With the number of electric vehicles (EVs) on the rise, there is a need for an adequate charging infrastructure to serve these vehicles. The emerging extreme fast-charging (XFC) technology has the potential to provide a refueling experience similar to that of gasoline vehicles. In this article, we review the state-of-the-art EV charging infrastructure and focus on the XFC technology, which will be necessary to support the current and future EV refueling needs. We present the design considerations of the XFC stations and review the typical power electronics converter topologies suitable to deliver XFC. We consider the benefits of using the solid-state transformers (SSTs) in the XFC stations to replace the conventional line-frequency transformers and further provide a comprehensive review of the medium-voltage SST designs for the XFC application.

Extreme Fast Charging of Electric Vehicles: A Technology Overview

The coexistence of ac and dc subgrids in a hybrid microgrid is likely given that modern distributed sources can either be ac or dc. Linking these subgrids is a power converter, whose topology should preferably be not too unconventional. This is to avoid unnecessary compromises to reliability, simplicity, and industry relevance of the converter. The desired operating features of the hybrid microgrid can then be added through this interlinking converter. To demonstrate, an appropriate control scheme is now developed for controlling the interlinking converter. The objective is to keep the hybrid microgrid in autonomous operation with active power proportionally shared among its distributed sources. Power sharing here should depend only on the source ratings and not their placements within the hybrid microgrid. The proposed scheme can also be extended to include energy storage within the interlinking converter, as already proven in simulation and experiment. These findings have not been previously discussed in the literature, where existing schemes are mostly for an ac or a dc microgrid, but not both in coexistence.

Autonomous Control of Interlinking Converters with Energy Storages in Hybrid AC-DC Microgrids

In the last decade, the concept of grid-forming (GFM) converters has been introduced for microgrids and islanded power systems. Recently, the concept has been proposed for use in wider interconnected transmission networks, and several control structures have thus been developed, giving rise to discussions about the expected behaviour of such converters. In this paper, an overview of control schemes for GFM converters is provided. By identifying the main subsystems in respect to their functionalities, a generalized control structure is derived and different solutions for each of the main subsystems composing the controller are analyzed and compared. Subsequently, several selected open issues and challenges regarding GFM converters, i. e. angle stability, fault ride-through (FRT) capabilities, and transition from islanded to grid connected mode are discussed. Perspectives on challenges and future trends are lastly shared.

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Grid-Forming Converters: Control Approaches, Grid-Synchronization, and Future Trends—A Review

With growing concern on climate change, widespread adoption of electric vehicles (EVs) is important. One of the main barriers to EV acceptance is range anxiety, which can be alleviated by fast charging (FC). The main technology constraints for enabling FC consist of high-charging-rate batteries, high-power-charging infrastructure, and grid impacts. Although these technical aspects have been studied in literature individually, there is no comprehensive review on FC involving all the perspectives. Moreover, the power quality (PQ) problems of fast charging stations (FCSs) and the mitigation of these problems are not clearly summarized in the literature. In this paper, the state-of-the-art technology, standards for FC (CHAdeMO, GB/T, CCS, and Tesla), power quality issues, IEEE and IEC PQ standards, and mitigation measures of FCSs are systematically reviewed.

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Grid Impact of Electric Vehicle Fast Charging Stations: Trends, Standards, Issues and Mitigation Measures - An Overview

Networked microgrids (NMGs) provide a promising solution for accommodating various distributed energy resources (DERs) and enhancing the system performance in terms of reliability, resilience, flexibility, and energy efficiency. With the penetration of MGs, the communication-based distributed control is playing an increasingly important role in NMGs for coordinating a multitude of heterogeneous and spatially distributed DERs, which feature enhanced efficiency, reliability, resilience, scalability, and privacy-preserving as compared with conventional centralized control. This article aims to provide a comprehensive survey of distributed control and communication strategies in NMGs. We provide thorough discussions and elaborate on: 1) Essential merits of MGs and NMGs, and their practical implementations; 2) Distributed communication network characteristics and specific operation objectives of NMGs; 3) Classifications of distributed control strategies in NMGs and their salient features; 4) Communication reliability issues concerning data timeliness, data availability, and data accuracy, and the development of countermeasures; 5) Advancements in several promising communication and computation technologies and their potential applications in NMGs.

Distributed Control and Communication Strategies in Networked Microgrids

Virtual oscillator control (VOC) is a nonlinear time domain controller that achieves significantly faster primary control response in islanded microgrids, compared to droop or virtual synchronous machine (VSM) control. Despite its superior performance, adoption of VOC is limited due to the lack of compatible secondary regulation or grid synchronization techniques. This is attributed to the nonlinear nature of VOC that complicates secondary control design, and the third-harmonic component in VOC output voltage that severely restricts grid-tied operation. To leverage the faster primary control response characteristics of VOC, we propose a compatible hierarchical control structure that enables operation and seamless transition between islanded and grid-connected modes. In the islanded mode, the controller achieves voltage and frequency regulation and grid synchronization; in the grid-tied mode, notch filters are used to suppress harmonic currents and tertiary level power reference tracking is achieved. The proposed controllers are validated through a series of real-time hardware-in-the-loop tests and hardware experiments using laboratory inverter prototype and state-of-the-art controls and communications hardware.

Hierarchical Control for Virtual Oscillator Based Grid-Connected and Islanded Microgrids

With the increasing integration of power electronics interfaced distributed generators, transient stability assessment of grid-connected inverters subjected to large grid disturbances is of vital importance for the secure and resilient operation of the power grid. Dispatchable virtual oscillator control (dVOC) is an emerging approach to implement nonlinear control of grid-forming inverters. Through coordinate transformation, a simple first-order nonlinear power angle dynamic equation is uncovered from the complex oscillator dynamics. Furthermore, this article proposes a concise and straightforward graphical approach to assess transient stability of dVOC using vector field on the circle. To provide a more in-depth analysis, a complete large-signal model is derived and the impact of dVOC voltage amplitude dynamics is analyzed. For comparison, transient stability of the currently prevalent droop control is also assessed using phase portraits. Salient transient stability features of dVOC and droop control during grid faults are summarized and compared. The theoretical analysis is validated by controller hardware-in-the-loop testbed using industry-grade hardware.

Comparative Transient Stability Assessment of Droop and Dispatchable Virtual Oscillator Controlled Grid-Connected Inverters

In contrast to conventional static microgrids (MGs), MGs with dynamic and adjustable territories (i.e., dynamic MGs) are proposed and implemented in this article. Dynamic MGs are commonly dominated by grid-forming inverters and nested in unbalanced distribution feeders. Unlike balanced systems where only positive-sequence components exist, proper operation of unbalanced dynamic MGs presents additional challenges. A distributed secondary control strategy is developed in this article for distributed generators (DGs) interfaced with grid-forming inverters in unbalanced dynamic MGs by providing coordinated regulations on both positive- and negative-sequence system models. System frequency and voltage are under constant regulation, along with voltage unbalance (VU) management for multiple critical load buses (CLBs). The proposed control strategy enables seamless system transition during unbalanced dynamic MGs reconfiguration and guarantees proportional positive- and negative-sequence power-sharing among connected DGs with respect to system topology variation. Detailed controller designs are provided and stability analyses are derived. The proposed control strategy is fully implemented in hardware controllers and validated on a hardware-in-the-loop (HIL) MG testbed.

Dynamic Microgrids With Self-Organized Grid-Forming Inverters in Unbalanced Distribution Feeders

Cascaded H-bridge topology has been used in grid-tied converters for battery energy storage system due to its modular structure. To fully utilize the converter's modularity, this article proposes a hierarchical distributed control architecture that consists of primary control, secondary control, and battery state-of-charge (SOC) balancing control. Primary control ensures accurate current tracking, whereas a distributed secondary control based on consensus algorithm is presented to regulate power sharing among modules and is proved to be stable theoretically. A distributed SOC balancing control is further introduced to improve energy efficiency of battery energy storage system. Finally, the hierarchical distributed control strategy is implemented using hardware controllers and a software platform. Besides, a carrier phase shift control is also implemented to achieve multilevel output voltage and harmonic reduction. The experimental results demonstrate the performance of the proposed control scheme effectively.

Hao Tu

Papers

Extreme Fast Charging of Electric Vehicles: A Technology Overview

Hierarchical Control for Virtual Oscillator Based Grid-Connected and Islanded Microgrids

Comparative Transient Stability Assessment of Droop and Dispatchable Virtual Oscillator Controlled Grid-Connected Inverters

Dynamic Microgrids With Self-Organized Grid-Forming Inverters in Unbalanced Distribution Feeders

A Distributed Control Architecture for Cascaded H-Bridge Converter With Integrated Battery Energy Storage