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

Although the concept of variable-speed drives, based on utilization of multiphase machines, dates back to the late 1960s, it was not until the mid- to late 1990s that multiphase drives became serious contenders for various applications. These include electric ship propulsion, locomotive traction, electric and hybrid electric vehicles, ldquomore-electricrdquo aircraft, and high-power industrial applications. As a consequence, there has been a substantial increase in the interest for such drive systems worldwide, resulting in a huge volume of work published during the last ten years. An attempt is made in this paper to provide a brief review of the current state of the art in the area. After addressing the reasons for potential use of multiphase rather than three-phase drives and the available approaches to multiphase machine designs, various control schemes are surveyed. This is followed by a discussion of the multiphase voltage source inverter control. Various possibilities for the use of additional degrees of freedom that exist in multiphase machines are further elaborated. Finally, multiphase machine applications in electric energy generation are addressed.

Multiphase Electric Machines for Variable-Speed Applications

The area of multiphase variable-speed motor drives in general and multiphase induction motor drives in particular has experienced a substantial growth since the beginning of this century. Research has been conducted worldwide and numerous interesting developments have been reported in the literature. An attempt is made to provide a detailed overview of the current state-of-the-art in this area. The elaborated aspects include advantages of multiphase induction machines, modelling of multiphase induction machines, basic vector control and direct torque control schemes and PWM control of multiphase voltage source inverters. The authors also provide a detailed survey of the control strategies for five-phase and asymmetrical six-phase induction motor drives, as well as an overview of the approaches to the design of fault tolerant strategies for post-fault drive operation, and a discussion of multiphase multi-motor drives with single inverter supply. Experimental results, collected from various multiphase induction motor drive laboratory rigs, are also included to facilitate the understanding of the drive operation.

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Multiphase induction motor drives - : a technology status review

This paper addresses to some of the latest contributions on the application of Finite Control Set Model Predictive Control (FCS-MPC) in Power Electronics. In FCS-MPC , the switching states are directly applied to the power converter, without the need of an additional modulation stage. The paper shows how the use of FCS-MPC provides a simple and efficient computational realization for different control objectives in Power Electronics. Some applications of this technology in drives, active filters, power conditioning, distributed generation and renewable energy are covered. Finally, attention is paid to the discussion of new trends in this technology and to the identification of open questions and future research topics.

State of the Art of Finite Control Set Model Predictive Control in Power Electronics

This paper gives an overview of medium-voltage (MV) multilevel converters with a focus on achieving minimum harmonic distortion and high efficiency at low switching frequency operation. Increasing the power rating by minimizing switching frequency while still maintaining reasonable power quality is an important requirement and a persistent challenge for the industry. Existing solutions are discussed and analyzed based on their topologies, limitations, and control techniques. As a preferred option for future research and application, an inverter configuration based on three-level building blocks to generate five-level voltage waveforms is suggested. This paper shows that such an inverter may be operated at a very low switching frequency to achieve minimum on-state and dynamic device losses for highly efficient MV drive applications while maintaining low harmonic distortion.

Medium-Voltage Multilevel Converters—State of the Art, Challenges, and Requirements in Industrial Applications

Multiphase variable-speed drives and generation systems (systems with more than three phases) have become one of the mainstream research areas during the last decade. The main driving forces are the specific applications, predominantly related to the green agenda, such as electric and hybrid electric vehicles (EVs), locomotive traction, ship propulsion, “more-electric” aircraft, remote offshore wind farms for electric energy generation, and general high-power industrial applications. As a result, produced body of significant work is substantial, making it impossible to review all the major developments in a single paper. This paper therefore surveys the recent progress in two specific areas associated with multiphase systems, namely power electronic supply control and innovative ways of using the additional degrees of freedom in multiphase machines for various nontraditional purposes.

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Advances in Converter Control and Innovative Exploitation of Additional Degrees of Freedom for Multiphase Machines

An induction motor is the most frequently used electric machine in high-performance drive applications. Control schemes of such drives require an exact knowledge of at least some of the induction motor parameters. Any mismatch between the parameter values used within the controller and actual parameter values in the motor leads to a deterioration in the drive performance. Numerous methods for induction machine on-line and off-line parameter estimation have been developed exclusively for application in high-performance drives. This paper aims at providing a review of the major techniques used for the induction motor parameter estimation. The paper is illustrated throughout with experimental and simulation examples related to various parameter estimation techniques.

A review of RFO induction motor parameter estimation techniques

The paper presents a study of postfault control for an asymmetrical six-phase induction machine with single and two isolated neutral points, during single open-phase fault. Postfault control is based on the normal decoupling (Clarke) transformation, so that reconfiguration of the controller is minimized. Effect of the single open-phase fault on the machine equations under this control structure is discussed. Different modes of postfault operation are analyzed and are further compared in terms of the achievable torque and stator winding losses. Validity of the analysis is verified using experimental results obtained from a six-phase induction motor drive prototype.

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Emil Levi

Papers

Multiphase Electric Machines for Variable-Speed Applications

Multiphase induction motor drives - : a technology status review

Advances in Converter Control and Innovative Exploitation of Additional Degrees of Freedom for Multiphase Machines

A review of RFO induction motor parameter estimation techniques

Post-fault operation of an asymmetrical six-phase induction machine with single and two isolated neutral points