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

http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

The author evaluates the state of the art in pulsewidth modulation for AC drives fed from three-phase voltage source inverters. Feedforward and feedback pulsewidth modulation schemes with relevance for industrial application are described and their respective merits and shortcomings are explained. Secondary effects such as the influence of load-current dependent switching time delay and transients in synchronized pulsewidth modulation schemes are discussed, and adequate compensation methods are presented. Recorded oscillograms illustrate the performance of the respective pulsewidth modulation principles. The author provides a guideline and quick reference for the practicing engineer to decide which methods should be considered for an application of a given power level, switching frequency, and dynamic response. >

Pulsewidth modulation-a survey

This paper presents a review of recently used direct torque and flux control (DTC) techniques for voltage inverter-fed induction and permanent-magnet synchronous motors. A variety of techniques, different in concept, are described as follows: switching-table-based hysteresis DTC, direct self control, constant-switching-frequency DTC with space-vector modulation (DTC-SVM). Also, trends in the DTC-SVM techniques based on neuro-fuzzy logic controllers are presented. Some oscillograms that illustrate properties of the presented techniques are shown.

Direct torque control of PWM inverter-fed AC motors - a survey

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

Electric Spring (ES) technology is an innovative user-level solution developed to stabilize the supply voltage of the loads against variations of the grid voltage. Two basic ES versions exist: reactive and active, whereas the former operates by exchanging only reactive power with the non-critical user loads whilst the latter exchanges both reactive and active power with them. Several strategies have been developed for the ES control, mainly considering loads of R type and focusing on the arrangement of the ES command. This paper deals with a user of RL type equipped with a reactive ES, and pursues the twofold objective of analyzing the dynamic behavior of the user quantities and designing an effective ES control system. The analysis reveals the slightly damped behavior of the user supply voltage in response to an ES command. The designed control system provides a fast and smooth voltage stabilization; to this end, a nested scheme is proposed and the individual controllers are properly synthesized. The performance of the ES control system is validated by Matlab/Simulink simulations.

Dynamic Behavior Analysis and Control of Reactive Electric Springs

With direct torque control (DTC), the stator flux magnitude and the torque of an induction motor (IM) drive are controlled in a closed-loop way. The actual values of the controlled variables are estimated by processing the inverter dc-link voltage and the stator currents by means of the IM voltage model. Any error in the model parameter (the stator resistance) or in the voltage and current transduction degrades the drive performance. In this paper, a theoretical investigation of the effects of the various errors on a DTC IM drive is carried out. The investigation gives a proof of the drive instability due to some errors and provides, for the other errors, the amount of the deviation produced on the controlled variables in steady state. The theoretical findings are substantiated by experimental and simulation results. Index Terms—Direct torque control (DTC), drive performance degradation, induction motor (IM) drives.

Steady-State Performance Degradation of a DTC IM

Electric Spring (ES) technique is a user-level solution developed to stabilize the supply voltage of a user under variations of the grid voltage. This paper analyzes the stabilization capabilities of a reactive ES that operates according to the demand-side power paradigm. By help of a convenient ES modeling, the extreme values of the active power that a user can draw under the ES action are first determined. Then, it is demonstrated that the demand-side power paradigm is fulfilled only if the distribution line impedance has a resistive component while its reactive component weakens such fulfillment. Lastly, the variations of the grid voltage that ES is able to cope with are worked out. All findings are formulated in terms of normalized quantities and consequently are of general validity. Computer-aided simulation of a case study exemplifies the theoretical findings.

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Demand-Side Power Paradigm-Oriented Analysis of Reactive Electric Spring Stabilization Capabilities

This paper deals with the optimal fault-tolerant operation of a 5-phase permanent magnet (PM) brushless DC (BLDC) drive. In particular, this paper (Part I) proposes the modelization used to optimize the motor current supply to obtain the maximum power in healthy conditions and its analytical solution is presented. In the companion paper (Part II) the drive is considered both in healthy and faulty conditions and the optimization problem is solved numerically. In all cases the solution is compared with the one obtained with a “Maximum Torque Per Ampere” objective.

5-Phase PM brushless DC motor current optimization — Part I: Modelization and analytical solution for a healthy drive

Electric drives with a three-phase structure are commonly utilized in road vehicles for propulsion and in aeronautics as control-surface actuators. In recent times, some concerns have emerged on the use of three-phase drives in such applications in regard to power capability, efficiency and fault-tolerance. A way to effectively cope with these concerns is the adoption of multiphase drives. In this sense, a five-phase drive is a promising solution as it does not imply an excessively complicated structure. In this paper a five-phase PM BLDC drive is considered and its torque capabilities are investigated for square-wave current supply. Specifically, the torque developed by a five-phase drive is first compared to the three-phase counterpart; torque comparison is carried out by keeping motor size and loading unchanged and by considering two hypotheses: equal phase back-emf and equal phase rms current. Then the torque available from a five-phase drive is determined under various supply modes, characterized by the simultaneous conduction of a reduced number of phases; the torque available is determined by imposing an equal rms phase current in the supply modes.

Giuseppe Buja

Papers

Dynamic Behavior Analysis and Control of Reactive Electric Springs

Steady-State Performance Degradation of a DTC IM

Demand-Side Power Paradigm-Oriented Analysis of Reactive Electric Spring Stabilization Capabilities

5-Phase PM brushless DC motor current optimization — Part I: Modelization and analytical solution for a healthy drive

Torque capabilities of a five-phase PM BLDC drive vs. a three-phase one and various supply modes