Modeling the controlled delivery power grid

Competitive energy markets, stricter regulation, and the integration of distributed renewable energy sources are forcing companies to reengineer energy production and distribution. The Controlled Delivery Power Grid is proposed as a novel approach to transport energy from generators to consumers. In this approach, energy distribution is performed in an asynchronous and distributed fashion. Much like the Internet, energy is delivered as addressable packets, which allow a controlled delivery of energy. As a proof-of-concept of the controllable delivery grid, two experimental test beds, one with integrated energy storage and another with no energy storage, were designed and built to evaluate the efficiency of a power distribution and scheduling scheme. Both test beds use a request-grant protocol where energy is supplied in discrete quantities. The performance of the system is measured in terms of the ability to satisfy requests from consumers. The results show high satisfaction ratios for distribution capacities that are smaller than the maximum demand. The distribution of energy is modelled with graph theory and as an Integer Linear Programming problem to minimize transmission losses and determine routes for energy flows in a network with distributed sources and consumers. The obtained results are compared with a heuristic approach based on the Dijkstra\u27s shortest path algorithm, which is proposed as a feasible approach to routing the transmission of packetized energy

Distributed Control Strategies for Microgrids: An Overview

There is an increasing interest and research effort focused on the analysis, design and implementation of distributed control systems for AC, DC and hybrid AC/DC microgrids. It is claimed that distributed controllers have several advantages over centralised control schemes, e.g., improved reliability, flexibility, controllability, black start operation, robustness to failure in the communication links, etc. In this work, an overview of the state-of-the-art of distributed cooperative control systems for isolated microgrids is presented. Protocols for cooperative control such as linear consensus, heterogeneous consensus and finite-time consensus are discussed and reviewed in this paper. Distributed cooperative algorithms for primary and secondary control systems, including (among others issues) virtual impedance, synthetic inertia, droop-free control, stability analysis, imbalance sharing, total harmonic distortion regulation, are also reviewed and discussed in this survey. Tertiary control systems, e.g., for economic dispatch of electric energy, based on cooperative control approaches, are also addressed in this work. This review also highlights existing issues, research challenges and future trends in distributed cooperative control of microgrids and their future applications

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

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

Technical solutions for low-voltage microgrid concept

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Wide-Area Time-Synchronized Closed-Loop Control of Power Systems And Decentralized Active Distribution Networks

The rapidly expanding power system grid infrastructure and the need to reduce the occurrence of major blackouts and prevention or hardening of systems against cyber-attacks, have led to increased interest in the improved resilience of the electrical grid. Distributed and decentralized control have been widely applied to computer science research. However, for power system applications, the real-time application of decentralized and distributed control algorithms introduce several challenges. In this dissertation, new algorithms and methods for decentralized control, protection and energy management of Wide Area Monitoring, Protection and Control (WAMPAC) and the Active Distribution Network (ADN) are developed to improve the resiliency of the power system. To evaluate the findings of this dissertation, a laboratory-scale integrated Wide WAMPAC and ADN control platform was designed and implemented. The developed platform consists of phasor measurement units (PMU), intelligent electronic devices (IED) and programmable logic controllers (PLC). On top of the designed hardware control platform, a multi-agent cyber-physical interoperability viii framework was developed for real-time verification of the developed decentralized and distributed algorithms using local wireless and Internet-based cloud communication. A novel real-time multiagent system interoperability testbed was developed to enable utility independent private microgrids standardized interoperability framework and define behavioral models for expandability and plug-and-play operation. The state-of-theart power system multiagent framework is improved by providing specific attributes and a deliberative behavior modeling capability. The proposed multi-agent framework is validated in a laboratory based testbed involving developed intelligent electronic device prototypes and actual microgrid setups. Experimental results are demonstrated for both decentralized and distributed control approaches. A new adaptive real-time protection and remedial action scheme (RAS) method using agent-based distributed communication was developed for autonomous hybrid AC/DC microgrids to increase resiliency and continuous operability after fault conditions. Unlike the conventional consecutive time delay-based overcurrent protection schemes, the developed technique defines a selectivity mechanism considering the RAS of the microgrid after fault instant based on feeder characteristics and the location of the IEDs. The experimental results showed a significant improvement in terms of resiliency of microgrids through protection using agent-based distributed communication

Multi-agent system based active distribution networks

This thesis gives a particular vision of the future power delivery system with its main requirements. An investigation of suitable concepts and technologies which creates a road map forward the smart grid has been carried out. They should meet the requirements on sustainability, efficiency, flexibility and intelligence. The so called Active Distribution Network (ADN) is introduced as an important element of the future power delivery system. With an open architecture, the ADN is designed to integrate various types of networks, i.e., MicroGrid or Autonomous Network, and different forms of operation, i.e., islanding or interconnection. By enabling an additional local control layer, these so called cells are able to reconfigure, manage local faults, support voltage regulation, or manage power flow. Furthermore, the Multi-Agent System (MAS) concept is regarded as a potential technology to cope with the anticipated challenges of future grid operation. Analysis of benefits and challenges of implementing MAS shows that it is a suitable technology for a complex and highly dynamic operation and open architecture as the ADN. By taking advantages of the MAS technology, the AND is expected to fully enable distributed monitoring and control functions. This MAS-based ADN focuses mainly on control strategies and communication topologies for the distribution systems. The transition to the proposed concept does not require an intensive physical change to the existing infrastructure. The main point is that inside the MAS-based ADN, loads and generators interact with each other and the outside world. This infrastructure can be built up of several cells (local areas) that are able to operate autonomously by an additional agent-based control layer. The ADN adapts a MAS hierarchical control structure in which each agent handles three functional layers of management, coordination, and execution. In the operational structure, the ADN addresses two main function parts: Distributed State Estimation (DSE) to analyze the network topology, compute the state estimation, and detect bad data; and Local Control Scheduling (LCS) to establish the control set points for voltage coordination and power flow management. Under the distributed context of the controls, an appropriate method for DSE is proposed. The method takes advantage of the MAS technology to compute iteratively the local state variables through neighbor data measurements. Although using the classical Weighted Least Square (WLS) as a core, the proposed algorithm based on an agent environment distributes drastically computation burden to subtasks of state estimation with only two interactive buses and an interconnection line in between. The accuracy and complexity of the proposed estimation are investigated through both off-line and on-line simulations. Distributed and parallel working of processors improves significantly the computation time. This estimation is also suitable for a meshed configuration of the ADN, which includes more than one interconnection between each pair of the cells. Depending on the availability of a communication infrastructure, it is able to work locally inside the cells or globally for the whole ADN. As a part of the LCS, the voltage control function is investigated in both steady-state and dynamic environments. The autonomous voltage control within each network area (cell) can be deployed by a combination of active and reactive power support of distributed generation (DG). The coordinated voltage control defines the optimal tap setting of the on-load tap changer (OLTC) while comparing amounts of control actions in each area. Based on the sensitivity factors, these negotiations are thoroughly supported in the distributed environment of the MAS platform. To verify the proposed method, both steady-state and dynamic simulations are developed. Simulation results show that the proposed function helps to integrate more DG while mitigating voltage violation effectively. The optimal solution can be reached within a small number of calculation iterations. It opens a possibility to apply the proposed method as an on-line application. Furthermore, a distributed approach for the power flow management function is developed. By converting the power network to a represented graph, the optimal power flow is understood as the well-known minimum cost flow problem. Two fundamental solutions for the minimum cost flow, i.e., the Successive Shortest Path (SSP) algorithm and the Cost-Scaling Push-Relabel (CS-PR) algorithm, are introduced. The SSP algorithm is augmenting the power flow along the shortest path until reaching the capacity of at least one edge. After updating the flow, it finds another shortest path and augments the flow again. The CS-PR algorithm approaches the problem in a different way which is scaling cost and pushing as much flow as possible at each active node. Simulations of both meshed and radial test networks are developed to compare their performances in various network conditions. Simulation results show that the two methods can allow both generation and power flow controller devices to operate optimally. In the radial test network, the CS-PR needs less computation effort represented by a number of exchanged messages among the MAS platform than the SSP. Their performances in the meshed network are, however, almost the same. Last but not least, this novel concept of MAS-based AND is verified under a laboratory environment. The lab set-up separates some local network areas by using a three-inverter system. The MAS platform is created on different computers and is able to retrieve data from and to hardware components, i.e., the three-inverter system. In this set-up, a configuration of the power router is established in a combination of the three-inverter system with the MAS platform. Three control functions of the inverters, AC voltage control, DC bus voltage control, and PQ control, are developed in a Simulink diagram. By assigning suitable operation modes for the inverters, the set-up successfully experiments on synchronizing and disconnecting a cell to the rest of the grid. In the MAS platform, an obvious power routing strategy is executed to optimally manage power flow in the lab set-up. The results show that the proposed concept of the ADN with the power router interface works well and can be used to manage electrical networks with distributed generation and controllable loads, leading to active networks

Control of distributed renewable energy generation systems in converter-dominated microgrid applications

Mención Internacional en el título de doctorThere is a growing interest in the use of renewable Distributed Energy Resources (DERs) that increase the efficiency of the transmission system and reduce the ecological impact of renewable energy infrastructures. At the same time, they reduce the associated capital requirements, thus increasing the potential installation of renewable energy. Microgrids have been proposed as a solution to improve the integration of renewable DERs. By the use of advanced control techniques, they provide a reliable frame for DERs to support the power system operation. As such, Microgrids can be a promising solution to increase renewable energy penetration. However, since renewable DERs are usually interfaced by Power Electronic Converters (PECs), they do not provide the common stabilization characteristics of traditional generation interfaced by Synchronous Generators (SGs). Therefore, there are concerns about the stability of converter-dominated Microgrids. This Thesis focus on the specific requirements of PEC-interfaced renewable DERs operating in Microgrids. An overview of available solutions show that, for PECs to support the Microgrid operation in both grid-connected and islanded modes, they require a synchronizing mechanism that does not rely on the measurement of an external frequency. A promising alternative is to emulate the behavior of traditional SGs in the PEC control system with the so-called Virtual Synchronous Machine (VSM) solutions. The synchronization system underlying to these proposals is analyzed. A comparison with the use of traditional frequency measurement systems, namely Phase-Locked Loops (PLLs), in the support of the Microgrid power balance is addressed, showing that the PEC synchronization system has a direct effect on the Microgrid stability. The Thesis includes a new proposal to ensure synchronous operation based on the use reactive power, instead of active power as in VSMs, that does not require frequency measurements. A dynamic model of a grid-connected PEC is used to demonstrate that reactive power can be used to ensure synchronism. This Reactive Power Synchronization system is used to propose a solution for the black-start of Wind Energy Conversion Systems (WECSs), so that they can contribute to the restoration of the power system following a blackout. The proposed control systems are validated with experimental results of a grid connected PEC and an isolated WECS.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y Automática por la Universidad Carlos III de MadridPresidente: Luis Rouco Rodríguez.- Secretario: Emilio José Bueno Peña.- Vocal: Roberto Alves Baraciart