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

As Photovoltaic (PV) penetration level rapidly increases in the modern power systems, the adoption of energy storage systems (ESS) is a preferred option to control the rapid and unpredictable PV power fluctuations. It is desired to develop new reliability evaluation methods that can model and evaluate the reliability impact of these new entries. This paper has proposed an area risk based method that can take into account the PV intermittence and ESS operations when evaluating the unit commitment risk (UCR). The rapid changes of PV power is accounted for every 5-minute interval during the lead time. A control algorithm for the ESS is also developed and integrated into the evaluation of UCR. The method is applied to quantify the effects of different ESS capacities on the system's load carrying capability. System planners can utilize this method to determine the proper ESS installation for a system with given PV penetration level. This method is also useful for system operators to decide on the required committed units as well as the charging and discharging power of ESS given PV fluctuation.

Unit commitment risk evaluation of power systems with PV and energy storage

Slew rate in a power supply gets limited by the inductance value and the voltage across the inductor. Slew rate can be increased by reducing the inductance, but it results in higher inductor current ripple and hence higher losses in the power converters. In this paper, we present a novel scheme for increasing the slew rate without reducing the inductance value. The scheme operates only when there is a demand in the slew rate and restores to the normal operating conditions during steady state operation. It provides reduced voltage undershoots and faster settling times in output voltage during load transients. The proposed scheme is tested on a 1 V/12.5 A buck converter switching at 1MHz and the experimental results are presented.

Input voltage switched dc-dc converter with improved transient performance

Plug-in Electric Vehicles will be proliferating in many urban societies around the world due to aggressive targets set-up by many Governments around the world. This trend will get a boost especially after the COVID-19 lockdown phase, when it was evident that Green House Gases in the atmosphere could be brought down by large extent in a very short-time. However, many urban areas are densely populated and many car owners do not even have a proper car park. In such case, how will they get an access to charging facility. In this paper, we present a model of an aggregated charging facility that is located as a work-place car park. The arrival and departure of PEVs are modeled using the office statistics. We show how this random variable can be represented as an electrical load to the grid. We then show charging strategies that can be used to ensure the PEVs have a full charge at the time of departure. This is to eliminate the need for residential charging as there may not be such a facility available. The charging of PEVs is carried out for minimize the loading on the grid and yet meet the constraints of the PEV users.

Evaluation of Workplace Car-park for Electric Vehicle charging in a dense Urban environment

Economic analysis of annual load loss due to voltage sags in industrial distribution networks with distributed PVs

Matrix Pencil Method (MPM) has been applied to sub-cycle information of space vector and zero-sequence voltage to classify voltage dip phenomena. The feature extraction performance of MPM using different sampling window width has been statistically analyzed and found to perform relatively well between 0.1 and 1 cycle of a damped complex sinusoidal signal. The results show that MPM is able to estimate the fundamental frequency space vector even in highly distorted signals. The space vector ellipse's estimation is further enhanced by augmenting an ellipse fitting algorithm to MPM. This enhanced method differentiated between two highly similar two-phase voltage dips using only a quarter-cycle of data, demonstrating the feasibility of this scheme. This is demonstrated in a fault simulation on IEEE 34-bus system using a 5 ms sampling window.

Ashwin M. Khambadkone

Papers

Unit commitment risk evaluation of power systems with PV and energy storage

Input voltage switched dc-dc converter with improved transient performance

Evaluation of Workplace Car-park for Electric Vehicle charging in a dense Urban environment

Economic analysis of annual load loss due to voltage sags in industrial distribution networks with distributed PVs

Application of Matrix pencil method in sub-cycle voltage dip classification