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

DC and AC Microgrids are key elements to integrate renewable and distributed energy resources as well as distributed energy storage systems. In the last years, efforts toward the standardization of these Microgrids have been made. In this sense, this paper present the hierarchical control derived from ISA-95 and electrical dispatching standards to endow smartness and flexibility to microgrids. The hierarchical control proposed consist of three levels: i) the primary control is based on the droop method, including an output impedance virtual loop; ii) the secondary control allows restoring the deviations produced by the primary control; and iii) the tertiary control manage the power flow between the microgrid and the external electrical distribution system. Results from a hierarchical-controlled microgrid are provided to show the feasibility of the proposed approach.

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Hierarchical control of droop-controlled DC and AC microgrids — a general approach towards standardization

Energy management means to optimize one of the most complex and important technical creations that we know: the energy system. While there is plenty of experience in optimizing energy generation and distribution, it is the demand side that receives increasing attention by research and industry. Demand Side Management (DSM) is a portfolio of measures to improve the energy system at the side of consumption. It ranges from improving energy efficiency by using better materials, over smart energy tariffs with incentives for certain consumption patterns, up to sophisticated real-time control of distributed energy resources. This paper gives an overview and a taxonomy for DSM, analyzes the various types of DSM, and gives an outlook on the latest demonstration projects in this domain.

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Demand Side Management: Demand Response, Intelligent Energy Systems, and Smart Loads

The enabling of ac microgrids in distribution networks allows delivering distributed power and providing grid support services during regular operation of the grid, as well as powering isolated islands in case of faults and contingencies, thus increasing the performance and reliability of the electrical system. The high penetration of distributed generators, linked to the grid through highly controllable power processors based on power electronics, together with the incorporation of electrical energy storage systems, communication technologies, and controllable loads, opens new horizons to the effective expansion of microgrid applications integrated into electrical power systems. This paper carries out an overview about microgrid structures and control techniques at different hierarchical levels. At the power converter level, a detailed analysis of the main operation modes and control structures for power converters belonging to microgrids is carried out, focusing mainly on grid-forming, grid-feeding, and grid-supporting configurations. This analysis is extended as well toward the hierarchical control scheme of microgrids, which, based on the primary, secondary, and tertiary control layer division, is devoted to minimize the operation cost, coordinating support services, meanwhile maximizing the reliability and the controllability of microgrids. Finally, the main grid services that microgrids can offer to the main network, as well as the future trends in the development of their operation and control for the next future, are presented and discussed.

Control of Power Converters in AC Microgrids

The increasing interest in integrating intermittent renewable energy sources into microgrids presents major challenges from the viewpoints of reliable operation and control. In this paper, the major issues and challenges in microgrid control are discussed, and a review of state-of-the-art control strategies and trends is presented; a general overview of the main control principles (e.g., droop control, model predictive control, multi-agent systems) is also included. The paper classifies microgrid control strategies into three levels: primary, secondary, and tertiary, where primary and secondary levels are associated with the operation of the microgrid itself, and tertiary level pertains to the coordinated operation of the microgrid and the host grid. Each control level is discussed in detail in view of the relevant existing technical literature.

Trends in Microgrid Control

In the literature, many studies on stability analysis of dc microgrids have been conducted. However, most of them mainly focus on small-signal stability. On the other hand, few works analyze large-signal stability, but the major part of these works is based on a single unit or a simple cascaded system as a case study. Different from those, this article aims to address the large-signal stability analysis of a dc microgrid from a system-level perspective. First, the equivalent model of a droop-controlled dc microgrid is developed. Subsequently, the Lyapunov-based large-signal stability analysis and the stability criterion are derived, and mixed potential theory is used to make comparisons to verify the effectiveness of the derived criterion. The equilibrium point stability for different operation stages was obtained by means of theoretical calculation. Furthermore, the instabilities principle as well as their physical interpretation is revealed. In this article, the bus voltage is used as the only index to assess the microgrid power balance. Hence, the power load limit can be obtained by taking into consideration the stability and voltage deviation constraints. Finally, simulation and experimental results from a four-converter dc microgrid system verify the feasibility of the proposed theoretical analysis.

System-Level Large-Signal Stability Analysis of Droop-Controlled DC Microgrids

Next-generation power systems will require innovative control strategies to exploit existing and potential capabilities of developing renewable-based microgrids. Cooperation of interconnected microgrids has been introduced recently as a promising solution to improve the operational and economic performance of distribution networks. In this paper, a hierarchical control structure is proposed for the integrated operation management of a multi-microgrid system. A central energy management entity at the highest control level is responsible for designing a reference trajectory for exchanging power between the multi-microgrid system and the main grid. At the second level, the local energy management system of individual microgrids adopts a two-stage stochastic model predictive control strategy to manage the local operation by following the scheduled power trajectories. An optimal solution strategy is then applied to the local controllers as operating set-points to be implemented in the system. To distribute the penalty costs resulted from any real-time power deviation systematically and fairly, a novel methodology based on the line flow sensitivity factors is proposed. Simulation and experimental analyses are carried out to evaluate the effectiveness of the proposed approach. According to the simulation results, by adopting the proposed operation management strategy, a reduction of about 47% in the average unplanned daily power exchange of the multi-microgrid system with the main grid can be achieved.

Stochastic Predictive Energy Management of Multi-Microgrid Systems

The protection of AC microgrids (MGs) is an issue of paramount importance to ensure their reliable and safe operation. Designing reliable protection mechanism, however, is not a trivial task, as many practical issues need to be considered. The operation mode of MGs, which can be grid-connected or islanded, employed control strategy and practical limitations of the power electronic converters that are utilized to interface renewable energy sources and the grid, are some of the practical constraints that make fault detection, classification, and coordination in MGs different from legacy grid protection. This article aims to present the state-of-the-art of the latest research and developments, including the challenges and issues in the field of AC MG protection. A broad overview of the available fault detection, fault classification, and fault location techniques for AC MG protection and coordination are presented. Moreover, the available methods are classified, and their advantages and disadvantages are discussed.

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Recent Developments and Challenges on AC Microgrids Fault Detection and Protection Systems–A Review

A coordinated secondary control approach based on an autonomous current-sharing control strategy for balancing the discharge rates of energy storage systems (ESSs) in islanded ac microgrids is proposed in this paper. The coordinated secondary controller can regulate the power outputs of distributed generation (DG) units according to their states-of-charge and ESS capacities by adjusting the virtual resistances of the paralleled voltage-controlled inverters. Compared with existing controllers, the proposed control strategy not only effectively prevents operation failure caused by overcurrent incidents and unintentional outages in DG units, but also aims to provide a fast transient response and an accurate output-current-sharing performance. A complete root locus analysis is given in order to achieve the system stability and parameter sensitivity. Experimental results are presented to show the performance of the whole system and to verify the effectiveness of the proposed controller.

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Coordinated Secondary Control for Balanced Discharge Rate of Energy Storage System in Islanded AC Microgrids

DC and dc/ac hybrid distribution and energy storage for shipboard power systems (SPSs) are becoming a major trend due to efficiency improvement, space saving, and maneuverability enhancement. This paper has taken a real hybrid-electric-ferry as a case-study to integrate battery units (BUs) to a dc bus for supplying the propulsion motors. Furthermore, two diesel generators (DGs) are connected to the ac bus to supply the hotel loads, and a bidirectional dc/ac converter with an LCL filter is responsible for the power flow between ac and dc buses. This power topology is flexible for this ferry operation in pure electrics, extended range, and shore power modes. The DC bus voltage is stabilized and its voltage ripple is limited by BUs’ interleaved three-phase bidirectional dc/dc converter with its controller considering the operation states of propulsion motors. A coordinated power flow control between DGs and BUs is presented that the system frequency is fixed for the optimal operational efficiency of the diesel engines and a ${Q}$ – ${V}$ droop control plus a virtual impedance loop is used to make different ac bus voltages. Synchronization with shore power and the dc/ac converter is facilitated by ${P}$ – ${f}$ droop control. Simulation results are presented to validate the proposed control approach in different missions.

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Juan C. Vasquez

Papers

System-Level Large-Signal Stability Analysis of Droop-Controlled DC Microgrids

Stochastic Predictive Energy Management of Multi-Microgrid Systems

Recent Developments and Challenges on AC Microgrids Fault Detection and Protection Systems–A Review

Coordinated Secondary Control for Balanced Discharge Rate of Energy Storage System in Islanded AC Microgrids

Coordinated Control of a Hybrid-Electric-Ferry Shipboard Microgrid