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

This paper presents a review of advanced control techniques for microgrids. This paper covers decentralized, distributed, and hierarchical control of grid-connected and islanded microgrids. At first, decentralized control techniques for microgrids are reviewed. Then, the recent developments in the stability analysis of decentralized controlled microgrids are discussed. Finally, hierarchical control for microgrids that mimic the behavior of the mains grid is reviewed.

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Advanced Control Architectures for Intelligent Microgrids—Part I: Decentralized and Hierarchical Control

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.

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

The analysis of the small-signal stability of conventional power systems is well established, but for inverter based microgrids there is a need to establish how circuit and control features give rise to particular oscillatory modes and which of these have poor damping. This paper develops the modeling and analysis of autonomous operation of inverter-based microgrids. Each sub-module is modeled in state-space form and all are combined together on a common reference frame. The model captures the detail of the control loops of the inverter but not the switching action. Some inverter modes are found at relatively high frequency and so a full dynamic model of the network (rather than an algebraic impedance model) is used. The complete model is linearized around an operating point and the resulting system matrix is used to derive the eigenvalues. The eigenvalues (termed "modes") indicate the frequency and damping of oscillatory components in the transient response. A sensitivity analysis is also presented which helps identifying the origin of each of the modes and identify possible feedback signals for design of controllers to improve the system stability. With experience it is possible to simplify the model (reduce the order) if particular modes are not of interest as is the case with synchronous machine models. Experimental results from a microgrid of three 10-kW inverters are used to verify the results obtained from the model

Modeling, Analysis and Testing of Autonomous Operation of an Inverter-Based Microgrid

This paper presents an energy management system (EMS) for a stand-alone droop-controlled microgrid, which adjusts generators output power to minimize fuel consumption and also ensures stable operation. It has previously been shown that frequency-droop gains have a significant effect on stability in such microgrids. Relationship between these parameters and stability margins are therefore identified, using qualitative analysis and small-signal techniques. This allows them to be selected to ensure stability. Optimized generator outputs are then implemented in real-time by the EMS, through adjustments to droop characteristics within this constraint. Experimental results from a laboratory-sized microgrid confirm the EMS function.

https://spiral.imperial.ac.uk:8443/bitstream/10044/1/15430/2/IEEE%20Transactions%20on%20Power%20Electronics_23_5_2008.pdf

Energy Management in Autonomous Microgrid Using Stability-Constrained Droop Control of Inverters

The paper investigates the use of ancillary services from inverter-interfaced distributed generators (DGs) to achieve harmonic mitigation across a network. The approach is to include the functionality of a resistive active-power filter (R-APF) within several DGs. The R-APF provides adjustable damping at harmonic frequencies. In a realistic network, which has feeder sections of different characteristic impedances, it is impractical to damp with a single value of resistance. Instead, feeders are split into harmonic sections based on the standing waves of the highest-order harmonic, and DG ancillary services are called up for each section. Co-ordination of services from each DG is arranged through adaptation of the harmonic resistance according to target THD levels. The primary purpose of each DG is the supply of real power and this is respected through a further aspect of the resistance adaptation which reduces the harmonic duty of an individual DG if it approaches the apparent-power rating of the inverter. The harmonic VA required of the inverter is dependent on both the chosen harmonic resistance and the harmonic-voltage component present at the connection bus. The system is demonstrated through a simulation of an irregular feeder using Simulink and PLECS.

/pdf/harmonic-mitigation-throughout-a-distribution-system-a-11elcy2vyo.pdf

Harmonic mitigation throughout a distribution system: a distributed-generator-based solution

This paper presents an energy management system (EMS) for a stand-alone droop-controlled microgrid, which adjusts generator outputs to minimize fuel consumption and also ensures stable operation. It has previously been shown that droop gains have a significant effect on stability in such microgrids. Approximate relationships between these parameters and stability margins are therefore identified, using qualitative analysis and small-signal techniques. This allows them to be selected to ensure stability. Optimized generator outputs are then implemented in real-time by the EMS, through adjustments to droop characteristics within this constraint. Experimental results from a laboratory-sized microgrid confirm the EMS function.

Energy Management System with Stability Constraints for Stand-alone Autonomous Microgrid

The presence of inverter-interfaced distributed generators in a network provides an opportunity to mitigate the harmonic voltages caused throughout the network by non-linear loads. DG can be given a resistive characteristic at harmonic frequencies to provide a low impedance path to harmonic current and to damp resonant interaction of line inductance and shunt capacitance. Approaches to choosing a suitable resistance (gain) are examined and a scheme for gain adaptation that works within the current limitation of the inverter is proposed. The harmonic VA deployed by each inverter can be measured and controlled through the choice of resistance. It is possible to use a common resistance value for all harmonics or individually set resistance for each harmonic. A common resistance is significantly easier to implement, but an individual resistance for a few lower order harmonics is more effective

N. Pogaku

Papers

Modeling, Analysis and Testing of Autonomous Operation of an Inverter-Based Microgrid

Energy Management in Autonomous Microgrid Using Stability-Constrained Droop Control of Inverters

Harmonic mitigation throughout a distribution system: a distributed-generator-based solution

Energy Management System with Stability Constraints for Stand-alone Autonomous Microgrid

Application of Inverter-Based Distributed Generators for Harmonic Damping Throughout a Distribution Network