STATE OF CHARGE BASED DROOP SURFACE FOR OPTIMAL CONTROL OF DC MICROGRIDS

For a microgrid with a high penetration level of renewable energy, energy storage use becomes more integral to the system performance due to the stochastic nature of most renewable energy sources. This thesis examines the use of droop control of an energy storage source in dc microgrids in order to optimize a global cost function. The approach involves using a multidimensional surface to determine the optimal droop parameters based on load and state of charge. The optimal surface is determined using knowledge of the system architecture and can be implemented with fully decentralized source controllers. The optimal surface control of the system is presented. Derivations of a cost function along with the implementation of the optimal control are included. Results were verified using a hardware-in-the-loop system

An Islanding Detection Method for Micro-Grids With Grid-Connected and Islanded Capability

With the increasing prevalence of renewable energy and distributed generation (DG) in distribution systems, micro-grids are becoming more popular and an attractive option for enhancing system operation and reliability. This can be attributed to the micro-grid ability to operate in both connected and disconnected modes. Equally important, micro-grids are the best solution to meet the increasing demand of electric power in a cost effective manner due to the close proximity to the load demand and thus minimizing system losses. Islanding detection methods have been proposed for inverter based distributed generation with only grid-connected capability. Micro-grids are composed of DGs that are capable of operating in two modes: grid connected and islanded. This thesis introduces and proposes the concept of micro-grid transition detection where the status of the micro-grid is detected based on adaptively modifying the droop slope. The droop coefficient is chosen such that the micro-grid is stable while grid connected and in the contrary Unstable once an islanded micro-grid operation is initiated. The droop coefficient is adaptively modified, once the micro-grid transitions from grid-connected to islanded operation, to stabilize the micro-grid for the islanded mode of operation. The proposed method is capable of detecting micro-grid transition in less than 600 ms under various active and reactive power mismatches. The proposed micro-grid transition detection method is tested on a micro-grid equipped with inverter based DGs controlled using the droop approach. The main objective of this thesis is to develop a novel islanding detection method for micro-grids with grid connected and islanded capability. A micro-grid model was developed using power system computer aided design/ electromagnetic transient and DC (PSCAD/EMTDC) as a platform for testing the proposed method. Simulation results were conducted considering the Institute of Electrical and Electronics Engineers Standard 1547(IEEE Std. 1547) standard islanding detection testing procedure

Cooperative Power Sharing control in Multi-terminal VSC-HVDC

The Multi-terminal high voltage DC (MTDC) system is a viable solution for increasing an electrical power generation to interconnect renewable resources into an AC grid. Using a voltage source converter (VSC) allows independent control of a reactive and an active power flow. Based on the literature, there is a trend to implement MTDC into a distribution grid system in the future. Power sharing control among MTDCs is an important and critical consideration from the point of view of stability and operation. MTDC systems consist of multi-input converters (rectifiers) and single or multi-output converters (inverters), thus controlling and operating MTDC systems pose many challenges due to their complexity. Since the DC link in MTDC systems might have several connection nodes all having a common DC voltage value, using the DC voltage value as a common reference for all terminal control loops makes it possible to get a cooperative control performance. An economical autonomous control to share active power among MTDC systems based on the availability of active power or power management policy is proposed in this thesis. Power sharing among MTDC systems has a priority or sequential procedural problem because of the use of the conventional droop strategy. On the other hand, using predefined or constant power sharing does not provide the available power that can be shared when it is not being consumed by another inverter. The proposed strategy solves these issues using different options. In this thesis, the test system consists of four simulated VSC terminals based on a detailed switching VSC model with two AC voltage levels. The MTDC system is simulated in a PSCAD/EMTDC environment. The simulation results show a significant decrease in operational costs and protection from overloading which had been an issue

Power Sharing in an Islanded Micro-grid with No Synchronous Generator

This thesis addresses a real power sharing method of electronically interfaced dis tributed generation (DG) units in the context of a multiple-DG micro-grid system where there is no synchronous generator. The emphasis is primarily on electronically interfaced DG (EI-DG) units like DFIG connected wind generator, PV system and battery storage. In this dissertation, the main goal is designing and testing of a new algorithm to distribute the load changes among intermittent distributed generators according to their power ratings. The power sharing by the commercial distributed energy resource (CDER) unit is mainly based on locally measured signals without communications. In the controller, proposed in this thesis, the voltage source inverter of the battery energy storage system (BESS) changes the frequency of the network with the change in load demand. The real power of each CDER unit is controlled based on a frequency-droop characteristic and a complimentary frequency restoration strategy. A systematic approach to develop a small-signal dynamic model of a multiple-DG micro grid, with a real power management method, is also presented. The model is used to investigate the sensitivity of the design to the changes in parameters and operatingpoint and to optimize performance of the micro grid system. Finally, the performance of the proposed algorithm is tested on a benchmark medium voltage network

Controle coordenado em microrredes de baixa tensão baseado no algoritmo power-based control e conversor utility interface

Orientadores: José Antenor Pomilio, Fernando Pinhabel MarafãoTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de ComputaçãoResumo: Esta tese apresenta uma possível arquitetura e sua respectiva estratégia de controle para microrredes de baixa tensão, considerando-se a existência de geradores distribuídos pela rede. A técnica explora totalmente a capacidade dos geradores distribuídos em ambos os modos de operação: conectado à rede e ilhado. Quando conectado à rede, sob o modo de otimização global, o controle busca a operação quase ótima da microrrede, reduzindo as perdas de distribuição e os desvios de tensão. Quando em modo ilhado, a técnica regula de forma eficaz os geradores distribuídos disponíveis, garantindo a operação autônoma, segura e suave da microrrede. A estratégia de controle é aplicada a uma estrutura de microrrede completamente despachável, baseada em uma arquitetura de controle mestre-escravo, em que as unidades distribuídas são coordenadas por meio do recém-desenvolvido algoritmo Power-Based Control. As principais vantagens da arquitetura proposta são a expansividade e a capacidade de operar sem sincronização ou sem conhecimento das impedâncias de linha. Além disso, a microrrede regula as interações com a rede por meio do conversor chamado de Utility Interface, o qual é um inversor trifásico com armazenador de energia. Esta estrutura de microrrede permite algumas vantagens como: compensação de desbalanço e reativo, rápida resposta aos transitórios de carga e de rede, e suave transição entre os modos de operação. Em contrapartida, para compartilhar a potência ativa e reativa proporcionalmente entre as unidades distribuídas, controlar a circulação de reativos, e maximizar a operação, a comunicação da microrrede requer em um canal de comunicação confiável, ainda que sem grandes exigências em termos de resolução ou velocidade de transmissão. Neste sentido, foi demonstrado que uma falha na comunicação não colapsa o sistema, apenas prejudica o modo de otimização global. Entretanto, o sistema continua a operar corretamente sob o modo de otimização local, que é baseado em um algoritmo de programação linear que visa otimizar a compensação de reativos, harmônicos e desbalanço de cargas por meio dos gerador distribuído, particularmente, quando sua capacidade de potência é limitada. Esta formulação consiste em atingir melhores índices de qualidade de energia, definidos pelo lado da rede e dentro de uma região factível em termos de capacidade do conversor. Baseado nas medições de tensão e corrente de carga e uma determinada função objetiva, o algoritmo rastreia as correntes da rede ótima, as quais são utilizadas para calcular os coeficientes escalares e finalmente estes são aplicados para encontrar as referências da corrente de compensação. Finalmente, ainda é proposta uma técnica eficiente para controlar os conversores monofásicos conectados arbitrariamente ao sistema de distribuição trifásico, sejam conectados entre fase e neutro ou entre fase e fase, com o objetivo de compensar o desbalanço de carga e controlar o fluxo de potência entre as diferentes fases da microrrede. Isto melhora a qualidade da energia elétrica no ponto de acoplamento comum, melhora o perfil de tensão nas linhas, e reduz as perdas de distribuição. A arquitetura da microrrede e a estratégia de controle foi analisada e validada através de simulações computacionais e resultados experimentais, sob condições de tensão senoidal/simétrica e não-senoidal/assimétrica, avaliando-se o comportamento em regime permanente e dinâmico do sistema. O algoritmo de programação linear que visa otimizar a compensação foi analisado por meio de resultados de simulaçãoAbstract: This thesis presents a flexible and robust architecture and corresponding control strategy for modern low voltage microgrids with distributed energy resources. The strategy fully exploits the potential of distributed energy resources, under grid-connected and islanded operating modes. In grid-connected mode, under global optimization mode, the control strategy pursues quasi-optimum operation of the microgrid, so as to reduce distribution loss and voltage deviations. In islanded mode, it effectively manages any available energy source to ensure a safe and smooth autonomous operation of the microgrid. Such strategy is applied to a fully-dispatchable microgrid structure, based on a master-slave control architecture, in which the distributed units are coordinated by means of the recently developed power-based control. The main advantages of the proposed architecture are the scalability (plug-and-play) and capability to run the distributed units without synchronization or knowledge of line impedances. Moreover, the proposed microgrid topology manages promptly the interaction with the mains by means of a utility interface, which is a grid-interactive inverter equipped with energy storage. This allows a number of advantages, including compensation of load unbalance, reduction of harmonic injection, fast reaction to load and line transients, and smooth transition between operating mode. On the other hand, in order to provide demand response, proportional power sharing, reactive power control, and full utilization of distributed energy resources, the microgrid employs a reliable communication link with limited bit rate that does not involve time-critical communications among distributed units. It has been shown that a communication failure does not jeopardize the system, and just impairs the global optimization mode. However, the system keeps properly operating under the local optimization mode, which is managed by a linear algorithm in order to optimize the compensation of reactive power, harmonic distortion and load unbalance by means of distributed electronic power processors, for example, active power filters and other grid-connected inverters, especially when their capability is limited. It consists in attain several power quality performance indexes, defined at the grid side and within a feasible power region in terms of the power converter capability. Based on measured load quantities and a certain objective function, the algorithm tracks the expected optimal source currents, which are thereupon used to calculate some scaling coefficients and, therefore, the optimal compensation current references. Finally, the thesis also proposes an efficient technique to control single-phase converters, arbitrarily connected to a three-phase distribution system (line-to-neutral or line-to-line), aiming for reduce unbalance load and control the power flow among different phases. It enhances the power quality at the point-of-common-coupling of the microgrid, improve voltage profile through the lines, and reduce the overall distribution loss. The master-slave microgrid architecture has been analyzed and validated by means of computer simulations and experimental results under sinusoidal/symmetrical and nonsinusoidal/asymmetrical voltage conditions, considering both the steady-state and dynamic performances. The local optimization mode, i.e., linear algorithm for optimized compensation, has been analyzed by simulation resultsDoutoradoEnergia EletricaDoutor em Engenharia Elétrica2012/24309-8, 2013/21922-3FAPES

Virtual power plants

El presente proyecto tiene como objetivo determinar posibles beneficios de la agregación de unidades de energía distribuida controlables en Virtual Power Plants (VPP). Se diseñan y se implementan con el programa GAMS® los modelos de programación utilizados para optimizar la operación de una agregación de sistemas VPP en diversos escenarios. En la primera parte del trabajo se expone una introducción teórica de las Virtual Power Plants y sus elementos asociados: unidades de generación distribuida, sistemas de almacenamiento y técnicas de gestión de la demanda, lo que permite destacar los beneficios que produce la agregación. En la segunda parte, se realiza un estudio del estado del arte de la tecnología, exponiendo los casos más destacados respecto a modelos de gestión, con el objetivo de identificar los principales retos y localizar puntos que puedan ser mejorados con la propuesta de un nuevo algoritmo de operación. Finalmente, se desarrollan los algoritmos para determinar la operación óptima de sistemas VPP en varios casos de estudio. Los casos de estudio se diferencian por la política energética aplicada para favorecer la integración de recursos renovables en las redes de distribución: Feed-in Tariff, balance neto y agregación como Virtual Power Plant a gran escala. La comparación de los resultados obtenidos para un escenario particular definido permite destacar cual de las estrategias sería más beneficiosa económicamente desde el punto de vista del prosumidor, y, por tanto, más favorecedora para integrar generación distribuida mediante recursos renovables

Control of AC/DC microgrids with renewables in the context of smart grids including ancillary services and electric mobility

Microgrids are a very good solution for current problems raised by the constant growth of load demand and high penetration of renewable energy sources, that results in grid modernization through “Smart-Grids” concept. The impact of distributed energy sources based on power electronics is an important concern for power systems, where natural frequency regulation for the system is hindered because of inertia reduction. In this context, Direct Current (DC) grids are considered a relevant solution, since the DC nature of power electronic devices bring technological and economical advantages compared to Alternative Current (AC). The thesis proposes the design and control of a hybrid AC/DC Microgrid to integrate different renewable sources, including solar power and braking energy recovery from trains, to energy storage systems as batteries and supercapacitors and to loads like electric vehicles or another grids (either AC or DC), for reliable operation and stability. The stabilization of the Microgrid buses’ voltages and the provision of ancillary services is assured by the proposed control strategy, where a rigorous stability study is made. A low-level distributed nonlinear controller, based on “System-of-Systems” approach is developed for proper operation of the whole Microgrid. A supercapacitor is applied to deal with transients, balancing the DC bus of the Microgrid and absorbing the energy injected by intermittent and possibly strong energy sources as energy recovery from the braking of trains and subways, while the battery realizes the power flow in long term. Dynamical feedback control based on singular perturbation analysis is developed for supercapacitor and train. A Lyapunov function is built considering the interconnected devices of the Microgrid to ensure the stability of the whole system. Simulations highlight the performance of the proposed control with parametric robustness tests and a comparison with traditional linear controller. The Virtual Synchronous Machine (VSM) approach is implemented in the Microgrid for power sharing and frequency stability improvement. An adaptive virtual inertia is proposed, then the inertia constant becomes a system’s state variable that can be designed to improve frequency stability and inertial support, where stability analysis is carried out. Therefore, the VSM is the link between DC and AC side of the Microgrid, regarding the available power in DC grid, applied for ancillary services in the AC Microgrid. Simulation results show the effectiveness of the proposed adaptive inertia, where a comparison with droop and standard control techniques is conducted.As Microrredes são uma ótima solução para os problemas atuais gerados pelo constante crescimento da demanda de carga e alta penetração de fontes de energia renováveis, que resulta na modernização da rede através do conceito “Smart-Grids”. O impacto das fontes de energia distribuídas baseados em eletrônica de potência é uma preocupação importante para o sistemas de potência, onde a regulação natural da frequência do sistema é prejudicada devido à redução da inércia. Nesse contexto, as redes de corrente contínua (CC) são consideradas um progresso, já que a natureza CC dos dispositivos eletrônicos traz vantagens tecnológicas e econômicas em comparação com a corrente alternada (CA). A tese propõe o controle de uma Microrrede híbrida CA/CC para integrar diferentes fontes renováveis, incluindo geração solar e frenagem regenerativa de trens, sistemas de armazenamento de energia como baterias e supercapacitores e cargas como veículos elétricos ou outras (CA ou CC) para confiabilidade da operação e estabilidade. A regulação das tensões dos barramentos da Microrrede e a prestação de serviços anciliares são garantidas pela estratégia de controle proposta, onde é realizado um rigoroso estudo de estabilidade. Um controlador não linear distribuído de baixo nível, baseado na abordagem “System-of-Systems”, é desenvolvido para a operação adequada de toda a rede elétrica. Um supercapacitor é aplicado para lidar com os transitórios, equilibrando o barramento CC da Microrrede, absorvendo a energia injetada por fontes de energia intermitentes e possivelmente fortes como recuperação de energia da frenagem de trens e metrôs, enquanto a bateria realiza o fluxo de potência a longo prazo. O controle por dynamical feedback baseado numa análise de singular perturbation é desenvolvido para o supercapacitor e o trem. Funções de Lyapunov são construídas considerando os dispositivos interconectados da Microrrede para garantir a estabilidade de todo o sistema. As simulações destacam o desempenho do controle proposto com testes de robustez paramétricos e uma comparação com o controlador linear tradicional. O esquema de máquina síncrona virtual (VSM) é implementado na Microrrede para compartilhamento de potência e melhoria da estabilidade de frequência. Então é proposto o uso de inércia virtual adaptativa, no qual a constante de inércia se torna variável de estado do sistema, projetada para melhorar a estabilidade da frequência e prover suporte inercial. Portanto, o VSM realiza a conexão entre lado CC e CA da Microrrede, onde a energia disponível na rede CC é usada para prestar serviços anciliares no lado CA da Microrrede. Os resultados da simulação mostram a eficácia da inércia adaptativa proposta, sendo realizada uma comparação entre o controle droop e outras técnicas de controle convencionais