Σκοπός της παρούσας διπλωματικής εργασίας είναι η προστασία ενός μικροδικτύου συνδεδεμένου στο δίκτυο διανομής χαμηλής τάσης από μεγάλα ρεύματα που οφείλονται σε βυθίσεις τάσης του δικτύου διανομής. Η μελέτη θα περιλαμβάνει όλα εκείνα τα στοιχεία που είναι απαραίτητα για τη σύνδεση σε ένα δίκτυο. Συγκεκριμένα, θα περιλαμβάνει τον ανυψωτή τάσης (boost converter) ο οποίος είναι υπεύθυνος για την ανύψωση της συνεχούς τάσης εξόδου της διεσπαρμένης παραγωγής, τον αντιστροφέα (inverter) ο οποίος θα μετατρέπει την συνεχή τάση εξόδου του ανυψωτή σε εναλλασσόμενη με το επιθυμητό πλάτος κ συχνότητα, και τέλος θα περιλαμβάνει το σύστημα ελέγχου το οποίο θα προσπαθεί να ικανοποιεί τις ενεργειακές ανάγκες κάθε φορτίου ρυθμίζοντας κατάλληλα τις παραμέτρους των προηγούμενων μερών. Επιπλέον σε περιπτώσεις βύθισης τάσης η διπλωματική εργασία προτείνει μία μέθοδο ελέγχου η οποία προσπαθεί να άρει την διανυσματική πτώση τάσης στην γραμμή διασύνδεσης μικροδικτύου και δικτύου διανομής. Επειδή η μελέτη ενός τέτοιου πραγματικού συστήματος δεν ήταν εφικτή στα πλαίσια μας διπλωματικής εργασίας, επιλέχθηκε η μέθοδος εξομοίωσης σε ηλεκτρονικό υπολογιστή. Για την εξομοίωση χρησιμοποιήθηκε το πρόγραμμα PSCAD,ένα από τα πλέον καταλληλότερα προγράμματα για την μελέτη των συστημάτων ηλεκτρικής ενέργειας. Η επιλογή του PDCAD στηρίχθηκε στο γεγονός ότι είναι εξαιρετικά εύχρηστο ενώ ταυτόχρονα έχει γρήγορες αποκρίσεις και χρησιμοποιείται κατά κόρον για την προσομοίωση τέτοιων συστημάτων. / The purpose of this diplomathesis is to protect a microgrid, which is connected to low voltage distribution network, of large currents due to voltage sags in the distribution network. This study includes all the elements needed to connect the microgrid to a network. Specifically, it includes the boost converter which is responsible for raising the voltage output of dispersed production, the inverter which will convert the voltage output of the enhancer into alternating with the desired width and frequency and at last it includes the control system which tries to satisfy the energy needs of each load by adjusting appropriate parameters of the previous parameters. Moreover, in cases of voltage sags, this diploma thesis proposes a control method that attempts to eliminate the vector voltage drop in interconnection microgrid and distribution network. Since the study of such a real system was not feasible in our thesis, the method was chosen to be simulated in a computer. The simulation program that was used is the PSCAD, one of the most appropriate programs for the study of electric power systems. The choice of PSCAD relied on the fact that it is extremely handy, with fast responses and is widely used for simulating such systems.

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Power converters are electronic circuits for conversion, control and regulation of electric power for various applications, such as from tablet computers in milliwatts to electric power systems at megawatts range. There are three basic types of power converters: buck (output voltage less than the input voltage), boost (output voltage higher than the input voltage) and buck-boost converters. The reliability of the power converters has become an essential focus of industrial applications. This research presents modeling and control of DC/DC boost converter using several control methods, such as Proportional-Integral (PI), Linear Quadratic Regulator (LQR) control, and nonlinear control concepts. Based on standard circuit laws, a mathematical model of the boost converter is derived which is expressed as a bilinear system. First a small signal model of the converter is derived to analyze the small deviations around the steady-state operating point which is used to develop closed loop control using the PI and the LQR methods. Simulation results show that the performance of the converter is good for operation around the operating state, however is unacceptable if there are large variations in the load or the reference input. To improve the performance of the closed loop system, the nonlinear control concept is used which shows excellent closed loop performance under large variations of load or setpoint. Comparative simulation results are presented for closed loop performance under various types of disturbances including random variations in load. / Electrical and Computer Engineering

Although the locomotive of a train is energized, in general, other railcars are not. This prevents commercial rail companies from installing sensor equipment on the railcars. Thus, several different solutions have been proposed to provide energy for commercial railcars. One such solution is a vibration-based energy harvester which can be mounted in the suspension coils of the railcar. The harvester translates the linear motion of the suspension vibration into rotational motion to turn a 3-phase AC generator. When subjected to real-world suspension displacements, the harvester is capable of generating peak energy levels in excess of 70 W, although the average energy harvested is much lower, around 1 W. A battery pack can be used to store the useful energy harvested. However, a power conditioning circuit is required to convert the 3-phase AC energy from the harvester into DC for the battery pack. The power converter should be capable of extracting maximum power from the energy harvester as well as acting as a battery manager. Experimental results with the energy harvester conclude that maximum power can be extracted if the harvester is loaded with 2 . In order to maintain a constant input impedance, the duty cycle of the power converter must be fixed. Conversely, output regulation requires the duty cycle to change dynamically. Consequently, there is a tradeoff between extracting maximum power and prolonging the battery life cycle. The proposed converter design aims to achieve both maximum power transfer and battery protection by automatically switching between control modes. The proposed converter design uses an inverting buck-boost converter operating in discontinuous conduction mode to maintain a constant input impedance through a fixed duty cycle. This constant input impedance mode is used to extract maximum power from the harvester when the battery is not close to fully charged. When the battery is near fully charged, extracting maximum power is not as important and the duty cycle can be controlled to regulate the output. Specifically, one-cycle control is used to regulate the output by monitoring the input voltage and adjusting the duty cycle accordingly. Finally, the converter is designed to shut down once the battery has been fully charged to prevent overcharging. The result is a power converter that extracts maximum power from the energy harvester for as long as possible before battery protection techniques are implemented. Previous related studies are discussed, tradeoffs in converter design are explained in detail, and an experimental prototype is used to confirm operation of the proposed control scheme. / Master of Science

As wearable technology and wireless sensor nodes become more and more ubiquitous, the batteries required to power them have become more and more unappealing as they limit lifetime and scalability. Energy harvesting from body heat provides a solution to these limitations. Energy can be harvested from body heat using thermoelectric generators, or TEGs. TEGs provide a continuous, scalable, solid-state energy source ideal for wearable and wireless electronics and sensors. Unfortunately, current TEG technology produces low power (< 1 mW) at a very low voltage (20-90 mV) and require the load to be matched to the TEG internal resistance for maximum power transfer to occur. This thesis research proposes a power management integrated circuit (PMIC) that steps up ultralow voltages generated by TEGs to a regulated 3 V, while matching the internal resistance. The proposed boost converter aims to harvest energy from body heat as efficiently and flexibly as possible by providing a regulated 3 V output that can be used by a variable load. A comparator-based burst mode operation affords the converter a high conversion ratio at high efficiency, while fractional open circuit voltage maximum power point tracking ensures that the controller can be used with a variety of TEGs and TEG setups. This control allows the converter to boost input voltages as low as 50 mV, while matching a range of TEG internal source resistances in one stage. The controller was implemented in 0.25 µm CMOS and taped out in February 2016. Since these fabricated chips will not be completed and delivered until May 2016, functionality has only been verified through simulation. Simulation results are promising and indicate that the peak overall efficiency is 81% and peak low voltage, low power efficiency is 73%. These results demonstrate the the proposed converter can achieve overall efficiencies comparable to current literature and low power efficiencies better than similar wide range converters in literature. / Master of Science

"In this thesis, based on the existing researches on power factor correction technology, I analyze, design and study the Boost type power factor correction technology, which is applied in the in-board two-stage battery charger. First I analyzed the basic working principle of the active power factor corrector. By comparing several different topologies of PFC converter main circuit and control methods, I specified the research object to be the average current control (ACM) boost power factor corrector. Then I calculated and designed the PFC circuit and the ACM controller applied in the first level charging of EVs. And I run the design in Simulink and study the important features like power factor, the input current waveform and the output DC voltage and the THD and odd harmonic magnitude."

Photovoltaic (PV) system is one of the most prominent energy sources, producing electricity directly from sunlight. In additionally, it is easy to install and is supported financially by many governments as part of their strategy to reduce CO2 gas emissions, and to achieve their agreed set of reduction targets by 2020. In the meantime, researchers have been working on the PV system to make it more efficient, easy to maintain, reliable to use and cost effective. In the stand-alone PV system, a battery is required. This is due to the fluctuating nature of the output energy delivered by the PV arrays owing to the weather conditions and the unpredictable behaviour of uses with regard to the consumption of energy. During the hours of sunshine, the PV system is directly feeding the load and any surplus electrical energy is stored in the battery at a constant current. During the night, or during a period of low solar irradiation, the energy is supplied to the load from the battery. However, the stand-alone PV system is designed to provide an acceptable balance between reliability and cost, which is a major challenge to the designer owing to the approaches used to size the PV arrays and the battery bank. As a result, the unpredictable, quick daily changes on the PV output is not dependable. Moreover, battery performance, length of life and energy efficiency depends on the rate at which it is discharged. Therefore, it is essential to use other methods to deal with any quick variation in energy. In this thesis, a super capacitor is used to solve this problem, as it can deal with the fast-changing weather, or a rapid variation in the energy requirements of the customer. A critical evaluation with in-depth analysis of the placement and the implementation for the super-capacitor in the PV standalone system has been carried out. The results show, super-capacitor capacitance and the converter efficiency affect the delivered load energy. However, the bi-directional topology performs better than uni-directional under the same conditions. Finally, a further improvement of the system at component level, has been developed through an energy recovery snubber for the switching transition and achieved a recovery of energy for the resistive load, 94.44% for the turn on transition and 92.86% for the turn off transition. Moreover, for the inductive load, 78.33% and 97.33% of energy has been recovered for the turn on and for the turn off transition respectively.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / Este trabalho aborda os efeitos de afundamentos de tensÃo em Acionamentos de Velocidade VariÃvel â AVVs para mÃquinas de corrente alternada, conversores estÃticos de dois estÃgios (retificador-inversor) amplamente utilizados no controle de velocidade e traÃÃo. O estudo tem como objetivo analisar o comportamento dos AVVs diante de afundamentos de tensÃo simÃtricos e assimÃtricos e tÃcnicas de aumento de suportabilidade para estes equipamentos, destacando-se o uso de conversores boost. SÃo apresentados resultados de simulaÃÃes computacionais e de ensaios laboratoriais de um conversor boost desenvolvido para aumentar a tolerÃncia de AVV a afundamentos de tensÃo. Nos ensaios de laboratÃrio foram levantadas as curvas de tolerÃncia do AVV para afundamentos de tensÃo dos tipos A, B e E e comparadas aos limites estabelecidos pela curva SEMI F47 0706 do instituto EPRI. O AVV mostrou-se sensÃvel aos afundamentos tipo A e E com imunidade de tensÃo remanescente de atà 0,7pu na barra CC do AVV. Foram ainda feitos ensaios experimentais com o conversor boost conectado ao AVV sob condiÃÃes de afundamentos de tensÃo severos dos tipos A e E. A anÃlise do conjunto AVV-MIT sob os demais tipos de afundamentos de tensÃo (B, C, D, F e G) foi feita por simulaÃÃo computacional usando um modelo previamente validado pela comparaÃÃo com os resultados experimentais. Os afundamentos dos tipos A, E, F e G, que podem provocar o desligamento do AVV, foram tambÃm analisados por simulaÃÃes computacionais com a conexÃo do conversor boost ao barramento CC do AVV. Para a avaliaÃÃo do comportamento do AVV com e sem o conversor boost foram obtidas as curvas de tensÃo do barramento CC, tensÃo de saÃda do AVV, tensÃo de entrada do conversor boost, e de correntes de entrada do AVV, do conversor boost e de saÃda da fonte. Conforme os resultados de simulaÃÃes e experimentais o conversor boost pode mitigar afundamentos do tipo A para atà 0,5pu e todos os outros tipos de afundamentos atà nÃveis de interrupÃÃo, isto Ã, 0pu. Comparados os resultados com as simulaÃÃes de outras soluÃÃes abordadas no trabalho, o conversor boost se mostrou como a melhor alternativa do ponto de vista tÃcnico. / This work investigates the effects of voltage sags on Adjustable Speed Drives â ASDs of AC machines, static converters of two stages (rectifier-inverter) widely used on speed and torque control. The goal is to analyse the behavior of the ASDs under symmetrical and asymmetrical voltage sags and the approaches to improve the ASD low voltage ride-through capability with emphasis to the boost converter technique. Computational simulations and experimental results of a boost converter designed to operate when the ASD is under voltage sags are presented. The tolerance curves of the ASD were obtained in laboratory for voltage sags types A, B and E and compared to the SEMI F47 0706 curve of the EPRI institute. The ASD was shown sensitive for voltage sags types A and E with immunity for remaining voltages up to 0.7pu on the DC link. Experimental tests were performed to evaluate the response of the boost converter operation when the ASD is under severe voltage sag conditions of types A and E. The set ASD-induction motor was modelled for the simulation tests and the model validation was performed by comparison with experimental results. The analysis of the ASD and the three-phase induction motor under voltage sags B, C, D, F e G were carried out by computational simulations. The voltage sag types A, E, F and G which can turn off the ASD were also analyzed by computational simulation with the boost converter connected to ASD DC bus. The curves of DC link voltage, the ASD output voltage, the boost input voltage and the input currents of the ASD, the boost converter and the source were plotted for evaluation of the ASD behavior with and without the boost converter. The simulation and experimental results have shown that the boost converter can mitigate voltage sags type A up to 0.5pu and all other types of voltage sags up to 0pu. The boost converter has proved a suitable solution to improve the ASD voltage sag ride through capability.

CoordenaÃÃo de AperfeÃoamento de Pessoal de NÃvel Superior / This work presents procedures for modeling a Buck-Boost converter based on offline parametric identification techniques, with employment of black box and gray box models. For the identification of the control-to-output-voltage transfer function, the nonlinear Hammerstein model is employed, a particularly interesting structure to identify DC-DC converters for its ability to incorporate nonlinear static characteristic aside from the dynamic behavior of the plant. The identification of the mentioned transfer function is achieved from input and output data, obtained in simulations. In order to identify transfer function parameters, a restricted least squares algorithm is used. As for the identification of the control-to-inductor-current transfer function, a linear black box first order model is considered, with its parameters being determined from systemâs frequency response. In order to show the modelâs utility, a control system is designed based on the identified expressions. The control system employed is the digital version of type 3 compensator for the voltage loop and type 2 compensator for the current loop, both operating under or logics. The identification results of the system presented excellent agreement between the obtained parametric models and the converterâs behavior, showing the reliability of the identification techniques employed in this work. Furthermore, the control system designed from the identified transfer functions presented good performance, providing stability and quick disturbance rejection, bolstering the validity of parametric identification methods applied to the Buck-Boost converter. / Este trabalho apresenta procedimentos para a modelagem de um conversor Buck-Boost com base em tÃcnicas de identificaÃÃo paramÃtricas offline com emprego de modelos matemÃticos tipo caixa preta e caixa cinza. Para a identificaÃÃo da funÃÃo de transferÃncia que relaciona a tensÃo de saÃda e a razÃo cÃclica, à empregado o modelo nÃo linear de Hammerstein, estrutura particularmente interessante para aplicaÃÃo em identificaÃÃo de conversores CC-CC por incorporar a caracterÃstica estÃtica nÃo linear da planta de forma dissociada ao seu comportamento dinÃmico. A identificaÃÃo da funÃÃo de transferÃncia citada à feita a partir de dados de entrada e saÃda do sistema, medidos em simulaÃÃo. Para determinaÃÃo dos parÃmetros da funÃÃo de transferÃncia que relaciona a tensÃo de saÃda e a razÃo cÃclica, à utilizado um algoritmo de mÃnimos quadrados nÃo recursivo com restriÃÃes. Quanto à identificaÃÃo da funÃÃo de transferÃncia que relaciona a corrente no indutor e a razÃo cÃclica, à empregado um modelo caixa preta linear de primeira ordem, sendo os parÃmetros de tal modelo determinados a partir da resposta em frequÃncia do sistema. Visando mostrar a utilidade dos modelos paramÃtricos, à realizado um projeto de controle com base nas expressÃes identificadas. O sistema de controle adotado à a versÃo digital de um compensador tipo 3 para a malha de tensÃo e de um compensador tipo 2 para a malha de corrente, que operam de forma alternada segundo a lÃgica ou. Os resultados de identificaÃÃo do sistema apresentam uma excelente concordÃncia entre os modelos paramÃtricos obtidos e o comportamento do conversor, mostrando a confiabilidade das tÃcnicas de identificaÃÃo empregadas nesse trabalho. Adicionalmente, o sistema de controle projetado a partir das funÃÃes de transferÃncia estimadas apresentou bom desempenho, garantindo estabilidade e rÃpida rejeiÃÃo a distÃrbios, reforÃando a validade dos mÃtodos de identificaÃÃo paramÃtrica aplicados ao conversor Buck-Boost.

Distributed generation is expected to play a significant role in remedying the many shortcomings in today’s energy market. In particular, fuel cell power generation will play a big part due to several advantages. Still, it is faced with its own challenges to tap into its potential as a solution to the crisis. The responsibilities of the Power Conditioning Unit (PCU), and thus its design, are therefore complex, yet critical to the fuel cell system’s performance and ability to meet the requirements. To this end, the dc-dc converter, considered the most critical component of the PCU for optimum performance, is closely examined. The selected converter is first modeled to gain insight into its behavior for the purpose of designing suitable compensators. MATLAB is then used to study the results using the frequency domain, and it was observed that the converter offers its own unique challenges in terms of closed-loop performance and stability. These limitations must therefore be carefully accounted for and compensated against when designing the control loops to achieve the desired objectives. Negative feedback control to ensure robustness is then discussed. The insertion of a second inner loop in Current Mode Control (CMC) offers several key advantages over single-loop Voltage Mode Control (VMC). Furthermore, the insertion of a Current Error Amplifier (CEA) in Average Current Mode Control (ACMC) helps overcome many of the problems present in Peak Current Mode Control (PCMC) whilst allowing much needed design flexibility. It is therefore well suited for this application in an attempt to improve the dynamic behavior and overcoming the shortcomings inherent in the converter. The modulator and controller for ACMC are then modeled separately and combined with the converter’s model previously derived to form the complete small-signal model. A suitable compensation network is selected based on the models and corresponding Bode plots used to assess the system’s performance and stability. The resulting Bode plot for the complete system verifies that the design objectives are clearly met. The complete system was also built in MATLAB/Simulink, and subjected to external disturbances in the form of stepped load changes. The results confirm the system’s excellent behavior despite the disturbance, and the effectiveness of the control strategy in conjunction with the derived models. To meet the demand in many applications for power sources with high energy density and high power density, it is constructive to combine the fuel cell with an Energy Storage System (ESS). The hybrid system results in a synergistic system that brings about numerous potential advantages. Nevertheless, in order to reap these potential benefits and avoid detrimental effects to the components, a suitable configuration and control strategy to regulate the power flow amongst the various sources is of utmost importance. A robust and flexible control strategy that allows direct implementation of the ACMC scheme is devised. The excellent performance and versatility of the proposed system and control strategy are once again verified using simulations. Finally, experimental tests are also conducted to validate the results presented in the dissertation. A scalable and modular test station is built that allows an efficient and effective design and testing process of the research. The results show good correspondence and performance of the models and control design derived throughout the thesis.