Control and stability analysis of islanded microgrids based on inner control loops approach

Islanded microgrids are developed nowadays as a new model for the generation, consumption, and supply of electrical energy and services in remote areas in an efficient, sustainable, flexible, and environmentally friendly manner. Among others, they are considered as good technical solutions for the electrification of developing countries, especially in (but not limited to) rural areas. However, the stable operation of islanded microgrids, which consist mainly of renewable generation units with different and non-predictable characteristics, remains a major challenge. This is due to the power electronic interfaces used by the renewable distributed energy sources connected into the microgrids, and the small size of microgrids. Adapted control strategies are required to ensure the integrity and stability of islanded microgrids and to maintain the magnitude and frequency of the system voltage within the allowable range. This thesis proposes a non-vertical hierarchical control structure composed of the level-zero control and level-one control. Level-zero control is the closest control level with the fastest dynamics. Its purpose is the close control of the distributed energy sources comprising the microgrid so as to maintain their operating point. The proposed level-zero control structures allow an inverter-based source to be controlled as a constant current source in grid-following inverter mode injecting controlled active and reactive current into the microgrid and, as a constant voltage source in grid-forming inverter mode controlling microgrid voltage magnitude and imposing the system frequency. The level-one control is performed through a distributed hybrid control approach without communication allowing the control of the voltage magnitude and frequency, and power sharing within islanded microgrids powered by multiple distributed energy sources. To accomplish these tasks, the proposed level-one control strategy implements inverters in grid-forming control mode and grid-following control mode. The proposed control strategy decouples power-sharing control from the references of the voltage and current applied to the inner voltage and current control loops. This allows the inverters with the fastest dynamics to be most closely controlled. In addition, the proposed level-one control strategy is more efficient than the conventional droop control approach, for performing instantaneous power sharing independently of the frequency control and output impedance of the inverter. The active and reactive power are adjusted with respect to the control role of each interface of the distributed energy sources connected to the microgrid. The dynamic response of the proposed hierarchical control structure is analyzed through small and large disturbances using eigenvalues analysis and time-domain simulations, respectively. The obtained simulation results prove the effective performance of the proposed control strategy.(FSA - Sciences de l'ingénieur) -- UCL, 202

Inner Current Control Loop Influence on Islanded Microgrid Dynamic Behavior

A stable islanded microgrid operation is generally dictated by the adopted control strategy into the system. Droop control and master-slave control are the most used into parallel-connected inverters. However, the choice between the two is often performed in function of their complexity and that of the system configuration. It is rarely achieved in function of the microgrid dynamics. This paper proposes the system dynamics and stability as a criterion for choosing a microgrid control strategy. This is explored into the microgrid composed of photovoltaic and battery energy sources, as well as constant impedance load. The influence of inner current control loop parameters on microgrid dynamics, for both droop control and hybrid V-f and PQ control, is achieved through Small-signal stability analysis. The obtained results determine the proper control strategy from stability limits of two control methods established in the left half complex plane

Local stability performance analysis of islanded microgrid based on inner control loops approach

High dynamical behavior and performance are required to maintain stable islanded microgrid operation based on inverters. However, this is strongly conditioned by the implemented control strategy as well as the characteristic parameters of the islanded microgrids. This paper investigates the influence of the controller characteristics and the microgrid parameters, on the small-signal stability of islanded microgrid. It makes the coupling between both factors in order to carry out the local stability model depending on the microgrid parameters. The islanded microgrid fed by a single inverter-based source whose control is based on inner control loops approach, is considered. The influence of each parameter is examined by analyzing the dynamical behavior of the eigenvalues in the complex plane, and the dynamical performance of the controller, with Matlab/Simulink. Accordingly, the modelling and simulation results obtained, showed that the effects of these parameters help identify and define the limits of local stability of islanded microgrids

Islanded Microgrid Voltage Control Structure Small-Signal Stability Analysis

Voltage and frequency stability is one major concern within inverter-based microgrid operating in islanded mode. Control of those interfaced inverters in grid-forming mode is one of the proper solutions, which has been widely discussed to improve the islanded microgrid stability. However, impacts of voltage control structure based on conventional proportional-integral controllers (PI) as suitable control technique, on islanded microgrid dynamics and stability, have not been distinctly investigated. Therefore, much control structures, with or without additional loops such as feedforward and decoupling loops, can be found in existing literature when the d-axis and q-axis synchronous reference frame is adopted. This paper proposes a thorough control structure analysis by considering the influence of each supplementary loop on inverter dynamics and stability, and on microgrid. System stability is investigated through computation and trajectory analysis of inverter small-signal model eigenvalues plotted in the complex plane. The obtained modelling results are compared with two model results reported in literature. These show that the effects of feedforward and decoupling loops strongly influence the microgrid dynamics and these help identify robust voltage control structure

Analyses des paramètres affectant la consommation d'énergie dans un four rotatif cimentier et solutions possibles d'optimisation énergétique

International audienceL'industrie cimentière consomme l'énergie allant de 30-40% du cout de production (Ahamed, 2012; Kabir, 2010 ; Madlool, 2012 ; Rasul, 2005). A cet effet, elle est considérée comme étant la plus énergivore, comparé à d'autres industries. Le présent article procède à l'analyse des paramètres affectant la consommation énergétique du four rotatif Unax de la Cimenterie Nationale de Kimpese en République Démocratique du Congo, et de donner des pistes d'optimisation énergétique. Le four rotatif Unax connait une usure avancée des matériaux réfractaires ; ce qui occasionne des pertes thermiques excessives. L'analyse thermodynamique du four est effectuée le long du circuit dudit four ; de l'entrée matières premières (et sortie gaz) à la sortie clinker (et entrée air de refroidissement), pour atteindre un système de gestion de l'énergie efficace et efficiente. Cette analyse permet de connaitre les endroits où les grandes pertes thermiques sont présentes afin d'apporter une solution satisfaisante. Les résultats trouvés montrent que la grande portion de pertes est localisée dans les zones de décarbonatation et de cuisson. La consommation spécifique d'énergie pour la production de clinker est de 3457,27 kJ/kg de clinker. Les rendements de la première et seconde lois de thermodynamiques valent respectivement 93 % et 27,97 %. Il se dégage une détérioration qualitative de 72,03 % de l'énergie. Pour ce qui est des pertes thermiques, les valeurs suivantes ont été trouvées respectivement par convection et par rayonnement 415,21 kJ/kg de clinker et 972,34 kJ/kg de clinker.Les deux premières solutions technologiques, celle d'utiliser de réfractaires neufs et/ou ayant des faibles coefficients de conductibilité thermique et celle d'un four à double couche de réfractaires, ont permis d'économiser respectivement 73 % et 67 % d'énergie perdue par convection et par rayonnement. Les grosses pertes sont localisées dans le tube du four, ainsi que dans le refroidisseur. Cela signifie qu'il faut plus se concentrer dans ces deux zones. Nomenclature Chaleur massique, J.kg-1 .K-1 ̅̅̅ Chaleur spécifique molaire, J.mol-1 .K-1 Exergie, J Exergie massique ou spécifique, J.kg-1 ℎ Enthalpie massique, J.kg-1 ℎ 0 Enthalpie spécifique de formation, J.kg-1 NA non analysé Pression, Pa Pertes, J Constante des gaz parfaits, J.mol-1 .K-1 ̇ Entropie, J.s-1 .K-1 Température, °C Température,K Symboles grecs Rendement, % Rendement exergétique, % Indices 0 Ambiance atmosphérique ℎ Chimique Détruite E Entrée Énergétique 2 Formation clinker généré poussière-gaz ℎ Physique s sortie Total entrée Total sorti

Inner control loops approach to control the islanded photovoltaic microgrid

The intermittent character of the photovoltaic generator, power electronic converters and load dynamic are the main factors leading operation instability in islanded microgrids. The necessity of the immediate control within the micro-source is very important since it impacts the interaction between micro-sources in the microgrid. This control level must be characterized by a fast and accurate response in order to control the operation point of the photovoltaic generator of the microgrid, and also the injected currents into the microgrid, as well as the amplitude and the frequency of the voltage at the photovoltaic connecting point at AC side. However, its performance depends on the control structure and the connecting mode of the photovoltaic generator. In this present paper, we propose a new structure of the internal control applied in islanded photovoltaic microgrids. It advocates the use of the close control multi-objective controller which is developed and implemented with Matlab/Simulink. Accordingly, the simulation results obtained show that this control approach improves the stability operation of the islanded microgrid while allowing maximum power transfer from DC to AC side

Voltage Unbalance Compensation for Islanded Microgrids with Hybrid Control Approach

Voltage unbalance is one of the challenges of low-voltage microgrids. It is often caused by unbalanced loads connected to the microgrid. Its compensation is required to ensure an efficient microgrid operation. In this paper, a voltage unbalance compensation mechanism based on modifying the control signals is explored. A hybrid control scheme is used to control and regulate the distributed generators (DGs) inverters in an islanded low voltage microgrid. The DGs inverters are either controlled as grid-forming or grid-feeding units. A voltage unbalance compensation mechanism using local measurements is used to reduce the voltage unbalance at the different points throughout the microgrid. This voltage unbalance compensation is performed only by the grid-forming controller which uses local voltage measurements only. The compatibility between the control algorithm and the compensation mechanism is shown through different test scenarios. The simulation results obtained using Matlab/SimscapePowerSystems prove the effective performance of the proposed approach