Integration of energy storage components with cascaded H-bridge multilevel converters
"In recent years, multilevel converters have gained considerable attention in medium-voltage motor drive and grid applications. This popularity is owed to their reduced voltage stress on the semiconductor devices used in their structure. In addition, multilevel converters generate near sinusoidal outputs with low harmonic distortions. Other advantages of such converters include inherent modularity and low dv/dt stresses. In general, multilevel power electronic converters are classified into three main topologies: diode-clamped, flying-capacitor, and cascaded H-bridge. A cascaded H-bridge multilevel converter is created when several H-bridge cells are placed in series. Each H-bridge cell must be fed by a stiff voltage source. In earlier implementations, every one of these voltage sources had to contribute to the overall power supplied to the load. Later, it was demonstrated that under certain operating conditions, one can replace all but one of these sources with energy storage devices, e.g., capacitors. In other words, the entire power can be supplied by only one source. The replacing capacitors must only maintain a constant dc voltage supplying zero net power. Although this approach benefits from cost reductions, balancing the voltages across the replacing capacitors turns out to be a challenge. In this thesis, the operating conditions under which the capacitor voltage regulation is feasible are first analytically investigated. The results show that the amplitude of the output voltage as well as the power factor of the load current determines the regulation range when the staircase modulation method is employed. In order to extend the regulation range for the replacing capacitors, a new control scheme - phase shift modulation - is proposed. This method is more robust when compared to existing methods. In this method, the main H-bridge cell of the multilevel converter operates at the fundamental frequency and the auxiliary cells run at the PWM frequency. Finally, the sigma-delta modulation method has been utilized to extend the capacitor voltage regulation range. This method benefits from simplicity in implementation in comparison to PWM techniques. The analytical and simulation results prove the effectiveness of the proposed approaches. They are also consistent with the results of the experiment"--Abstract, page iv.
Sigma-delta modulator, Electrical and Computer Engineering
Missouri University of Science and Technology
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