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

A decentralized control method is proposed for a system of multiple voltage source converters (VSC) stacked in series on the AC side. Using the proposed method, a group of low-voltage rated VSC modules can be utilized to implement converters for medium/high voltage grid-tied applications. Each converter module can maintain synchronization using only its local measurements without any communication or any form of centralized synchronization signal. A mathematical model of such a system consisting of $N$ number of VSC modules employing the proposed control scheme is presented and a linearized model is developed for small signal stability assessment. The proposed method is robust against grid frequency variation and can naturally correct power sharing errors among modules caused by sensor inaccuracies or reference mismatches. Real time simulation results are presented to validate the proposed controller.

Decentralized Synchronization of AC-Stacked Voltage Source Converters

FERC Order 755 requires transmission system operators to provide performance-based payment for ancillary services Battery Energy Storage Systems (BESS) have the capacity to play a more vital role in regulation market because of their ability to provide an accurate and fast response to automatic generation control (AGC) dispatch signals However, since batteries are complex electrochemical systems, it is difficult to understand how the battery degradation phenomena will affect the total revenue from BESS providing the regulation service In this paper, we show that bid-in strategies that consider the battery degradation as a function of the service provided have a substantial effect on the total revenue for the BESS operators Different bid-in strategies aiming at enhancing BESS's total revenue before it reaches its end of life are analyzed and compared with nominal capacity based bid-in approaches Finally, we propose a practical bid-in strategy that provides substantial improvement over the nominal capacity based approach

Effects of battery degradation on economic viability of energy storage systems participating in regulation markets

This paper proposes an approach for implementing the restart for Synchronous Reluctance Machines (SynRMs) driving a high inertia load. The proposed method uses a simple identification algorithm to estimate the rotor speed and position of the machine so that the appropriate voltage vector can be applied to the machine, thus minimizing the inrush current at the instant of the restart. The novelty is that its implementation only requires nameplate machine parameters and a single DC-link current sensor. The proposed method is suitable for implementation with both high-inertia vector and scalar-controlled SynRMs.

Restart Strategy for Scalar Controlled Synchronous Reluctance Machine Driving a High-Inertia Load with a Single DC-link Current Sensor

Certain embodiments involve a modular medium voltage fast charger. The modular medium voltage fast charger can include: (1) a predictive power factor correction (“PFC”) controller (or predictive controller) for series-interleaved multi-cell three-level boost (“SIMCB”) converters, (2) an active neutral point clamped (“NPC”) dual active bridge (“DAB”) modulation scheme to achieve soft switching, (3) an auxiliary capacitor to reduce NPC DAB turn-off voltages, (4) a comprehensive and scalable protection circuit, and (5) a high-isolation pulse transformer with a bobbin for reducing coupling capacitance of the pulse transformer.

Modular medium voltage fast chargers

DC-link decoupling snubber circuit is usually placed close to the power switches in power converters to suppress voltage ringing on the devices during the device turn-off transient. In this paper, the snubber circuit performance is analyzed numerically based on a detailed forth-order high-frequency equivalent circuit of a double pulse test circuit. The approach to optimize the snubber circuit, not only for the voltage spike suppression but also for the good thermal performance, is proposed. The thermal analysis and simulation results reveal that an additional resistor in the snubber circuit can release the thermal stress on the capacitors, which makes it more reliable at high switching frequency and high-power applications. Finally, the performance of an optimized snubber circuit is confirmed in a 100-kW traction inverter system operating at 40 kHz switching frequency and 105°C ambient temperature.

Srdjan Lukic

Papers

Decentralized Synchronization of AC-Stacked Voltage Source Converters

Effects of battery degradation on economic viability of energy storage systems participating in regulation markets

Restart Strategy for Scalar Controlled Synchronous Reluctance Machine Driving a High-Inertia Load with a Single DC-link Current Sensor

Modular medium voltage fast chargers

Optimization of DC-Link Decoupling Snubber Circuit for SiC-based EV Traction Inverters