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.

Multilevel inverters: a survey of topologies, controls, and applications

Multilevel converters have been under research and development for more than three decades and have found successful industrial application. However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers with a comprehensive and insightful review of where multilevel converter technology stands and is heading. This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry. In addition, new promising topologies are discussed. Recent advances made in modulation and control of multilevel converters are also addressed. A great part of this paper is devoted to show nontraditional applications powered by multilevel converters and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities.

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Recent Advances and Industrial Applications of Multilevel Converters

This paper presents a technology review of voltage-source-converter topologies for industrial medium-voltage drives. In this highly active area, different converter topologies and circuits have found their application in the market. This paper covers the high-power voltage-source inverter and the most used multilevel-inverter topologies, including the neutral-point-clamped, cascaded H-bridge, and flying-capacitor converters. This paper presents the operating principle of each topology and a review of the most relevant modulation methods, focused mainly on those used by industry. In addition, the latest advances and future trends of the technology are discussed. It is concluded that the topology and modulation-method selection are closely related to each particular application, leaving a space on the market for all the different solutions, depending on their unique features and limitations like power or voltage level, dynamic performance, reliability, costs, and other technical specifications.

Multilevel Voltage-Source-Converter Topologies for Industrial Medium-Voltage Drives

Neutral-point-clamped (NPC) inverters are the most widely used topology of multilevel inverters in high-power applications (several megawatts). This paper presents in a very simple way the basic operation and the most used modulation and control techniques developed to date. Special attention is paid to the loss distribution in semiconductors, and an active NPC inverter is presented to overcome this problem. This paper discusses the main fields of application and presents some technological problems such as capacitor balance and losses.

A Survey on Neutral-Point-Clamped Inverters

Review of the maximum power point tracking algorithms for stand-alone photovoltaic systems

A converter includes a full-wave bridge network across which operating voltages of +E and -E volts are applied, diagonal legs of the bridge each including two semi-conductor switches connected in series between individual operating voltage and output terminals, two diodes polarized in one direction being connected for conducting current from a point of reference potential to the centralmost switches of the two upper legs, respectively, and another two diodes like polarized in another direction being connected for conducting current from the centralmost switches of the two lower legs, respectively, to the point of reference potential. Each diode and its associated switch form a unidirectional gated current path and the vertically opposing centralmost switch and diode pairs of the upper and lower legs form a bidirectional gated current path. The switches are operable for providing zero, +E, +2E, -E, and -2E volts across a load in some predetermined sequence for producing a desired AC voltage waveform across the load. The diodes also serve to clamp the common connections between their associated two semiconductor switches to within a diode drop of the reference potential, thereby protecting the switches from overvoltage.

Bridge converter circuit

The d.c. power supplied by a photovoltaic solar panel to a load is controlled by monitoring the slope of the panel voltage vs. current characteristic and adjusting the current supplied by the panel to the load so that the slope is approximately unity. The slope is monitored by incrementally changing the panel load and indicating whether the resulting change in current derived from the panel is above or below a reference value, indicative of the panel voltage. In response to the change in the monitored current being above the reference value, the slope of a voltage vs. current curve is greater than unity and the load is adjusted to decrease the current supplied by the panel to the load. Conversely, in response to the current being less than the reference value, the slope of the voltage vs. current curve is less than unity and the load is adjusted to increase the current supplied by the panel to the load.

Method of and apparatus for enabling output power of solar panel to be maximised

An inverter circuit includes a DC voltage supply having a plurality of voltage taps coupled via (1) a first plurality of diodes for unidirectional current flow therefrom to successive common connections between the upper half of a plurality of first transistors connected in cascade across the voltage supply, (2) a second plurality of diodes for unidirectional current flow therefrom to successive common connections between the upper half of a plurality of capacitors, of like number to the first transistors, connected in series and across the DC voltage supply via a pair of diodes providing unidirectional current between the voltage supply and capacitors, (3) a third plurality of diodes for unidirectional current flow thereto from successive common connections between the lower half of the plurality of first transistors, (4) a fourth plurality of diodes for unidirectional current flow thereto from the successive common connections between the lower half of the plurality of capacitors; and a plurality of second transistors connected in cascade across the series string of capacitors, their centralmost common connection being connected to an output terminal, individual ones of a fifth plurality of diodes being connected between successive common connections of the capacitors to and for unidirectional current flow (1) therefrom to the successive common connections between the upper half of, and (2) thereto from the successive common connections between the lower half, of the second transistors, the pluralities of first and second transistors being operable to different combinations of their conductive and nonconductive states for predetermined periods of time for producing a multitiered AC waveform at the output terminal.

High-voltage converter circuit

A DC to AC converter capable of producing up to three-tier waveforms includes a first transistorized switching amplifier operable to a first condition, for charging a first capacitor to +2E volts, concurrent with applying +E volts to a first terminal, and connecting a DC supply of -E volts to the positive plate of, and in series circuit with, a second capacitor (previously charged to have a voltage drop thereacross of -2E volts), the series circuit being connected between a reference and second terminals, for applying -3E volts to the second terminal. The first switching amplifier is operable to a second condition for charging the second capacitor to -2E volts, concurrent with applying -E volts to the second terminal, and applying +E volts to the negative plate of the first capacitor, the positive plate of which is connected to the first terminal, for applying +3E volts to the first terminal. A second transistorized switching amplifier is operable to a first or second condition, for individually connecting the first and second terminals to an output terminal, respectively. A control circuit is used to operate the first and second switching amplifiers in various combinations of their respective first and second conditions, over a period of time, for producing a desired waveform at the output terminal. Two or more of the converters can be interconnected and driven by the control circuit for providing polyphase voltage waveforms.

Synthesizer circuit for generating three-tier waveforms

A relatively high power switching device is provided via the combination on a common substrate of a VMOS transistor having a gate electrode for receiving a control signal, a drain electrode, and a source electrode, individually connected to the collector and base electrodes of a bipolar transistor, respectively, the collector-emitter current path of the latter being the main current carrying path of the switching device.

Richard H. Baker

Papers

Bridge converter circuit

Method of and apparatus for enabling output power of solar panel to be maximised

High-voltage converter circuit

Synthesizer circuit for generating three-tier waveforms

VMOS/Bipolar power switching device