its energy from roof mounted solar panels. The existing system was analyzed to see what equipment can remain, what needs to be upsized, and what needs to be added to the system. Two power systems are considered for the residence: a fully AC power system and a hybrid AC/DC power system. Simulations were run in PSCAD to compare the efficiencies of the two proposed systems at varying load. The results of the simulations showed the hybrid power system to be generally less efficient when supplying AC and DC loads, but greater than 5% more efficient when only supplying DC load. Although the hybrid AC/DC system is approximately 70% more expensive, it is still the final recommended design due to potential efficiency gains and in an effort to provide educational opportunities that may lead to further efficiency gains in future hybrid AC/DC power systems.

DC power distribution has gained popularity in sustainable buildings, renewable energy utilization, transportation electrification and high-efficiency data centers. This dissertation focuses on two aspects of facilitating the application of dc systems: (a) system-level control to improve load sharing, voltage regulation and efficiency; (b) design of a high-efficiency interface converter to connect dc microgrids with the existing low-voltage ac distributions, with a special focus on common-mode (CM) voltage attenuation. Droop control has been used in dc microgrids to share loads among multiple sources. However, line resistance and sensor discrepancy deteriorate the performance. The quantitative relation between the droop voltage range and the load sharing accuracy is derived to help create droop design guidelines. DC system designers can use the guidelines to choose the minimum droop voltage range and guarantee that the sharing error is within a defined range even under the worst cases. A nonlinear droop method is proposed to improve the performance of droop control. The droop resistance is a function of the output current and increases when the output current increases. Experiments demonstrate that the nonlinear droop achieves better load sharing under heavy load and tighter bus voltage regulation. The control needs only local information, so the advantages of droop control are preserved. The output impedances of the droop-controlled power converters are also modeled and measured for the system stability analysis. Communication-based control is developed to further improve the performance of dc microgrids. A generic dc microgrid is modeled and the static power flow is solved. A secondary control system is presented to achieve the benefits of restored bus voltage, enhanced load sharing and high system efficiency. The considered method only needs the information from its adjacent node; hence system expendability is guaranteed. A high-efficiency two-stage single-phase ac-dc converter is designed to connect a 380 V bipolar dc microgrid with a 240 V split-phase single-phase ac system. The converter efficiencies using different two-level and three-level topologies with state-of-the-art semiconductor devices are compared, based on which a two-level interleaved topology using silicon carbide (SiC) MOSFETs is chosen. The volt-second applied on each inductive component is analyzed and the interleaving angles are optimized. A 10 kW converter prototype is built and achieves an efficiency higher than 97% for the first time. An active CM duty cycle injection method is proposed to control the dc and low-frequency CM voltage for grounded systems interconnected with power converters. Experiments with resistive and constant power loads in rectification and regeneration modes validate the performance and stability of the control method. The dc bus voltages are rendered symmetric with respect to ground, and the leakage current is reduced. The control method is generalized to three-phase ac-dc converters for larger power systems. / Ph. D.