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This dissertation is the result of research and development of a power conditioning system for Superconductive Magnetic Energy Storage System. The dominant challenge of this research was to develop the power conditioning system that can match slowly varying dc voltage and dc current on the super conductive magnet side with the ac voltages and ac currents on the utility side. At the same time the power conditioning system was required to provide a bi-directional power flow to the superconductive magnet. The focus of this dissertation is a three-level diode clamped dc-ac converter which is a principle part of the power conditioning system. Accordingly, this dissertation deals with the space vector modulation of three-level converters and introduces a computationally very efficient three-level space vector modulation algorithm that is experimentally verified. Furthermore, the proposed space vector modulation algorithm is successfully generalized to allow equally efficient, real time implementation of space vector modulation to dc-ac converters with virtually any number of levels. The most important advantage of the proposed concept is in the fact that the number of instructions required to implement the algorithm is almost independent from the number of levels in a multilevel converter. More on the side of the control of multilevel converters, the particular attention in this dissertation is paid to the problem of charge balance in the split dc-link capacitors of three-level neutral-point-clamped converters. It is a known fact that although the charge balance in the neutral point can be maintained on a line cycle level, a significant third harmonic current flows into the neutral point for certain loading conditions, causing the neutral point voltage ripple. The logical consequence of that ripple is the deteriorated quality of the output voltage waveforms as well as the increased voltage stress on the switching devices. This was the motivation to more carefully explore the loading conditions that cause the unbalance, as well as to study the fundamental limitations of dc-link capacitor charge balancing algorithms. As a part of that work, a new model of the neutral point current in the rotating coordinate frame is developed as a tool in investigation of theoretical limitations and in providing some intuitive insight into the problem. Additionally, the low frequency ripple is quantified and guidelines are offered that can help size the dc-link capacitors. Because the study of the neutral point balance identified the loading conditions, that under some possible system constraints, cause an unavoidable neutral point voltage ripple, a feed forward type of control method is developed next. The proposed feed forward algorithm can effectively prevent the neutral point voltage ripple from creating distortions in the converter output voltage under all loading conditions and without causing additional disturbance in the neutral point voltage. The feed forward method is developed for a sine triangle as well as for the space vector type PWM algorithm. The simulation results that include the full dynamic model of the converter and load system validate the feed forward approach and prove that the feed forward algorithm can effectively compensate the effect of the neutral point voltage ripple. The simulation results are than experimentally verified. / Ph. D.

This thesis focuses on the study, implementation and improvement of a digital controller for a power factor correction (PFC) converter. The development of the telecommunications industry and the Internet demands reliable, cost-effective and intelligent power. Nowadays, the telecommunication power systems have output current of up to several kilo amperes, consisting of tens of modules. The high-end server system, which holds over 100 CPUs, consumes tens of kilowatts of power. For mission-critical applications, communication between modules and system controllers is critical for reliability. Information about temperature, current, and the total harmonic distortion (THD) of each module will enable the availability of functions such as dynamic temperature control, fault diagnosis and removal, and adaptive control, and will enhance functions such as current sharing and fault protection. The dominance of analog control at the modular level limits system-module communications. Digital control is well recognized for its communication ability. Digital control will provide the solution to system-module communication for the DC power supply. The PFC converter is an important stage for the distributed power system (DPS). Its controller is among the most complex with its three-loop structure and multiplier/divider. This thesis studies the design method, implementation and cost effectiveness of digital control for both a PFC converter and for an advanced PFC converter. Also discussed is the influence of digital delay on PFC performance. A cost-effective solution that achieves good performance is provided. The effectiveness of the solution is verified by simulation. The three level PFC with range switch is well recognized for its high efficiency. The range switch changes the circuit topology according to the input voltage level. Research literature has discussed the optimal control for both range-switch-off and range-switch-on topologies. Realizing optimal analog control requires a complex structure. Until now optimal control for the three-level PFC with analog control has not been achieved. Another disadvantage of the three-level PFC is the output capacitor voltage imbalance. This thesis proposes an active balancing solution to solve this problem. / Master of Science

Published Article / Several forecasting models are available for research in predicting the shape of electric load curves. The development of Artificial Intelligence (AI), especially Artificial Neural Networks (ANN), can be applied to model short term load forecasting. Because of their input-output mapping ability, ANN's are well-suited for load forecasting applications. ANN's have been used extensively as time series predictors; these can include feed-forward networks that make use of a sliding window over the input data sequence. Using a combination of a time series and a neural network prediction method, the past events of the load data can be explored and used to train a neural network to predict the next load point. In this study, an investigation into the use of ANN's for short term load forecasting for Bloemfontein, Free State has been conducted with the MATLAB Neural Network Toolbox where ANN capabilities in load forecasting, with the use of only load history as input values, are demonstrated.

Electro-Hydraulic (EH) systems are commonly used in the industry for applications that require high power-weight ratios and large driving forces. The EH system studied in this master thesis have recently been upgraded with new hardware components and as a part of this upgrade a new controller was requested. The system consists of a controller that computes a control signal for an electric motor. The motor drives a gear pump that generates a flow of hydraulic fluid. The flow is then directed to a cylinder. The movements of a piston in the cylinder is affected by the flow and the piston position can be measured. The measured piston position is then fed back to the controller and the control loop is complete. The system was previously controlled using a Proportional-Integral-Derivative (PID) controller and the purpose of this thesis is to compare the old controller with alternative control strategies suitable for this application. The evaluation of the controllers is based on both software and hardware simulations and results in a recommendation for final implementation of the best suited controller. The control strategies chosen for investigation are: a retuned PID controller, a PID controller with feed forward from reference, a PID based cascade controller, a Linear Quadratic (LQ) controller, and a Model Predictive Controller (MPC). To synthesize the controllers an approximate model of the system is formed and implemented in the software environment Matlab Simulink. The model is tuned to fit recorded data and provides a decent estimation of the actual system. The proposed control strategies are then simulated and evaluated in Simulink with the model posing as the real system. These simulations resulted in the elimination of the cascade controller as a possible candidate since it proved unstable for large steps in the reference signal. The remaining four controllers were all selected for simulation on the real hardware system. Unfortunately the MPC was never successfully implemented on the hardware due to some unknown compatibility error and hence eliminated as a possible candidate. The three remaining control strategies, PID, PID with feed forward from reference and the LQ controller, were all successfully implemented and simulated on hardware. The results from the hardware simulations compared to simulations made with the old controller, as well as the results from the software simulations, were then evaluated. Depending on the purpose one of two control strategies is recommended for this application. The LQ controller achieved the best overall performance and is presented as the control strategy best suited for this application.

The Dynamic Positioning (DP) systems currently available employ a feedback controller based on the position and heading error, associated with a wind feed-forward control action to keep the vessel position. This last technology improves the eficiency of the DP by anticipating wind loads. However, there is no consolidated technology to feed the controller with wave loads information and the major issue is related to the wave measurement process. The present thesis aims at filling this technological gap by proposing an alternative approach for the measuring of waves near the vessel. Image-based measurement methods are noninvasive and can produce a spatial and temporal description of the surface, making them suitable for recovering the geometry of liquid surfaces. Nevertheless, these surface reconstruction methods, particularly those applying stereoscopic approaches, have dificulty in measuring waves produced in laboratory facilities, mainly because, in such conditions, the water surface is smooth, translucent and highly specular. Alternatively, intensity based image methods are capable of dealing with these surface characteristics, if employed under controlled conditions and if a suitable reflectance model is selected. In the present study, a well-known reflectance model is applied to recover the parameters of regular waves produced in an offshore basin. Firstly, an experimental setup is proposed, designed to grant appropriate conditions for the application of the reflectance model, even under conventional laboratory illumination. Later, the second set of experiments with a model scale DP vessel are presented, which applied the image-based method developed before as a wave feed-forward system. Three control methods are evaluated, namely: PID, PD (Proportional-Derivative) and the PD with the wave feed-forward. Results demonstrated that the presence of the wave feed-forward reduced the steady error of the PD controller. These are the first steps towards the practical use of the wave feed-forward, and several aspects remain pending. However, the promising results and discussions regarding the future steps define the contribution of this work. / Os sistemas atuais de Posicionamento Dinâmico (DP) empregam técnicas de controle baseadas na realimentação da posição e do aproamento, associadas à compensação antecipada das cargas de vento (wind feed-forward) para manter a posição da embarcação. Esta tecnologia melhora a experiência do DP, pois antecipa a ação do vento. Entretanto, não há tecnologia consolidada para a pré-compensação de forças ondas (wave feed-forward) e o maior desafio reside na medição desses agentes ambientais. A presente tese tem como objetivo preencher essa lacuna tecnológica propondo uma abordagem alternativa para a medição das ondas próxima ao casco. Métodos de medição baseados em imagem são não invasivos e produzem descrições espaciais e temporais da superfície analisada, tornando-os particularmente adequados à medição de superfícies líquidas. Entretanto, os métodos comumente empregados, principalmente aqueles baseados em imagens estéreo, são incapazes de medir ondas produzidas em ambiente de laboratório, pois, nestas condições, a superfície da água é lisa, apresenta transparência e comporta-se como um espelho. Por outro lado, métodos baseados na intensidade de luz são capazes de lidar com tais características, se aplicados em condições adequadas e quando empregam modelos apropriados. Neste trabalho, um modelo de reetância amplamente conhecido é empregado para extrair os parâmetros principais de ondas regulares produzidas em um tanque de provas offshore. Inicialmente, propõe-se um arranjo experimental que permita a aplicação do modelo de refetância construído, mesmo sob as condições de iluminação naturais do laboratório. Posteriormente, um segundo conjunto de experimentos com um modelo de embarcação DP é proposto, no qual se aplica o método baseado em imagem, testado anteriormente, como sistema de pré-compensação de forças de onda. Três métodos de controle são avaliados, quais sejam: PID (Proporcional-Integral-Derivativo), PD (Proporcional-Derivativo) e o PD-WFF (PD associado ao wave feed-forward). Os resultados demonstram que a presença pré-conpensação de forças de onda reduz o erro em regime do controlador PD. Estes são os primeiros passos em direção da incorporação da pré-compensação de forças de onda nos sistemas DP reais e diversos aspectos técnicos ainda estão pendentes. Entretanto, os resultados promissores e discussões acerca dos futuros passos da pesquisa definem a contribuição do presente trabalho no campo do Posicionamento Dinâmico.

Les neurones pyramidaux sont constamment bombardés par une activité GABAergique spontanée qui régule le comportement de la décharge neuronale. Des résultats récents ont montré que cette activité spontanée GABAergique pouvait moduler l'excitabilité mais aussi la fidélité temporelle de décharge d'un neurone définie comme sa capacité à reproduire à l'identique un patron de décharge lors de la présentation répétée d'un même stimulus. D'autre part, de nombreuses études ont caractérisé l'existence d'une plasticité à court-terme de l'activité GABAergique médiée par les endocannabinoïdes. Ce phénomène, connu sous le nom de DSI (Depolarization-induced Suppression of Inhibition) a été décrit dans de nombreuses structures comme le cervelet, le cortex ou encore l'hippocampe.Au cours de ma thèse, j'ai étudié quelles pouvaient être les conséquences fonctionnelles de la production d'endocannabinoïdes sur l'activité neuronale et en particulier sur la fidélité temporelle de la décharge. Dans un premier temps nous avons montré que le profil de décharge in vivo des cellules de lieu de l'hippocampe pouvait induire, quand il était rejoué in vitro, le phénomène de DSI. Nous avons observé ensuite que cette diminution transitoire de la transmission GABAergique était associée à une amélioration de la fidélité temporelle de la décharge.En conclusion, nos travaux suggèrent que l'activité des cellules de lieu de la région CA1 de l'hippocampe peut provoquer, par la synthèse et la libération rétrograde d'endocannabinoïdes, une diminution à court-terme de l'activité GABAergique reçue par ces cellules avec pour conséquence des modifications de la précision temporelle de la décharge neuronale. / Pyramidal neurons are constantly bombarded by spontaneous GABAergic activity that regulates their firing behaviour. Recent results have shown that this spontaneous GABAergic activity can modulate both the excitability and the temporal fidelity of action potential discharge (Caillard, 2011). Many studies have characterized the existence of short-term plasticity of GABAergic activity mediated by endocannabinoids. This phenomenon, known as DSI (Depolarization-induced Suppression of Inhibition) has been described in many brain structures such as the cerebellum, cortex or hippocampus (for review see Freund et al., 2003; Kano et al. 2009).During my PhD thesis, I have evaluated the functional consequences of the endocannabinoid production on neuronal activity and in particular on the spike-time precision of the CA1 pyramidal neurons. As a first step we have shown that the in vivo firing pattern of place cells could induce, when replayed in vitro, a decrease in spontaneous GABA release by the endocannabinoid signalling pathway. We then observed that this transient depression of GABAergic transmission improved spike-time precision of CA1 pyramidal neurons.In conclusion, our work suggests that, in the hippocampus, CA1 place cell firing can induce, following the synthesis and retrograde release of endocannabinoids, a short-term decrease in the GABAergic activity received by these cells that consequently affects their spike-time precision.

This thesis focuses on simulating, detecting, localizing and classifying the power quality disturbances using advanced signal processing techniques and neural networks. Primarily discrete wavelet and Fourier transforms are used for feature extraction, and classification is achieved by using neural network algorithms. The proposed feature vector consists of a combination of features computed using multi resolution analysis and discrete Fourier transform. The proposed feature vectors exploit the benefits of having both time and frequency domain information simultaneously. Two different classification algorithms based on Feed forward neural network and adaptive resonance theory neural networks are proposed for classification. This thesis demonstrates that the proposed methodology achieves a good computational and error classification efficiency rate.

Elevation axis model of a barrel stabilization system is constructed. The nonlinearities which are considered in the model are orifice flow characteristics, coulomb friction, hard-stop limits, kinematics of the system and unbalance on the barrel. A Simulink&reg / model for the servo valve, actuation system and barrel is constructed. Servo valve identification is made via the actual test data. Compressibility of the hydraulic fluid is taken into consideration while modeling the actuation system. Friction model is simulated for different cases. Controller of the system is constructed by two PIDs, one for each of the velocity and the position loops. Velocity feed forward can reduce the time to make a quick move by the system. The disturbance is evaluated from a given road profile and disturbance feed forward is applied to the system.