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

This paper describes a novel control strategy based on driving cycle recognition. A Driving Cycle Recognition Algorithm (DCRA) is firstly presented. It allows switching between three driving modes: urban, suburban and highway. A real-time control strategy is then defined based on fuzzy logic with DCRA. Results are drawn and compared to fuzzy logic controllers parametrized for urban or highway cycles.

Control strategy of fuel cell hybrid electric vehicle based on driving cycle recognition

Two distinct approaches, one droop based phasor domain modeled and the other nonlinear oscillator based time domain modeled, have emerged for the analysis and control of power electronic converters at the system interface layer where these converters are integrating distributed energy resources (DERs). While the droop type controllers are based on distinct time-scale separation of control loops, purposefully slowing down the response of the DERs, the oscillator based controllers deliver fast dynamic response with accurate power sharing capability as well as stability guarantee. In this paper, we analyze both the droop and oscillator type converters in the context of grid forming converters with respect to steady state terminal response, transient stability, and harmonic compensation in converter output current or in network voltage. Simulation and experimental results are provided to demonstrate the easier implementation of oscillator based controls that can also achieve supplementary control objectives pertinent to power quality.

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Droop and Oscillator Based Grid-Forming Converter Controls: A Comparative Performance Analysis

Demands for higher fuel economy, performance, reliability, convenience, as well as reduced emissions push the automotive industry to seek electrification of ancillaries and engine augmentations. In cars of the future, throttle actuation, steering, anti-lock braking, rear-wheel steering, active suspension and ride-height adjustment, air-conditioning, and electrically heated catalyst will all benefit from the electrical power system. Therefore, a higher system voltage, such as the proposed 42V, is necessary to handle these new introduced loads. In this paper, an overview of the systems that will benefit from the 42V bus is presented. Effects of the new introduced electrical loads on the electrical power systems of conventional cars are described. Dynamic characteristics of each load for a typical drive cycle are defined. In addition, system level issues and vehicle performances such as fuel economy are addressed.

Effects of Electrical Loads on 42V Automotive Power Systems

Substantial induced eddy-current losses and the need for skin-depth-size strands leave litz wire out as an option for inductive applications at multi-megahertz frequencies. To address this, we analyze and minimize the litz-wire eddy-current losses by adding a coating layer to each strand while keeping the wire dimensions and packing factor constant. The skin and proximity effects of isolated and in-bundle coated strands are studied for various coating properties and dimensions. A new method is proposed for calculation of proximity losses inside coated-strand litz wire, which enables simple systematic approach for proximity loss and ac resistance ( ${R} _{\mathrm{ac}}$ ) determination. Three types of pure copper (Cu), silver-coated copper (Ag/Cu), and nickel-coated copper (Ni/Cu) strands are studied in isolation and inside a litz wire. The litz wires associated with these strands are fabricated and their power dissipations are measured. Analytical results followed by finite-element modeling and experimental results show 26% reduction in Ni/Cu litz wire ${R} _{\mathrm{ac}}$ , and 3% reduction in Ag/Cu litz wire ${R} _{\mathrm{ac}}$ at 13.56 MHz compared with same-size Cu litz wire.

Coated-Strand Litz Wire for Multi-Megahertz Frequency Applications

This work presents a platform for decentralized distributed computing called Resilient Information Architecture for the Smart Grid (RIAPS) through a transactional energy and a traffic application.

Srdjan Lukic

Papers

Control strategy of fuel cell hybrid electric vehicle based on driving cycle recognition

Droop and Oscillator Based Grid-Forming Converter Controls: A Comparative Performance Analysis

Effects of Electrical Loads on 42V Automotive Power Systems

Coated-Strand Litz Wire for Multi-Megahertz Frequency Applications

Transactive energy demo with RIAPS platform