Rozprawy doktorskie na temat „Control system- AC and DC microgrids”

Les Microgrids sont une excellente solution aux problèmes actuels soulevés par la croissance constante de la demande de charge et la forte pénétration des sources d’énergie renouvelables, qui se traduisent par une modernisation du réseau grâce au concept de “Smart-Grids”. L’impact des sources d’énergie distribuées basées sur l’électronique de puissance est une préoccupation importante pour les systèmes d’alimentation, où la régulation naturelle de la fréquence du système est entravée en raison de la réduction de l’inertie. Dans ce contexte, les réseaux à courant continu (CC) sont considérés comme une solution pertinente, car la nature CC des appareils électroniques de puissance apporte des avantages technologiques et économiques par rapport au courant alternatif (CA). La thèse propose la conception et le contrôle d’une Microgrid hybride AC/DC pour intégrer différentes sources renouvelables, y compris la récupération d’énergie solaire et de freinage des trains, aux systèmes de stockage d’énergie sous forme de batteries et de supercondensateurs et à des charges telles que les véhicules électriques ou d’autres réseaux (AC ou DC), pour un fonctionnement et une stabilité fiables. La stabilisation des tensions des bus du Microgrid et la fourniture de services systèmes sont assurées par la stratégie de contrôle proposée, où une étude de stabilité rigoureuse est réalisée. Un contrôleur non linéaire distribué de bas niveau, basé sur une approche “Systemof- Systems”, est développé pour un fonctionnement correct de l’ensemble du Microgrid. Un supercondensateur est appliqué pour faire face aux transitoires, équilibrant le bus CC du Microgrid et absorbant l’énergie injectée par des sources d’énergie intermittentes et possiblement très fortes comme celle provenant du freinage régénératif de trains ou metros, tandis que la batterie réalise le flux de puissance à long terme. Un contrôle de linéarisation par bouclage dynamique basé sur une analyse par perturbation singulière est développé pour les supercondensateurs et les trains. Des fonctions de Lyapunov sont construites en tenant compte des dispositifs interconnectés au Microgrid pour assurer la stabilité de l’ensemble du système. Les simulations mettent en évidence les performances du contrôle proposé avec des tests de robustesse paramétriques et une comparaison avec le contrôleur linéaire traditionnel. L’approche VSM (Virtual Synchronous Machine) est implémentée dans le Microgrid pour le partage de puissance et l’amélioration de la stabilité de fréquence. Une inertie virtuelle adaptative est proposée, puis la constante d’inertie devient une variable d’état du système qui peut être conçue pour améliorer la stabilité de fréquence et le support inertiel, où l’analyse de stabilité est effectuée. Par conséquent, le VSM est la connexion de liaison entre les côtés DC et AC du Microgrid, où la puissance disponible dans le réseau DC est utilisée pour les services système dans les Microgrids AC. Les résultats de la simulation montrent l’efficacité de l’inertie adaptative proposée, où une comparaison avec la solution de statisme et le contrôle standard est effectuée
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

Battery storage devices have been widely utilized for different applications. However, for high power applications, battery storage systems come with several challenges, such as the thermal issue, low power density, low life span and high cost. Compared with batteries, supercapacitors have a lower energy density but their power density is very high, and they offer higher cyclic life and efficiency even during fast charge and discharge processes. In this dissertation, new techniques for the control and energy management of the hybrid battery-supercapacitor storage system are developed to improve the performance of the system in terms of efficiency, power quality and reliability. To evaluate the findings of this dissertation, a laboratory-scale DC microgrid system is designed and implemented. The developed microgrid utilizes a hybrid lead-acid battery and supercapacitor energy storage system and is loaded under various grid conditions. The developed microgrid has also real-time monitoring, control and energy management capabilities. A new control scheme and real-time energy management algorithm for an actively controlled hybrid DC microgrid is developed to reduce the adverse impacts of pulsed power loads. The developed control scheme is an adaptive current-voltage controller that is based on the moving average measurement technique and an adaptive proportional compensator. Unlike conventional energy control methods, the developed controller has the advantages of controlling both current and voltage of the system. This development is experimentally tested and verified. The results show significant improvements achieved in terms of enhancing the system efficiency, reducing the AC grid voltage drop and mitigating frequency fluctuation. Moreover, a novel event-based protection scheme for a multi-terminal DC power system has been developed and evaluated. In this technique, fault identification and classifications are performed based on the current derivative method and employing an artificial inductive line impedance. The developed scheme does not require high speed communication and synchronization and it transfers much less data when compared with the traditional method such as the differential protection approach. Moreover, this scheme utilizes less measurement equipment since only the DC bus data is required.

Modern power networks incorporate communications and information technology infrastructure into the electrical power system to create a smart grid in terms of control and operation. The smart grid enables real-time communication and control between consumers and utility companies allowing suppliers to optimize energy usage based on price preference and system technical issues. The smart grid design aims to provide overall power system monitoring, create protection and control strategies to maintain system performance, stability and security. This dissertation contributed to the development of a unique and novel smart grid test-bed laboratory with integrated monitoring, protection and control systems. This test-bed was used as a platform to test the smart grid operational ideas developed here. The implementation of this system in the real-time software creates an environment for studying, implementing and verifying novel control and protection schemes developed in this dissertation. Phasor measurement techniques were developed using the available Data Acquisition (DAQ) devices in order to monitor all points in the power system in real time. This provides a practical view of system parameter changes, system abnormal conditions and its stability and security information system. These developments provide valuable measurements for technical power system operators in the energy control centers. Phasor Measurement technology is an excellent solution for improving system planning, operation and energy trading in addition to enabling advanced applications in Wide Area Monitoring, Protection and Control (WAMPAC). Moreover, a virtual protection system was developed and implemented in the smart grid laboratory with integrated functionality for wide area applications. Experiments and procedures were developed in the system in order to detect the system abnormal conditions and apply proper remedies to heal the system. A design for DC microgrid was developed to integrate it to the AC system with appropriate control capability. This system represents realistic hybrid AC/DC microgrids connectivity to the AC side to study the use of such architecture in system operation to help remedy system abnormal conditions. In addition, this dissertation explored the challenges and feasibility of the implementation of real-time system analysis features in order to monitor the system security and stability measures. These indices are measured experimentally during the operation of the developed hybrid AC/DC microgrids. Furthermore, a real-time optimal power flow system was implemented to optimally manage the power sharing between AC generators and DC side resources. A study relating to real-time energy management algorithm in hybrid microgrids was performed to evaluate the effects of using energy storage resources and their use in mitigating heavy load impacts on system stability and operational security.

In un impianto fotovoltaico connesso alla rete elettrica, l’ integrazione di un sistema di accumulo permette di raccogliere l’ energia dal solare nelle ore di minor richiesta di rete (di giorno), ed erogarla nei momenti di bassa produzione e di maggiore richiesta di rete (la sera). In collaborazione con ENGIE Eps, è sorta l’ esigenza di confrontare tre diverse tipologie di accoppiamento delle batterie in un impianto ibrido PV+Batteria connesso alla rete elettrica. La prima architettura è chiamata AC coupling poiché il BESS (Battery Energy Storage System) è connesso tramite opportuni convertitori, direttamente alla rete elettrica. La terza e la seconda architettura sono denominate DC Coupling poiché il BESS è collegato tramite un convertitore o senza, al lato DC dell’ impianto. Il confronto è stato realizzato analizzando i flussi di potenza dell’ impianto facendo riferimento a dati di produzione reali forniti da ENGIE Eps. Più in particolare, sono stati forniti i dati di produzione e di irraggiamento di un impianto reale di potenza massima pari a 285 MW, con storage di capacità pari a 275 MWh. La valutazione della potenza richiesta all’ impianto è stata ottenuta dall’analisi del segnale AGC relativo alla rete nella quale l’ impianto è inserito. Tale segnale `e stato generato a partire da dati di frequenza di rete forniti dall’ azienda. Dall’ analisi precedentemente descritta si è individuata l’ architettura migliore in termini di rendimento, che risulta essere la DC coupling con DC/DC sulla batteria. Nell’ ultima parte della tesi si è inoltre svolto su richiesta di ENGIE Eps, lo studio del controllo dei convertitori relativi all’ architettura in esame. La strategia di controllo individuata è descritta nel dettaglio in questo documento.

DC power distribution has gained popularity in sustainable buildings, renewable energy utilization, transportation electrification and high-efficiency data centers. This dissertation focuses on two aspects of facilitating the application of dc systems: (a) system-level control to improve load sharing, voltage regulation and efficiency; (b) design of a high-efficiency interface converter to connect dc microgrids with the existing low-voltage ac distributions, with a special focus on common-mode (CM) voltage attenuation. Droop control has been used in dc microgrids to share loads among multiple sources. However, line resistance and sensor discrepancy deteriorate the performance. The quantitative relation between the droop voltage range and the load sharing accuracy is derived to help create droop design guidelines. DC system designers can use the guidelines to choose the minimum droop voltage range and guarantee that the sharing error is within a defined range even under the worst cases. A nonlinear droop method is proposed to improve the performance of droop control. The droop resistance is a function of the output current and increases when the output current increases. Experiments demonstrate that the nonlinear droop achieves better load sharing under heavy load and tighter bus voltage regulation. The control needs only local information, so the advantages of droop control are preserved. The output impedances of the droop-controlled power converters are also modeled and measured for the system stability analysis. Communication-based control is developed to further improve the performance of dc microgrids. A generic dc microgrid is modeled and the static power flow is solved. A secondary control system is presented to achieve the benefits of restored bus voltage, enhanced load sharing and high system efficiency. The considered method only needs the information from its adjacent node; hence system expendability is guaranteed. A high-efficiency two-stage single-phase ac-dc converter is designed to connect a 380 V bipolar dc microgrid with a 240 V split-phase single-phase ac system. The converter efficiencies using different two-level and three-level topologies with state-of-the-art semiconductor devices are compared, based on which a two-level interleaved topology using silicon carbide (SiC) MOSFETs is chosen. The volt-second applied on each inductive component is analyzed and the interleaving angles are optimized. A 10 kW converter prototype is built and achieves an efficiency higher than 97% for the first time. An active CM duty cycle injection method is proposed to control the dc and low-frequency CM voltage for grounded systems interconnected with power converters. Experiments with resistive and constant power loads in rectification and regeneration modes validate the performance and stability of the control method. The dc bus voltages are rendered symmetric with respect to ground, and the leakage current is reduced. The control method is generalized to three-phase ac-dc converters for larger power systems.
Ph. D.

With ambitions of reducing the environmental pollution, power systems integrate larger shares of Renewable Energy Sources (RES) to phase out conventional thermal and nuclear generators. Since RES (such as wind and solar power) are connected to the grid through power electronics devices, they do not inherently contribute to system inertia. With decreasing inertia, the Instantaneous Frequency Deviation (IFD), which follows a power unbalance, is significantly impacted. Frequency Containment Reserves (FCR) are designed to provide a fast dynamic response, counteract power imbalances and stabilize the frequency within a short time interval. Besides inertia, the significant factors affecting frequency behavior are the available amount of FCR and the capability of their fast and stable response. System operators define the list of requirements that a generating unit has to satisfy to participate in FCR. Generators, which are the major part of FCR, have different governors and turbines properties. This study assesses the dynamical performance of typical generators in both open-loop testing and closed-loop varying inertia systems. The goal is to evaluate if specific FCR requirements present a sufficient condition for the desired response, and which governor properties are capable of satisfying them. As an additional, and sometimes necessary, support to FCR, HVDC interconnections are utilized in the form of Emergency Power Control (EPC). This thesis investigates which of the EPC methods performs appropriately in terms of IFD improvement, closed-loop stability, and power and energy provided. The analysis is a continuation from the previous investigation on FCR, and mainly compare two EPC methods related to Nordic Power System (NPS) test case: ramp/step method which is currently implemented in the NPS, and droop frequency-based EPC, proposed by this study for the future operation in the NPS. Apart from ensuring a proper system frequency response, the influence of implemented HVDC supplementary active power control is analyzed to rotor angle stability. In further, this thesis presents a comprehensive analysis of the impact that proposed HVDC supplementary power control has on the linearized dynamics of power systems. By building a generic system, this analytical study is the first of its kind that includes both higher order generator dynamics, and local angle/frequency input of the controller. The methodological approach here analytically formulates the impact the HVDC supplementary control has mainly on the generator synchronizing and damping torque components. The positive impact of the droop frequency-based HVDC power support is highlighted using both single and multi-machine systems. In that way, the implementation of desired droop frequency-based HVDC control to mainly improve system frequency is motivated furthermore. It shows that a proper HVDC supplementary control may impose the various positive impacts for future variable and low inertia scenarios, and ensure a proper power system sustainability.

QC 20200907

multiDC - Advanced Control and Optimization Methods for AC and HVDC Grids

Energy storage attracts attention nowadays due to the critical role it will play in the power generation and transportation sectors. Electric vehicles, as moving energy storage, are going to play a key role in the terrestrial transportation sector and help reduce greenhouse emissions. Bulk hybrid energy storage will play another critical role for feeding the new types of pulsed loads on ship power systems. However, to ensure the successful adoption of energy storage, there is a need to control and optimize the charging/discharging process, taking into consideration the customer preferences and the technical aspects. In this dissertation, novel control and optimization algorithms are developed and presented to address the various challenges that arise with the adoption of energy storage in the electricity and transportation sectors. Different decentralized control algorithms are proposed to manage the charging of a mass number of electric vehicles connected to different points of charging in the power distribution system. The different algorithms successfully satisfy the preferences of the customers without negatively impacting the technical constraints of the power grid. The developed algorithms were experimentally verified at the Energy Systems Research Laboratory at FIU. In addition to the charge control of electric vehicles, the optimal allocation and sizing of commercial parking lots are considered. A bi-layer Pareto multi-objective optimization problem is formulated to optimally allocate and size a commercial parking lot. The optimization formulation tries to maximize the profits of the parking lot investor, as well as minimize the losses and voltage deviations for the distribution system operator. Sensitivity analysis to show the effect of the different objectives on the selection of the optimal size and location is also performed. Furthermore, in this dissertation, energy management strategies of the onboard hybrid energy storage for a medium voltage direct current (MVDC) ship power system are developed. The objectives of the management strategies were to maintain the voltage of the MVDC bus, ensure proper power sharing, and ensure proper use of resources, where supercapacitors are used during the transient periods and batteries are used during the steady state periods. The management strategies were successfully validated through hardware in the loop simulation.

The development of voltage source converter (VSC) based high voltage direct current (HVDC) transmission has progressed rapidly worldwide over the past few years. The UK transmission system is going through a radical change in the energy landscape which requires a number of VSC HVDC installations to connect large Round 3 windfarms and for interconnections to other countries. For bulk power long distance transmission, VSC HVDC technology offers flexibility and controllability in power flow, which can benefit and strengthen the conventional AC system. However, the associated uncertainties and potential problems need to be identified and addressed. To carry out this research, integrated mathematical dynamic AC/DC system models are developed in this thesis for small disturbance stability analysis. The fidelity of this research is further increased by developing a dynamic equivalent representative Great Britain (GB) like system, which is presented as a step-by-step procedure with the intention of providing a road map for turning a steady-state load flow model into a dynamic equivalent. This thesis aims at filling some of the gaps in research regarding the integration of VSC HVDC technology into conventional AC systems. The main outcome of this research is a systematic assessment of the effects of VSC controls on the stability of the connected AC system. The analysis is carried out for a number of aspects which mainly orbit around AC/DC system stability issues, as well as the control interactions between VSC HVDC and AC system components. The identified problems and interactions can mainly be summarized into three areas: (1) the effect of VSC HVDC controls on the AC system electromechanical oscillations, (2) the potential control interactions between VSC HVDC and flexible alternating current transmission systems (FACTS) and (3) the active power support capability of VSC HVDC for improving AC system stability. The effect of VSC controls on the AC system dynamics is assessed with a parametric sensitivity analysis to highlight the trade-offs between candidate VSC HVDC outer control schemes. A combination of analysis techniques including relative gain array (RGA) and modal analysis, is then applied to give an assessment of the interactions – within the plant model and the outer controllers – between a static synchronous compensator (STATCOM) and a VSC HVDC link operating in the same AC system. Finally, a specific case study is used to analyse the capability of VSC HVDC for providing active power support to the connected AC system through a proposed frequency droop active power control strategy.

Les nouvelles politiques adoptées par les autorités nationales ont encouragé pendant les dernières années l'intégration à grande échelle des systèmes d'énergie renouvelable (RES). L'intégration à grande échelle des RES aura inévitablement des conséquences sur le réseau de transport d'électricité tel qu'il est conçu aujourd'hui, car le transport de l'électricité massif sur de longues distances pourrait amener les réseaux de transport à fonctionner près de leurs limites, réduisant ainsi leurs marges de sécurité. Des systèmes de transport d’électricité plus complexes seront donc nécessaires.Dans ce scénario, les systèmes de transmission à Courant Continu Haute Tension (HVDC) constituent la solution la plus intéressante pour le renforcement et l'amélioration des réseaux à Courant Alternatif (AC) existants, non seulement en utilisant des configurations point à point, mais aussi dans des configurations multi-terminales. L'introduction des systèmes HVDC aboutira à terme à un réseau électrique hybride haute tension AC/DC, qui doit être analysé comme un système unique afin de mieux comprendre les interactions entre le réseau AC et le réseau DC.Cette thèse porte sur l'analyse de la stabilité transitoire des systèmes de transmission électrique hybrides AC/DC. Plus particulièrement, deux questions ont été abordées: Quel est l'impact d'un défaut du réseau DC sur la stabilité transitoire du réseau AC? Comment est-il possible de se servir des systèmes de transmission DC en tant qu'actionneurs afin d'améliorer la stabilité transitoire AC ?Dans la première partie de ce travail, les modèles mathématiques du réseau hybride AC/DC sont décrits ainsi que les outils nécessaires à l'analyse du système en tenant compte de sa nature non linéaire. Ensuite, une analyse approfondie de la stabilité transitoire du réseau électrique dans le cas particulier d'un court-circuit dans le réseau DC et l'exécution des stratégies de protection correspondantes sont effectuées. En complément, des indicateurs de stabilité et des outils pour dimensionner les futurs réseaux de la MTDC afin de respecter les contraintes des stratégies de protection existantes sont proposés.La deuxième partie de la thèse porte sur les propositions de commande pour la modulation des références de puissance des systèmes de transmission HVDC dans le but d'améliorer la stabilité transitoire du système AC connecté à ce réseau DC. Tout d'abord, nous axons notre étude sur le contrôle non linéaire des liaisons HVDC point à point dans des liaisons hybrides AC/DC. La compensation rapide des perturbations de puissance, l'injection de puissance d'amortissement et l'injection de puissance de synchronisation sont identifiées comme des mécanismes par lesquels les systèmes HVDC peuvent améliorer les marges de stabilité des réseaux AC.Enfin, une stratégie de contrôle pour l'amélioration de la stabilité transitoire par injection de puissance active dans par un réseau MTDC est proposée. Grâce à la communication entre les stations, la commande décentralisée proposée injecte la puissance d'amortissement et de synchronisation entre chaque paire de convertisseurs en utilisant uniquement des mesures au niveau des convertisseurs. L'implémentation proposée permet d'utiliser au maximum la capacité disponible des convertisseurs en gérant les limites de puissance d'une manière décentralisée
The new policy frameworks adopted by national authorities has encouraged the large scale-integration of Renewable Energy Systems (RES) into bulk power systems. The large-scale integration of RES will have consequences on the electricity transmission system as it is conceived today, since the transmission of bulk power over long distances could lead the existing transmission systems to work close to their limits, thus decreasing their dynamic security margins. Therefore more complex transmissions systems are needed.Under this scenario, HVDC transmission systems raise as the most attractive solution for the reinforcement and improvement of existing AC networks, not only using point-to-point configurations, but also in a Multi-Terminal configuration. The introduction of HVDC transmission systems will eventually result in a hybrid high voltage AC/DC power system, which requires to be analyzed as a unique system in order to understand the interactions between the AC network and the DC grid.This thesis addresses the transient stability analysis of hybrid AC/DC electric transmission systems. More in particular, two questions sought to be investigated: What is the impact of a DC contingency on AC transient stability? How can we take advantage of the of DC transmission systems as control inputs in order to enhance AC transient stability?In the first part of this work, the mathematical models of the hybrid AC/DC grid are described as well as the necessary tools for the analysis of the system taking into account its nonlinear nature. Then, a thorough analysis of transient stability of the power system in the particular case of a DC fault and the execution of the corresponding protection strategies is done. As a complement, stability indicators and tools for sizing future MTDC grids in order to respect the constraints of existing protection strategies are proposed.The second part of the thesis addresses the control proposals for the modulation of power references of the HVDC transmission systems with the purpose of transient stability enhancement of the surrounding AC system. Firstly, we focus our study in the nonlinear control of point-to-point HVDC links in hybrid corridors. Fast power compensation, injection of damping power and injection of synchronizing power are identified as the mechanisms through which HVDC systems can improve stability margins.Finally, a control strategy for transient stability enhancement via active power injections of an MTDC grid is proposed. Using communication between the stations, the proposed decentralized control injects damping and synchronizing power between each pair of converters using only measurements at the converters level. The proposed implementation allows to fully use the available headroom of the converters by dealing with power limits in a decentralized way

Isolated electric energy generation systems are often needed to supply electric loads where the electrical network is not available. This could be caused due to geographic isolation, the necessity of load mobility, demanded values of voltage and current that are not compatible with the local networks, etc. This makes the design and construction of stand-alone energy generation systems a must.

Modern designs are being pushed towards cleaner technologies. The experience has shown that the usual methods employed to produce electrical energy are not sustainable, especially because of environmental concerns. Usual stand-alone energy generation systems employ batteries and fuel engines. Batteries offer a cheap mean to feed the generation system but need rigorous maintenance routines, the substances used in their construction are strong pollutants, offer relatively low durability and the ratio charge time/discharge time is too high. Fuel engines extract their energy from petroleum based fuels, and as its well known, pollute their surrounding environment in several ways producing smoke, noise and heat.

Polymer electrolyte membrane type fuel cells are among the new technologies that are being considered as a good alternative to the traditional power sources used for stand-alone energy generation systems.
AIthough the basic principles of operation of the fuel cells are known since 1839, this is a technology that is far from being mature. More work needs to be done in order to make of the fuel cells systems with, high reliability, with maximum efficiency, and capable of providing electrical energy with quality comparable to the quality achieved using usual methods.

The problems when working with fuel cells can be split in two big groups of interest, the first, being the handling and control of the electrochemical variables, and the second, the handling and control of the electrical variables taking care of the limits imposed by the dynamics of the fuel cell unit. This work deals with the second group of concerns, looking at the fuel cell as a black-box dc power supply with certain current/voltage characteristics. The energy provided by the fuel cells needs to be conditioned to the levels and characteristics required by the loads to be fed. In Europe, for single-phase ac loads, the specifications are a sinusoidal output voltage with 230 V ac rms and a frequency of 50 Hz. This work presents the the analysis, design, construction, and control of the electric energy conditioning system for a polymer eIectrolyte membrane type fuel cell to act as an stand-alone dc-ac inverter to feed linear or nonlinear loads with big variations.
Los sistemas de generación de energía eléctrica "en isla" son necesarios en muchas ocasiones para alimentar cargas donde la red eléctrica no está disponible. Esto puede deberse a diversos factores como: aislamiento geográfico, necesidad de movilidad de la carga, requerimientos de corriente y voltaje que no son compatibles con las redes locales, etc. Todas estas razones hacen del diseño y construcción de sistemas autónomos de generación de energía una necesidad.
En la actualidad, los diseños de este tipo de dispositivos están tendiendo hacia tecnologías más limpias.
La experiencia ha enseñado que los métodos habituales para producir energía eléctrica no son los más apropiados, especialmente por motivos medioambientales. Los sistemas autónomos de generación de energía eléctrica típicos utilizan baterías y máquinas de combustión. Las baterías ofrecen una fuente barata para alimentar el sistema de generación de energía eléctrica, pero necesitan de rigurosas rutinas de mantenimiento, algunas de las sustancias utilizadas en su construcción son altamente contaminantes, ofrecen una relativamente baja durabilidad y la razón tiempo de carga/tiempo de descarga es grande.
Por otro lado, las máquinas de combustión extraen la energía de combustibles a base de petróleo, como es bien conocido, contaminan el entorno produciendo humo, ruido y calor.
Las pilas de combustible de membrana de electrolito polimérico están entre las nuevas tecnologías que se consideran como una buena alternativa a las fuentes que se utilizan usualmente para alimenta sistemas autónomos de generación de energía. Aunque los principios básicos de operación de las pilas de combustible son conocidos desde 1839, esta es una tecnología que está aún lejos de pode considerarse madura. Aún es necesario realizar más esfuerzos con el objetivo de hacer de las pilas de combustible fuentes de energía de alta confiabilidad, de máxima eficiencia y capaces de proveer energía con niveles de calidad comparables a los alcanzados al utilizar los métodos tradicionales.
La problemática que se presenta al trabajar con pilas de combustible puede ser dividida en dos grandes grupos de interés, el primero, sería el control de las variables electroquímicas, y el segundo, el manejo control de las variables eléctricas tomando en cuenta los límites impuestos por la dinámica de la pila de combustible. Éste trabajo trata con el segundo, viendo la pila de combustible como una "caja negra" que constituye una fuente de potencia de corriente continua con ciertas características particulares de voltaje/corriente. La energía provista por la pila de combustible debe ser acondicionada a los niveles características requeridas por las cargas a ser alimentadas. En Europa, para sistemas de monofásico de corriente alterna, las especificaciones son un voltaje sinusoidal con 230 V efectivos y una frecuencia de 50 Hz. Éste trabajo presenta el análisis, diseño, construcción y control del sistema de acondicionamiento de energía eléctrica para una pila de combustible de membrana de electrolito polimérico, que actúa como un sistema autónomo de inversión de corriente continua-corriente alterna para alimentar cargas lineales o no lineales que pueden experimentar grandes variaciones.

This thesis introduces a new method especially designed to control the instantaneous power in voltage sourced converters operating under unbalanced conditions, including positive, negative and zero sequence content. A transformation technique, labelled mno transformation, was developed to enable the decomposition of the total instantaneous power flowing on three-phase transmission topologies into constant and oscillating terms. It is applied to three-wire and four-wire schemes, especially accommodating zero sequence unlike previous approaches. Classical and modern electric power theories are presented, particularly focusing on their definitions for adverse AC scenarios. The main mathematical transformations conceived to analyze such situations are summarized, showing their respective advantages and disadvantages. An enhanced instantaneous power theory is introduced. The novel proposed power equations, named mno instantaneous power components, expands the application of the p-q theory, which is attached to the αβ0 transformation. The mno instantaneous power theory is applied to develop an innovative power control method for grid connected voltage sourced converters in order to minimize power oscillations during adverse AC scenarios, particularly with zero sequence content. The method permits to sustain constant instantaneous three-phase power during unbalanced conditions by controlling independently the constant and the oscillating terms related to the instantaneous power. The effectiveness of the proposed control approach and the proposed power conditioning scheme was demonstrated using electromagnetic transient simulation of a VSC connected to an AC system.
May 2017

Le développement rapide de la production d'énergie renouvelable intermittente et des liaisons HVDC entraîne une augmentation importante du taux de pénétration des convertisseurs statiques dans les réseaux de transport. Aujourd'hui, les convertisseurs statiques ont pour fonction principale d'injecter une puissance dans le réseau tout en s'appuyant sur des machines synchrones qui garantissent tous les besoins du système électrique. Ce mode de fonctionnement est appelé «Grid-following». Les convertisseurs contrôlés en Grid-following ont plusieurs limitations: leur incapacité à fonctionner en mode autonome, leurs problèmes de stabilité dans des réseaux faibles et en cas de défaut ainsi que leur effet négatif sur l'inertie équivalent du système. Pour relever ces défis, le contrôle en Grid-forming est une bonne solution pour répondre aux besoins du système électrique et permettre un fonctionnement stable et sûr du système même avec un taux de pénétration des convertisseurs statique de 100%. Tout d'abord, trois stratégies de contrôle en Grid-forming sont proposées pour garantir quatre fonctionnalités principales: contrôle de tension, contrôle de puissance, émulation d'inertie et le support de la fréquence. La dynamique et la robustesse du système basées sur chaque contrôle ont été analysées et discutées. Ensuite, selon la topologie du convertisseur, la connexion avec le réseau AC peut nécessiter des filtres et des boucles de contrôle supplémentaires. Dans le cadre de cette thèse, deux topologies de convertisseur ont été envisagées (VSC à 2-niveaux et VSC-MMC) et l'implémentation associée à chacune a été discutée. Enfin, les questions de la protection des convertisseurs Grid-forming contre les surintensités et leur synchronisation post-défaut ont été étudiées, puis, des algorithmes de limitation de courant et de resynchronisation ont été proposés pour améliorer la stabilité transitoire du système. Un banc d'essai a été développé pour confirmer les approches théoriques proposées
The rapid development of intermittent renewable generation and HVDC links yields an important increase of the penetration rate of power electronic converters in the transmission systems. Today, power converters have the main function of injecting power into the main grid, while relying on synchronous machines that guaranty all system needs. This operation mode of power converters is called "Grid-following". Grid-following converters have several limitations: their inability to operate in a standalone mode, their stability issues under weak-grids and faulty conditions and their negative side effect on the system inertia.To meet these challenges, the grid-forming control is a good solution to respond to the system needs and allow a stable and safe operation of power system with high penetration rate of power electronic converters, up to a 100%. Firstly, three grid-forming control strategies are proposed to guarantee four main features: voltage control, power control, inertia emulation and frequency support. The system dynamics and robustness based on each control have been analyzed and discussed. Then, depending on the converter topology, the connection with the AC grid may require additional filters and control loops. In this thesis, two converter topologies have been considered (2-Level VSC and VSC-MMC) and the implementation associated with each one has been discussed. Finally, the questions of the grid-forming converters protection against overcurrent and their post-fault synchronization have been investigated, and then a hybrid current limitation and resynchronization algorithms have been proposed to enhance the transient stability of the system. At the end, an experimental test bench has been developed to confirm the theoretical approach

The Queensland University of Technology (QUT) allows the presentation of a thesis for the Degree of Doctor of Philosophy in the format of published or submitted papers, where such papers have been published, accepted or submitted during the period of candidature. This thesis is composed of seven published/submitted papers, of which one has been published, three accepted for publication and the other three are under review. This project is financially supported by an Australian Research Council (ARC) Discovery Grant with the aim of proposing strategies for the performance control of Distributed Generation (DG) system with digital estimation of power system signal parameters. Distributed Generation (DG) has been recently introduced as a new concept for the generation of power and the enhancement of conventionally produced electricity. Global warming issue calls for renewable energy resources in electricity production. Distributed generation based on solar energy (photovoltaic and solar thermal), wind, biomass, mini-hydro along with use of fuel cell and micro turbine will gain substantial momentum in the near future. Technically, DG can be a viable solution for the issue of the integration of renewable or non-conventional energy resources. Basically, DG sources can be connected to local power system through power electronic devices, i.e. inverters or ac-ac converters. The interconnection of DG systems to power system as a compensator or a power source with high quality performance is the main aim of this study. Source and load unbalance, load non-linearity, interharmonic distortion, supply voltage distortion, distortion at the point of common coupling in weak source cases, source current power factor, and synchronism of generated currents or voltages are the issues of concern. The interconnection of DG sources shall be carried out by using power electronics switching devices that inject high frequency components rather than the desired current. Also, noise and harmonic distortions can impact the performance of the control strategies. To be able to mitigate the negative effect of high frequency and harmonic as well as noise distortion to achieve satisfactory performance of DG systems, new methods of signal parameter estimation have been proposed in this thesis. These methods are based on processing the digital samples of power system signals. Thus, proposing advanced techniques for the digital estimation of signal parameters and methods for the generation of DG reference currents using the estimates provided is the targeted scope of this thesis. An introduction to this research – including a description of the research problem, the literature review and an account of the research progress linking the research papers – is presented in Chapter 1. One of the main parameters of a power system signal is its frequency. Phasor Measurement (PM) technique is one of the renowned and advanced techniques used for the estimation of power system frequency. Chapter 2 focuses on an in-depth analysis conducted on the PM technique to reveal its strengths and drawbacks. The analysis will be followed by a new technique proposed to enhance the speed of the PM technique while the input signal is free of even-order harmonics. The other techniques proposed in this thesis as the novel ones will be compared with the PM technique comprehensively studied in Chapter 2. An algorithm based on the concept of Kalman filtering is proposed in Chapter 3. The algorithm is intended to estimate signal parameters like amplitude, frequency and phase angle in the online mode. The Kalman filter is modified to operate on the output signal of a Finite Impulse Response (FIR) filter designed by a plain summation. The frequency estimation unit is independent from the Kalman filter and uses the samples refined by the FIR filter. The frequency estimated is given to the Kalman filter to be used in building the transition matrices. The initial settings for the modified Kalman filter are obtained through a trial and error exercise. Another algorithm again based on the concept of Kalman filtering is proposed in Chapter 4 for the estimation of signal parameters. The Kalman filter is also modified to operate on the output signal of the same FIR filter explained above. Nevertheless, the frequency estimation unit, unlike the one proposed in Chapter 3, is not segregated and it interacts with the Kalman filter. The frequency estimated is given to the Kalman filter and other parameters such as the amplitudes and phase angles estimated by the Kalman filter is taken to the frequency estimation unit. Chapter 5 proposes another algorithm based on the concept of Kalman filtering. This time, the state parameters are obtained through matrix arrangements where the noise level is reduced on the sample vector. The purified state vector is used to obtain a new measurement vector for a basic Kalman filter applied. The Kalman filter used has similar structure to a basic Kalman filter except the initial settings are computed through an extensive math-work with regards to the matrix arrangement utilized. Chapter 6 proposes another algorithm based on the concept of Kalman filtering similar to that of Chapter 3. However, this time the initial settings required for the better performance of the modified Kalman filter are calculated instead of being guessed by trial and error exercises. The simulations results for the parameters of signal estimated are enhanced due to the correct settings applied. Moreover, an enhanced Least Error Square (LES) technique is proposed to take on the estimation when a critical transient is detected in the input signal. In fact, some large, sudden changes in the parameters of the signal at these critical transients are not very well tracked by Kalman filtering. However, the proposed LES technique is found to be much faster in tracking these changes. Therefore, an appropriate combination of the LES and modified Kalman filtering is proposed in Chapter 6. Also, this time the ability of the proposed algorithm is verified on the real data obtained from a prototype test object. Chapter 7 proposes the other algorithm based on the concept of Kalman filtering similar to those of Chapter 3 and 6. However, this time an optimal digital filter is designed instead of the simple summation FIR filter. New initial settings for the modified Kalman filter are calculated based on the coefficients of the digital filter applied. Also, the ability of the proposed algorithm is verified on the real data obtained from a prototype test object. Chapter 8 uses the estimation algorithm proposed in Chapter 7 for the interconnection scheme of a DG to power network. Robust estimates of the signal amplitudes and phase angles obtained by the estimation approach are used in the reference generation of the compensation scheme. Several simulation tests provided in this chapter show that the proposed scheme can very well handle the source and load unbalance, load non-linearity, interharmonic distortion, supply voltage distortion, and synchronism of generated currents or voltages. The purposed compensation scheme also prevents distortion in voltage at the point of common coupling in weak source cases, balances the source currents, and makes the supply side power factor a desired value.

Dissertação de mestrado integrado em Engenharia Eletrónica e de Computadores
A crescente motivação para a utilização de fontes de energias renováveis prende-se essencialmente com a redução das emissões de CO2 e com a melhoria da qualidade de vida da humanidade. Isto é especialmente verdade em comunidades pequenas, isoladas e autónomas, onde o acesso a fontes de energia renováveis é a única solução para satisfazer as suas necessidades energéticas. Nesta dissertação são estudados os sistemas de eletrónica de potência e os restantes constituintes de uma microrrede que permitem o funcionamento de um Centro Social Sustentável numa aldeia rural isolada, sem ligação a uma rede de energia elétrica. Estes incluem a produção de energia elétrica por fontes renováveis e por um motor-gerador (a biodiesel), um sistema de armazenamento de energia, e um conjunto de cargas locais. Especificamente, nesta dissertação é desenvolvido o sistema de alimentação da microrrede isolada. Este sistema é constituído por um conversor de potência CC-CA de três estágios que pode ser dividido num conversor CC-CC em ponte completa isolado de alta-frequência do tipo buck, controlado a MOSFETs e por um conversor CC-CA em ponte completa controlado a IGBTs. O sistema de controlo dos conversores é completamente digital e tem por base o DSC TMS320F28335 da Texas Instruments. Para validar a solução apresentada, foi desenvolvido um protótipo do sistema que engloba os dois conversores mencionados. Posteriormente, foram realizados diversos testes experimentais em ambiente laboratorial ao protótipo implementado. Este foi testado com uma tensão de entrada de 30 V que foi elevada para uma tensão constante de 160 V por intermédio do conversor CC-CC. Esta tensão retificada é posteriormente convertida para uma tensão sinusoidal de 132 V de pico a uma frequência de 50 Hz, através do inversor monofásico. Nestas condições, foi comprovado o conceito e validado o funcionamento do sistema de alimentação da microrrede isolada. Ao mesmo tempo, os resultados obtidos permitiram validar tanto o bom funcionamento do controlador proporcional-integral aplicado ao conversor CC-CC, como o controlador preditivo deadbeat aplicado ao conversor CC-CA.
The increasing motivation for the use of renewable energy sources is mainly due to the reduction of CO2 emissions and to the improvement of mankind life quality. This is particularly true in small, isolated and standalone communities, where the access to renewable energy sources may be the only solution to meet their energy needs. This master thesis aims to study the power electronic systems that allow the operation of a sustainable Social Centre in an islanded rural village without connection to a main power grid. These include the production of electricity from renewable sources and a motorgenerator set (biodiesel), an energy storage system, and a local power supply. Specifically in this work, the developed power converter needed for the microgrid power system is a three stage DC-AC converter. This power converter can be separated into a DC-DC full-bridge high-frequency isolated MOSFET-based converter and a DC-AC full-bridge IGBT-based converter. The control system is fully digital and implemented using the Texas Instruments microcontroller DSC TMS320F28335. In order to validate the presented solution was developed a prototype of the microgrid power system constituted by the converters mentioned and several experimental tests were carried out in laboratory environment. The developed prototype was tested with a 30 V DC input voltage which resulted in a constant 160 V DC voltage controlled by the DC-DC MOSFET-based converter. This rectified voltage is then converted to a sinusoidal wave with 132 V of peak at 50 Hz frequency by the single-phase inverter. Under these conditions, the concept has been proven and the proper functioning of the island microgrid power system was validated. At the same time, the obtained results allowed to validate not only the proper functioning of the proportional integral controller applied to the DC-DC converter but also the deadbeat predictive controller applied to the DC-AC converter.

University of Technology Sydney. Faculty of Engineering and Information Technology.
The future trend of the power system is to ensure reliable, flexible, affordable and efficient power supply for customers with lower emissions. Conventional AC or DC microgrid suffers from increased losses and lower efficiency due to several AC-DC and DC-AC conversions. Therefore, hybrid microgrid (HMG) is getting popular to meet the growing penetration of modern DC loads and renewable energy sources with DC outputs into the existing AC power systems. The main objective of this dissertation is to develop and implement improved power management and control strategy to improve the performance of the hybrid microgrid. The first study proposes an improved power management and control coordination strategy for an autonomous HMG. The HMG considered in this part consists of multiple AC and DC sub-microgrids (SMGs) with different voltage levels. The hierarchical coordination of power management and control strategy for the autonomous HMG is introduced and analyzed. The designed system incorporates both the primary and secondary control levels to ensure a seamless and accurate transfer of power among the SMGs. A new technique for transferring power with a focus on the secondary control level is presented. The second study proposed in this thesis is a novel approach of distributed coordination control for multiple SMGs within the HMG. The traditional control method for power flow management among AC and DC SMGs is based on the proportional power-sharing principle. The proposed method suggests a distributed control system that ensures total controllability for the parallel interlinking converters (ILCs). It overcomes the total dependency on a specific variable for power exchange. The proposed method not only enables control of the power flow between SMGs but also ensures the continuity of power transfer in the event of a single SMG failure. The third study in this work focuses on coordinating the control and power management strategy for the multiple parallel ILCs that link the AC and DC SMGs together. The proposed new approach aims to manage the power flow across the HMG while regulating the voltage and frequency for the SMGs as part of the process. The main objective of the proposed method is to keep the HMG in autonomous operation with active power proportionally shared among its ILCs and distributed sources. The presented outer control loop is a modified arrangement that could not only ensure accurate power-sharing but also suppresses the circulating current at the DC side.

Dissertação de mestrado em Engenharia Eletrónica e de Computadores
A energia elétrica tem um papel extremamente importante no desenvolvimento de uma região, bem como na melhoria da qualidade de vida do ser humano. É assim fundamental eletrificar regiões isoladas e com elevadas taxas de subdesenvolvimento. A eletrificação da grande maioria destas regiões isoladas, passa pela produção local de energia recorrendo a fontes de energia renovável, quer por razões logísticas quer ambientais. Nesta dissertação são apresentados e descritos, os sistemas de eletrónica de potência que permitem implementar uma microrrede isolada com produção a partir de fontes renováveis e capacidade de armazenamento local de energia. O foco principal deste trabalho é o desenvolvimento e validação do sistema responsável pela implementação de uma microrrede isolada monofásica com tensão alternada sinusoidal de 230 V/50 Hz. O desenvolvimento do sistema responsável pela produção e armazenamento de energia encontra-se descrito numa dissertação com o tema complementar, realizada pelo colega de laboratório João Silva. Para a realização da presente dissertação foi necessário o estudo, dimensionamento e implementação do conversor CC-CA de três estágios. Este conversor pode ser dividido num conversor CC-CC em ponte completa isolado de alta frequência do tipo buck e num conversor CC-CA em ponte completa. Para ambos os conversores foi desenvolvido um controlador digital, sendo utilizado um controlador Proporcional Integral no conversor CC-CC e um controlador Preditivo no conversor CC-CA monofásico. Depois de desenvolvido o protótipo do sistema de alimentação da mirorrede isolada, foram realizados alguns ensaios laboratoriais de modo a validar a solução apresentada. Os ensaios realizados, permitiram analisar o comportamento do sistema para três tipos de carga diferentes: carga linear puramente resistiva; carga linear predominantemente indutiva; e carga não linear do tipo retificador com filtro capacitivo. Por último, foram ainda realizados alguns ensaios ao sistema desenvolvido na presente dissertação, interligado com o sistema complementar desenvolvido pelo colega João Silva. Perante os resultados obtidos, foi possível comprovar o conceito e validar o funcionamento do sistema de alimentação da microrrede isolada.
Electric power plays an essential role in the development of an isolated rural region, as well as in improving the overall human well-being. Therefore, the electrification of isolated and underdeveloped zones is a matter of extreme importance. Due to either logistic or environmental reasons, in most of these isolated regions, the required electrification involves the local energy production through renewable energy sources. In this dissertation, the power electronics system that allows the implementation of an islanded microgrid with local production and storage of energy is presented and described in detail. The main focus of this work is the development and validation of the power electronics system to implement a single-phase islanded microgrid with 230 V/50 Hz sinusoidal voltage. The electronic system responsible for energy production and storage is being developed within the dissertation work of the classmate João Silva. In order to complete this dissertation, it was required to study, sizing and implement a three-stage DC-AC converter. This converter is composed of a high-frequency isolated buck type full-bridge DC-DC converter and of a single-phase full-bridge DC-AC converter. A digital controller was developed for both converters, namely a Proportional Integral in the case of the DC-DC converter and a Predictive Deadbeat in the case of the DC-AC converter. After the development of the islanded microgrid power system prototype, some laboratorial tests were carried out to analyze the response of the presented solution. The elaborated tests allowed the analysis of the converters toward three different loads: a purely resistive linear load; a predominantly inductive linear load; and a rectifier with capacitive filter nonlinear load. Finally, a few tests were done on the system developed in the presented dissertation combined with the complementary system developed by João Silva. Concerning the final results, it was possible to prove the concept and to validate the operation of the islanded microgrid power system.

碩士
國立雲林科技大學
電子工程系
103
With the changing times and the advancement of technology, electronic products continue to developat a rapid pace. Every electronic product needs supply driver. This research designs LED lamp power management system with AC to DC power conversion mode. This researchadopt the process of TSMC 0.25 um CMOS HIGH VOLTAGE MIXED SIGNAL to complete two chips, and be off the assembly line of TSMC through CIC. The chips are (1) with level shift High-voltage converter chip (2) AC / DC voltage (current) feedback PWM controller chip. With level shift High-voltage converter chip contains huge Power Management System, which saves the costs and reduces the volume. The chip’s input voltage is designed below 60V. It can automatically adjust the load output voltage. AC / DC voltage (current) feedback PWM controller chiphas been used in the design of buck system in Flyback circuit architecture. It can use primary or secondary ring mode feedback loop to control the output voltage and current and over-voltage and over-current protection, and the frequency can be adjusted in accordance with the design requirements. In addition, this research accomplishes two system circuits which are (1) AC to DC voltage automatic tracking system and (2) Isolated AC-DC converter step-down system. AC to DC voltage automatic tracking system avoids applyingmismatched voltages tandem to different LED driving. Too much driving voltage will cause excess loss of LED or drive circuit. As a result, this research designs voltage automatic tracking system to automatically find the drive voltage to supply corresponding loads and conserve power. Isolated AC-DC converter step-down system use PT6917, PT6919 and LD7830 driving chips in the Flyback circuit architecture to design a buck system. Actuating the four-color LED lights as a goal, the current system’s output voltage is 30V, current 1A, power factor 0.97, and efficiency 89%.

碩士
國立臺北科技大學
電機工程系所
97
The main purpose of this thesis focuses on implementation of a three-phase AC-DC converter with bi-directional power flow control for elevator’s traction system. The converter not only provides the power regeneration to grid when elevator breaking, but also ensures the power factor is near 1 to meet the international standards and regulations. The control strategy of this thesis is based on cascade control of current and voltage loop to force three-phase currents as sinusoidal, power factor near 1, and stable DC-link voltage. Good current command tracking and better load regulation of DC-link voltage are yielded using synchronous frame vector control. Moreover, a paralleled three-phase AC-DC converter for energy regeneration is developed to reduce power capacity of the converter and fulfill the requirements for different applications. All the control schemes are full-digitally implemented using a RISC-based MCU - Renesas SH 7137. To achieve the purpose for increasing the software execution efficiency, the proposed control strategies are all coded by assembly language. Finally, the converter was fully tested on elevator tower to show the effectiveness of proposed control scheme.

The objective of an Energy Control Center (ECC) is to ensure secure and economic operation of power system. The challenge to optimize power system operation, while maintaining system security and quality of power supply to customers, is increasing. Growing demand without matching expansion of generation and transmission facilities and more tightly interconnected power systems contribute to the increased complexity of system operation. Rising costs due to inflation and increased environmental concerns has made transmission, as well as generation systems to be operated closure to design limits, with smaller safety margins and hence greater exposure to unsatisfactory operating conditions following a disturbance. Investigations of recent blackouts indicate that the root cause of most of these major power system disturbances is voltage collapse. Information gathered and preliminary analysis, from the most recent blackout incident in North America on 14th August 2003, is pointing the finger on voltage instability due to some unexpected contingency. In this incident, reports indicate that approximately 50 million people were affected interruption from continuous supply for more than 15 hours. Most of the incidents are related to heavily stressed system where large amounts of real and reactive power are transported over long transmission lines while appropriate real and reactive power resources are not available to maintain normal system conditions. Hence, the problem of voltage stability and voltage collapse has become a major concern in power system planning and operation. Reliable operation of large scale electric power networks requires that system voltages and currents stay within design limits. Operation beyond those limits can lead to equipment failures and blackouts. In the last few decades, the problem of reactive power control for improving economy and security of power system operation has received much attention. Generally, the load bus voltages can be maintained within their permissible limits by reallocating reactive power generations in the system. This can be achieved by adjusting transformer taps, generator voltages, and switchable Ar sources. In addition, the system losses can be minimized via redistribution of reactive power in the system. Therefore, the problem of the reactive power dispatch can be optimized to improve the voltage profile and minimize the system losses as well. The Instability in power system could be relieved or at least minimized with the help of most recent developed devices called Flexible AC Transmission System (FACTS) controllers. The use of Flexible AC Transmission System (FACTS) controllers in power transmission system have led to many applications of these controllers not only to improve the stability of the existing power network resources but also provide operating flexibility to the power system. In the past, transmission systems were owned by regulated, vertically integrated utility companies. They have been designed and operated so that conditions in close proximity to security boundaries are not frequently encountered. However, in the new open access environment, operating conditions tend to be much closer to security boundaries, as transmission use is increasing in sudden and unpredictable directions. Transmission unbundling, coupled with other regulatory requirements, has made new transmission facility construction more difficult. In fact, there are numerous technical challenges emerging from the new market structure. There is an acute need for research work in the new market structure, especially in the areas of voltage security, reactive power support and congestion management. In the last few decades more attention was paid to optimal reactive power dispatch. Since the problem of reactive power optimization is non-linear in nature, nonlinear programming methods have been used to solve it. These methods work quite well for small power systems but may develop convergence problems as system size increases. Linear programming techniques with iterative schemes are certainly the most promising tools for solving these types of problems. The thesis presents efficient algorithms with different objectives for reactive power optimization. The approach adopted is an iterative scheme with successive power-flow analysis using decoupled technique, formulation and solution of the linear-programmingproblem with only upper-bound limits on the state variables. Further the thesispresents critical analysis of the three following objectives, Viz., •Minimization of the sum of the squares of the voltage deviations (Vdesired) •Minimization of sum of the squares of the voltage stability L indices (Vstability) •Minimization of real power losses (Ploss) Voltage stability problems normally occur in heavily stressed systems. While the disturbance leading to voltage collapse may be initiated by a variety of causes, the underlying problem is an inherent weakness in the power system. The factors contributing to voltage collapse are the generator reactive power /voltage control limits, load characteristics, characteristics of reactive compensation devices, and the action of the voltage control devices such as transformer On Load Tap Changers (OLTCs). Power system experiences abnormal operating conditions following a disturbance, and subsequently a reduction in the EHV level voltages at load centers will be reflected on the distribution system. The OLTCs of distribution transformers would restore distribution voltages. With each tap change operation, the MW and MVAR loading on the EHV lines would increase, thereby causing great voltage drops in EHV levels and increasing the losses. As a result, with each tap changing operation, the reactive output of generators throughout the system would increase gradually and the generators may hit their reactive power capability limits, causing voltage instability problems. Thus, the operation of certain OLTCs has a significant influence on voltage instability under some operating conditions. These transformers can be made manual to avoid possible voltage instability due to their operation during heavy load conditions. Tap blocking, based on local measurement of high voltage side of load tap changers, is a common practice of power utilities to prevent voltage collapse. The great advantage of this method is that it can be easily implemented, but does not guarantee voltage stability. So a proper approach for identification of critical OLTC s based on voltage stability criteria is essential to guide the operator in ECC, which has been proposed in this thesis. It discusses the effect of OLTCs with different objectives of reactive power dispatch and proposes a technique to identify critical OLTCs based on voltage stability criteria. The fast development of power electronics based on new and powerful semiconductor devices has led to innovative technologies, such as High Voltage DC transmission (HVDC) and Flexible AC Transmission System (FACTS), which can be applied in transmission and distribution systems. The technical and economicalBenefits of these technologies represent an alternative to the application in AC systems. Deregulation in the power industry and opening of the market for delivery of cheaper energy to the customers is creating additional requirements for the operation of power systems. HVDC and FACTS offer major advantages in meeting these requirements. .A method for co-ordinated optimum allocation of reactive power in AC/DC power systems by including FACTS controller UPFC, with an objective of minimization of the sum of the squares of the voltage deviations of all the load buses has been proposed in this thesis. The study results show that under contingency conditions, the presence of FACTS controllers has considerable impact on over all system voltage stability and also on power loss minimization.minimization of the sum of the squares of the voltage deviations of all the load buses has been proposed in this thesis. The study results show that under contingency conditions, the presence of FACTS controllers has considerable impact on over all system voltage stability and also on power loss minimization. As power systems grow in their size and interconnections, their complexity increases. For secure operation and control of power systems under normal and contingency conditions, it is essential to provide solutions in real time to the operator in ECC. For real time control of power systems, the conventional algorithmic software available in ECC are found to be inadequate as they are computationally very intensive and not organized to guide the operator during contingency conditions. Artificial Intelligence (AI) techniques such as, Expert systems, Neural Networks, Fuzzy systems are emerging decision support system tools which give fast, though approximate, but acceptable right solutions in real time as they mostly use symbolic processing with a minimum number of numeric computations. The solution thus obtained can be used as a guide by the operator in ECC for power system control. Optimum real and reactive power dispatch play an important role in the day-to-day operation of power systems. Existing conventional Optimal Power Flow (OPF) methods use all of the controls in solving the optimization problem. The operators can not move so many control devices within a reasonable time. In this context an algorithm using fuzzy-expert approach has been proposed in this thesis to curtail the number of control actions, in order to realize real time objectives in voltage/reactive power control. The technique is formulated using membership functions of linguistic variables such as voltage deviations at all the load buses and the voltage deviation sensitivity to control variables. Voltage deviations and controlling variables are translated into fuzzy set notations to formulate the relation between voltage deviations and controlling ability of controlling devices. Control variables considered are switchable VAR compensators, OLTC transformers and generator excitations. A fuzzy rule based system is formed to select the critical controllers, their movement direction and step size. Results show that the proposed approach is effective for improving voltage security to acceptable levels with fewer numbers of controllers. So, under emergency conditions the operator need not move all the controllers to different settings and the solution obtained is fast with significant speedups. Hence, the proposed method has the potential to be integrated for on-line implementation in energy management systems to achieve the goals of secure power system operation. In a deregulated electricity market, it may not be always possible to dispatch all of the contracted power transactions due to congestion of the transmission corridors. System operators try to manage congestion, which otherwise increases the cost of the electricity and also threatens the system security and stability. An approach for alleviation of network over loads in the day-to-day operation of power systems under deregulated environment is presented in this thesis. The control used for overload alleviation is real power generation rescheduling based on Relative Electrical Distance (RED) concept. The method estimates the relative location of load nodes with respect to the generator nodes. The contribution of each generator for a particular over loaded line is first identified , then based on RED concept the desired proportions of generations for the desired overload relieving is obtained, so that the system will have minimum transmission losses and more stability margins with respect to voltage profiles, bus angles and better transmission tariff. The results obtained reveal that the proposed method is not only effective for overload relieving but also reduces the system power loss and improves the voltage stability margin. The presented concepts are better suited for finding the utilization of resources generation/load and network by various players involved in the day-to-day operation of the system under normal and contingency conditions. This will help in finding the contribution by various players involved in the congestion management and the deviations can be used for proper tariff purposes. Suitable computer programs have been developed based on the algorithms presented in various chapters and thoroughly tested. Studies have been carried out on various equivalent systems of practical real life Indian power networks and also on some standard IEEE systems under simulated conditions. Results obtained on a modified IEEE 30 bus system, IEEE 39 bus New England system and four Indian power networks of EHV 24 bus real life equivalent power network, an equivalent of 36 bus EHV Indian western grid, Uttar Pradesh 96 bus AC/DC system and 205 Bus real life interconnected grid system of Indian southern region are presented for illustration purposes.

The objective of an Energy Control Center (ECC) is to ensure secure and economic operation of power system. The challenge to optimize power system operation, while maintaining system security and quality of power supply to customers, is increasing. Growing demand without matching expansion of generation and transmission facilities and more tightly interconnected power systems contribute to the increased complexity of system operation. Rising costs due to inflation and increased environmental concerns has made transmission, as well as generation systems to be operated closure to design limits, with smaller safety margins and hence greater exposure to unsatisfactory operating conditions following a disturbance. Investigations of recent blackouts indicate that the root cause of most of these major power system disturbances is voltage collapse. Information gathered and preliminary analysis, from the most recent blackout incident in North America on 14th August 2003, is pointing the finger on voltage instability due to some unexpected contingency. In this incident, reports indicate that approximately 50 million people were affected interruption from continuous supply for more than 15 hours. Most of the incidents are related to heavily stressed system where large amounts of real and reactive power are transported over long transmission lines while appropriate real and reactive power resources are not available to maintain normal system conditions. Hence, the problem of voltage stability and voltage collapse has become a major concern in power system planning and operation. Reliable operation of large scale electric power networks requires that system voltages and currents stay within design limits. Operation beyond those limits can lead to equipment failures and blackouts. In the last few decades, the problem of reactive power control for improving economy and security of power system operation has received much attention. Generally, the load bus voltages can be maintained within their permissible limits by reallocating reactive power generations in the system. This can be achieved by adjusting transformer taps, generator voltages, and switchable Ar sources. In addition, the system losses can be minimized via redistribution of reactive power in the system. Therefore, the problem of the reactive power dispatch can be optimized to improve the voltage profile and minimize the system losses as well. The Instability in power system could be relieved or at least minimized with the help of most recent developed devices called Flexible AC Transmission System (FACTS) controllers. The use of Flexible AC Transmission System (FACTS) controllers in power transmission system have led to many applications of these controllers not only to improve the stability of the existing power network resources but also provide operating flexibility to the power system. In the past, transmission systems were owned by regulated, vertically integrated utility companies. They have been designed and operated so that conditions in close proximity to security boundaries are not frequently encountered. However, in the new open access environment, operating conditions tend to be much closer to security boundaries, as transmission use is increasing in sudden and unpredictable directions. Transmission unbundling, coupled with other regulatory requirements, has made new transmission facility construction more difficult. In fact, there are numerous technical challenges emerging from the new market structure. There is an acute need for research work in the new market structure, especially in the areas of voltage security, reactive power support and congestion management. In the last few decades more attention was paid to optimal reactive power dispatch. Since the problem of reactive power optimization is non-linear in nature, nonlinear programming methods have been used to solve it. These methods work quite well for small power systems but may develop convergence problems as system size increases. Linear programming techniques with iterative schemes are certainly the most promising tools for solving these types of problems. The thesis presents efficient algorithms with different objectives for reactive power optimization. The approach adopted is an iterative scheme with successive power-flow analysis using decoupled technique, formulation and solution of the linear-programmingproblem with only upper-bound limits on the state variables. Further the thesispresents critical analysis of the three following objectives, Viz., •Minimization of the sum of the squares of the voltage deviations (Vdesired) •Minimization of sum of the squares of the voltage stability L indices (Vstability) •Minimization of real power losses (Ploss) Voltage stability problems normally occur in heavily stressed systems. While the disturbance leading to voltage collapse may be initiated by a variety of causes, the underlying problem is an inherent weakness in the power system. The factors contributing to voltage collapse are the generator reactive power /voltage control limits, load characteristics, characteristics of reactive compensation devices, and the action of the voltage control devices such as transformer On Load Tap Changers (OLTCs). Power system experiences abnormal operating conditions following a disturbance, and subsequently a reduction in the EHV level voltages at load centers will be reflected on the distribution system. The OLTCs of distribution transformers would restore distribution voltages. With each tap change operation, the MW and MVAR loading on the EHV lines would increase, thereby causing great voltage drops in EHV levels and increasing the losses. As a result, with each tap changing operation, the reactive output of generators throughout the system would increase gradually and the generators may hit their reactive power capability limits, causing voltage instability problems. Thus, the operation of certain OLTCs has a significant influence on voltage instability under some operating conditions. These transformers can be made manual to avoid possible voltage instability due to their operation during heavy load conditions. Tap blocking, based on local measurement of high voltage side of load tap changers, is a common practice of power utilities to prevent voltage collapse. The great advantage of this method is that it can be easily implemented, but does not guarantee voltage stability. So a proper approach for identification of critical OLTC s based on voltage stability criteria is essential to guide the operator in ECC, which has been proposed in this thesis. It discusses the effect of OLTCs with different objectives of reactive power dispatch and proposes a technique to identify critical OLTCs based on voltage stability criteria. The fast development of power electronics based on new and powerful semiconductor devices has led to innovative technologies, such as High Voltage DC transmission (HVDC) and Flexible AC Transmission System (FACTS), which can be applied in transmission and distribution systems. The technical and economicalBenefits of these technologies represent an alternative to the application in AC systems. Deregulation in the power industry and opening of the market for delivery of cheaper energy to the customers is creating additional requirements for the operation of power systems. HVDC and FACTS offer major advantages in meeting these requirements. .A method for co-ordinated optimum allocation of reactive power in AC/DC power systems by including FACTS controller UPFC, with an objective of minimization of the sum of the squares of the voltage deviations of all the load buses has been proposed in this thesis. The study results show that under contingency conditions, the presence of FACTS controllers has considerable impact on over all system voltage stability and also on power loss minimization.minimization of the sum of the squares of the voltage deviations of all the load buses has been proposed in this thesis. The study results show that under contingency conditions, the presence of FACTS controllers has considerable impact on over all system voltage stability and also on power loss minimization. As power systems grow in their size and interconnections, their complexity increases. For secure operation and control of power systems under normal and contingency conditions, it is essential to provide solutions in real time to the operator in ECC. For real time control of power systems, the conventional algorithmic software available in ECC are found to be inadequate as they are computationally very intensive and not organized to guide the operator during contingency conditions. Artificial Intelligence (AI) techniques such as, Expert systems, Neural Networks, Fuzzy systems are emerging decision support system tools which give fast, though approximate, but acceptable right solutions in real time as they mostly use symbolic processing with a minimum number of numeric computations. The solution thus obtained can be used as a guide by the operator in ECC for power system control. Optimum real and reactive power dispatch play an important role in the day-to-day operation of power systems. Existing conventional Optimal Power Flow (OPF) methods use all of the controls in solving the optimization problem. The operators can not move so many control devices within a reasonable time. In this context an algorithm using fuzzy-expert approach has been proposed in this thesis to curtail the number of control actions, in order to realize real time objectives in voltage/reactive power control. The technique is formulated using membership functions of linguistic variables such as voltage deviations at all the load buses and the voltage deviation sensitivity to control variables. Voltage deviations and controlling variables are translated into fuzzy set notations to formulate the relation between voltage deviations and controlling ability of controlling devices. Control variables considered are switchable VAR compensators, OLTC transformers and generator excitations. A fuzzy rule based system is formed to select the critical controllers, their movement direction and step size. Results show that the proposed approach is effective for improving voltage security to acceptable levels with fewer numbers of controllers. So, under emergency conditions the operator need not move all the controllers to different settings and the solution obtained is fast with significant speedups. Hence, the proposed method has the potential to be integrated for on-line implementation in energy management systems to achieve the goals of secure power system operation. In a deregulated electricity market, it may not be always possible to dispatch all of the contracted power transactions due to congestion of the transmission corridors. System operators try to manage congestion, which otherwise increases the cost of the electricity and also threatens the system security and stability. An approach for alleviation of network over loads in the day-to-day operation of power systems under deregulated environment is presented in this thesis. The control used for overload alleviation is real power generation rescheduling based on Relative Electrical Distance (RED) concept. The method estimates the relative location of load nodes with respect to the generator nodes. The contribution of each generator for a particular over loaded line is first identified , then based on RED concept the desired proportions of generations for the desired overload relieving is obtained, so that the system will have minimum transmission losses and more stability margins with respect to voltage profiles, bus angles and better transmission tariff. The results obtained reveal that the proposed method is not only effective for overload relieving but also reduces the system power loss and improves the voltage stability margin. The presented concepts are better suited for finding the utilization of resources generation/load and network by various players involved in the day-to-day operation of the system under normal and contingency conditions. This will help in finding the contribution by various players involved in the congestion management and the deviations can be used for proper tariff purposes. Suitable computer programs have been developed based on the algorithms presented in various chapters and thoroughly tested. Studies have been carried out on various equivalent systems of practical real life Indian power networks and also on some standard IEEE systems under simulated conditions. Results obtained on a modified IEEE 30 bus system, IEEE 39 bus New England system and four Indian power networks of EHV 24 bus real life equivalent power network, an equivalent of 36 bus EHV Indian western grid, Uttar Pradesh 96 bus AC/DC system and 205 Bus real life interconnected grid system of Indian southern region are presented for illustration purposes.

Dissertação de mestrado em Engenharia Eletrónica e de Computadores
Devido à previsível escassez e às desvantagens inerentes ao uso de combustíveis fósseis na produção de energia elétrica, tem-se vindo a apostar em fontes de energia renovável, nomeadamente a eólica e a solar fotovoltaica. Contudo, estes tipos de fontes apresentam como principais desvantagens a intermitência na produção de energia, bem como a necessidade de um equipamento de interface com a rede elétrica, que tipicamente introduz problemas de qualidade de energia elétrica (QEE). Assim, de forma a possibilitar a integração de fontes de energia renovável na rede elétrica e explorar os incentivos referentes aos períodos de menor procura de energia, surgem novas oportunidades relativamente aos sistemas de armazenamento e gestão de energia, nomeadamente, o conceito de load shifting. Neste sentido, a presente dissertação de mestrado encontra-se inserida no desenvolvimento de um sistema de load-shift com características avançadas. Este dispositivo permite a interface de um sistema de armazenamento de energia com a rede elétrica, simultaneamente com a compensação de problemas de qualidade de energia elétrica relacionados com a corrente, nomeadamente, harmónicos, desequilíbrios e potência reativa. Nesta dissertação é abordado o desenvolvimento do conversor CC-CA trifásico, responsável pela interface com a rede elétrica, absorvendo ou injetando energia elétrica, dependendo do modo de operação do sistema de armazenamento. Além disso, este conversor CC-CA permite a operação como filtro ativo paralelo (FAP) trifásico, sendo esta uma das principais contribuições para a operação do sistema. Ao longo do desenvolvimento desta dissertação, foi possível validar a topologia de conversor CC-CA trifásico com quatro braços a operar em diferentes modos, bem como o seu sistema de controlo. Estes modos envolveram um estudo teórico necessário à execução dos testes de simulação e validação experimental do desempenho do sistema implementado.
Recently, there has been a focus on renewable energy sources, namely wind and solar photovoltaic, due to the predictable scarcity and inherent drawbacks of the use of fossil fuels in the production of electric energy. However, these types of energy sources have their own shortcomings: the intermittence of energy production as well as the need for an electrical network interface device that typically introduces power quality (PQ) problems. Thus, to allow the integration of renewable energy sources in the grid and to explore the incentives for periods of lower energy demand, new opportunities arise regarding energy storage and management systems, namely the load shifting concept. In this context, the present master dissertation is focused on developing a system of load-shift with advanced characteristics. This device allows the interface of a system of energy storage with the electric network, while fixing the electrical power quality problems related to current harmonics, unbalances and reactive power. This investigation discusses the development of three-phase DC-AC converter, which is responsible for interfacing with the grid, absorbing or injecting electrical energy, depending on the mode of operation of the storage system. Additionally, one of the main contributions to the system operation is that the DC-AC converter allows operation as a three-phase shunt active filter (APF). Throughout the development of this dissertation, it was possible to validate the topology of the three-phase DC-AC converter with four legs operating in different modes, as well as its control system. These modes involved a theoretical study, which is necessary to perform the simulation tests and experimental validation of the performance of the implemented system.
Este trabalho de mestrado está enquadrado no projeto de IC&DT “Quality4Power - Enhancing the Power Quality for Industry 4.0 in the era of Microgrids”, financiado pela Fundação para a Ciência e Tecnologia, com a referência PTDC/EEI-EEE/28813/2017.
Este trabalho de mestrado está enquadrado no projeto de IC&DT “newERA4GRIDs – New Generation of Unified Power Conditioner with Advanced Control, Integrating Electric Mobility, Renewables, and Active Filtering Capabilities for the Power Grid Improvement”, financiado pela Fundação para a Ciência e Tecnologia, com a referência POCI-01-0145-FEDER-030283.