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

The enabling of ac microgrids in distribution networks allows delivering distributed power and providing grid support services during regular operation of the grid, as well as powering isolated islands in case of faults and contingencies, thus increasing the performance and reliability of the electrical system. The high penetration of distributed generators, linked to the grid through highly controllable power processors based on power electronics, together with the incorporation of electrical energy storage systems, communication technologies, and controllable loads, opens new horizons to the effective expansion of microgrid applications integrated into electrical power systems. This paper carries out an overview about microgrid structures and control techniques at different hierarchical levels. At the power converter level, a detailed analysis of the main operation modes and control structures for power converters belonging to microgrids is carried out, focusing mainly on grid-forming, grid-feeding, and grid-supporting configurations. This analysis is extended as well toward the hierarchical control scheme of microgrids, which, based on the primary, secondary, and tertiary control layer division, is devoted to minimize the operation cost, coordinating support services, meanwhile maximizing the reliability and the controllability of microgrids. Finally, the main grid services that microgrids can offer to the main network, as well as the future trends in the development of their operation and control for the next future, are presented and discussed.

Control of Power Converters in AC Microgrids

The increasing interest in integrating intermittent renewable energy sources into microgrids presents major challenges from the viewpoints of reliable operation and control. In this paper, the major issues and challenges in microgrid control are discussed, and a review of state-of-the-art control strategies and trends is presented; a general overview of the main control principles (e.g., droop control, model predictive control, multi-agent systems) is also included. The paper classifies microgrid control strategies into three levels: primary, secondary, and tertiary, where primary and secondary levels are associated with the operation of the microgrid itself, and tertiary level pertains to the coordinated operation of the microgrid and the host grid. Each control level is discussed in detail in view of the relevant existing technical literature.

Trends in Microgrid Control

This paper presents a technology review of voltage-source-converter topologies for industrial medium-voltage drives. In this highly active area, different converter topologies and circuits have found their application in the market. This paper covers the high-power voltage-source inverter and the most used multilevel-inverter topologies, including the neutral-point-clamped, cascaded H-bridge, and flying-capacitor converters. This paper presents the operating principle of each topology and a review of the most relevant modulation methods, focused mainly on those used by industry. In addition, the latest advances and future trends of the technology are discussed. It is concluded that the topology and modulation-method selection are closely related to each particular application, leaving a space on the market for all the different solutions, depending on their unique features and limitations like power or voltage level, dynamic performance, reliability, costs, and other technical specifications.

Multilevel Voltage-Source-Converter Topologies for Industrial Medium-Voltage Drives

Cascaded multilevel inverters synthesize a medium-voltage output based on a series connection of power cells which use standard low-voltage component configurations. This characteristic allows one to achieve high-quality output voltages and input currents and also outstanding availability due to their intrinsic component redundancy. Due to these features, the cascaded multilevel inverter has been recognized as an important alternative in the medium-voltage inverter market. This paper presents a survey of different topologies, control strategies and modulation techniques used by these inverters. Regenerative and advanced topologies are also discussed. Applications where the mentioned features play a key role are shown. Finally, future developments are addressed.

A Survey on Cascaded Multilevel Inverters

Stability issue of DC bus in power system with multi-sources such as renewable and distributed energy sources is investigated. A simple design with cascade control for individual Buck converter, Boost converter and ICFFB converter to operate stably under constant power load is presented. Integration of these converters into the DC power system and solution for subsystem dynamic interactions are analyzed and verified by simulation.

A simple design of DC power system with multiple source-side converters to operate stably underconstant power load

In this paper, the small signal model analysis of an asymmetrical half-bridge DC-DC converter (AHBC) operating in the continuous conduction mode (CCM) is presented. The influence of the non-ideal components to the system performance is discussed. Based on the small signal model, the close loop feedback is designed in the frequency domain for a PI controller and a lead-lag controller. The design criteria for the close loop feedback are described and a clear design process for the feedback compensator is presented. How to achieve the optimum bandwidth and phase margin under the influence of the complicated system structure is investigated. The close loop system load dynamic response from a 12 V/4 A experiment circuit is obtained.

An asymmetrical half bridge DC-DC converter: close loop design in frequency domain

A simple and accurate fuel cell model is required for fuel cell based power electronic applications An artificial neural network (ANN) model is developed in this paper to model some nonlinear structures within the hybrid model of a PEM fuel cell It improves accuracy and allows the model to work even under varying operating conditions What is more, temperature effect on the fuel cell stack are finally represented as current effect by using ANN to represent the relationship between current and temperature Real-time implementation of the proposed ANN model is realized via a dSPACE processor Experimental results are provided to verify the validity of the proposed model

ANN Modelling of Nonlinear Subsystem of a PEMFC Stack for Dynamic and Steady State Operation

Power electronics devices play a key role in utilizing energy efficiently. When the power demand is highly dynamic and variable, the key requirement is to transfer power to the load with the most appropriate voltage, current, phase and frequency so as to achieve the most efficient conversion of power under all load conditions. IGBT behavior under short circuit and overload turn off condition is thus quite important. In this paper, an improved and comprehensive thermal model for power electronics building block (PEBB) is presented, which provides a higher degree of predictability of temperature, in not only steady state, especially at high current conditions but also under different short circuit failure modes.

Improved comprehensive thermal model for power electronics building block applications

Simulation of power electronic circuits remains a problem due to the high level of stiffness brought about by the modelling of switches as biresistors i.e. very low turn-on resistance and very high turn-off resistance. The merits and drawbacks of two modelling methods that address this problem are discussed. A modelling solution for ensuring numerically stable, accurate and fast simulation of power electronic systems is proposed. The solution enables easy connectivity between power electronic elements in the simulation model. It involves the modelling of virtual capacitance at switching nodes to soften voltage discontinuity due to the switch current suddenly going to zero. Undesirable ringing effects that may arise due to the interaction between the virtual capacitance and circuit inductance are eliminated by modelling virtual damping resistors in parallel to inductors that are adjacent to switching elements. A midpoint configuration method is also introduced for modelling shunt capacitors. A DC traction system is simulated using this modelling strategy and the results are included. Simulation results obtained using this modelling strategy are validated by comparison with the established mesh analysis technique of modelling. The simulation performance is also compared with the Power System Blockset commercial software.

Ashwin M. Khambadkone

Papers

A simple design of DC power system with multiple source-side converters to operate stably underconstant power load

An asymmetrical half bridge DC-DC converter: close loop design in frequency domain

ANN Modelling of Nonlinear Subsystem of a PEMFC Stack for Dynamic and Steady State Operation

Improved comprehensive thermal model for power electronics building block applications

Fast, accurate and stable simulation of power electronic systems using virtual resistors and capacitors