Browsing by Author "Essakiappan, Somasundaram"
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Item Bi-directional Current-fed Medium Frequency Transformer Isolated AC-DC Converter(2011-08-08) Essakiappan, SomasundaramThe use of high power converters has increased tremendously. Increased demand for transportation, housing and industrial needs means that more number of power converters interact with the utility power grid. These converters are non-linear and they draw harmonic currents, significantly affecting power quality. To reduce harmonics, filters, power factor correction circuits and capacitor banks are required. And the development of hybrid technologies and renewable energy power stations trigger a demand for power converters with bi-directional capabilities. The objective of this thesis is to develop a high power quality, bi-directional AC-DC power converter that is a solution to the aforementioned problems. This thesis studies an existing topology for a high power AC-DC power conversion with transformer isolation. The topology consists of an uncontrolled rectifier followed by a DC-DC converter to produce a set voltage output. A design example of the topology is simulated using the PSIM software package (version 6). Critical performance characteristics such as power factor and total harmonic distortion are analyzed. Following that study a new topology is proposed, which is an improvement over the older design, with reduced power conversion stages. The new topology has a fully controlled current source Pulse Width Modulation (PWM) rectifier at the front end to replace the uncontrolled rectifier and DC-DC combination. This topology has multiquadrant operational capabilities and the controller employs Selective Harmonic Elimination techniques to produce the programmed PWM switching functions for the rectifier. A design example of the converter and the digital controller are simulated in PSIM environment. The converter input current THD (Total Harmonic Distortion) and input power factor are within IEEE 519 and DoE standards. The converter is simulated in both first and fourth quadrant operations. A side-by-side comparison of the two topologies is done with respect to design and performance features such as power factor, THD, filter size, etc. The new topology converter provides performance superior to that of the older topology. Finally the thesis explores possible applications for the converter in power supplies, renewable energy and hybrid technologies.Item Multilevel Converter Topologies for Utility Scale Solar Photovoltaic Power Systems(2014-04-30) Essakiappan, SomasundaramRenewable energy technologies have been growing in their installed capacity rapidly over the past few years. This growth in solar, wind and other technologies is fueled by state incentives, renewable energy mandates, increased fossil fuel prices and environmental consciousness. Utility scale systems form a substantial portion of electricity capacity addition in modern times. This sets the stage for research activity to explore new efficient, compact and alternative power electronic topologies to integrate sources like photovoltaics (PV) to the utility grid, some of which are multilevel topologies. Multilevel topologies allow for use of lower voltage semiconductor devices than two-level converters. They also produce lower distortion output voltage waveforms. This dissertation proposes a cascaded multilevel converter with medium frequency AC link which reduces the size of DC bus capacitor and also eliminates power imbalance between the three phases. A control strategy which modulates the output voltage magnitude and phase angle of the inverter cells is proposed. This improves differential power processing amongst cells while keeping the voltage and current ratings of the devices low. A battery energy storage system for the multilevel PV converter has also been proposed. Renewable technologies such as PV and wind suffer from varying degrees of intermittency, depending on the geographical location. With increased installation of these sources, management of intermittency is critical to the stability of the grid. The proposed battery system is rated at 10% of the plant it is designed to support. Energy is stored and extracted by means of a bidirectional DC-DC converter connected to the PV DC bus. Different battery chemistries available for this application are also discussed. In this dissertation, the analyses of common mode voltages and currents in various PV topologies are detailed. The grid integration of PV power employs a combination of pulse width modulation (PWM) DC-DC converters and inverters. Due to their fast switching nature a common mode voltage is generated with respect to the ground, inducing a circulating current through the ground capacitance. Common mode voltages lead to increased voltage stress, electromagnetic interference and malfunctioning of ground fault protection systems. Common mode voltages and currents present in high and low power PV systems are analyzed and mitigation strategies such as common mode filter and transformer shielding are proposed to minimize them.