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.

/pdf/recent-advances-and-industrial-applications-of-multilevel-26fluutoc9.pdf

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

Inductive Wireless Power Transfer (WPT) is advantageously used to charge the battery pack of the electric vehicles (EVs). The key component of any inductive WPT system is the coupling setup of the two coils, one buried in the road pavement and the other one embedded into the EV, since it plays an important role in the power transfer. This paper refers to the study case of a WPT system for charging the battery pack of an electric city-car and evaluates the inductive characteristics of different coil-coupling setups. The evaluation begins with a formula-based approach and continues with a computer-assisted analysis of the coil-coupling inductive parameters. The evaluation includes the study of the dependence of the parameters on coil distance, axial misalignment, and turn distance. The outcomes of the analysis are used to compare various coil-coupling setups in view of the implementation of a WPT system in the electric citycar of the study case.

Inductive characteristics of different coupling setups for wireless charging of an electric city-car

In the last decades the voltage regulation has been challenged by the increase of power variability in the electric grid, due to the spread of non-dispatchable generation sources. This paper introduces a Smart Transformer (ST)-based Medium Voltage (MV) grid support by means of active power control in the ST-fed Low Voltage (LV) grid. The aim of the proposed strategy is to improve the voltage profile in MV grids before the operation of On-Load Tap Changer in the primary substation transformer, which needs tens of seconds. This is realized through reactive power injection by the AC/DC MV converter and simultaneous decrease of the active power consumption of voltage-dependent loads in ST-fed LV grid, controlling the ST output voltage. The last feature has two main effects: the first is to reduce the active power withdrawn from MV grid, and consequently the MV voltage drop caused by the active current component. At the same time, higher reactive power injection capability in the MV converter is unlocked, due to the lower active power demand. As result, the ST increases the voltage support in MV grid. The analysis and simulation results carried out in this paper show improvements compared to similar solutions, i.e. the only reactive power compensation. The impact of the proposed solution has been finally evaluated under different voltage-dependence of the loads in the LV grid.

/pdf/smart-transformer-based-medium-voltage-grid-support-by-means-4ohxmtxg8w.pdf

Smart transformer-based medium voltage grid support by means of active power control

PM brushless DC (BLDC) motors should be fed by square-wave currents synchronized with the back emfs to develop a constant torque with the required value. In practice, phase inductances and limited source voltage produce non ideal square-wave currents and unbalanced phase currents during commutations. Such phenomena affect the torque developed by the PM BLDC drives by deviating its value from the requested one and by giving rise to torque ripple. In this paper, an analytical study of the torque characteristics, namely motor torque and torque ripple, of PM BLDC drives is carried out as a function of the motor speed and is supported by experimental data. Torque characteristics are expressed in terms of a quantity distinctive of the motor, thus giving a generalized formulation to the results.

Analytical study of Torque vs. speed characteristics of PM brushless DC drives

Nonstationary distorted signals need to be analyzed in both the time and frequency domains to determine their characteristics. In this paper, a technique based on a neural network (NN) is presented which has the merit of providing such an analysis in real time. After arranging a suitable NN, the algorithm utilized to carry out the analysis is illustrated. Then, expressions assessing the dynamic behavior and the steady-state accuracy of the technique are derived. From the expressions, the influence of the NN parameters on the technique performance is readily recognized. As an example, the technique is applied to the analysis of the time evolution of the current harmonics absorbed by a diode rectifier and the results are compared with those obtained by the short-time Fourier transform.

Neural network technique for the joint time-frequency analysis of distorted signal

The paper is concerned with the soft-switching capabilities of the Single Active Bridge (SAB) converter. At first, soft-switching of the double active bridge converter is reviewed. Then, operation of the SAB converter is investigated with the end of assessing its soft-switching capabilities in both the discontinuous and continuous conduction mode. It is found that the SAB converter experiences either full or incomplete soft-switching depending on the working conditions. Afterwards, the procedure to size the capacitors for implementing soft-switching is illustrated. At last, an under-development power supply equipment exploiting the theoretical findings of the paper is presented together with a preliminary design of its components.

Giuseppe Buja

Papers

Inductive characteristics of different coupling setups for wireless charging of an electric city-car

Smart transformer-based medium voltage grid support by means of active power control

Analytical study of Torque vs. speed characteristics of PM brushless DC drives

Neural network technique for the joint time-frequency analysis of distorted signal

Soft-switching capabilities of SAB vs. DAB converters