There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

This work is devoted to review and analyze the most relevant characteristics of multilevel converters, to motivate possible solutions, and to show that we are in a decisive instant in which energy companies have to bet on these converters as a good solution compared with classic two-level converters. This article presents a brief overview of the actual applications of multilevel converters and provides an introduction of the modeling techniques and the most common modulation strategies. It also addresses the operational and technological issues.

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The age of multilevel converters arrives

This paper presents a hardware/software co-simulation approach to rapidly develop and validate processor firmware for modern power electronic converters The methodology models in precise detail all significant electronic circuitry of the target power converter, and then executes the actual software source code of the physical digital signal processor in the simulation environment This creates a near real-world firmware debugging capability that saves considerable code development time while still being safe to execute The approach has been used by research students and undergraduate/postgraduate students to develop firmware for a wide variety of power electronic converter applications Matching simulation and experimental results of an exemplary single-phase active rectifier application are provided to illustrate the effectiveness of the approach

A hardware/software co-simulation approach for power converter firmware design and debugging

Conventionally, multiple Distributed Generation systems operating as a standalone microgrid use droop control concepts to match load demand and share power. However, it is challenging to maintain a high quality microgrid voltage using droop control alone, particularly when nonlinear loads and load transients are considered. Recently, a resonant voltage regulation system integrated with a synchronous reference frame virtual impedance loop and a phase locked loop, has been shown to achieve significantly improved performance both in terms of power sharing and dynamic response. This paper now applies this approach to a single phase microgrid system operating with significant levels of nonlinear loads, showing how similar performance benefits can be achieved in this particularly difficult operating context. The advantages of the approach are shown by detailed simulation and experimental studies across a variety of load conditions.

High Quality Voltage Regulation of Single Phase Autonomous Microgrids Under Nonlinear Load Conditions

Energy storage systems are one of the essential components of a renewable energy based microgrid. However, for a fuel-cell based energy storage systems, the constraints imposed by the internal dynamics of a fuel-cell must be taken into account by its power interface converter system. In particular, a fuel-cell requires a constrained rate-of-change-of-current because of its slow dynamic capability. This introduces a slew rate nonlinearity between the primary and secondary feedback loops within a conventional cascaded control system, which can adversely impact the transient performance of a classical proportional integral (PI) controller. Hence such a controller cannot ensure safe operating conditions for a fuel cell, and may in fact damage it. This paper now investigates the design of an improved current regulated compensator for a fuel-cell energy storage system that takes the slew rate non linearity into account during microgrid load transients, while avoiding the need for more complex variable gain control system architectures. The resulting cascade control system for the fuel-cell dc-dc converter is implemented in the discrete time domain.

Improved Controller Design for a Microgrid Fuel-Cell Based Energy Storage System

This paper proposes a new autonomous current sharing control strategy for islanded microgrids that is simple and robust under unbalanced load conditions. The strategy estimates the distributed generator output voltage and current positive sequence components from the internal variables of a resonant current and resonant voltage regulator, and then uses them to create an integrated virtual impedance sharing mechanism. This avoids control complexity and the need for output current measurements, while maintaining a balanced voltage and accurate current sharing. The approach has been verified using a detailed switched simulation.

Autonomous Current Sharing Control with Integrated Virtual Impedance and Self Synchronisation for Unbalanced Islanded Microgrids

Dual Active Bridge (DAB) converters offer an unmatched capability to transfer energy in either direction between two DC sources, especially when they are operated under Zero Voltage Switching (ZVS) conditions. However, the parasitic commutation inductance within the bridge phase leg devices can still lead to significant energy losses at device turn-off despite operating under ZVS conditions. These losses can become significant when one bridge has a low DC bus voltage and a consequential high magnitude output current. This paper shows how harmonic decomposition analysis can be used to constrain the bridge circulating currents to their minimum possible values at the point of phase leg commutation while still ensuring that ZVS operation is maintained. This can be used to improve the overall DAB performance under low voltage, high current operating conditions of one DC port. The capability of the theoretical analysis process is confirmed by matching experimental results for a reference DAB system.

Brendan McGrath

Papers

A hardware/software co-simulation approach for power converter firmware design and debugging

High Quality Voltage Regulation of Single Phase Autonomous Microgrids Under Nonlinear Load Conditions

Improved Controller Design for a Microgrid Fuel-Cell Based Energy Storage System

Autonomous Current Sharing Control with Integrated Virtual Impedance and Self Synchronisation for Unbalanced Islanded Microgrids

Analytically constrained ZVS operation to reduce commutation losses for high boost dual active bridge converters