Access to clean, affordable and reliable energy has been a cornerstone of the world's increasing prosperity and economic growth since the beginning of the industrial revolution. Our use of energy in the twenty–first century must also be sustainable. Solar and water–based energy generation, and engineering of microbes to produce biofuels are a few examples of the alternatives. This Perspective puts these opportunities into a larger context by relating them to a number of aspects in the transportation and electricity generation sectors. It also provides a snapshot of the current energy landscape and discusses several research and development opportunities and pathways that could lead to a prosperous, sustainable and secure energy future for the world.

Opportunities and challenges for a sustainable energy future

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

Overview of current development in electrical energy storage technologies and the application potential in power system operation

This paper reviews the current status and implementation of battery chargers, charging power levels, and infrastructure for plug-in electric vehicles and hybrids. Charger systems are categorized into off-board and on-board types with unidirectional or bidirectional power flow. Unidirectional charging limits hardware requirements and simplifies interconnection issues. Bidirectional charging supports battery energy injection back to the grid. Typical on-board chargers restrict power because of weight, space, and cost constraints. They can be integrated with the electric drive to avoid these problems. The availability of charging infrastructure reduces on-board energy storage requirements and costs. On-board charger systems can be conductive or inductive. An off-board charger can be designed for high charging rates and is less constrained by size and weight. Level 1 (convenience), Level 2 (primary), and Level 3 (fast) power levels are discussed. Future aspects such as roadbed charging are presented. Various power level chargers and infrastructure configurations are presented, compared, and evaluated based on amount of power, charging time and location, cost, equipment, and other factors.

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Review of Battery Charger Topologies, Charging Power Levels, and Infrastructure for Plug-In Electric and Hybrid Vehicles

Wireless power transfer (WPT) using magnetic resonance is the technology which could set human free from the annoying wires. In fact, the WPT adopts the same basic theory which has already been developed for at least 30 years with the term inductive power transfer. WPT technology is developing rapidly in recent years. At kilowatts power level, the transfer distance increases from several millimeters to several hundred millimeters with a grid to load efficiency above 90%. The advances make the WPT very attractive to the electric vehicle (EV) charging applications in both stationary and dynamic charging scenarios. This paper reviewed the technologies in the WPT area applicable to EV wireless charging. By introducing WPT in EVs, the obstacles of charging time, range, and cost can be easily mitigated. Battery technology is no longer relevant in the mass market penetration of EVs. It is hoped that researchers could be encouraged by the state-of-the-art achievements, and push forward the further development of WPT as well as the expansion of EV.

Wireless Power Transfer for Electric Vehicle Applications

Accurate prediction of electric load is critical to optimally controlling and operating buildings. It provides the opportunities to reduce building energy consumption and to implement advanced functionalities such as demand response in the context of smart grid. However, buildings are nonstationary and it is important to consider the underlying concept changes that will affect the load pattern. In this paper we present an online learning method for predicting building electric load during concept changes such as COVID-19. The proposed methods is based on online Long Short-Term Memory (LSTM) recurrent neural network. To speed up the learning process during concept changes and improve prediction accuracy, an ensemble of multiple models with different learning rates is used. The learning rates are updated in realtime to best adapt to the new concept while maintaining the learned information for the prediction.

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An LSTM-based online prediction method for building electric load during COVID-19

The 12-pulse diode rectifier has been widely used in high power applications. Although the dominant 5th and 7th harmonics of the input current are canceled out, the total harmonic distortion is still well above 5%. Therefore additional filtering is required to meet the pertinent standards. In this paper, several methods that deal with the input current harmonics are reviewed. The fundamentals behind these methods are discussed. By utilizing the concept of virtual resistor, a new approach is proposed in this paper to, not only eliminate the AC input current harmonics, but also eliminate DC output voltage harmonics. Moreover, the energy storage integration is possible with the proposed method. We analyze the control and operation of the proposed method. Simulation and experiment results verify the analysis.

A 12-pulse diode rectifier with energy storage integration and high power quality on both AC and DC side

This study focuses on the implementation of ferriteless shielding for wireless power transfer applications. Shorted litz-wire based shielding is used to reduce the electromagnetic field (EMF) emissions in both lateral and vertical directions. Furthermore, the inclusion of this shielding type reduces the size and weight of EMF shielding, a critical constraint in some applications. Analysis, simulation and hardware results are presented for three systems that utilize: aluminum, aluminumferrite, and shorted litz-wire based shielding. It is found that the shorted litz-wire based shielding is able to adequately reduce EMF emissions, operate a comparable efficiency and is lighter than the other two designs.

Light Weight and Efficient Litz-wire Based Ferrite-less Shielding for Wireless Power Transfer

The medium-voltage (MV) electric vehicle (EV) fast charger is foreseen as a feasible way to improve efficiency and power density of future EV charging infrastructure. The emerging 10 kV SiC MOSFET enables direct connection to MV utility using a single-module in basic topology, achieving a noticeable enhancement of system simplicity. In this paper, an active neutral- point-clamped (ANPC) based dual-active-bridge (DAB) converter using 10 kV SiC MOSFET is presented. The design and implementation of critical components including busbar, isolated gate driver and MV transformer are elaborated subsequently. The tradeoff between insulation of MV system against power density is discussed. The dc-transformer (DCX) is tested under a 10 kV to 700 V voltage ratio, with 99.1 % efficiency at 30 kW power throughput.

Design and Implementation of DC-Transformer using 10 kV SiC MOSFET for Medium-Voltage Extreme Fast Charger

Advancements in power electronics necessitate new developments in technologies interfacing it correspondingly. Signal generation and measurement are among these peripheral technologies. Higher performance of new power devices have made higher switching frequencies and higher ratings of power converters realistic. These new opportunities require high-speed switching transients, which causes new settings at source and measurement ends. In this study, we have introduced a new figure of merit for high-speed signals, which are generated or measured. This figure of merit improves accuracy in comparisons and rigidizes specifications for signals. In addition, we studied high speed signal generation methods. Most methods cannot generate a signal with high figure of merit without increasing power consumption and area. We verified capability of a leading choice. The demonstrated signal is ultra-narrow with duty cycle less than 2%. Subsequently, high-speed signals need superior considerations for accurate measurement. Effects and solutions in accurate measurement of high horizontal resolution and high speed signals have been presented. Furthermore, considerations for noise in measurements is discussed.

Srdjan Lukic

Papers

An LSTM-based online prediction method for building electric load during COVID-19

A 12-pulse diode rectifier with energy storage integration and high power quality on both AC and DC side

Light Weight and Efficient Litz-wire Based Ferrite-less Shielding for Wireless Power Transfer

Design and Implementation of DC-Transformer using 10 kV SiC MOSFET for Medium-Voltage Extreme Fast Charger

High Resolution Signal Generation and Management in Fast Switching Power Electronic Systems