Design And Implementation Of Co-Operative Control Strategy For Hybrid AC/DC Microgrids

This thesis is mainly divided in two major sections: 1) Modelling and control of AC microgrid, DC microgrid, Hybrid AC/DC microgrid using distributed co-operative control, and 2) Development of a four bus laboratory prototype of an AC microgrid system. At first, a distributed cooperative control (DCC) for a DC microgrid considering the state-of-charge (SoC) of the batteries in a typical plug-in-electric-vehicle (PEV) is developed. In DC microgrids, this methodology is developed to assist the load sharing amongst the distributed generation units (DGs), according to their ratings with improved voltage regulation. Subsequently, a DCC based control algorithm for AC microgrid is also investigated to improve the performance of AC microgrid in terms of power sharing among the DGs, voltage regulation and frequency deviation. The results validate the advantages of the proposed methodology as compared to traditional droop control of AC microgrid. The DCC-based control methodology for AC microgrid and DC microgrid are further expanded to develop a DCC-based power management algorithm for hybrid AC/DC microgrid. The developed algorithm for hybrid microgrid controls the power flow through the interfacing converter (IC) between the AC and DC microgrids. This will facilitate the power sharing between the DGs according to their power ratings. Moreover, it enables the fixed scheduled power delivery at different operating conditions, while maintaining good voltage regulation and improved frequency profile. The second section provides a detailed explanation and step-by-step design and development of an AC/DC microgrid testbed. Controllers for the three-phase inverters are designed and tested on different generation units along with their corresponding inductor-capacitor-inductor (LCL) filters to eliminate the switching frequency harmonics. Electric power distribution line models are developed to form the microgrid network topology. Voltage and current sensors are placed in the proper positions to achieve a full visibility over the microgrid. A running average filter (RAF) based enhanced phase-locked-loop (EPLL) is designed and implemented to extract frequency and phase angle information. A PLL-based synchronizing scheme is also developed to synchronize the DGs to the microgrid. The developed laboratory prototype runs on dSpace platform for real time data acquisition, communication and controller implementation

Active power sharing and frequency regulation in inverter-based islanded microgrids subject to clock drifts, damage in power links and loss of communications

Tesi en modalitat de compendi de publicacions; hi ha diferents seccions retallades per drets de l'editorMicrogrids (MGs) are small-scale power systems containing storage elements, loads and distributed generators that are interfaced with the electric network via power electronic inverters. When an MG is in islanded mode, its dynamics are no longer dominated by the main grid. Then, inverters, driven by digital processors that may exchange data over digital communication, must act as voltage source inverters (VSIs) to take coordinated actions to ensure power quality and supply. The scope of this thesis is bounded to control strategies for active power sharing and frequency regulation in islanded MGs. The focus is on the analysis of prototype control policies when operating conditions are no longer ideal. In particular, the thesis covers the effect that a) clock drifts of digital processors, b) damage in power transmission lines, and c) failures in digital communications have in control performance. The work is submitted as a compendium of publications, including journal and international conference papers, where two main areas of research can be distinguished. The first area refers to the analysis of the effect that clock drifts have on frequency regulation and active power sharing. VSIs digital processors are equipped with oscillators, which run at not necessarily identical frequencies. As consequence, the local clocks in the physically distributed VSIs may differ. This part, reported in two conference papers and one journal paper, investigates state-of-the-art control policies when clocks of the computational devices drift. The contributions related to this part are a) the reformulation of existing control policies in terms of clock drifts, b) the steady-state analysis of these policies that offers analytical expressions to quantify the impact that drifts have on frequency and active power equilibrium points, c) the closed-loop model capable of accommodating all the policies, d) the stability analysis of the equilibrium points, and e) the experimental results. The second area copes with the analysis of the effect that electrical and communication failures have on frequency regulation and active power sharing. This investigation focuses on distributed/cooperative control policies where each inverter control action is computed using both local measures and data received from other inverters within the MG. This part, reported in one conference paper and two journal papers, investigates two control policies when the considered failures in terms of damage in power links and/or loss of communication between inverters provoke partitions within the MG. The contributions related to this part are a) the formulation of the MG as two connected graphs corresponding to the electrical and communication networks where both type of failures lead to disconnected electrical/communication sub-graphs, named partitions, that co-exist within the MG, b) the closed-loop model integrating the two graph Laplacian matrices, c) the stability analysis that identifies which type of partitions may lead to MG instability, d) the steady-state analysis that indicates how to compute the equilibrium points for the case of stable dynamics, e) a new control strategy based on switched control principles that permits avoiding the instability scenario, and f) the experimental results. For the purpose of verifying the operational performance of the analytical results, diverse experiments on a laboratory MG have been performed. The outcomes obtained are discussed and analyzed in terms of the objectives sought. Finally, conclusions and future research lines complete the thesis.Las microredes (MG) son sistemas de energía a pequeña escala que contienen elementos de almacenamiento, cargas y generadores distribuidos que están conectados con la red eléctrica a través de inversores de potencia. Cuando una MG está en modo aislado, su dinámica no está dominada por la red principal. Así, los inversores, comandados por procesadores digitales que pueden intercambiar información a través de comunicaciones digitales, deben actuar como fuentes de voltaje para ejecutar acciones coordinadas que garanticen el suministro de energía. Esta tesis se enmarca dentro de estrategias de control de última generación para compartir potencia activa y regular frecuencia en MG aisladas basadas en inversores. Su enfoque se centra en analizar estas políticas cuando las condiciones de operación no son ideales. En particular, la tesis cubre el efecto que a) desviaciones del reloj de los procesadores digitales, b) daños en las líneas de transmisión de energía, y c) fallas en las comunicaciones digitales, provocan en el rendimiento de control. El trabajo se presenta como un compendio que incluye publicaciones de revistas y de conferencias internacionales, donde se pueden distinguir dos temas principales de investigación. El primer tema comprende el análisis del efecto que tienen las desviaciones de reloj sobre la regulación de frecuencia y la compartición de potencia activa. Los procesadores de los inversores están equipados con osciladores que funcionan a frecuencias no necesariamente idénticas. Como consecuencia, los relojes locales en los inversores distribuidos físicamente, pueden diferir. Esta parte, descrita a través de dos artículos de conferencia y uno de revista, analiza el comportamiento de las políticas de control cuando los relojes de los dispositivos computacionales se desvían. Las contribuciones relacionadas con este tema son a) reformulación de las políticas de control de última generación en términos de desviaciones de reloj, b) análisis de estado estacionario de estas estrategias que ofrece expresiones analíticas para cuantificar el impacto que las desviaciones de reloj tienen sobre los puntos de equilibrio de frecuencia y potencia activa, c) modelo de lazo cerrado adaptable a todas las políticas, d) análisis de estabilidad de los puntos de equilibrio, y e) resultados experimentales. El segundo tema hace frente al análisis del efecto que las fallas eléctricas y de comunicaciones tienen sobre la regulación de frecuencia y el uso compartido de potencia activa. Esta parte se centra en políticas de control distribuido/cooperativo donde cada acción de control del inversor se calcula utilizando medidas locales y datos recibidos de otros inversores de la MG. Esta parte, descrita a través de un artículo de conferencia y dos de revista, investiga dos políticas de control cuando particiones en la MG son provocadas por daños en los enlaces de alimentación y/o por pérdida de comunicación entre inversores. Las contribuciones relacionadas con este tema son a) formulación de la MG como dos grafos correspondientes a las redes eléctrica y de comunicación donde ambos tipos de fallas conducen a sub-grafos eléctricos/comunicacionales desconectados, llamados particiones, que coexisten dentro de la MG, b) modelo de lazo cerrado que integra las matrices Laplacianas de los dos grafos, c) análisis de estabilidad que identifica las particiones que pueden conducir a inestabilidad en la MG, d) análisis de estado estacionario para calcular puntos de equilibrio cuando la dinámica es estable, e) nueva estrategia basada en principios de control conmutado para evitar el escenario de inestabilidad, y f) resultados experimentales. Con el fin de verificar el rendimiento operativo de los resultados analíticos, se han realizado diversos experimentos sobre una microred de laboratorio, los mismos que se discuten en términos de los objetivos de la tesis. El trabajo finaliza con las conclusionesPostprint (published version

A flexible experimental laboratory for distributed generation networks based on power inverters

In the recently deregulated electricity market, distributed generation based on renewable sources is becoming more and more relevant. In this area, two main distributed scenarios are focusing the attention of recent research: grid-connected mode, where the generation sources are connected to a grid mainly supplied by big power plants, and islanded mode, where the distributed sources, energy storage devices, and loads compose an autonomous entity that in its general form can be named a microgrid. To conduct a successful research in these two scenarios, it is essential to have a flexible experimental setup. This work deals with the description of a real laboratory setup composed of four nodes that can emulate both scenarios of a distributed generation network. A comprehensive description of the hardware and software setup will be done, focusing especially in the dual-core DSP used for control purposes, which is next to the industry standards and able to emulate real complexities. A complete experimental section will show the main features of the system.Peer ReviewedPostprint (published version