Fuel cells are a clean, high-efficiency source for power generation. This innovative technology is going to penetrate all aspects in our life, from utility distributed power, transportation applications, down to power sources for portable devices such as laptop computer and cell phones. To enable the usage of fuel cell, developing power converters dedicated for fuel cells becomes imminent. Currently, the full-bridge converter is the dominating topology in high power dc/dc applications. Although multiphase converters have been proposed, most of them are dealing with high input-voltage systems, and their device characteristic is not suitable for a low voltage source such as a fuel cell. For a high power fuel cell system, high voltage conversion ratios and high input currents are the major obstacles to achieving high-efficiency power conversions. This dissertation proposes a novel 3-phase 6-leg dc/dc power converter with transformer isolation to overcome these obstacles. Major features of the proposed converter include: (1) Increase converter power rating by paralleling phases, not by paralleling multiple devices; (2) Double the output voltage by transformer delta-wye connection, thus lowering the turns-ratio; (3) Reduce the size of output filter and input dc bus capacitor with interleaved control; (4) Achieve Zero-Voltage Zero-Current Switching (ZVZCS) over a wide load range without auxiliary circuitry. High conversion efficiency above 96% is verified with different measurement approaches in experiments. This dissertation also presents the power stage and control design for the proposed converter. Control design guideline is provided and the design result is confirmed with both simulation and hardware experiments. When using the fuel cell for stationary utility power applications, a low-frequency ripple interaction was identified among fuel cell, dc/dc converter and dc/ac inverter. This low frequency ripple tends to not only damage the fuel cell, but also reduce the source capability. This dissertation also investigates the mechanism of ripple current propagation and exploits the solutions. A linearized ac model is derived and used to explain the ripple propagation. An active ripple reduction technique by the use of the current loop control is proposed. This active current loop control does not add extra converters or expensive energy storage components. Rather, it allows a reduction in capacitance because the ripple current flowing into the capacitor is substantially reduced, and less capacitance can be used while maintaining a clean dc bus voltage. The design process and guideline for the proposed control is suggested, and the effectiveness of this active control is validated by both simulation and experimental results. / Ph. D.

Yes / In this paper, an improved Shiono-Knight model (SKM) has been proposed to calculate the rectangular compound open channel flows by considering a Multi-Zonal (MZ) approach in modelling turbulence and secondary flows across lateral flow direction. This is an effort to represent natural flows with compound shape more closely. The proposed model improves the estimation of secondary flow by original SKM model to increase the accuracy of depthaveraged velocity profile solution formed within the transitional region between different sections (i.e. between main-channel and floodplain) of compound channel. This proposed MZ model works by sectioning intermediate zones between floodplain and main-channel for running computation in order to improve the modelling accuracy. The modelling results have been validated using the experimental data by national UK Flood Channel Facility (FCF). It has been proven to work reasonably well to model secondary flows within the investigated compound channel flow cases and hence produce better representation to their flow lateral velocity profile.

In order to increase the power density, the discontinuous conducting mode (DCM) and small inductance is adopted for high power bidirectional dc-dc converter. The DCM related current ripple is minimized with multiphase interleaved operation. The turn-off loss caused by the DCM induced high peak current is reduced by snubber capacitor. The energy stored in the capacitor needs to be discharged before device is turned on. A complementary gating signal control scheme is employed to turn on the non-active switch helping discharge the capacitor and diverting the current into the anti-paralleled diode of the active switch. This realizes the zero voltage resonant transition (ZVRT) of main switches. This scheme also eliminates the parasitic ringing in inductor current. This work proposes an inductance and snubber capacitor optimization methodology. The inductor volume index and the inductor valley current are suggested as the optimization method for small volume and the realization of ZVRT. The proposed capacitance optimization method is based on a series of experiments for minimum overall switching loss. According to the suggested design optimization, a high power density hardware prototype is constructed and tested. The experimental results are provided, and the proposed design approach is verified. In this dissertation, a general-purposed power stage model is proposed based on complementary gating signal control scheme and derived with space-state averaging method. The model features a third-order system, from which a second-order model with resistive load on one side can be derived and a first-order model with a voltage source on both sides can be derived. This model sets up a basis for the unified controller design and optimization. The Δ-type model of coupled inductor is introduced and simplified to provide a more clearly physical meaning for design and dynamic analysis. These models have been validated by the Simplis ac analysis simulation. For power flow control, a unified controller concept is proposed based on the derived general-purposed power stage model. The proposed unified controller enables smooth bidirectional current flow. Controller is implemented with digital signal processing (DSP) for experimental verification. The inductor current is selected as feedback signal in resistive load, and the output current is selected as feedback signal in battery load. Load step and power flow step control tests are conducted for resistive load and battery load separately. The results indicate that the selected sensing signal can produce an accurate and fast enough feedback signal. Experimental results show that the transition between charging and discharging is very smooth, and there is no overshoot or undershoot transient. It presents a seamless transition for bidirectional current flow. The smooth transition should be attributed to the use of the complementary gating signal control scheme and the proposed unified controller. System simulations are made, and the results are provided. The test results have a good agreement with system simulation results, and the unified controller performs as expected. / Ph. D.

Ce mémoire s'intéresse à l'étude des comportements dynamiques dans les systèmes d'électronique de puissance. La nature des cycles définis par les variables d'état du système peut être établie soit grâce à un diagramme de bifurcation soit grâce aux multiplieurs de Floquet obtenus par le modèle discret du convertisseur. Cependant, pour certaines structures de convertisseur statique, un modèle discret ne peut être obtenu sans introduire de fortes hypothèses. Les bifurcations de type flip (à l'échelle des hautes fréquences) ne peuvent pas être prédites par le modèle moyen alors qu'elles apparaissent dans de nombreux convertisseurs statiques. Une nouvelle formulation d'un modèle moyen continu permettant d'estimer le premier point de bifurcation (flip, Hopf...) a été présentée. La méthodologie proposée a été appliquée à un convertisseur boost, en mode de conduction continu, contrôlé en courant par différents types de contrôleur à fréquence fixe de commutation. Ensuite, ce modèle moyen a été utilisé pour le dimensionnement des paramètres du régulateur de courant associé à un système de filtrage actif. Les résultats obtenus par le modèle proposé ont été validés par simulation numérique ainsi que par des résultats expérimentaux.Les méthodes d'analyse des cycles limites ont été étendues pour étudier la stabilité des réseaux DC lorsque la fréquence de coupure du filtre LC reliant la source de tension avec le convertisseur statique est proche de la fréquence apparente de commutation des convertisseurs utilisés. Différents modèles discrets ont été développés pour pouvoir considérer ou non le caractère échantillonné de la commande des convertisseurs / This thesis discusses the study of dynamic behavior in power electronics systems. The nature of cycles defined by the state variables of the system can be settled either by a bifurcation diagram or by Floquet multipliers obtained from the discrete model of the converter. However, for certain structures of static converters, a discrete model cannot be obtained without introducing large assumptions. The flip bifurcations (fast-scale) cannot be predicted by the averaged model whereas they appear in many statics converters. A new formulation of a continuous averaged model is presented to estimate the first bifurcation point (flip, Hopf ...). The proposed methodology is applied to a boost converter, operated in continuous conduction mode, controlled by different types of fixed frequency switching current controllers. Then, this averaged model is used for the design of the current controller parameters associated with an active filter system. The variables obtained by the proposed model have been validated by numerical simulation and experimental results. Methods of analysis of limit cycles are extended to study the stability of DC networks when the cutoff frequency of the LC filter between the voltage source and the converter is close to the switching frequency of converters. Different discrete-time models have been developed in order to consider whether to use sampled or no-sampled converter control