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 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

Modular multilevel converter (MMC) is one of the most promising topologies for medium to high-voltage high-power applications. The main features of MMC are modularity, voltage and power scalability, fault tolerant and transformer-less operation, and high-quality output waveforms. Over the past few years, several research studies are conducted to address the technical challenges associated with the operation and control of the MMC. This paper presents the development of MMC circuit topologies and their mathematical models over the years. Also, the evolution and technical challenges of the classical and model predictive control methods are discussed. Finally, the MMC applications and their future trends are presented.

Evolution of Topologies, Modeling, Control Schemes, and Applications of Modular Multilevel Converters

The evolution of power electronics and its control has been mainly driven by industry applications and influenced by the development achieved in several technologies, such as power semiconductors, converter topologies, automatic control, and analog and digital electronics. Digital signal processors (DSPs), in particular, have experienced an exponential development in processing power, which until now has not been fully exploited for control purposes in power converters and drive applications. Presently, the control system technology finds itself in a paradigm-changing tipping point, in which more demanding control goals, system flexibility, and functionalities required by emerging applications are driving the control system technology development, in addition to stabilization and robustness, which was the main focus in the past. This article walks briefly through the history of the mainstream power converter control scene, with an emphasis on the more recent introduction of predictive control, and gives a glimpse on the challenges and possibilities ahead. Special attention is given to finite control set (FCS)-model predictive control (MPC), because of its simplicity, flexibility, inherent adaptation to power electronic circuits and their discrete nature, both in the finite amount of switching states and the digital implementation with microprocessors.

Model Predictive Control: MPC's Role in the Evolution of Power Electronics

In conventional direct torque controlled (DTC) permanent magnet synchronous motor drive, there is usually undesired torque and flux ripple. The existing literature have proposed some methods to reduce torque and flux ripple by optimizing the duty ratio of the active vector. However, these methods are usually complicated and parameter dependent. This paper first compares the performances of three duty determination methods in detail and then proposes a very simple but effective method to obtain the duty ratio. The novel method is superior to the existing methods in terms of simplicity and robustness. By appropriately arranging the sequence of the vectors, the commutation frequency is reduced effectively without performance degradation. To further improve the performance of system, a low-pass filter-based voltage model with compensations of amplitude and phase is employed to acquire accurate stator flux estimation. The proposed scheme is able to reduce the torque and flux ripple significantly while maintaining the simplicity and robustness of conventional DTC at the most. Simulations and presented experimental results validate the effectiveness of the proposed schemes in this paper.

Direct Torque Control of Permanent Magnet Synchronous Motor With Reduced Torque Ripple and Commutation Frequency

This paper presents a detailed description of Finite Control Set Model Predictive Control (FCS-MPC) applied to power converters. Several key aspects related to this methodology are in depth presented and compared with traditional power converter control techniques, such as linear controllers with PWM based modulation methods. The basic concepts, operating principles, control diagrams and results are used to provide a comparison between the different control strategies. The analysis is performed on a traditional three-phase voltage source inverter, used as simple and comprehensive reference frame. However, additional topologies and power systems are addressed to highlight differences, potentialities and challenges of FCS-MPC. Among the conclusions are the feasibility and great potential of FCSMPC due to today's signal processing capability, specially for power systems with a reduced number of switching states and more complex operating principles, such as matrix converters. In addition, the possibility to address different or additional control objectives easily in a single cost function, enables a simple, flexible and improved performance controller for power conversion systems.

Marian Kazrnierkowski

Papers

Model predictive control -- a simple and powerful method to control power converters