Modelagem e análise da dinâmica de microrredes de distribuição de energia elétrica

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia de Automação e Sistemas, Florianópolis, 2017.Uma das tendências dentro da eletrônica de potência atualmente é a aplicação de conversores de energia para a operação em microrredes, o que na essência é eletrônica de potência aplicada a sistemas de distribuição de energia elétrica. Tipicamente, microrredes são redes elétricas compostas por diversas fontes e cargas, o que implica em diversas interações fontes-fontes e fontes-cargas. O propósito deste trabalho é modelar, propor técnicas de controle e analisar a estabilidade paramétrica de pequenas microrredes CC e CA. Um dos grandes problemas que existem na operação de microrredes é quando diversas fontes operam em paralelo e compartilham um conjunto de cargas. O controle por droop é a técnica mais popular para lidar com este tipo de problema, mas possui o contraponto de introduzir não linearidades no sistema. Por conta disso, este trabalho propõe o uso de ferramentas matemáticas como a teoria de bifurcações para compreender o comportamento dinâmico de microrredes. Este procedimento permite um entendimento mais amplo quando comparado com técnicas tradicionais baseadas em modelos linearizados, além de fornecer mais informações que permitem otimizar o funcionamento do sistema garantindo a sua estabilidade. Uma parte significativa deste trabalho possui uma natureza teórica, com exceção de um caso de estudo envolvendo o paralelismo de conversores CC-CA, em que resultados experimentais são apresentados.Abstract : Nowadays, one of the trends within power electronics is the application of power converters in microgrids, which in essence means power electronics applied to electric power distribution systems. As microgrids can be dc, ac, or even hybrid networks, they are complex systems with many source-source and source-load-type interactions. The focus of this work is to model, propose control techniques, and to verify the stability of a small ac and dc microgrids through parametric diagrams. Load sharing is one of the major issues within microgrids, and it is defined as how several voltage sources operate in parallel feeding a given load set. Droop control is the standard approach towards the load sharing problem in microgrids, but this control can make the system unstable due to some nonlinearities introduced into the microgrid?s operation. For that reason, nonlinear dynamic analysis techniques are used here to understand the microgrid?s dynamic behavior. In particular, bifurcation theory offers a broader insight into the system dynamics when compared to usual techniques based on linear models. Moreover, bifurcation theory can optimize the microgrid operation and, thus, increase the system stability range due to a better understanding on how a group of parameters can influence the microgrid operation.A significant part of this thesis consists of a theoretical study, with the exception of one case study involving the parallelism of two dc-ac power converters, where experimental results are shown to validate the theoretical premises

Global stabilization for nonlinear two-port characteristics of bidirectional DC/DC converter and its application to peer-to-peer energy transfer

A global stabilization method for the conversion characteristics of a bidirectional DC/DC converter and its application in peer-to-peer energy transfer systems is described. Peer-to-peer energy transfer is a control strategy in which the supply and load cooperate to transmit power, and it requires the global operation of the converter. According to the power relation, the bidirectional DC/DC converter has two equilibrium points. To realize global stability, a unique equilibrium point is achieved by eliminating the untargeted equilibrium point using the power relationship between the ports. Global stability is realized by setting feedback gains to converge globally to this equilibrium point. The experimental results demonstrate the global stability of the proposed method when applied to a stand-alone system and a peer-to-peer energy transfer system

Stability, control, and optimization of nonlinear dynamical systems with applications in electric power networks

Electric power systems in recent years have witnessed an increasing adoption of renewable energy sources as well as restructuring of distribution systems into multiple microgrids. These trends, together with an ever-growing electricity demand, are making power networks operate closer to their stability margins, thereby raising numerous challenges for power system operators. In this thesis, we focus on two major challenges: How to efficiently assess and certify the stability of power systems; and how to optimize the operation of multiple microgrids while maintaining their stability. In the first part of the thesis, we focus on the first question, and study one of the most fundamental models of power systems, namely the swing equation model. We develop sufficient conditions under which the equilibrium points of swing equations are asymptotically stable. We also discuss the connection between the stability of equilibrium points and the network structure. This for example reveals an analog of Braess’s Paradox in power system stability, showing that adding power lines to the system may decrease the stability margin. Based on the developed theories, we also introduce several distributed control schemes for maintaining the stability of the system. Since swing equations belong to a more general class of second-order ordinary differential equations (ODEs) which are the cornerstone of studying many other physical and engineering systems, a considerable part of this thesis is devoted to the study of this general class of ODEs, where we investigate the impact of damping as a system parameter on the stability, hyperbolicity, and bifurcation in such systems. In the second part of the thesis, we address the second question and provide a computationally efficient method for optimizing multi-microgrid operation while ensuring its stability. Our goal is to maintain the frequency stability of multi-microgrid networks under an islanding event and to achieve optimal load shedding and network topology control with AC power flow constraints. Attaining this goal requires solving a challenging optimization problem with stability constraints. To cope with this challenge, we develop a strong mixed-integer second-order cone programming (MISOCP)-based reformulation and a cutting plane algorithm for scalable computation of the problem. The optimization frameworks and stability certificates developed in this thesis can be used as powerful decision support tools for power system operators.Ph.D

Controle de microrredes de distribuição de energia elétrica em corrente contínua

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia de Automação e Sistemas, Florianópolis, 2015.As microrredes (MR) CC se apresentam como uma solução para determinadas aplicações de distribuição de energia em que se exige expansão modular, eficiência e integração de energias renováveis. A arquitetura da microrrede CC baseia-se no agrupamento de diversas fontes de energia distribuída, dispositivos de armazenamento e cargas, todos acoplados por conversores de potência. A interação dinâmica provocada por essa estrutura de múltiplos estágios de conversores traz problemas de estabilidade, cujas causas são discutidas, bem como as possíveis soluções. Neste trabalho, propõe-se uma modelagem não linear da MR em que abstrai-se a diversidade de fontes/cargas e topologias de conversores, visando obter uma modelagem compacta do sistema. Tal modelagem permite a análise de estabilidade de grandes sinais do sistema de forma analítica, além de prever possíveis comportamentos dinâmicos de caráter oscilatório e de instabilidade que não são possíveis por meio da análise de modelos lineares. Adicionalmente, propõem-se dois controladores por modos deslizantes, integral e washout, para os conversores responsáveis pelo controle de tensão com o objetivo de adicionar amortecimento ativo durante perturbações. Dessa forma, estabelecem-se as regiões seguras de operação por meio da avaliação de diagramas de bifurcação e as diretrizes para o projeto de MR CC robustas.Abstract : DC micro-grids (MG) are presented as a solution for power distribution applications which requires modular expansion, efficiency and integration of renewable energy. DC MG architecture is based on the grouping of distributed energy resources, storage devices and loads, all coupled by power converters. The dynamic interaction caused by such multi-stage converter structure brings stability problems whose causes and solutions are discussed. It is proposed a nonlinear modeling of the MG which abstracts the diversity of sources/loads and power converters topologies in order to obtain a compact modeling of the system. This modeling allows the large signal stability analysis and it is capable to predict possible oscillatory behaviors and instabilities that are not possible through the analysis of linear models. It is further proposed sliding mode controllers, integral and washout, to the power converters responsible for voltage control in order to add active damping during disturbances. Thus, it is set up safe operating regions through the evaluation of bifurcation diagrams and guidelines for designing robust DC MGs

Producció científica de l'ETSEIB a Futur. Articles publicats per investigadors de l'ETSEIB l'any 2018

Postprint (author's final draft