Hierarchical control of droop-controlled DC and AC microgrids — a general approach towards standardization
01 Nov 2009-Vol. 58, Iss: 1, pp 158-172
TL;DR: The hierarchical control derived from ISA-95 and electrical dispatching standards to endow smartness and flexibility to MGs is presented and results are provided to show the feasibility of the proposed approach.
Abstract: 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.
Citations
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TL;DR: In this paper, a detailed analysis of the main operation modes and control structures for power converters belonging to micro-grids is carried out, focusing mainly on grid-forming, grid-feeding, and grid-supporting configurations.
Abstract: 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.
2,621 citations
Cites background from "Hierarchical control of droop-contr..."
...controlling the voltage and frequency of the ac waveforms provided to the microgrid in stand-alone mode [59], [60] and gridsupporting inverters will contribute to guarantee a proper voltage profile along the microgrid extension....
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...Microgrid power converters can competently participate in regulating the grid voltage profile, mainly in LV grids, by means of controlling the active and reactive power delivery through droop control algorithms [47], [60], [70]....
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TL;DR: 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).
Abstract: 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.
2,358 citations
Cites background from "Hierarchical control of droop-contr..."
...A hierarchical control scheme is proposed in [50] to improve the flexibility and expansibility of droop-based microgrids....
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...gent performance requirements [50]–[53]....
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...However, the conventional droop control method has several disadvantages [50], [78], [80], [81]: • Poor transient performance or instability issues due to the use of average values of active and reactive power over a cycle....
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TL;DR: Decentralized, distributed, and hierarchical control of grid-connected and islanded microgrids that mimic the behavior of the mains grid is reviewed.
Abstract: 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.
1,702 citations
Cites background from "Hierarchical control of droop-contr..."
...This three-level hierarchical control is organized as follows [33]....
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TL;DR: This paper reviews the status of hierarchical control strategies applied to microgrids and discusses the future trends.
Abstract: 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.
1,234 citations
Cites background from "Hierarchical control of droop-contr..."
...Thus, (3) is not valid for microgrid applications [12], [45]....
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...Block diagram of the secondary and tertiary controls [12]....
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...Then, the error signals are processed by individual controllers as in (52); the resulting signals ( and ) are sent to the primary controller of the DER to compensate for the frequency and voltage deviations [12]...
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...facilitates an economically optimal operation [12], [13]....
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...where , and are the controllers parameters [12]....
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TL;DR: In this paper, a review of control strategies, stability analysis, and stabilization techniques for dc microgrids is presented, where overall control is systematically classified into local and coordinated control levels according to respective functionalities in each level.
Abstract: 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.
1,131 citations
Cites background or methods from "Hierarchical control of droop-contr..."
...either output power or output current can be selected as the feedback signal in droop control [3], [29]....
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...1) Decentralized control: DCLs do not exist and power lines are used as the only channel of communication....
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...MGs can operate autonomously or be grid-connected, and depending on the type of voltage in the point of common coupling (PCC), ac, and dc MGs can be distinguished [3]....
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...Some of the functionalities that can be accomplished by using DCLs include secondary/tertiary control, real-time optimization, unit commitment, and internal operating mode changing (see Fig....
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...the conventional approach uses a centralized controller which collects information from all units via low-bandwidth DCLs [3], a very active field of research is focused on resolution of these problems via distributed control1 [22], [23]....
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References
More filters
TL;DR: Adapt virtual output impedance is proposed in order to achieve a proper reactive power sharing regardless of the line impedance unbalances and can be properly shared due to the addition of a current harmonic loop in the control strategy.
Abstract: In this paper, a method for the parallel operation of inverters in an ac-distributed system is proposed. This paper explores the control of active and reactive power flow through the analysis of the output impedance of the inverters and its impact on the power sharing. As a result, adaptive virtual output impedance is proposed in order to achieve a proper reactive power sharing, regardless of the line-impedance unbalances. A soft-start operation is also included, avoiding the initial current peak, which results in a seamless hot-swap operation. Active power sharing is achieved by adjusting the frequency in load transient situations only, owing to which the proposed method obtains a constant steady-state frequency and amplitude. As opposed to the conventional droop method, the transient response can be modified by acting on the main control parameters. Linear and nonlinear loads can be properly shared due to the addition of a current harmonic loop in the control strategy. Experimental results are presented from a two-6-kVA parallel-connected inverter system, showing the feasibility of the proposed approach
676 citations
TL;DR: The most important control schemes for the parallel operation of UPS systems, including active load-sharing techniques and droop control approaches, are depicted.
Abstract: In the last years, the use of distributed uninterruptible power supply (UPS) systems has been growing into the market, becoming an alternative to large conventional UPS systems. In addition, with the increasing interest in renewable energy integration and distributed generation, distributed UPS systems can be a suitable solution for storage energy in micro grids. This paper depicts the most important control schemes for the parallel operation of UPS systems. Active load-sharing techniques and droop control approaches are described. The recent improvements and variants of these control techniques are presented.
666 citations
TL;DR: In this article, the authors discuss the definitions of a distributed energy system and evaluate political, economic, social, and technological dimensions associated with regional energy systems on the basis of the degree of decentralization.
Abstract: Conventionally, power plants have been large, centralized units. A new trend is developing toward distributed energy generation, which means that energy conversion units are situated close to energy consumers, and large units are substituted by smaller ones. A distributed energy system is an efficient, reliable and environmentally friendly alternative to the traditional energy system. In this article, we will first discuss the definitions of a distributed energy system. Then we will evaluate political, economic, social, and technological dimensions associated with regional energy systems on the basis of the degree of decentralization. Finally, we will deal with the characteristics of a distributed energy system in the context of sustainability. This article concludes that a distributed energy system is a good option with respect to sustainable development.
640 citations
"Hierarchical control of droop-contr..." refers background in this paper
...AC MGs are now in the cutting edge of the state of the art [20]–[25]....
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TL;DR: A novel control for voltage-source inverters with the capability to flexibly operate in grid-connected and islanded modes based on the droop method, which uses some estimated grid parameters such as the voltage and frequency and the magnitude and angle of the grid impedance.
Abstract: This paper proposes a novel control for voltage-source inverters with the capability to flexibly operate in grid-connected and islanded modes. The control scheme is based on the droop method, which uses some estimated grid parameters such as the voltage and frequency and the magnitude and angle of the grid impedance. Hence, the inverter is able to inject independently active and reactive power to the grid. The controller provides a proper dynamics decoupled from the grid-impedance magnitude and phase. The system is also able to control active and reactive power flows independently for a large range of impedance grid values. Simulation and experimental results are provided in order to show the feasibility of the control proposed.
602 citations
TL;DR: This paper explains the control structure required for the converters to permit the use of DBS, and explains a procedure for implementing a system-wide control law through independent control of the source/storage interface converters.
Abstract: A dc nanogrid is a hybrid renewable system since renewable sources supply the average load demand, while storage and nonrenewable generation maintain the power balance in the presence of the stochastic renewable sources. The system is power electronic based, with converters being used to interface both the sources and loads to the system. The nanogrid is controlled using dc-bus signaling (DBS), a distributed control strategy in which the control nodes, the source/storage interface converters, induce voltage-level changes to communicate with the other control nodes. This paper explains the control structure required for the converters to permit the use of DBS, and explains a procedure for implementing a system-wide control law through independent control of the source/storage interface converters. Experimental results are presented to demonstrate the operation of this novel control strategy
538 citations