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

Dynamic wireless power transfer systems require to supply many transmitting coils deployed under the road surface and arranged along the so-called track. This layout entails the use of a large number of inverters or of devices that switch the power to the proper coils. This article presents a technique that uses a single three-phase inverter to supply two coils with voltages having different and independently adjustable amplitudes of their first harmonic component. Differently from the well-known phase shift technique, the amplitude and the phase of the voltages are not correlated. Moreover, the presented technique has the ability of inherently reducing the phase difference between the two output currents when the supplied loads are partially reactive. This feature enhances the power transfer capability of the inverter when both the track coils are coupled with the same pickup. After presenting this technique, this article analyzes the functioning of the dual-output inverter in different load conditions recognizing the boundaries of four different modes of operation. For each of them the analytical expression of the amplitude and phase of the generated voltages are given. The theoretical findings are validated by experiments performed on a prototypal setup that implements the presented modulation technique.

Analysis and Experimentation of a Novel Modulation Technique for a Dual-Output WPT Inverter

The paper is concerned with the wireless power (WP) systems feeding in-motion electric vehicles (EVs) and focuses on the power sizing of three key WP system components, namely the track coils, the pickup and the voltage supply. It is assumed that the track is made of lumped type with no overlapped coupling of the pickup, and that the track coils and the pickup are compensated for by series resonating capacitors. Power sizing equations are formulated for a WP system that is controlled in the way of transferring a constant power to EVs during their motion over a large part of the track coils. Due to the variation of the coupling between the pickup and a track coil, the transfer of a constant power requires the adjustment of the currents in the WP system, obtained by commanding the embedded power conversion units. Two solutions are examined: by one solution, the pickup side current is controlled at a proper value whilst the track side current is regulated at a constant level; by the other solution, the opposite occurs. Power sizing equations are expressed in terms of specifications and parameters of the WP system. The equations outline that the power sizing is higher for the components in the side where the current control is executed. At last, the case study of a WP system is examined, showing the convenience of the track-side control.

Power sizing of wireless power systems feeding in-motion electric vehicles at constant power

With the expansion of power grid scale, abnormal disturbance of power quality will bring serious harm. In view of the failure of traditional power management to effectively explore the potential capability of multiple power electronic equipment, a multilevel and synergistic management strategy for power quality based on data driven is proposed in this paper. Firstly, based on the local operation characteristics of the power quality control device, the important features in the power quality time series of each node of the distribution network are extracted. In order to improve the data quality, the data cleaning method based on k-nearest neighbor is adopted to construct isochronous dimension sequence array, and the correlation of each node data is analyzed by improved gray relational analysis (IGRA). The nodes are sorted according to the power quality correlation degree and the dominant nodes are determined by k-means++. The global governance under the power quality disturbance is realized through the regional regulation. Finally, the IEEE14-node simulation example is used to verify the accuracy of the proposed method and the rationality of improving the power supply quality in the whole network.

Multilevel and Synergistic Management Strategy for Power Quality Based on Data Driven

The direct torque control (DTC) of an induction motor drive carries out the closed-loop control of the stator flux magnitude and the motor torque. The feedback values are estimated by processing the inverter dc-link voltage and the stator currents through the motor voltage model. Any error in the model parameter (i.e. the stator resistance) and in transducing voltage and currents yields an incorrect estimation of the controlled variables, thus altering the drive operation. In this paper the various errors are considered and their effect on the drive operation is studied analytically. The study shows that some errors lead to instability of the drive whilst some others deteriorate its steady-state performance. The theoretical findings are substantiated by experimental results.

Effect of parameter and sensing errors on DTC IM drive operation

Direct current electric spring (DCES), as a new solution for the power quality issue caused by the unstable output of photovoltaic (PV) and wind power integrated into the DC microgrid, has the advantages of flexible configuration, smart and small size. This paper proposes an improved topology based on the recent proposed DCES with parallel structure, defined as DCES-3, for residential rooftop PV power generation, and proposes a control strategy based on the small-signal modeling. The proposed topology consists of a triple active bridge (TAB) and a Buck-Boost converter (BBC), and adopts the overall control scheme of three-dimensional decoupling based on single phase-shifting (SPS) control, and uses the BBC to connect the non-critical load (NCL) in parallel to form an equivalent variable load. The DCES passes the power fluctuations to this variable load by changing the duty cycle of the BBC, while ensuring the power on the CL stable. Compared with DCES-3, it has the advantages of clearer concept of ‘spring’, easier analysis of transmission power and zero-voltage switching condition, controllable power quality of the NCL, lower scene transformation and production costs. Finally, the correctness and effectiveness of the proposed DCES topology and its control have been validated by both simulation and experimental results.

Giuseppe Buja

Papers

Analysis and Experimentation of a Novel Modulation Technique for a Dual-Output WPT Inverter

Power sizing of wireless power systems feeding in-motion electric vehicles at constant power

Multilevel and Synergistic Management Strategy for Power Quality Based on Data Driven

Effect of parameter and sensing errors on DTC IM drive operation

A Novel Topology for DC Electric Spring With Parallel Structure