A hybrid ac/dc microgrid is a more complex but practical network that combines the advantages of an AC and a DC system. The main advantage of this network is that it connects both alternating current and direct current networks via an interlinking converter (IC) to form a unified distribution grid. The hybrid microgrid (HMG) will enable the direct integration of both alternating current (AC) and direct current (DC) distributed generators (DGs), energy storage systems (ESS), and alternating current and direct current (DC) loads into the grid. The alternating current and direct current sources, loads, and ESS are separated and connected to their respective subgrids primarily to reduce power conversion and thus increase overall system efficiency. As a result, the HMG architecture improves power quality and system reliability. Planning a hybrid microgrid entails estimating the capacities of DGs while taking technical, economic, and environmental factors into account. The hybrid ac-dc microgrid is regarded as the distribution network of the future, as it will benefit from both ac and dc microgrids. This thesis presents a general architecture of a hybrid ac-dc microgrid, which includes both planning and design. The goal of the Hybrid ac-dc microgrid planning problem is to maximise social welfare while minimising total planning costs such as investment, maintenance, and operation costs. This configuration will assist Hybrid microgrid planners in estimating planning costs while allowing them to consider any type of load ac/dc and DER type. Finally, this thesis identifies the research questions and proposes a future research plan.

A proteção de Sistemas de Distribuição (SDs) é de fundamental importância considerando as condições adversas de operação e faltas elétricas a que os mesmos podem estar submetidos. A crescente utilização de Geradores Distribuídos (GDs) conectados aos SDs e as possíveis operações em ilhamento promovem interferências consideráveis aos dispositivos de proteção. Dessa forma, evidencia-se a importância de estudos para esses novos cenários. Neste contexto, esta pesquisa tem como proposta analisar o desempenho do sistema de proteção, quando na presença de uma elevada contribuição de geração distribuída, e aprimorar a proteção tradicional, quando tem-se a mudança do cenário de operação para o modo ilhado. Sendo assim, evidencia-se através de simulações no Real-Time Digital Simulator (RTDS) os efeitos da perturbação nos Relés de Sobrecorrente (RSC). Na metodologia desenvolvida implementa-se um sistema de proteção adaptativo, cujos parâmetros são definidos por um algoritmo de otimização. Também são adicionadas as proteções associadas aos GDs, recomendadas por norma. Tal prática é dificilmente abordada em trabalhos correlatos a pesquisa, no entanto através dos resultados observou-se que esses dispositivos podem influenciar na respostas dos elementos de proteção implementados nos SDs. Como diferencial também buscou-se implementar todo o sistema de controle dos GDs e utilizou-se da simulação em malha fechada com relés comerciais, afim de aproximar a modelagem da realidade encontrada nos SDs. Por fim, evidenciou-se que a utilização de relés adaptativos pode viabilizar a operação em ilhamento. / The protection of Distribution Systems (DSs) is of fundamental importance, considering the adverse operating conditions and electrical faults to which they may be submitted. The increasing usage of Distributed Generators (DGs) connected to DSs, and possible islanded operations promote considerable interference to protective devices. Thus, it is evident the importance of studies for these new scenarios.This research aims to analyze the performance of the protection system, when in the presence of a massive contribution of distributed generation, and to improve the traditional protection when there is a change from the operating scenario to island mode. Thus, via simulations, carried out in the Real-Time Digital Simulator (RTDS), the effects of the disturbance on the Overcurrent Relays (OCs) were evident. The developed methodology implements an adaptive protection system, whose parameters are defined by an optimization algorithm. The protections associated with DGs were also added, as recommended by the standard. This practice is hardly approached in related studies however, through the results, it was observed that these devices could influence the responses of the protective elements implemented. The closed-loop simulations carried out considering the standard controls systems regarding DGs and commercial intelligent eletronic devices, ensuring the test bed verisimilitude to actual DSs. This approach is not standard on the literature related. Finally, it was evidenced that the use of adaptive relays can contribute to the islanded operation.

A conexão de Geradores Distribuídos (GDs) no sistema de distribuição vem se intensificando nos últimos anos. Neste cenário, o aumento de GDs pode trazer alguns benefícios, como a redundância da geração e redução das perdas elétricas. Por outro lado, o problema do ilhamento também vem se destacando. Atualmente, existem técnicas já consolidadas para a detecção do ilhamento, sendo que as técnicas passivas estão entre as mais utilizadas. Entretanto, as técnicas passivas são bastante dependentes do desbalanço de potência entre a geração e as cargas no momento de ocorrência do ilhamento para atuarem corretamente. Caso o desbalanço de potência seja pequeno, as técnicas passivas tendem a não identificar o ilhamento, gerando as chamadas Zonas de Não Detecção (ZNDs). Para mitigar este problema, a pesquisa por técnicas passivas inteligentes baseadas em aprendizagem de máquina vem se tornando cada vez mais comum. Neste trabalho foi modelada uma proteção anti-ilhamento baseada em Redes Neurais Artificiais (RNAs). A classificação do ilhamento é feita com base no espectro de frequência das tensões nos terminais do GD com o uso da Transformada de Stockwell, ou apenas Transformada S (TS). Outro ponto importante da metodologia foi a implementação de uma etapa de detecção de eventos, também baseada nas energias do espectro de frequência das tensões, para evitar a constante execução do classificador. Assim, a RNA apenas irá classificar o evento após receber um sinal de trigger da etapa de detecção de evento. Para o treinamento da RNA foram testados dois algoritmos diferentes, o clássico Backpropagation (BP) e a Máquina de Aprendizado Extremo, do inglês Extreme Learning Machine (ELM). Ressalta-se o melhor desempenho obtido com as redes treinadas pelo ELM, que apresentaram uma capacidade de generalização muito maior, logo, resultando em taxas de acerto mais elevadas. De modo geral, depois de comparada com métodos passivos convencionais para a detecção de ilhamento, a proteção proposta se mostrou mais precisa e com um tempo de detecção muito menor, sendo inferior a 2 ciclos. Por fim, ainda foi realizada a análise das ZNDs para a proteção proposta e as técnicas convencionais, por ser uma característica muito importante para a proteção antiilhamento, mas que não é comumente abordada para técnicas passivas inteligentes. Nesta análise, o método para a detecção de ilhamento proposto novamente se sobressaiu às técnicas convencionais, apresentado uma ZND muito menor. / The connection of distributed generators (DG) in the distribution system has been intensified in the recent years. In this scenario, the increase of DG can bring some benefits, such as generation redundancy and reduction of power losses. On the other hand, the problem of islanding is also been highlighted. Currently, there are already consolidated techniques for islanding detection, and passive techniques are among the most used ones. However, the passive techniques are very dependent of the power unbalance between the generation and the loads at the moment of the islanding in order to actuate properly. If the power mismatch is small, the passive techniques tend to not identify the islanding, generating the so called Non-Detection Zones (NDZ). To mitigate this issue, the research of intelligent passive techniques based in machine learning is becoming more common. In this study, an anti-islanding protection based on Artificial Neural Networks (ANN) was modelled. The islanding classification is done based on the frequency spectrum of the DG\'s terminal voltages using the Stockwell Transform, or just S-Transform (ST). Another important point of the methodology was the implementation of an event detection stage, also based on the energies of the voltages frequency spectrum, to avoid the constant execution of the classifier. Therefore, the ANN will only classify the event after receiving a trigger signal from the event detection stage. To train the ANN, two different algorithms were tested: the classic Backpropagation and the Extreme Learning Machine (ELM). It is noteworthy the better performance obtained with the neural networks trained by the ELM, which had a greater capacity of generalization, hence resulting in higher success rates. In general, after being compared with conventional passive techniques for islanding detection, the proposed protection was more accurate and with a much smaller detection time, being less than 2 cycles. Finally, the analysis of the NDZ for the proposed protection and the conventional techniques was carried out, as it is a very important feature for anti-islanding protection, but is not commonly addressed for intelligent passive techniques. In this analysis, the islanding detection method proposed again overcame the conventional techniques, presenting a much smaller NDZ.

Les travaux de la thèse se focalisent sur la protection des réseaux de distribution HTA en présence des générateurs distribués (éoliennes, fermes solaires,...). Dans un premier temps, un état de l’art a été réalisé sur les comportements des générateurs en creux de tension, leurs impacts sur le système de protection et les pistes de solution proposées pour y remédier. L’étape suivante est la mise au point d’algorithmes directionnels de détection de défauts, sans mesure de tension. Ces algorithmes s’appuient sur la décomposition en composantes symétriques des courants mesurés. Ces relais doivent empêcher le déclenchement intempestif de protections de surintensité dû au courant de défaut provenant des générateurs distribués. Ils sont moins coûteux par rapport à ceux traditionnels car les capteurs de tension, qui sont indispensables pour ces derniers, peuvent être enlevés. Après détection d’un défaut sur critère de seuil simple (max de I ou max de I résiduel), la direction est évaluée à l’aide d’un algorithme en delta basé sur les rapports courants inverse-homopolaire ou inverse-direct, selon le type de défaut (monophasé ou biphasé). En se basant sur ces rapports, un classifieur SVM (Support Vector Machines), entrainé préalablement à partir des simulations, donne ensuite l’estimation de la direction du défaut (amont ou aval par rapport au relais). La bonne performance de ces algorithmes a été montrée dans la thèse pour différents paramètres du réseau et en présence de différents types de générateurs. Le développement de tels algorithmes favorise la mise en œuvre des protections en réseau, qui pourraient être installées dans les futurs Smart Grids. / This research focuses on the protection of MV distribution networks with Distributed Generators (DGs), such as wind farms or photovoltaic farms, etc. First, the state of art is carried out on fault behaviour of DGs, their impacts on protection system and the mitigation solutions. Next, algorithms are developed for directional relays without voltage sensors. Based on the symmetrical component method, these algorithms help the overcurrent protections to avoid the false tripping issue due to fault contribution of DGs. With the suppression of voltage sensors, such directional relays become cheaper in comparison with the traditional ones. Following the fault detection (the phase or residual current reaches the pick-up value) and depending on fault type (line-to-ground or line-to-line fault), the ratios between the variation (before and during fault) of negative-zero sequence or negative-positive sequence currents are calculated. From these ratios, a SVM (Support Vector Machines) classifier estimates the fault direction (upstream or downstream the detector). The classifier is trained beforehand from transient simulations. This survey shows good performances of the directional algorithms with different network parameters and different kinds of DGs. Such algorithms could be implemented in protections along the feeders in the future smart grids.

Orientador: Jônatas Boás Leite / Resumo: A análise de curto-circuito é um método extremamente importante para os sistemas de energia elétrica (SEE), pois fornece os parâmetros necessários para o dimensionamento adequado dos equipamentos elétricos e ajuste dos dispositivos de proteção que garantem a segurança do SEE e contribuem para a redução dos riscos associados às pessoas próximas ao SEE. Assumindo o atual cenário de intensa inserção de fontes de energia renováveis no SEE, este trabalho propõe métodos e modelos matemáticos para representar os geradores distribuídos de fontes renováveis e não renováveis na análise de curto-circuito em sistemas de distribuição. As características da corrente de curto-circuito de quatro tecnologias de geradores distribuídos são apresentadas: síncrono, indução, fotovoltaico e gerador de indução duplamente alimentado (DFIG, do inglês, Doubly Fed Induction Generator). Neste trabalho considera-se um sistema elétrico trifásico desbalanceado real onde são realizadas simulações de faltas mono-, bi- e trifásicas permitindo, assim, avaliação e compreensão dos efeitos dessas tecnologias nas correntes de curto-circuito em redes de distribuição. Na análise de curto-circuito proposta, o método de varredura para cálculo do fluxo de potência em sistemas radiais, e/ou fracamente malhados, é utilizado na obtenção das condições de operação do sistema de distribuição. A determinação da matriz de impedância nodal com redução de Kron para cada gerador presente no SEE e o princípio da superposição são ut... (Resumo completo, clicar acesso eletrônico abaixo) / Mestre

Doctor of Philosophy / Electrical and Computer Engineering / Anil Pahwa / Noel N. Schulz / Microgrids with distributed generators have changed how protection and control systems are designed. Protection systems in conventional U.S. distribution systems are radial with the assumption that current flows always from the utility source to the end user. However, in a microgrid with distributed generators, currents along power lines do not always flow in one direction. Therefore, protection systems must be adapted to different circuit paths depending on distributed generator sites in the microgrid and maximum fuse ampere ratings on busses. Adaptive overcurrent protection focuses on objectives and constraints based on operation, maximum load demand, equipment, and utility service limitations. Adaptive overcurrent protection was designed to protect the power lines and bus feeders of the microgrid with distributed generators by coordinating fuses and relays in the microgrid. Adaptive overcurrent protection was based on the relay setting group and protection logic methods. Non-real-time simulator (NRTS) and real-time simulator (RTS) experiments were performed with computer-based simulators. Tests with two relays in the loop proved that primary relays tripped faster than backup relays for selectivity coordination in the adaptive overcurrent protection system. Relay test results from tripping and non-tripping tests showed that adaptive inverse time overcurrent protection achieved selectivity, speed, and reliability. The RTS and NRTS with two relays in the loop techniques were described and compared in this work. The author was the first graduate student to implement real-time simulation with two relays in the loop at the Burns & McDonnell - K-State Smart Grid Laboratory. The RTS experimental circuit and project are detailed in this work so other graduate students can apply this technique with relays in the loop in smart grid research areas such as phasor measurement units, adaptive protection, communication, and cyber security applications.

Distributed generators (DGs) are proposed as a possible solution to supply economic and reliable electricity to customers. It is adapted to overcome the challenges that are characterized by centralized generation such as transmission and distribution losses, high cost of fossil fuels and environmental damage. This work presents the basic principles of integrating renewable DGs in low voltage distribution networks and particularly focuses on the operation of DG installations and their impacts on active and reactive power. In this thesis, a novel technique that applies the stochastic approach for the operation of distribution networks with considering active network management (ANM) schemes and demand response (DR) within a joint active and reactive distribution market environment is proposed. The projected model is maximized based on social welfare (SW) using market-based joint active and reactive optimal power flow (OPF). The intermittent behaviour of renewable sources (such as solar irradiance and wind speed) and the load demands are modelled through Scenario-Tree technique. The distributed network frame is recast using mixed-integer linear programming (MILP) that is solved by using the GAMS software and then the obtained results are being analysed and discussed. In addition, the impact of wind and solar power penetration on the active and reactive distribution locational prices (D-LMPs) within the distribution market environment is explored in terms of the maximization of SW considering the uncertainty related to solar irradiance, wind speed and load demands. Finally, a realistic case study (16-bus UK generic medium voltage distribution system) is used to demonstrate the effectiveness of the proposed method. Results show that ANM schemes and DR integration lead to an increase in the social welfare and total dispatched active and reactive power and consequently decrease in active and reactive D-LMPs. / Ministry of Higher Education and Scientific Research - Iraq / The selected author's publications, the published versions of which were attached at the end of the thesis, have been removed due to copyright.

[ES] Dos aspectos críticos en la operación de una microrred son las estrategias de control y gestión de potencia implementadas, las cuales son esenciales para proporcionar su buen funcionamiento. La aplicación adecuada de dichas estrategias permite compensar los desequilibrios de potencia causados por la discontinuidad de la generación y de la demanda de energía en las microrredes. En este sentido, el objetivo global de estas estrategias de gestión es equilibrar adecuadamente el flujo de potencia en la microrred, mediante la aplicación de diferentes algoritmos que permiten cumplir con los criterios de estabilidad, protección, balance de potencia, transiciones, sincronización con la red y gestión adecuada de la microrred. En el caso de microrredes de pequeña escala de potencia con bajo número de generadores y sistemas de almacenamiento distribuidos, las estrategias de control centralizado ofrecen un alto nivel de flexibilidad para lograr funcionalidades avanzadas en la microrred y una adecuada distribución de la potencia entre los convertidores que la conforman. Esta tesis se ha enmarcado en el contexto de algoritmos de gestión centralizada de potencia de una microrred de generación distribuida en modo conectado a red. Los algoritmos presentados se pueden aplicar a los convertidores de potencia conectados al bus DC de una microrred AC/DC híbrida o en una microrred de DC, donde el despacho de potencia es observado y gestionado por un controlador central. Este último adquiere datos del sistema mediante una infraestructura de comunicaciones y estima la potencia que gestionará cada uno de los convertidores de potencia, sistemas de almacenamiento y cargas en funcionamiento. En este estudio se muestra la validación experimental de las estrategias de gestión aplicadas en la microrred desde el enfoque del comportamiento de los convertidores de potencia, de las baterías y las cargas ante dicha gestión. Se verifica la estabilidad de la microrred sometiendo a los convertidores a diferentes escenarios de funcionamiento. Estos escenarios pueden ser fluctuaciones en la irradiación, la demanda, el estado de carga de las baterías, los límites máximos de exportación/importación de potencia desde/hacia la microrred hacia/desde la red principal y de la tarifa eléctrica. Adicionalmente, se propone un sistema de almacenamiento de energía en baterías encargado de mantener el equilibrio de potencia en el bus de DC de la microrred que permite aprovechar las fuentes de generación renovables presentes en la microrred y maximizar el tiempo de servicio de las baterías mediante la aplicación de un algoritmo de carga de las baterías. Este último se ajusta al procedimiento de carga especificado por el fabricante, estableciendo las tasas de carga en función de los escenarios en que la microrred se encuentre. El procedimiento de carga en las baterías es fundamental para garantizar las condiciones adecuadas de operación de las mismas, ya que toman en consideración los parámetros establecidos por el fabricante, como son: tasas de carga/descarga, tensión máxima de carga, temperaturas de operación, etc. / [CAT] Dos dels aspectes crítics en l'operació d'una micro-xarxa són les estratègies de control i gestió de potència implementades, les quals són essencials per proporcionar el seu bon funcionament. L'aplicació adequada de dites estratègies permet compensar els desequilibris de potència causats per la discontinuïtat de la generació i demanda d'energia en les micro-xarxes. En aquest sentit, l'objectiu global de les nomenades estratègies de gestió és equilibrar adequadament el flux de potència en la micro-xarxa mitjançant l'aplicació de diferents algoritmes que permeten complir amb els criteris d'estabilitat, protecció, balanç de potència, transicions, sincronització amb la xarxa i gestió adequada de la micro-xarxa. En el cas de micro-xarxes de potència a petita escala i amb baix nombre de generadors i sistemes d'emmagatzematge distribuïts, les estratègies de control centralitzades ofereixen un alt nivell de flexibilitat per aconseguir funcionalitats avançades en la micro-xarxa i una adequada distribució de la potència entre els convertidors que la conformen. Aquesta tesi s'ha emmarcat al context d'algoritmes de gestió centralitzada de potència d'una micro-xarxa de generació distribuïda en mode de connexió a xarxa. Els algoritmes presentats es poden aplicar als convertidors de potència connectats al bus DC d'una micro-xarxa AC/DC hibrida o en una micro-xarxa de DC, on el despatx de potència és observat i gestionat per un controlador central. Aquest últim adquireix dades del sistema mitjançant una infraestructura de comunicacions i estima la potència que gestionarà cadascun dels convertidors de potència, sistemes d'emmagatzematge i càrregues en funcionament. En aquest estudi es mostren la validació experimental de les estratègies de gestió aplicades en la micro-xarxa des d'un enfocament dels convertidors de potència, de les bateries i les càrregues davant d'aquesta gestió. Es verifica l'estabilitat de la micro-xarxa exposant als convertidors a diferents escenaris de funcionament. Aquest escenaris poden ser fluctuants en la irradiació, la demanda, l'estat de càrrega de les bateries, els límits màxims d'exportació/importació de potència des de/cap a la micro-xarxa cap a/des de la xarxa principal i de la tarifa elèctrica. Addicionalment, es proposa un sistema d'emmagatzematge d'energia en bateries encarregats de mantindre l'equilibri de potència al bus DC de la micro-xarxa i que permet aprofitar les fonts de generació renovables presents en la micro-xarxa i maximitzar el temps de servei de les bateries mitjançant l'aplicació d'un algoritme de càrrega de bateries. Aquest últim s'ajusta al procediment de càrrega especificat pel fabricant, establint les taxes de càrrega en funció dels escenaris en que la micro-xarxa es trobe. El procediment de càrrega a les bateries es fonamental per garantir les condicions adequades d'operació de les mateixes, ja que prenen en consideració els paràmetres establerts pel fabricant, com ara són: taxes de càrrega/descàrrega, tensió màxima de càrrega, temperatures d'operació, etc. / [EN] Two critical aspects in microgrids operation are the control and power management strategies, which are essential for their efficient operation. The adequate application of these strategies allows compensating the power imbalance caused by the discontinuity in the energy generation or changes in the power demand of the microgrid. In this sense, the overall objective of these power management strategies is to keep the power balance between the generation and the demand in the microgrid through the application of different algorithms that fulfill the criteria of stability, protection, smooth transitions and synchronization with the main grid. In the case of small-scale microgrids with a low number of distributed generators and energy storage systems, the centralized control strategies offer a higher level of flexibility to achieve advanced features in the microgrid and for the suitable power sharing between the converters that compose it. This thesis has been focused on centralized power management algorithms of a microgrid working in grid connected mode. These algorithms can be applied to the power converters connected to the DC bus of both hybrid AC/DC and DC microgrids, where the power dispatch is controlled by a central controller which acquires system data through a communication infrastructure and sets the power to be managed by each of the converters under operation. In this thesis, the experimental validation of the power management strategies of the microgrid is presented, from the point of view of the behavior of the power converters, batteries and loads. It is provided with a realistic evaluation under different microgrid operation scenarios. These scenarios were sudden changes of the irradiation, load, state of charge, the maximum power to be exported/imported from/to the microgrid to/from the grid, and the electricity tariff. Additionally, it is proposed a battery energy storage system that keeps the power balance at the DC bus of the microgrid, taking advantage from the renewable energy sources and adjusting the battery energy storage through a suitable charging procedure specified by the manufacturer. The proposed procedure changes the charging parameters of the batteries depending on the microgrid states. Its goal is to extend the service time of batteries and to allow proper energy management in the system. / Salas Puente, RA. (2019). Gestión eficiente de los convertidores de potencia conectados al bus DC de una Microrred híbrida de generación distribuida [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/118658 / TESIS