Date of Award

12-2018

Degree Name

Doctor of Philosophy

Department

Electrical and Computer Engineering

First Advisor

Dr. Ikhlas M. Abdel-Qader

Second Advisor

Dr. Pablo Gomez

Third Advisor

Dr. Azim Houshyar

Keywords

multilevel converter, multilevel inverter, AFE rectifier, h-bridge topology, improved control strategies

Abstract

Rising worldwide demand for energy, pressing economic constraints, and substantial environmental concerns have led to the harvesting of clean, renewable energy sources such as solar PV and wind energy. To integrate these new resources into the power grid, power electronic converters play a crucial role and have become indispensable devices.

Multilevel converters are considered to be state-of-the-art, efficient solutions for medium- and high-voltage industrial applications, due to the difficulty of connecting traditional two-level converters to high- and medium-voltage grids, since the single power switch cannot stand such high voltage. The standard multilevel converter topologies, such as the neutral point clamped (NPC), flying-capacitor multilevel (FCM), and cascaded H-bridge (CHB), are currently used; however, the need for higher efficiency multilevel topologies that require the lowest number of components These benefits make the proposed. The cascade H-bridge multilevel converter topology has been the preferred solution over other standard multilevel converter topologies because each level has the same structure, with no extra clamping diodes or capacitors. Therefore, this study proposes a CHB converter with a new structure that requires fewer number of the insulated-gate bipolar transistors (IGBTs) for generating AC voltage at the output stage of the converter, using a modified phase shift pulse width modulation (PWM) control system. The reduction in the number of required IGBTs will decrease the converter cost, size, and installation area, while also improving its reliability. These benefits make the proposed topology a good candidate for renewable energy applications, especially for photovoltaic integration. The ability of the proposed inverter to generate the desired output voltage waveform has been validated through a laboratory low-power prototype. A comparative analysis with the other typologies is provided, which supports the capability of the proposed topology for reducing the number of high-frequency IGBTs and isolated DC-link.

Since one of the most commonly and extensively used converter topologies in power electronics are rectifiers, a grid-connected, active front-end (AFE) rectifier based on the suggested reduced-switch-count CHB converter family is also proposed. The bidirectional capabilities of the proposed multilevel converter verified through simulation and operation in the inverting and rectifying modes. It was shown that the propose typology is able to inject the commanded active and reactive power into the grid in addition to the abilities to absorb power from the grid. The voltage-oriented control (VOC) method has been implemented on the grid-tied bidirectional multilevel converter, and simulation results verify the benefits of the new typology.

The proposed converter, modulated with the selective harmonic elimination method (SHEM), has inherited complexity due to the set of nonlinear equations derived to determine the switching angles for the CHB converter with different modulation indices, voltage levels, and various harmonics selected for elimination. Therefore, a generalized solution to address total harmonic distortion (THD) is also proposed.

Access Setting

Dissertation-Campus Only

Restricted to Campus until

12-2028

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