Decentralized control for parallel operation of distributed generation inverters using resistive output impedance
01 Dec 2005-pp 1665636
TL;DR: In this paper, a novel wireless load sharing controller for islanding parallel inverters in an ac distributed system is proposed, where the resistive output impedance of the parallel-connected inverters is explored.
Abstract: In this paper, a novel wireless load-sharing controller for islanding parallel inverters in an ac distributed system is proposed. The paper explorers the resistive output impedance of the parallel-connected inverters in an island microgrid
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01 Nov 2009
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.
4,145 citations
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 "Decentralized control for parallel ..."
...Reference [83] introduces a virtual output resistance to achieve automatic harmonic power sharing and re-...
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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 or methods from "Decentralized control for parallel ..."
...pedances have a significant resistive component [16]–[23]....
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...issue is presented in [16], in which virtual resistive output impedance is introduced by modifying the output voltage reference based on output current feedback....
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...However, those have an inherent trade of between P/Q sharing and frequency/amplitude regulation [16]–[19]....
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...impedance of the units by adding output virtual reactors [17] or resistors [16] have been included into the droop method, with the purpose of sharing the harmonic current content properly....
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...Usually, the virtual impedance ZD is designed to be bigger than the output impedance of the inverter plus the line impedance; this way, the total equivalent output impedance is mainly dominated by ZD [16]....
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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 "Decentralized control for parallel ..."
...Thus, the voltage-active power droop and frequency-reactive power boost (VPD/FQB) characteristics are alternatively considered [40]...
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TL;DR: In this article, a power control strategy for a low-voltage microgrid is proposed, where the mainly resistive line impedance, the unequal impedance among distributed generation (DG) units, and the microgrid load locations make the conventional frequency and voltage droop method unpractical.
Abstract: In this paper, a power control strategy is proposed for a low-voltage microgrid, where the mainly resistive line impedance, the unequal impedance among distributed generation (DG) units, and the microgrid load locations make the conventional frequency and voltage droop method unpractical. The proposed power control strategy contains a virtual inductor at the interfacing inverter output and an accurate power control and sharing algorithm with consideration of both impedance voltage drop effect and DG local load effect. Specifically, the virtual inductance can effectively prevent the coupling between the real and reactive powers by introducing a predominantly inductive impedance even in a low-voltage network with resistive line impedances. On the other hand, based on the predominantly inductive impedance, the proposed accurate reactive power sharing algorithm functions by estimating the impedance voltage drops and significantly improves the reactive power control and sharing accuracy. Finally, considering the different locations of loads in a multibus microgrid, the reactive power control accuracy is further improved by employing an online estimated reactive power offset to compensate the effects of DG local load power demands. The proposed power control strategy has been tested in simulation and experimentally on a low-voltage microgrid prototype.
1,060 citations
Cites background from "Decentralized control for parallel ..."
...Another way to avoid the power coupling is to properly control the interfacing inverter with virtual output impedance [12]–[14]....
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References
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28 Sep 1991
TL;DR: In this article, a control scheme for parallel-connected inverters in a standalone AC supply system is presented, which uses feedback of only those variables that can be measured locally at the inverter and does not need communication of control signals between the inverters.
Abstract: A scheme for controlling parallel-connected inverters in a standalone AC supply system is presented. This scheme is suitable for control of inverters in distributed source environments such as in isolated AC systems, large and distributed uninterruptible power supply (UPS) systems, photovoltaic systems connected to AC grids, and low-voltage DC power transmission meshes. A key feature of the control scheme is that it uses feedback of only those variables that can be measured locally at the inverter and does not need communication of control signals between the inverters. This is essential for the operation of large AC systems, where distances between inverters make communication impractical. It is also important in high-reliability UPS systems where system operation can be maintained in the face of a communication breakdown. Real and reactive power sharing between inverters can be achieved by controlling two independent quantities: the power angle and the fundamental inverter voltage magnitude. Simulation results are presented. >
1,550 citations
TL;DR: This paper deals with the design of the output impedance of uninterruptible power system (UPS) inverters with parallel-connection capability, and proposes novel control loops to achieve both stable output impedance and proper power balance.
Abstract: This paper deals with the design of the output impedance of uninterruptible power system (UPS) inverters with parallel-connection capability. In order to avoid the need for any communication among modules, the power-sharing control loops are based on the P/Q droop method. Since in these systems the power-sharing accuracy is highly sensitive to the inverters output impedance, novel control loops to achieve both stable output impedance and proper power balance are proposed. In this sense, a novel wireless controller is designed by using three nested loops: 1) the inner loop is performed by using feedback linearization control techniques, providing a good quality output voltage waveform; 2) the intermediate loop enforces the output impedance of the inverter, achieving good harmonic power sharing while maintaining low output voltage total harmonic distortion; and 3) the outer loop calculates the output active and reactive powers and adjusts the output impedance value and the output voltage frequency during the load transients, obtaining excellent power sharing without deviations in either the frequency or the amplitude of the output voltage. Simulation and experimental results are reported from a parallel-connected UPS system sharing linear and nonlinear loads.
1,076 citations
"Decentralized control for parallel ..." refers background in this paper
...The output impedance of the closed-loop inverter affects the power-sharing accuracy and also determines the droopcontrol strategy [13]....
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...Besides, the integral term adds an inductive behavior to the output impedance [13], which is not desirable for our approach....
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...However, this is not always true since the output impedance of the inverter depends also on the control strategy [13], [14], and the line impedance is predominantly resistive for low-voltage cabling....
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TL;DR: In this paper, a novel control strategy for parallel inverters of distributed generation units in an AC distribution system is presented, based on the droop control method, using only locally measurable feedback signals.
Abstract: This paper presents a novel control strategy for parallel inverters of distributed generation units in an AC distribution system. The proposed control technique, based on the droop control method, uses only locally measurable feedback signals. This method is usually applied to achieve good active and reactive power sharing when communication between the inverters is difficult due to its physical location. However, the conventional voltage and frequency droop methods of achieving load sharing have a slow and oscillating transient response. Moreover, there is no possibility to modify the transient response without the loss of power sharing precision or output-voltage and frequency accuracy. In this work, a great improvement in transient response is achieved by introducing power derivative-integral terms into a conventional droop scheme. Hence, better controllability of the system is obtained and, consequently, correct transient performance can be achieved. In addition, an instantaneous current control loop is also included in the novel controller to ensure correct sharing of harmonic components when supplying nonlinear loads. Simulation and experimental results are presented to prove the validity of this approach, which shows excellent performance as opposed to the conventional one.
1,003 citations
"Decentralized control for parallel ..." refers methods in this paper
...After studying the eigenvalues behavior of the system (λ1, λ2, and λ3) through a series of root-locus diagrams, as in [11] and [12], and using the values listed in Table II, we can conclude that the output impedance RD has little effect on the location of the roots in comparison with the md and nd coefficients....
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...In [12], a wireless controller was proposed in order to enhance the dynamic performance of the paralleled inverters by adding integral–derivative power terms to the droop-control method....
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Book•
01 Jan 1986
TL;DR: In this article, the authors present a model of a single machine-infinite bus (SIB) with three-phase transformer connections for the purpose of detecting faults in a generator.
Abstract: 1. Background. Introduction. Electric Energy. Fossil-Fuel Plant. Nuclear Power Plant. Hydroelectric Power Plant. Other Energy Sources. Transmission and Distribution Systems. The Deregulated Electric Power Industry. 2. Basic Principles. Introduction. Phasor Representation. Complex Power Supplied to a One-Port. Conservation of Complex Power. Balanced Three-Phase. Per Phase Analysis. Balanced Three-Phase Power. Summary. 3. Transmission-Line Parameters. Introduction. Review of Magnetics. Flux Linkages of Infinite Straight Wire. Flux Linkages Many-Conductor Case. Conductor Bundling. Transposition. Impedance of Three Phase lines Including Ground Return. Review of Electric Fields. Line Capacitance. Determination of Line Parameters Using Tables. Typical Parameter Values. Summary. 4. Transmission-Line Modeling. Introduction. Derivation of Terminal V, I Relations. Waves on Transmission Lines. Transmission Matrix. Lumped-Circuit Equivalent. Simplified Models. Complex Power Transmission (Short Line). Complex Power Transmission (Radial Line). Complex Power Transmission (Long or Medium Lines). Power-Handling Capability of Lines. Summary. 5. Transformer Modeling and the Per Unit System. Introduction. Single-Phase Transformer Model. Three-Phase Transformer Connections. Per Phase Analysis. Normal Systems. Per Unit Normalization. Per Unit Three-Phase Quantities. Change of Base. Per Unit Analysis of Normal System. Regulating Transformers for Voltage and Phase Angle Control. Autotransformers. Transmission Line and Transformers. Summary 6. Generator Modeling I (Machine Viewpoint). Introduction. Classical Machine Description. Voltage Generation. Open-Circuit Voltage. Armature Reaction. Terminal Voltage. Power Delivered by Generator. Synchronizing Generator to an Infinite Bus. Synchronous Condensor. Role of Synchronous Machine Excitation in Controlling Reactive Power. Summary. 7. Generator Modeling II (Circuit Viewpoint). Introduction. Energy Conversion. Application to Synchronous Machine. The Park Transformation. Park's Voltage Equation. Park's Mechanical Equation. Circuit Model. Instantaneous Power Output. Applications. Synchronous Operation. Steady-State Model. Simplified Dynamic Model. Generator Connected to Infinite Bus (Linear Model). Summary 8. Generator Voltage Control. Introduction. Exciter System Block Diagram. Generator Models. Stability of Excitation System. Voltage Regulation. Generator Connected to Infinite Bus. Summary. 9. Network Matrices. Introduction. Bus Admittance Matrix. Network Solution. Network Reduction (Kron Reduction). YBUS Structure and Manipulation. Bus Impedance Matrix. Inverse Elements to Determine Columns of ZBUS. Summary. 10. Power Flow Analysis. Introduction. Power Flow Equations. The Power Flow Problem. Solution by Gauss Iteration. More General Iteration Scheme. Newton-Raphson Iteration. Application to Power Flow Equations. Decoupled Power Flow. Control Implications. Regulating Transformers in Power Flow Analysis, Power Flow Solutions for Large Power Systems. Summary. 11. Automatic Generation Control and the New Market Environment. Introduction. Power Control System Modeling. Application to Single Machine-Infinite Bus System. Simplified Analysis of Power Control System. Power Control, Multigenerator Case. Special Case Two Generating Units. Division of Power System Into Control Areas. Formulation of the Economic Dispatch Problem. Classical Economic Dispatch (Line Losses Neglected). Generator Limits Included. Line Losses Considered. Calculation of Penalty Factors. Economic Issues and Mechanisms in the New Market Environment. Transmission Issues and Effects in the New Market Environment. Summary. 12. Unbalanced System Operation. Introduction. Symmetrical Components. Use of Symmetrical Components for Fault Analysis. Sequence Network Connections for Different Types of Faults. More General Fault Circuit Analysis. Power From Sequence Variables. Sequence Representation of Y and ...D Connected Circuits. Generator Models for Sequence Networks. Transformer Models for Sequence Networks. Sequence Representation of Transmission Lines. Assembly of Sequence Networks. Fault Analysis for Realistic Power System Model. Matrix Methods. Summary. 13. System Protection. Introduction. Protection of Radial Systems. System with Two Sources. Impedance (Distance) Relays. Modified-Impedance Relays. Differential Protection of Generators. Differential Protection of Transformers. Differential Protection of Buses and Lines. Overlapping Zones of Protection. Sequence Filters. Computer Relaying. Summary. 14. Power System Stability. Introduction. Model. Energy Balance. Linearization of Swing Equation. Solution of Nonlinear Swing Equation. Other Applications. Extension to Two-Machine Case. Multimachine Application. Multimachine Stability Studies. Summary. Appendices. Reluctance. Force Generation in a Solenoid. Method of Lagrange Multipliers. Root-Locus Method. Negative- and Zero-Sequence Impedances of Synchronous Machines. Inversion Formula. Modification of Impedance Matrices. Conductor Characteristics. Selected Bibliography. Index.
749 citations
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
"Decentralized control for parallel ..." refers background in this paper
...However, this is not always true since the output impedance of the inverter depends also on the control strategy [13], [14], and the line impedance is predominantly resistive for low-voltage cabling....
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