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Journal ArticleDOI

A wireless controller to enhance dynamic performance of parallel inverters in distributed generation systems

03 Sep 2004-IEEE Transactions on Power Electronics (IEEE)-Vol. 19, Iss: 5, pp 1205-1213
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.
Citations
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Journal ArticleDOI
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

Journal ArticleDOI
TL;DR: In this paper, the authors developed a model for autonomous operation of inverter-based micro-grids, where each sub-module is modeled in state-space form and all are combined together on a common reference frame.
Abstract: 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

2,482 citations


Cites methods from "A wireless controller to enhance dy..."

  • ...The modeling approach presented in [7] concentrates on stability issues for an individual inverter connected to a stiff ac bus....

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Journal ArticleDOI
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 "A wireless controller to enhance dy..."

  • ...Small signal stability and eigenvalue analysis of converter-fed microgrids that are controlled based on droop characteristics have also been reported in the literature [82], [85], [93], [94], concluding that the dominant modes of the closed loop system are determined by droop controllers, and the overall stability of the system mainly depends on droop control gains, system loading conditions, and network parameters....

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  • ...inaccuracy and active/reactive power coupling in distribution networks, the control loop is augmented with a virtual inductive output impedance in [79], [84], [85]....

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Journal ArticleDOI
TL;DR: In this paper, an adaptive decentralized droop controller of paralleled inverter-based distributed generation (DG) units is presented to preserve the power sharing stability, which is based on the static droop characteristics combined with an adaptive transient droop function.
Abstract: This paper addresses the low-frequency relative stability problem in paralleled inverter-based distributed generation (DG) units in microgrids. In the sense of the small-signal dynamics of a microgrid, it can be shown that as the demanded power of each inverter changes, the low-frequency modes of the power sharing dynamics drift to new locations and the relative stability is remarkably affected, and eventually, instability can be yielded. To preserve the power sharing stability, an adaptive decentralized droop controller of paralleled inverter-based DG units is presented in this paper. The proposed power sharing strategy is based on the static droop characteristics combined with an adaptive transient droop function. Unlike conventional droop controllers, which yield 1-DOF tunable controller, the proposed droop controller yields 2-DOF tunable controller. Subsequently, the dynamic performance of the power sharing mechanism can be adjusted, without affecting the static droop gain, to damp the oscillatory modes of the power sharing controller. To account for the power modes immigration at different loading conditions, the transient droop gains are adaptively scheduled via small-signal analysis of the power sharing mechanism along the loading trajectory of each DG unit to yield the desired transient and steady-state response. The gain adaptation scheme utilizes the filtered active and reactive powers as indices; therefore, a stable and smooth power injection performance can be obtained at different loading conditions. The adaptive nature of the proposed controller ensures active damping of power oscillations at different operating conditions, and yields a stable and robust performance of the paralleled inverter system.

1,130 citations


Cites background from "A wireless controller to enhance dy..."

  • ...An enhanced droop control featuring a transient droop performance is proposed in [11]....

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  • ...To improve the active and reactive power decoupling performance, improved droop controllers with virtual output impedance are reported [11], [12]....

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Journal ArticleDOI
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


Cites background from "A wireless controller to enhance dy..."

  • ...This is useful to share linear and nonlinear loads [21], [22]....

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References
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Journal ArticleDOI
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

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

Journal ArticleDOI
T. Kawabata1, S. Higashino1
TL;DR: In this paper, the authors discuss the problem of parallel operating systems of voltage source inverters with other inverters or with the utility source and propose several methods for protection against failure.
Abstract: Parallel operating systems of voltage source inverters with other inverters or with the utility source are sensitive to disturbances from the load or other sources and can easily be damaged by overcurrent. Thus extremely careful attention should be given to the system design of parallel operating inverters. Types of system configuration, control methods, and means of protection against failure are summarized, and several methods are proposed. Features and problems of these systems are discussed. >

404 citations


Additional excerpts

  • ...active and reactive power unbalances [12]....

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Journal ArticleDOI
01 Jan 2000
TL;DR: In this article, a new control method is presented which enables equal sharing of linear and nonlinear loads in three-phase power converters connected in parallel, without communication between the converters.
Abstract: In this paper, a new control method is presented which enables equal sharing of linear and nonlinear loads in three-phase power converters connected in parallel, without communication between the converters. The paper focuses on solving the problem that arises when two converters with harmonic compensation are connected in parallel. Without the new solution, they are normally not able to distinguish the harmonic currents that flow to the load and harmonic currents that circulate between the converters. Analysis and experimental results on two 90-kVA 400-Hz converters in parallel are presented. The results show that both linear and nonlinear loads can be shared equally by the proposed concept.

357 citations


"A wireless controller to enhance dy..." refers methods in this paper

  • ...In another approach [ 18 ], every single term of the harmonic current is used to produce a proportional droop in the corresponding harmonic voltage term, which is added to the output-voltage reference....

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Proceedings ArticleDOI
23 Feb 1997
TL;DR: In this article, the authors developed a control technique for operating two or more single phase inverter modules in parallel with no auxiliary interconnections, using frequency, fundamental voltage, and harmonic voltage droop to allow independent inverters to share the load in proportion to their capacities.
Abstract: To provide reliable power under scheduled and unscheduled outages requires an uninterruptible power supply (UPS) which can be easily expanded to meet the needs of a growing demand. A system suck as this should also be fault tolerant and include the capability for redundancy. These goals can be met by paralleling together smaller inverters if a control scheme can be designed to allow them to operate independently yet still share the load. We have developed a control technique for operating two or more single phase inverter modules in parallel with no auxiliary interconnections. This technique uses frequency, fundamental voltage, and harmonic voltage droop to allow independent inverters to share the load in proportion to their capacities. Simulation results are provided to prove the concept.

332 citations


"A wireless controller to enhance dy..." refers background or methods in this paper

  • ...voltage regulation [15], which can be acceptable if, for instance, the frequency and amplitude deviations are mostly at 2% and 5%, respectively (see Figs....

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  • ...In [15], a controller was proposed to share nonlinear loads by adjusting the output voltage bandwidth with the delivered harmonic power....

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  • ...the coefficients and can be chosen as in the conventional droop method to ensure steady state control objectives [15] as follows:...

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