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

Creating an open source platform to develop embedded control systems, and performing a control application on a real-time distributed system

In this project, it has been created a bootloader for the dsPIC33-based board named as Full FLEX, which works jointly with the Erika Enterprise RTOS. In addition, was designed a firmware to perform USB/UART bridging on a PIC18F microcontroller that is also placed inside this board. A python-based software, which decodes both ELF or HEX files into binary source and sends it via USB, from any computer towards the bootloader on the controller, has been created. Furthermore, at runtime the same communication channel can be used to establish a transparent bidirectional communication between the controller and any other software. With this implementation, at development stage, it has been removed an additional IDE and one programming-tool used in the past to program the controller. Furthermore, two additional devices utilized to establish communication between the board and any computer software at runtime stage, are also avoided. So it was possible to reduce both development time and cost spent on tools. It was decided to apply this tool for designing a control system of an inverted pendulum. This application is based on four nodes linked via a CAN. Each node has implemented the bootloader and also runs the above cited real-time operating system. Testing and analyzing results lead to conclude that the system is reliable, efficient and works in harmony beside the RTOS. In addition, the pendulum control system is stable and robust, plus it has achieved the desired control objectives. The development was carried out in the context of having a complete new platform that will be used to facilitate the creation of real-time and/or distributed control sys- tems that can be used both by researchers and students at the Real-Time Distributed Control Systems Laboratory in the Automation Department of the Technical University of Catalonia. The whole project and its documentation are available at github.com/xavierrosero/ FlexboardBootloader

Analysis of the effect of clock drifts on frequency regulation and power sharing in inverter-based islanded microgrids

© 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Local hardware clocks in physically distributed computation devices hardly ever agree because clocks drift apart and the drift can be different for each device. This paper analyses the effect that local clock drifts have in the parallel operation of voltage source inverters (VSIs) in islanded microgrids (MG). The state-of-the-art control policies for frequency regulation and active power sharing in VSIs-based MGs are reviewed and selected prototype policies are then re-formulated in terms of clock drifts. Next, steady-state properties for these policies are analyzed. For each of the policies, analytical expressions are developed to provide an exact quantification of the impact that drifts have on frequency and active power equilibrium points. In addition, a closed-loop model that accommodates all the policies is derived, and the stability of the equilibrium points is characterized in terms of the clock drifts. Finally, the implementation of the analyzed policies in a laboratory MG provides experimental results that confirm the theoretical analysis.Peer ReviewedPostprint (author's final draft

Local frequency restoration for droop-controlled parallel inverters in islanded microgrids

© 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksIn islanded microgrids, voltage source inverters working in parallel are expected to provide regulation of the local frequency while granting active power sharing. This paper presents a local control approach at each inverter based on an event-driven operation of a parameter-varying filter. It ensures perfect active power sharing and controllable accuracy for frequency restoration without requiring the exchange of control data between inverters over the communication network. The paper includes stability analysis and design guidelines for the control parameters using a modeling approach that considers the interaction between inverters. Selected experimental results on a three-inverter laboratory microgrid corroborate the effectiveness of the proposed control scheme, and outlines its advantages with respect to previous similar schemes and the performance cost that implies not using communicationsPeer ReviewedPostprint (author's final draft

Analysis of consensus-based islanded microgrids subject to unexpected electrical and communication partitions

© 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksMicrogrids are power systems consisting of an electrical network composed by distributed loads and generation units that may include a communication network for improved operation. The considered microgrid in islanded mode is driven by voltage source inverters implementing decentralized droop control for active power sharing together with a communication-based consensus algorithm for frequency regulation. This paper analyses the microgrid performance subject to network failures that provoke network partitions. It is considered that the electrical partition leads to several sub-microgrids working in parallel where the power demand can be always guaranteed by the generation units, and the communication partition leads to several consensus algorithms also working in parallel. The double partitioning is analyzed through a closed-loop system model derived using the power flow equations that includes the electrical and communication connectivity. Analytical expressions for the steady-state values for both frequency and active power depending on the partitioning are derived. Selected experimental results on a low-scale laboratory microgrid illustrate the (undesirable) impact that unexpected partitions have in system performancePeer ReviewedPostprint (author's final draft

Validación de instrumento para evaluar multitarea y actitudes tecnológicas en universitarios de ciencias de la salud

The technological habits, attitudes, and aptitudes of university students in a cybernetic society are increasingly consumed by technology. The relationship between everyday life and the academic environment generates several factors, such as multitasking management. This study aimed to validate an instrument for measuring multitasking and attitudes towards technology in health students through factor analysis. To achieve this, 442 observations were collected; an approach with factorial statistical methods was used in the data analysis for this exploratory case. Previously, teachers in the area evaluated the instrument with a reliability coefficient of 0,90. Finally, the multitasking and attitudes towards technologies scale instrument was structured, consisting of 17 items grouped into six factors: Positive Attitude, Negative attitude, Anxiety/Dependency, Perceptions, Multitasking/Social Networks, and Multitasking/Working. The results showed a model that explained 63% of the variance, with a root mean square of the residuals of 0,07. The factor structure identified and validated by the statistical method of exploratory and confirmatory factor analysis has provided a solid basis for evaluating these dimensions in the context of Ecuadorian health sciences university students in future research.Los hábitos tecnológicos de los estudiantes universitarios, actitudes y aptitudes de una sociedad cibernética cada vez consumida por la tecnología, la relación entre la cotidianidad y el ambiente académico generan varios factores como es el manejo de la multitarea. El objetivo de este estudio fue validar un instrumento de medición de la multitarea y las actitudes hacia la tecnología en estudiantes del área de salud mediante el análisis factorial. Para lograrlo se recolectaron 442 observaciones, se utilizó en el análisis de datos un enfoque con métodos estadísticos factoriales para este caso el exploratorio. Previamente el instrumento fue valorado por docentes del área, el mismo que tuvo un coeficiente de confiabilidad de 0,90. Finalmente, se estructuró el instrumento de escala de multitarea y de actitudes hacia las tecnologías constituido por 17 ítems, que se agruparon en 6 factores como la actitud positiva; actitud negativa; ansiedad/dependencia; percepciones; multitarea/redes sociales; multitarea/trabajo. los resultados arrojaron un modelo que explicó el 63% de la varianza, con una raíz cuadrada media de los residuos de 0,07. La estructura factorial identificada y validada por el método estadístico de análisis factorial exploratorio y confirmatorio, ha logrado brindar una base sólida para evaluar estas dimensiones en el contexto de universitarios ecuatorianos de ciencias de la salud en futuras investigaciones

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 conclusione

Creating an open source platform to develop embedded control systems, and performing a control application on a real-time distributed system

In this project, it has been created a bootloader for the dsPIC33-based board named as Full FLEX, which works jointly with the Erika Enterprise RTOS. In addition, was designed a firmware to perform USB/UART bridging on a PIC18F microcontroller that is also placed inside this board. A python-based software, which decodes both ELF or HEX files into binary source and sends it via USB, from any computer towards the bootloader on the controller, has been created. Furthermore, at runtime the same communication channel can be used to establish a transparent bidirectional communication between the controller and any other software. With this implementation, at development stage, it has been removed an additional IDE and one programming-tool used in the past to program the controller. Furthermore, two additional devices utilized to establish communication between the board and any computer software at runtime stage, are also avoided. So it was possible to reduce both development time and cost spent on tools. It was decided to apply this tool for designing a control system of an inverted pendulum. This application is based on four nodes linked via a CAN. Each node has implemented the bootloader and also runs the above cited real-time operating system. Testing and analyzing results lead to conclude that the system is reliable, efficient and works in harmony beside the RTOS. In addition, the pendulum control system is stable and robust, plus it has achieved the desired control objectives. The development was carried out in the context of having a complete new platform that will be used to facilitate the creation of real-time and/or distributed control sys- tems that can be used both by researchers and students at the Real-Time Distributed Control Systems Laboratory in the Automation Department of the Technical University of Catalonia. The whole project and its documentation are available at github.com/xavierrosero/ FlexboardBootloader

Metodología para evaluación de usabilidad del entorno de desarrollo integrado de Arduino

Los sistemas embebidos de hardware libre (OSHW, Open Source Hardware) han sido ampliamente aplicados en la enseñanza de sistemas microprocesados; la edición, compilación y programación de aplicaciones para OSHW se realizan a través de entornos de desarrollo integrado (IDEs, Integrated Development Environments). Este artículo presenta una metodología para evaluar la usabilidad de IDEs de OSHW y se centra en la plataforma de Arduino por ser una de las más utilizadas hoy en día en la academia; conocer qué tan fácil y confortable es aprender a programar microcontroladores en Arduino es un objetivo colateral pero de igual importancia. La principal contribución de este trabajo consiste en utilizar enfoques de efectividad, eficiencia y satisfacción del usuario para la evaluación de usabilidad de IDEs de programación de OSHW. La evaluación se centra en la realización de programas con complejidad dentro de una escala ascendente. La efectividad se determina en relación al cumplimiento satisfactorio de tareas; la eficiencia se evalúa en base al tiempo empleado para el desarrollo de programas. El grado de satisfacción del usuario se diagnostica usando encuestas de escala de usabilidad del sistema y de facilidad de uso. Los resultados asociados muestran que la efectividad y eficiencia del usuario utilizando el IDE dependen de la complejidad de las tareas y que el manejo de la interfaz es fácil y cómodo

On the performance improvement of the optimal-sampling-inspired self-triggered control at implementation stage

The self-triggered control includes a sampling strategy that focuses on decreasing the use of computational resources (processor and network) while preserving the same control performance as the one obtained via a controller with periodic sampling. Within this framework it has been developed recently a self-triggered control technique inspired by a sampling pattern whose optimal density minimizes the control cost, this approach is called “optimal-sampling inspired self-triggered control”. However, the strategies used to implement it on microprocessor-controlled systems working under perturbation are still unclear; this paper addresses some techniques to organize and improve the implementation on actual controllers. The proposed solution comprises both the formulation of two algorithms to organize the implementation and the insertion of a closed-loop observer to deal with the perturbation that normally appears on real plants. Regarding the former, certain computationally expensive processes involved in the implementation of this control technique are treated through their replacement by lightweight polynomials fitted at design stage. Simulations and practical experiments confirm the solution is effective and there could be an open research topic concerning observation in optimal-sampling self-triggered control strategies