IEEE Transactions on Industrial Electronics

Multilevel inverter technology has emerged recently as a very important alternative in the area of high-power medium-voltage energy control. This paper presents the most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-clamped (flying capacitor), and cascaded multicell with separate DC sources. Emerging topologies like asymmetric hybrid cells and soft-switched multilevel inverters are also discussed. This paper also presents the most relevant control and modulation methods developed for this family of converters: multilevel sinusoidal pulsewidth modulation, multilevel selective harmonic elimination, and space-vector modulation. Special attention is dedicated to the latest and more relevant applications of these converters such as laminators, conveyor belts, and unified power-flow controllers. The need of an active front end at the input side for those inverters supplying regenerative loads is also discussed, and the circuit topology options are also presented. Finally, the peripherally developing areas such as high-voltage high-power devices and optical sensors and other opportunities for future development are addressed.

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The use of a cascaded multilevel inverter for transformerless sag/dip compensation is investigated. This topology is investigated as a cost-effective means for series sag compensation by eliminating the large injection transformer and output filter components that are used in conventional series injection devices. This prototype inverter is designed for sag compensation of a 250-kVA load. In this design, cost effectiveness plays a major role in the selection of the energy storage and the switching components. Control schemes are discussed for series sag compensation with this multilevel inverter. New control methods for sag compensation and injection are also introduced. A prototype is developed and the control schemes of this sag compensator are successfully verified in the practical results and show successful compensation for sags for different types of loads. The performance of this compensator makes it promising for future power rating upgrade and industrialization.

This paper presents a novel switch-mode power amplifier based on a multicell multilevel circuit topology. The total output voltage of the system is formed by series connection of several switching cells having a low DC-link voltage. Therefore, the cells can be realized using modern low-voltage high-current power MOSFET devices and the DC link can easily be buffered by rechargeable batteries or "super" capacitors to achieve very high amplifier peak output power levels ("flying-battery" concept). The cells are operated in a phase-shifted interleaved pulsewidth-modulation mode, which, in connection with the low partial voltage of each cell, reduces the filtering effort at the output of the total amplifier to a large extent and, consequently, improves the dynamic system behavior. The paper describes the operating principle of the system, analyzes the fundamental relationships being relevant for the circuit design, and gives guidelines for the dimensioning of the control circuit. Furthermore, simulation results as well as results of measurements taken from a laboratory setup are presented.

Component counts and oversimplified reliability rules may lead to the conclusion that multilevel converters are less safe than two-level converters, just because they use more components. A better approach might be to consider that they use a different arrangement of components and also that the consequence of faults may be very different. This paper is focused on the study of the consequences of faults in hard-switching and soft-switching multicell converters. Solutions to minimize the consequences of major faults are described.