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

This paper presents the latest comprehensive literature review of AC and DC microgrid (MG) systems in connection with distributed generation (DG) units using renewable energy sources (RESs), energy storage systems (ESS) and loads. A survey on the alternative DG units' configurations in the low voltage AC (LVAC) and DC (LVDC) distribution networks with several applications of microgrid systems in the viewpoint of the current and the future consumer equipments energy market is extensively discussed. Based on the economical, technical and environmental benefits of the renewable energy related DG units, a thorough comparison between the two types of microgrid systems is provided. The paper also investigates the feasibility, control and energy management strategies of the two microgrid systems relying on the most current research works. Finally, the generalized relay tripping currents are derived and the protection strategies in microgrid systems are addressed in detail. From this literature survey, it can be revealed that the AC and DC microgrid systems with multiconverter devices are intrinsically potential for the future energy systems to achieve reliability, efficiency and quality power supply.

AC-microgrids versus DC-microgrids with distributed energy resources: A review

DC microgrids (MGs) have been gaining a continually increasing interest over the past couple of years both in academia and industry. The advantages of dc distribution when compared to its ac counterpart are well known. The most important ones include higher reliability and efficiency, simpler control and natural interface with renewable energy sources, and electronic loads and energy storage systems. With rapid emergence of these components in modern power systems, the importance of dc in today's society is gradually being brought to a whole new level. A broad class of traditional dc distribution applications, such as traction, telecom, vehicular, and distributed power systems can be classified under dc MG framework and ongoing development, and expansion of the field is largely influenced by concepts used over there. This paper aims first to shed light on the practical design aspects of dc MG technology concerning typical power hardware topologies and their suitability for different emerging smart grid applications. Then, an overview of the state of the art in dc MG protection and grounding is provided. Owing to the fact that there is no zero-current crossing, an arc that appears upon breaking dc current cannot be extinguished naturally, making the protection of dc MGs a challenging problem. In relation with this, a comprehensive overview of protection schemes, which discusses both design of practical protective devices and their integration into overall protection systems, is provided. Closely coupled with protection, conflicting grounding objectives, e.g., minimization of stray current and common-mode voltage, are explained and several practical solutions are presented. Also, standardization efforts for dc systems are addressed. Finally, concluding remarks and important future research directions are pointed out.

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DC Microgrids—Part II: A Review of Power Architectures, Applications, and Standardization Issues

A cooperative control paradigm is used to establish a distributed secondary/primary control framework for dc microgrids. The conventional secondary control, that adjusts the voltage set point for the local droop mechanism, is replaced by a voltage regulator and a current regulator. A noise-resilient voltage observer is introduced that uses neighbors’ data to estimate the average voltage across the microgrid. The voltage regulator processes this estimation and generates a voltage correction term to adjust the local voltage set point. This adjustment maintains the microgrid voltage level as desired by the tertiary control. The current regulator compares the local per-unit current of each converter with the neighbors’ and, accordingly, provides a second voltage correction term to synchronize per-unit currents and, thus, provide proportional load sharing. The proposed controller precisely handles the transmission line impedances. The controller on each converter communicates with only its neighbor converters on a communication graph. The graph is a sparse network of communication links spanned across the microgrid to facilitate data exchange. The global dynamic model of the microgrid is derived, and design guidelines are provided to tune the system's dynamic response. A low-voltage dc microgrid prototype is set up, where the controller performance, noise resiliency, link-failure resiliency, and the plug-and-play capability features are successfully verified.

Distributed Cooperative Control of DC Microgrids

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 examines a boundary control for dc-dc buck converters subject to instantaneous constant-power loads. These loads introduce a destabilizing nonlinear effect on the converter through an inverse voltage term that leads to significant oscillations in the main bus voltage or its collapse. Converter dynamics are analyzed in both switching states and the various operating regions of switch interaction with a first-order switching surface (boundary) are identified. Furthermore, sufficient conditions for large-signal stability of the closed-loop system are established. The analysis indicates that a linear switching surface with a negative slope passing through the desired operation point yields a stable operation. It is also shown that instability as well as system-stalling, which we term the invariant-set problem, may still occur in reflective mode. However, a hysteresis band that contains the designed boundary may be used to prevent system-stalling. This hysteresis band also allows for a practical implementation of the controller by avoiding chattering. Design considerations are included and recommendations are given. The theoretical analysis is verified by simulations and experimental results.

Analysis of Boundary Control for Buck Converters With Instantaneous Constant-Power Loads

In this paper, a switching strategy for multiple-input converters (MICs) is presented and analyzed. MICs have been identified to provide a cost-effective approach for energy harvesting in hybrid systems, and for power distribution in micro- and nanogrids. The basic principle of the proposed switching strategy is that the effective duty ratio of each switch is an integer multiple of a common duty ratio (CDR), the CDR being the duty ratio of a common switching function that is generated at a higher frequency by frequency division. The proposed strategy enables switching functions for MICs that have a greater number of input legs to be generated with relative ease. Another benefit of this scheme is that it allows an MIC's output voltage to be regulated by employing the CDR as the only control variable, irrespective of the number of input legs present. Essentially, the strategy transforms an MIC into an equivalent single-input single-output system for analysis, which simplifies controller design and implementation. Without loss of generality, this technique is demonstrated by analyzing a multiple-input buck-boost converter. A PI controller is shown to regulate the MIC's operating point. The analysis is verified by simulations and experiments.

A Modified-Time-Sharing Switching Technique for Multiple-Input DC–DC Converters

Cascade distributed power architectures include tightly-regulated point-of-load converters that exhibit instantaneous-constant-power-load (CPLs) characteristics. Boundary control is investigated for dc-dc boost and buck-boost converters that feed these instantaneous (CPLs). Without adequate controls, the destabilizing nonlinear effect of the CPL inverse-voltage term leads to significant oscillations in output voltage of these converters, and possible voltage collapse. The analysis presented in this paper reveals important characteristics of CPL effects on these two basic converter topologies. In order to avoid issues related with the fact that the state-dependent switching function is undefined on the switching surface, in reflective mode solutions to both converter systems are defined in the sense of Filippov. Consequently, Lyapunov's direct method is used to identify stable reflective regions that guarantee sliding-mode operation towards the desired operating point for both converters. It is shown that first-order switching surfaces with negative slopes achieve large signal stability for both converter systems, while positive slopes lead to instability. For the boost converter, it is illustrated via simulation and experiment that positive slopes may lead to another closed-loop limit cycle. Design considerations are included and recommendations are given. Simulations and experimental results verify the analysis.

Analysis of boundary control for boost and buck-boost converters in distributed power architectures with constant-power loads

This paper discusses stability issues in dc micro-grids with cascade distributed architectures. In these architectures point-of-load converters behave as instantaneous constant-power loads to line regulating converters located upstream. Constant-power loads introduce a destabilizing effect into a dc micro-grid that may cause its main bus voltages to show significant oscillations or to collapse. Mitigating strategies such as load shedding, use of linear and nonlinear (geometric) controllers, or addition of resistive loads, filters or energy storage directly connected to the main bus are explained. Advantages and disadvantages of these methods are discussed and the analysis is verified through simulations and experiments with prototypes.

Chimaobi N. Onwuchekwa

Papers

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

Analysis of Boundary Control for Buck Converters With Instantaneous Constant-Power Loads

A Modified-Time-Sharing Switching Technique for Multiple-Input DC–DC Converters

Analysis of boundary control for boost and buck-boost converters in distributed power architectures with constant-power loads

Effects of instantaneous constant-power loads on DC micro-grids for sustainable power systems