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Proceedings ArticleDOI

Quadratic regulator based optimal state feedback controller for stability improvement of inverter based microgrids

TL;DR: In this work, an optimal state feedback controller is designed to improve the stability of the two inverter based test microgrid and improves the dynamic response of the individual inverter by providing sufficient damping to the oscillatory modes.
Abstract: Small signal stability of inverter based droop controlled microgrid is a major problem from past decade. In this work, an optimal state feedback controller is designed to improve the stability of the two inverter based test microgrid. The detailed full order small signal model of the test microgrid is developed in state space considering network and load dynamics. Eigen value and sensitivity analysis is done to find out the states which affects the stability of the system and their respective participation in shaping the critical modes of the microgrid. Linear quadratic regulator (LQR) based an optimal controller is then implemented into the single inverter model which provides a state feedback to the open loop system to enhance stability. Eigen value analysis is done using MATLAB to prove that the designed controller improves the dynamic response of the individual inverter by providing sufficient damping to the oscillatory modes. Hence it increases the overall stability of the microgrid.
Citations
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Proceedings ArticleDOI
01 Sep 2018
TL;DR: This method of EMD and prony analysis overcomes the requirement of a detailed small signal model which is widely used for the calculation of damping of low frequency oscillation (LFO) in inverter based microgrid.
Abstract: This paper shows the application of empirical mode decomposition (EMD) and prony analysis for identification of low frequency modes present in droop controlled inverter based microgrid for stability analysis. This method of EMD and prony analysis overcomes the requirement of a detailed small signal model which is widely used for the calculation of damping of low frequency oscillation (LFO) in inverter based microgrid. The small signal analysis requires solution of large number of differential and algebraic equations which makes it computationally difficult and time taking. Signal processing techniques have already been used in conventional power systems for frequency identification of LFO. This paper shows its application in inverter based microgrid to avoid the use of full order small signal model. EMD is used for decomposing the time varying signal into different intrinsic mode functions (IMFs) and then prony analysis is used to find the frequency and damping of individual mode. Comparison of calculated frequency of LFO with those obtained from eigen value analysis of small signal model is also presented for verification of the presented method.

Cites background from "Quadratic regulator based optimal s..."

  • ...The two inverter based DGs are connected through a line and they are feeding a load [15]....

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  • ...State feedback based optimal controller and second order derivative based controllers are also proposed in [15] and [16] for stability improvement and improved power sharing respectively....

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References
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Book
01 Jan 1994
TL;DR: In this article, the authors present a model for the power system stability problem in modern power systems based on Synchronous Machine Theory and Modelling, and a model representation of the synchronous machine representation in stability studies.
Abstract: Part I: Characteristics of Modern Power Systems. Introduction to the Power System Stability Problem. Part II: Synchronous Machine Theory and Modelling. Synchronous Machine Parameters. Synchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems. High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems, High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Oscillations. Mid-Term and Long-Term Stability. Methods of Improving System Stability.

13,467 citations


"Quadratic regulator based optimal s..." refers background in this paper

  • ...When the grid is present, then all the system dynamics are determined by grid itself due to the fact that DG’s are very small in comparison to the grid and hence does not play significant role in shaping the dynamics [2], [3]....

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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


"Quadratic regulator based optimal s..." refers background or methods in this paper

  • ...Detailed small signal analysis of droop controlled autonomous microgrid is done in [11], [12]....

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  • ...Compelete small signal modelling procedure is given in [11]....

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  • ...Due to the limitation of space, the linearised equations are not shown in this paper but can be found in [11]....

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  • ...The dynamic modelling given in [11] is used to construct the overall system state matrix on a common reference frame....

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  • ...Here, all the coefficient matrices are found in [11] and δ is defined as the angle between the individual inverter frame and common reference frame....

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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


"Quadratic regulator based optimal s..." refers methods in this paper

  • ...An adaptive decentralized droop controller was proposed in [16], in which an adaptive transient droop function is combined with static droop characteristics....

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Journal ArticleDOI
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


"Quadratic regulator based optimal s..." refers methods in this paper

  • ...In [17] significant improvement in transient performance is obtained by developing power derivative-integral controller which is included in the conventional droop scheme....

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Journal ArticleDOI
TL;DR: The paper explorers the resistive output impedance of the parallel-connected inverters in an island microgrid and proposes a novel wireless load-sharing controller for islanding parallel invertes in an ac distributed system.
Abstract: In this paper, a novel wireless load-sharing controller for islanding parallel inverters in an ac-distributed system is proposed This paper explores the resistive output impedance of the parallel-connected inverters in an island microgrid The control loops are devised and analyzed, taking into account the special nature of a low-voltage microgrid, in which the line impedance is mainly resistive and the distance between the inverters makes the control intercommunication between them difficult In contrast with the conventional droop-control method, the proposed controller uses resistive output impedance, and as a result, a different control law is obtained The controller is implemented by using a digital signal processor board, which only uses local measurements of the unit, thus increasing the modularity, reliability, and flexibility of the distributed system Experimental results are provided from two 6-kVA inverters connected in parallel, showing the features of the proposed wireless control

928 citations