There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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.

/pdf/review-of-battery-charger-topologies-charging-power-levels-85yy2igx2w.pdf

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

http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

Advanced control strategies are vital components for realization of microgrids. This paper reviews the status of hierarchical control strategies applied to microgrids and discusses the future trends. This hierarchical control structure consists of primary, secondary, and tertiary levels, and is a versatile tool in managing stationary and dynamic performance of microgrids while incorporating economical aspects. Various control approaches are compared and their respective advantages are highlighted. In addition, the coordination among different control hierarchies is discussed.

Hierarchical Structure of Microgrids Control System

This paper presents a review of control strategies, stability analysis, and stabilization techniques for dc microgrids (MGs). Overall control is systematically classified into local and coordinated control levels according to respective functionalities in each level. As opposed to local control, which relies only on local measurements, some line of communication between units needs to be made available in order to achieve the coordinated control. Depending on the communication method, three basic coordinated control strategies can be distinguished, i.e., decentralized, centralized, and distributed control. Decentralized control can be regarded as an extension of the local control since it is also based exclusively on local measurements. In contrast, centralized and distributed control strategies rely on digital communication technologies. A number of approaches using these three coordinated control strategies to achieve various control objectives are reviewed in this paper. Moreover, properties of dc MG dynamics and stability are discussed. This paper illustrates that tightly regulated point-of-load converters tend to reduce the stability margins of the system since they introduce negative impedances, which can potentially oscillate with lightly damped power supply input filters. It is also demonstrated that how the stability of the whole system is defined by the relationship of the source and load impedances, referred to as the minor loop gain. Several prominent specifications for the minor loop gain are reviewed. Finally, a number of active stabilization techniques are presented.

/pdf/dc-microgrids-part-i-a-review-of-control-strategies-and-3oi6nfb2fy.pdf

DC Microgrids—Part I: A Review of Control Strategies and Stabilization Techniques

This paper explores stability issues in dc microgrids with instantaneous constant-power loads (CPLs). DC microgrids typically have distributed power architectures in which point-of-load converters behave as instantaneous CPLs to line regulating converters located upstream. Constant-power loads introduce a destabilizing effect in dc microgrids that may cause their main bus voltages to show significant oscillations or to collapse. This paper also discusses stabilization strategies to prevent these undesired behaviors from occurring. Mitigating strategies such as load shedding, addition of resistive loads, filters, or energy storage directly connected to the main bus, and control methods are investigated. Advantages and disadvantages of these methods are discussed and recommendations are made. The analysis is verified with simulations and hardware-based experiments.

Dynamic Behavior and Stabilization of DC Microgrids With Instantaneous Constant-Power Loads

This paper presents a spatial and temporal model of electric vehicle charging demand for a rapid charging station located near a highway exit. Most previous studies have assumed a fixed charging location and fixed charging time during the off-peak hours for anticipating electric vehicle charging demand. Some other studies have based on limited charging scenarios at typical locations instead of a mathematical model. Therefore, from a distribution system perspective, electric vehicle charging demand is still unidentified quantity which may vary by space and time. In this context, this study proposes a mathematical model of electric vehicle charging demand for a rapid charging station. The mathematical model is based on the fluid dynamic traffic model and the M/M/s queueing theory. Firstly, the arrival rate of discharged vehicles at a charging station is predicted by the fluid dynamic model. Then, charging demand is forecasted by the M/M/s queueing theory with the arrival rate of discharged vehicles. This mathematical model of charging demand may allow grid's distribution planners to anticipate a charging demand profile at a charging station. A numerical example shows that the proposed model is able to capture the spatial and temporal dynamics of charging demand in a highway charging station.

Spatial and Temporal Model of Electric Vehicle Charging Demand

This paper presents a quantitative method to evaluate dc microgrids availability by identifying and calculating minimum cut sets occurrence probability for different microgrid architectures and converter topologies. Hence, it provides planners with an essential tool to evaluate downtime costs and decide technology deployments based on quantitative risk assessments by allowing to compare the effect that converter topologies and microgrid architecture choices have on availability. Conventional architectures with single-input converters and alternative configurations with multiple-input converters (MICs) are considered. Calculations yield that all microgrid configurations except those utilizing center converters achieve similar availability of 6-nines. Three converter topologies are used as representatives of many other circuits. These three benchmark circuits are the boost, the isolated SEPIC (ISEPIC), and the current-source half-bridge. Marginal availability differences are observed for different circuit topology choices, although architectures with MICs are more sensitive to this choice. MICs and, in particular, the ISEPIC, are identified as good compromise options for dc microgrids source interfaces. The analysis also models availability influence of local energy storage, both in batteries and generators' fuel. These models provide a quantitative way of comparing dc microgrids with conventional backup energy systems. Calculations based on widely accepted data in industry supports the analysis.

https://users.ece.utexas.edu/~kwasinski/05677476%20published.pdf

Quantitative Evaluation of DC Microgrids Availability: Effects of System Architecture and Converter Topology Design Choices

This letter studies single-input dc-dc converter topologies that are suitable to be expanded into their multiple-input converter version. The analysis is based on several assumptions, restrictions, and conditions, including the goal of minimizing the total number of components and the use of at least one forward-conducting bidirectional blocking switch in each input leg. These conditions may affect the outcome of the multiple-input converter synthesis and lead to different configurations from those suggested before in the literature. Although simultaneous power delivery from all sources is not required, it should be possible to independently control the power drained from each input with some degree of freedom. The letter lists four rules that must be observed in order to be able to realize a multiple-input converter from its single-input version. These rules are used to identify the only feasible input cell that complies with all assumptions and conditions. Unfeasible input cells are also shown. The letter also identifies some additional feasible input cells if some of the assumptions and conditions are relaxed. These input cells are used to suggest several multiple-input converter topologies. Among them, six topologies, including versions of the single-ended primary inductance converter and the Cuk converters, are introduced through their circuit schematic.

Identification of Feasible Topologies for Multiple-Input DC–DC Converters

This paper presents a dynamic modeling and control strategy for a sustainable microgrid primarily powered by wind and solar energy. A current-source-interface multiple-input dc-dc converter is used to integrate the renewable energy sources to the main dc bus. Potential suitable applications range from a communication site or a residential area. A direct-driven permanent magnet synchronous wind generator is used with a variable speed control method whose strategy is to capture the maximum wind energy below the rated wind speed. This study considers both wind energy and solar irradiance changes in combination with load power variations. As a case study a 30-kW wind/solar hybrid power system dynamic model is explored. The examined dynamics shows that the proposed power system is a feasible option for a sustainable microgrid application.

Alexis Kwasinski

Papers

Dynamic Behavior and Stabilization of DC Microgrids With Instantaneous Constant-Power Loads

Spatial and Temporal Model of Electric Vehicle Charging Demand

Quantitative Evaluation of DC Microgrids Availability: Effects of System Architecture and Converter Topology Design Choices

Identification of Feasible Topologies for Multiple-Input DC–DC Converters

Dynamic Modeling and Operation Strategy for a Microgrid With Wind and Photovoltaic Resources