Distributed generators systems and Microgrid are becoming more important to increase the renewable energy penetration in the public utility. This paper presents a mathematical model for connected inverters in Microgrid systems with large range variations in operating conditions. No-lineal tools and computer simulations, phase-plane trajectory analysis, method of Lyapunov and bifurcations analysis for evaluate the limits of the small signal models are used, and conclusion suggested utilizing models that can permit to analysis of the system when subjected to a severe transient disturbance such as loss a large load or loss of generation. The study of transient stability for Microgrid systems in stand-alone of the utility grid is useful to improve the design of Microgrid's architecture.
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... The dynamic response of different DGs during large disturbances , characteristics of DG fault current contribution with different control strategies , and transient process simulations of grid connected Microgrid  are mainly discussed. And the transient stability analysis of islanded Microgrid was shown in . ...
... Thus the transient behaviors of islanded Microgrid to large disturbance are much sensitive to that of the grid connected Microgrid. More and more researches have been focused on this field . ...
... The transient stability of Microgrid is closely related to the load characteristics. Most of the researches focused on the RCL load, even though the transient behaviors of the motor load, the active load are used more widely [77,78]. A transient stability model based on controlled current source was proposed in . ...
Microgrid is becoming an attractive concept to meet the increasing demands for energy and deal with air pollutions. Distributed energy sources (DERs) in Microgrid are usually interfaced with the utility grid by inverters, so the characteristics of Microgrid stability are much different from that of a traditional grid. However, the classifications, guidelines, and analysis method of Microgrid stability are well behind of the Microgrid development. In this paper, a Microgrid stability classification methodology is proposed on the basis of the of Microgrid characteristics investigation, which considers the Microgrid operation mode, types of disturbance and time frame. Then a comprehensive review of the body of research on Microgrid stability is presented in order to identify and advance the field. Finally, some challenges and suggestions of Microgrid stability for further researches are discussed.
This paper proposes a new methodology for coordinated voltage support in distribution networks with large integration of distributed generation and microgrids. Given the characteristics of the LV networks, it is shown that traditional control strategies using only reactive power control may not be sufficient in order to perform efficient voltage control. Therefore, microgeneration shedding must also be employed, especially in scenarios with extreme microgeneration penetration. An optimisation tool based on a meta-heuristic approach was developed to address the voltage control problem. In addition, neural networks were employed in order to decrease computational time, thus enabling the use of the tool for online operation. The results obtained revealed good performance of this control approach.
In this paper, thorough analysis is carried out on a microgrid system with a constant power load. Constant power loads make the system to be unstable due to their negative impedance characteristics and this is proved for the microgrid system by deriving the small signal state space model for the system and then observing the location of the system poles. Subsequently, a sensitivity analysis is carried out to check whether it is possible to eliminate the instability of the system by tuning the voltage and current controller gain values of the microgrid inverter system. Also by transforming small signal state space model to Laplace domain, stability conditions are derived for the microgrid with a constant power load connected parallel with other loads. From these conditions, the loading limit of the constant power load for making the system stable is obtained. These theoretical loading limits obtained are proven using a model of microgrid simulated in Matlab/Simulink platform.
Technological advances and environmental pressures are driving the interconnection of renewable energy sources to the distribution network. The interconnection of large amounts of non-traditional generation however causes problems in a network designed for 'conventional' operation. The use of power electronics interfaces and the 'bundling' of micro-generation and loads into so-called Microgrids, offers a potential solution. Each Microgrid is designed to operate as a 'good citizen' or near ideal conventional load. This paper discusses the various elements of the new Microgrid concept and presents suggestions for some typical control strategies for the various system elements.
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
In this paper, an algorithm is presented, suitable for simulating the dynamic behavior of LV Microgrids both under grid connected and autonomous operation. The algorithm follows the stability approach, focusing on low-frequency dynamics, and adjusts the standard methodology so that the dynamic analysis of the system can be carried out, even in the absence of a synchronous machine when all the sources are interfaced to the network with inverters. Proper network representation allows for the modeling of all the characteristic unbalances of the LV network. The capability of the algorithm to simulate the operating modes of a Microgrid is demonstrated by representative study cases
This book presents a modern and self-contained treatment of the Liapunov method for stability analysis, in the framework of mathematical nonlinear control theory. A Particular focus is on the problem of the existence of Liapunov functions (converse Liapunov theorems) and their regularity, whose interest is especially motivated by applications to automatic control. Many recent results in this area have been collected and presented in a systematic way. Some of them are given in extended, unified versions and with new, simpler proofs. In the 2nd edition of this successful book several new sections were added and old sections have been improved, e.g., about the Zubovs method, Liapunov functions for discontinuous systems and cascaded systems. Many new examples, explanations and figures were added making this book accessible and well readable for engineers as well as mathematicians.
The state-space model and implementation results of a power conditioning system are presented in this paper. Eigenvalues with different controller gains and load conditions for grid-tie mode and standalone mode are utilized to analyze the system stability. For standalone mode, higher controller gain and higher load resistance tend to make system more unstable. In grid-tie mode, higher controller gain is also found to make the system more unstable but load variation will not change the system stability much. Experimental and simulation results also verify the model. The state-space model is extended to a parallel-inverter system for investigation of the load variation and current-sharing controller effects to the system stability. A time-domain current ripple criterion is also suggested for light-load operation of the parallel-inverter microgrid system.
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.