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

Modeling, control and design of AC microgrids in islanded mode

Tesi per compendi de publicacions, amb diferents seccions retallades pels dret de l'editorPremi Extraordinari de Doctorat, promoció 2018-2019. Àmbit de les TICThe present doctoral thesis is focused on the analysis and design of control strategies for the secondary control layer of islanded AC microgrids without the use of communications. The work is submitted as a compendium of publications, composed by journals and international conference papers. The first contribution is a control strategy for the secondary control layer based on a switchable configuration, that does not require the use of communications. For stability analysis purposes, a closed-loop system modeling is presented, which is also used to determine design considerations for the control parameters. The second contribution is a complementary control strategy that improves the frequency regulation of the previous proposed control, using a dynamic droop gain in the primary layer. For this purpose, a time protocol that drives the variable parameters is proposed which guarantees an effectively reduction of the maximum frequency error without relying on complex techniques, maintaining the simplicity of the basis strategy and the non-use of communications. The third contribution is a multi-layer hierarchical control scheme that is composed by a droop-based primary layer, a time-driven secondary layer and an optimized power dispatch tertiary layer. The proposed control guarantees an excellent performance in terms of frequency restoration and power sharing. The fourth contribution is an improved secondary control layer strategy without communications, which presents superior operating performance compared with the previous proposals. The scheme is based on a event-driven operation of a parameter-varying filter which ensures perfect active power sharing and controllable accuracy for frequency restoration. A complete modeling that considers the topology of the MG and the electrical interaction between the DGs is derived for the stability analysis and to determine design guidelines for the key control parameters. For the purpose of analyzing and verifying the operational performance of the control schemes, an experimental MG was implemented, where selected tests were carried out. The obtained results are discussed and its relation with the doctoral thesis objectives analyzed. The thesis ends presenting conclusions and future research lines.La presente tesis doctoral se enfoca en el análisis y diseño de estrategias de control para la capa de control secundaria en microrredes aisladas de corriente alterna, sin el uso de comunicaciones. El trabajo se presenta en la modalidad de compendio, por lo que está compuesto por publicaciones previamente aceptadas en revistas y congresos científicos internacionales. La primera contribución es un estrategia de control para la capa secundaria basada en una configuración conmutable, que no requiere el uso de comunicaciones. Con el propósito de analizar la estabilidad, se presenta el modelado del sistema de lazo cerrado, que también es usado para determinar reglas de diseño de los parámetros de control. La segunda contribución es una estrategia de control complementaria que mejora la regulación de frecuencia de la propuesta anterior, usando una ganancia dinámica en la capa de control primaria. Se propone la variación de los parámetros siguiendo un protocolo de tiempo, garantizando la reducción del error máximo de frecuencia sin depender de técnicas complejas, manteniendo la simplicidad de la estrategia base y sin requerir comunicaciones. La tercera contribución es un esquema de control jerárquico compuesto por una capa primaria basada en el método de la pendiente, una capa secundaria controlada por un protocolo de tiempo y una capa terciaria que optimiza el despacho de potencias. El control propuesto garantiza un excelente desempeño en términos de la regulación de la frecuencia y la compartición de potencias. La cuarta contribución es una estrategia de control para la capa secundaria que no usa comunicaciones, la cual presenta un comportamiento operativo superior comparado con las propuestas anteriores. El esquema está basado en una operación controlada por eventos, de un filtro con parámetros variables que garantiza una perfecta compartición de potencias y una precisa restauración de frecuencia. Además, para el análisis de la estabilidad y la determinación de pautas de diseño de los parámetros se presenta un modelo que considera la topología de la microrred y las interacciones eléctricas de los generadores. Con el objetivo de analizar y verificar el desempeño operativo de los esquemas de control, se implementó una microrred experimental donde se llevaron a cabo las pruebas requeridas. Se discutieron los resultados obtenidos y se analizó su relación con los objetivos de la tesis doctoral. El documento termina presentado las conclusiones así como futuras líneas de investigaciónAward-winningPostprint (published version

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

Inter-Microgrid Operation: Power Sharing, Frequency Restoration, Seamless Reconnection and Stability Analysis

Electrification in the rural areas sometimes become very challenging due to area accessibility and economic concern. Standalone Microgrids (MGs) play a very crucial role in these kinds of a rural area where a large power grid is not available. The intermittent nature of distributed energy sources and the load uncertainties can create a power mismatch and can lead to frequency and voltage drop in rural isolated community MG. In order to avoid this, various intelligent load shedding techniques, installation of micro storage systems and coupling of neighbouring MGs can be adopted. Among these, the coupling of neighbouring MGs is the most feasible in the rural area where large grid power is not available. The interconnection of neighbouring MGs has raised concerns about the safety of operation, protection of critical infrastructure, the efficiency of power-sharing and most importantly, stable mode of operation. Many advanced control techniques have been proposed to enhance the load sharing and stability of the microgrid. Droop control is the most commonly used control technique for parallel operation of converters in order to share the load among the MGs. But most of them are in the presence of large grid power, where system voltage and frequency are controlled by the stiff grid. In a rural area, where grid power is not available, the frequency and voltage control become a fundamental issue to be addressed. Moreover, for accurate load sharing a high value of droop gain should be chosen as the R/X ratio of the rural network is very high, which makes the system unstable. Therefore, the choice of droop gains is often a trade-off between power-sharing and stability. In the context, the main focus of this PhD thesis is the fundamental investigations into control techniques of inverter-based standalone neighbouring microgrids for available power sharing. It aims to develop new and improved control techniques to enhance performance and power-sharing reliability of remote standalone Microgrids. In this thesis, a power management-based droop control is proposed for accurate power sharing according to the power availability in a particular MG. Inverters can have different power setpoints during the grid-connected mode, but in the standalone mode, they all need their power setpoints to be adjusted according to their power ratings. On the basis of this, a power management-based droop control strategy is developed to achieve the power-sharing among the neighbouring microgrids. The proposed method helps the MG inverters to share the power according to its ratings and availability, which does not restrict the inverters for equal power-sharing. The paralleled inverters in coupled MGs need to work in both interconnected mode and standalone mode and should be able to transfer between modes seamlessly. An enhanced droop control is proposed to maintain the frequency and voltage of the MGs to their nominal value, which also helps the neighbouring MGs for seamless (de)coupling. This thesis also presents a mathematical model of the interconnected neighbouring microgrid for stability and robustness analysis. Finally, a laboratory prototype model of two MGs is developed to test the effectiveness of the proposed control strategies

Identification and development of microgrids emergency control procedures

Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 200

EcoBlock: Grid Impacts, Scaling, and Resilience

Widespread deployment of EcoBlocks has the potential to transform today's electricity system into one that is more resilient, flexible, efficient and sustainable. In this vision, the system will consist of self- su cient, renewable-powered, block-scale entities that can deliberately adjust their net power exchange and can optimize performance, maintain stability, support each other, or disconnect entirely from the grid as needed. This report is intended as an independent analysis of the potential relationships, both constructive and adverse, between EcoBlocks and the grid

Cooperative Strategies for Management of Power Quality Problems in Voltage-Source Converter-based Microgrids

The development of cooperative control strategies for microgrids has become an area of increasing research interest in recent years, often a result of advances in other areas of control theory such as multi-agent systems and enabled by emerging wireless communications technology, machine learning techniques, and power electronics. While some possible applications of the cooperative control theory to microgrids have been described in the research literature, a comprehensive survey of this approach with respect to its limitations and wide-ranging potential applications has not yet been provided. In this regard, an important area of research into microgrids is developing intelligent cooperative operating strategies within and between microgrids which implement and allocate tasks at the local level, and do not rely on centralized command and control structures. Multi-agent techniques are one focus of this research, but have not been applied to the full range of power quality problems in microgrids. The ability for microgrid control systems to manage harmonics, unbalance, flicker, and black start capability are some examples of applications yet to be fully exploited. During islanded operation, the normal buffer against disturbances and power imbalances provided by the main grid coupling is removed, this together with the reduced inertia of the microgrid (MG), makes power quality (PQ) management a critical control function. This research will investigate new cooperative control techniques for solving power quality problems in voltage source converter (VSC)-based AC microgrids. A set of specific power quality problems have been selected for the application focus, based on a survey of relevant published literature, international standards, and electricity utility regulations. The control problems which will be addressed are voltage regulation, unbalance load sharing, and flicker mitigation. The thesis introduces novel approaches based on multi-agent consensus problems and differential games. It was decided to exclude the management of harmonics, which is a more challenging issue, and is the focus of future research. Rather than using model-based engineering design for optimization of controller parameters, the thesis describes a novel technique for controller synthesis using off-policy reinforcement learning. The thesis also addresses the topic of communication and control system co-design. In this regard, stability of secondary voltage control considering communication time-delays will be addressed, while a performance-oriented approach to rate allocation using a novel solution method is described based on convex optimization

The Role of Inverter-based Generation in Future Energy Systems: An Oriented Decentralized Strategy for Reactive Power Sharing in Islanded AC Microgrids and a Techno-Economic Approach to Inertia Requirements Assessment of the Italian Transmission Network

One of the most impacting changes in the electricity energy scenario of the latest decades is the extensive increase of Distributed Energy Resources (DER) including Electrical Storage Systems (EES), fuel cells and Renewable Energy Sources (RES), such as Photovoltaic (PV) and Wind Turbines (WT). The integration of a rapidly increasing share of inverter-based generation poses relevant challenges in terms of frequency and voltage control both in Islanded Microgrids (MG) and traditional transmission networks. For the sake of complementarity, the thesis focuses on reactive power and voltage regulation in MG and frequency instability problems in a future Italian transmission network. In MG with converter-based energy production, one of the main problems is the proper reactive power sharing among DER and related voltage regulation. In this concern the most used approach is based on the conventional droop control; however, it presents some relevant drawbacks. In SECTION A an Advanced Droop Control strategy (ADC) and an Advanced Boost Control strategy (ABC) are proposed, to approach primary voltage control and reactive power sharing among Grid-Supporting inverters in islanded MG. The strategies are presented defining their control laws and the control schemes together with the relevant stability analysis. Then, an analytical procedure is developed for each control methods to set proper control parameters. Next, a comparison between the new strategies and droop conventional control is performed with simulations on a common benchmark MG, in order to show that new strategies, according to their specific control logics, are able to guarantee improved performance in terms of the combined regulation of voltage and reactive power. Considering the traditional electric system, one of the main consequences of the increasing penetration of RES is, besides of the decrease of the system short-circuit power, the reduction of the electric system inertia: this could lead to frequency instability problems in case of severe perturbations, especially for what concerns the Rate of Change of Frequency (RoCoF)and the frequency nadir. In SECTION B, the thesis provides a technical-economic methodology for the estimation of the amount of additional inertia that will be needed in the Italian Transmission Network in a prospective 2030 scenario, in order to limit the RoCoF within sustainable values. Moreover, the algorithm optimally commits synthetic inertia contribution from RES and Battery Energy Storage Systems (BESS) and installation of Synchronous Compensators (SC) among the Italian market areas. The method is designed to be sufficiently simple to process a relevant number of working scenarios in order to exploit the relevant quantity of information owned by the TSO. Results have shown to be highly accurate as outline by comparison with detailed time domain simulations