Rapidly rising fossil fuel costs along with increased environmental awareness has encouraged the development of alternative energy sources. Such sources include fuel cells, wind, solar and ocean tide power. Among them, fuel cells have received increased interest in the recent years. This is mainly due to their high efficiency, modularity, and simple construction. However, due to their low output voltage and wide variation from no load to full load, a power electronics converter is required to interface the fuel cell with its loads. This dissertation focuses on developing a set of considerations that will assist designers of the power electronics converter in the design and optimization of the system. These design considerations are obtained analytically and verified experimentally and allow obtaining an efficient and stable fuel cell – power converter system. In addition to the design guidelines this dissertation presents new power converter topologies that do not require the use of transformers to achieve a large voltage gain. Further a new modular fuel cell power converter system that divides the fuel cell stack to optimize power generation is proposed. It is shown by means of mathematical analysis and experimental prototypes that the proposed solutions contribute to the reduction of size and cost of the power converter as well to increase the efficiency of the system.

Thesis ( Ph.D. ) -- University of Texas at Arlington, 2008.

This dissertation examines single-phase nonlinear power electronic loads. The transient response of power electronic loads is unlike traditional linear loads. Therefore, a composite power electronic transient load model is developed. The load model dynamics are validated with actual utility voltage sag response data, laboratory controlled load response testing, and power electronic load dynamic simulations. The power electronic load model is applied in the University of Texas at Austin power system. The system transient response is unique and considerably different from what it would be with traditional linear loads. The power electronic load can be friendly or unfriendly to the system depending on the fault and system configuration. / text

This thesis explores some new applications in power electronics for supercapacitors. This involves the design and development of dc-dc converters to interface the supercapacitor banks with the rest of the power electronic system. Two applications for supercapacitors are proposed and analyzed. The first application is aimed at high power applications such as motor drives. The proposed approach compensates the effect of voltage sags in the dc link of typical adjustable speed drives, thus reducing speed fluctuations in the motor and eliminating the possibility of nuisance tripping on the drive control board. The second approach presented in this thesis explores the use of supercapacitors to extend run-time for mobile devices such as laptop computers and hand held devices. Three possible approaches are explored: a) Supercapacitors connected directly across the battery; b) Battery-inductor-supercapacitor connection; and c) Supercapacitor, and battery connected via a DC-DC converter. Analytical models, simulation and experimental results on a typical laptop computer are presented.

This thesis presents topologies and control methods to improve the efficiency and dynamic response of Electric Vehicle (EV) power converters. There are three main converters in an EV power conditioning system: a plug-in AC/DC converter, a low-voltage DC/DC converter, and a three-phase inverter. The focus of this thesis is to improve the plug-in AC/DC converter and the low-voltage DC/DC converter. A new topology is proposed to improve the efficiency and increase the reliability of the plug-in AC/DC converter. The plug-in AC/DC converter consists of a Power Factor Correction (PFC) stage, which is followed by a high voltage DC/DC converter for galvanic isolation. The proposed approach includes a simple and effective auxiliary circuit for the PFC stage, which guarantees soft-switching for the power switches. Next, a current-driven full-bridge topology is proposed for the high-voltage DC/DC conversion stage, which guarantees soft-switching and eliminates voltage spikes across the output diodes. Also, two control approaches are proposed in order to improve the dynamic response of the AC/DC converter. The first controller is based on nonlinear differential flatness theory, which can be used to improve the transient response of the AC/DC converter. The second controller is based on an optimized stabilizing control-Lyapunov function, which extends the stability margins and improves reliability. An optimized variable-frequency phase-shift controller is proposed for the low voltage DC/DC converter, which adaptively controls the amount of reactive current required to maintain soft-switching throughout the whole range of operation and minimizes the switching and conduction losses of the converter. Mathematical analysis, simulation, and experimental results are presented to verify the performance of the proposed techniques. / Thesis (Ph.D, Electrical & Computer Engineering) -- Queen's University, 2012-01-18 11:41:33.654

This thesis presents adaptive control techniques to improve the stability and performance of grid-tied single-phase inverter systems. There are three main control systems involved in the conversion of DC to AC power: a synchronizer, a current-controller, and a DC-bus controller. The focus of this thesis is to create a control system which responds to the changing operating conditions. In order to adapt to changing operating conditions, estimations of the variables which influence stability and performance are necessary. A synchronization system is proposed, which delivers very accurate estimation of the grid frequency and voltage amplitude across all operating conditions. The synchronization system also provides an accurate and normalized set of orthogonal vectors for the AC output current reference phase. An adaptive parameter estimation technique is proposed to passively estimate the LCL-filter parameters and DC input current. Using the parameter estimations of the output filter, an adaptive state-feedback system is proposed. Next, a frequency-adaptive current controller is proposed, which dynamically tunes to the frequency provided by the synchronizer. Finally, the DC-bus controller is enhanced with the grid-voltage estimate and DC current estimator. The proposed techniques are verified for performance and stability though simulation and experimental results. / Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2014-05-05 15:25:47.342

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

M.Ing. (Electrical and Electronic Engineering) / Many applications need a high power (> 500W) at a relatively low voltage. The known technology of this class of converters is primitive and such converters are big as a result thereof. An investigation is made into the suitability of matrix transformer technology in the supply of large currents at low voltages. A layout for this topology which make optimal use of its characteristics is proposed . This includes modularity, possible application in distributed power supplies and equal current sharing. A system is designed and models are created to optimize the design. The models are extended to accommodate the effects of unavoidable parasitics which was observed in the practical system. A system consisting of sixteen modular units with a total output current capability of 320A at a voltage of between 0 and 5V is developed. The output capability and thus the number of modules needed is dependent on the application. Provision is thus made for a large power range. Power is transformed at a frequency of 100kHz to keep the physical dimensions of the system small.

M.Ing. (Electrical & Electronic Engineering) / Switching losses in power electronic converters can be reduced by using snubbers, or resonant circuits. Simple snubbers can be implemented without much effort, but then energy is still dissipated, which reduces the efficiency of the converter. Regenerative snubbers are known, but these usually require complicated additional circuits. The use of nonlinear capacitors as turn-off snubbers has been investigated, and proved profitable. The main reason for this is the much smaller amount of energy which is stored in the the nonlinear capacitors, compared to linear capacitors. Resonant circuits reduce switching losses by allowing switchings to occur when the voltage across the switching device, or the current through the switching device is zero. This effect is called soft-switching and to obtain this, resonance between an inductor and capacitor is usually involved. Resonant circuits have the disadvantage that the installed switching power cannot be used optimally. This drawback has been reduced by using a saturable inductor in the resonant circuit. This thesis considers the use of both nonlinear capacitors and nonlinear inductors in a resonant phase arm. It was found that the nonlinear resonant elements lead to very low overdimensioning of the switching devices, and minimal switching losses in such a phase arm. Very few additional components are used, which keeps the cost of the converter down. A 1.5kW converter was constructed in which the double nonlinear resonant phase arm was implemented. This converter was investigated both experimentaly, and by means of computer simulations. The converter is also compared with the current state of the art. It is concluded that resonant phase arms with both. nonlinear inductors and nonlinear capacitors can function effectively in power electronic converters. Some important advantages are also assosiated with the use of the double nonlinear resonant circuit.

University of Macau / Faculty of Science and Technology / Department of Electrical and Computer Engineering