Date of Award
Doctor of Philosophy (PhD)
Electrical and Computer Engineering (Holcomb Dept. of)
William Rod Harrell
An unremitting and persistent research for developing advanced multilevel converter topologies with improved characteristics, performance, modulation methods, and control techniques still goes on. The multilevel converters are well-suited for power conversion areas demanding more power density, reliablity, efficiency, and power quality. The emergence of new power conversion areas such as grid integration of large-scale renewable energy sources and large-capacity energy storage necessitate the evolution of the classic multilevel converters.
This dissertation proposes innovative logic-equation-based modulation techniques and control methods for new multilevel converter topologies. Chapter I reviews the classic multilevel converter configurations. Chapter II proposes an enhanced topology for the flying-capacitor multilevel converters to reduce the voltage diversity and stored energy of the flying-capacitors within switching-power-cells in order to improve the multilevel converter integrity and modularity. Chapter III provides general solutions for nonlinear transcendental equations of the cascaded H-bridge converters modulated under the selective harmonic elimination method to eliminate the significant low-order line-to-line voltage harmonics. Chapters IV, V, VI investigate the conduction and switching power losses in the flying-capacitor-based multilevel converter topologies utilizing the proposed closed-form formulas. Chapter VII proposes a duo-active-neutral-point-clamped multilevel converter topology with an innovative control technique and modulation method. The proposed converter reduces the number of the high-frequency medium-voltage semiconductor power switches in the active-neutral-point-clamped multilevel converter family by 50%. The substantial reduction in the number of the switching-power-cells by 50% in comparison with the classic active-neutral-point-clamped converter along with a drastic decrease in the total voltage rating and the stored energy of the flying-capacitors are the paramount advantages of the proposed multilevel converter topology. Chapter IIX suggests an improved configuration for the active-neutral-point-clamped multilevel converters to reduce the flying-capacitors in this breed. Moreover, logic-equation-based control technique is developed in chapter IIX to modulate the proposed multilevel converter topology. Chapter IX proposes a novel multilevel converter topology which is realized by the cascaded connection of the classic two-level or multilevel converters and modular-concatenated-cell H-bridge converters. This results in a significant improvement in the harmonic spectra of the generated voltage at the converter output stage along with a reduction in the dv/dt-stress on the semiconductor power switches. Utilizing the flying-capacitors instead of the isolated dc-voltage sources is the major distinction between the proposed modular-concatenated-cell multilevel converter topology and the cascaded H-bridge inverters. The voltage rating of the power switches and the flying-capacitors are 1 p.u. in the proposed configuration. This boosts the integrity and modularity of the flying-capacitor-based multilevel converters. Moreover, chapter IX derives an innovative modulation method exploiting logic-equations for balancing the flying-capacitor voltages at their reference voltage-levels within modular-concatenated-cells of the proposed multilevel converter. Chapter X proposes a novel active capacitor voltage balancing technique for flying-capacitor multilevel converters using logic-equations. The foremost advantage of the proposed active control technique is that it does not require any complex computations.
Dargahi, Vahid, "Innovative Control And Modulation Methods Exploiting Logic-Equations For Novel Multilevel Converter Topologies" (2017). All Dissertations. 2547.