Performance evaluation of secondary control policies with respect to digital communications properties in inverter-based islanded microgrids

A key challenge for inverted-based microgrids working in islanded mode is to maintain their own frequency and voltage to a certain reference values while regulating the active and reactive power among distributed generators and loads. The implementation of frequency and voltage restoration control policies often requires the use of a digital communication network for real-time data exchange (tertiary control covers the coordi- nated operation of the microgrid and the host grid). Whenever a digital network is placed within the loop, the operation of the secondary control may be affected by the inherent properties of the communication technology. This paper analyses the effect that properties like transmission intervals and message dropouts have for four existing representative approaches to secondary control in a scalable islanded microgrid. The simulated results reveals pros and cons for each approach, and identifies threats that properly avoided or handled in advance can prevent failures that otherwise would occur. Selected experimental results on a low- scale laboratory microgrid corroborate the conclusions extracted from the simulation study.Peer ReviewedPostprint (author's final draft

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

Reactive power sharing and voltage harmonic distortion compensation of droop controlled single phase islanded microgrids

When paralleling multiple inverters that are capable of operating as an island, the inverters typically employ the droop control scheme. Traditional droop control enables the decentralized regulation of the local voltage and frequency of the microgrid by the inverters. The droop method also enables the inverters to share the real and reactive power required by the loads. This paper focuses on some of the limitations of parallel islanded single phase inverters using droop control. Algorithms with the aim to address the following limitations in islanded operation were proposed: reactive power sharing and reduction of the voltage harmonic distortion at the point of common coupling (PCC). Experimental results were then presented to show the suitability of the proposed algorithms in achieving reactive power sharing and in improving the voltage harmonic distortion at the PCC.peer-reviewe

Distributed secondary voltage regulation for autonomous microgrid

© 2017 IEEE. This research addresses the control problem of microgrids and presents a robust distributed secondary control system for voltage regulation of an islanded microgrid with droop-controlled and inverter-based distributed generators (DGs). A consensus-based distributed control approach is proposed to restore the voltage and frequency of the islanded microgrid to the reference values for all DGs within a very short time. The proposed method is flexible to system topology variations which AIDS the plug-and-play operation of microgrid. An autonomous micogrid test system consisting of four DGs is constructed in MATLAB using SimPowerSystem Toolbox to test the proposed design method, and the simulation results show the effectiveness of the proposed control strategy. The performance of the proposed controller is shown through several test case studies

Single-phase microgrid with seamless transition capabilities between modes of operation

Microgrids are an effective way to increase the penetration of DG into the grid. They are capable of operating either in grid-connected or in islanded mode thereby increasing the supply reliability for the end user. This paper focuses on achieving seamless transitions from islanded to grid-connected and vice versa for a single phase microgrid made up from voltage controlled voltage source inverters (VC-VSIs) and current controlled voltage source inverters (CC-VSIs) working together in both modes of operation. The primary control structures for the VC-VSIs and CC-VSIs is considered together with the secondary control loops that are used to synchronize the microgrid as a single unit to the grid. Simulation results are given that show the seamless transitions between the two modes without any disconnection times for the CC-VSIs and VC-VSIs connected to the microgrid.peer-reviewe

Design and control of parallel three phase voltage source Inverters in low voltage AC microgrid

Design and hierarchical control of three phase parallel Voltage Source Inverters are developed in this paper. The control scheme is based on synchronous reference frame and consists of primary and secondary control levels. The primary control consists of the droop control and the virtual output impedance loops. This control level is designed to share the active and reactive power correctly between the connected VSIs in order to avoid the undesired circulating current and overload of the connected VSIs. The secondary control is designed to clear the magnitude and the frequency deviations caused by the primary control. The control structure is validated through dynamics simulations.The obtained results demonstrate the effectiveness of the control structure

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