Dissertations / Theses on the topic 'Flexible AC Transmission Systems (FACTS)'

This thesis describes the implementation of the Flexible AC Transmission Systems (FACTS) devices to develop a market-based approach to the problem of transmission congestion management in a Balancing Market. The causes, remedies and pricing methods of transmission congestion are briefly reviewed. Balancing Market exists in markets in which most of the trading is done via decentralized bilateral contracts. In these markets only final adjustments necessary to ensure secure system operation is carried out at a centralized Balancing Market. Each market player can participate in the Balancing Market by submitting offers and bids to increase and decrease its initially submitted active generation output. In this research a method is proposed to reduce costs associated with congestion re-dispatch in a Balancing Market by optimal placement of FACTS devices, and in particular Thyristor Controlled Phase Shifter Transformers (TCPST). The proposed technique is applicable to both Mixed Integer Linear Programming (MILP) and Mixed Integer Non-Linear Programming (MINLP). In the MILP a power system network is represented by a simplified DC power flow under a MILP structure and the Market participants' offers and bids are also represented by linear models. Results show that applications of FACTS devices can significantly reduce costs of congestion re-dispatch. The application of the method based on the MINLP creates a nonlinear and non-convex AC OPF problem that might be trapped in local sub-optima solutions. The reliability of the solution that determines the optimal placement of FACTS devices is an important issue and is carried out by investigation of alternative solvers. The behavior of the MINLP solvers is presented and finally the best solvers for this particular optimization problem are introduced. The application of DC OPF is very common in industry. The accuracy of the DC OPF results is investigated and a comparison between the DC and AC OPF is presented.

As novas tecnologias FACTS aplicadas ao sistema de transmissão, com base em eletrônica de potência, podem também ser úteis à distribuição. Para tal é preciso conduzir um procedimento de consolidação da utilização e do desempenho destas, para sua aplicação sem riscos. Neste trabalho, dois aspectos serão contemplados. O primeiro se refere à aplicação do dispositivo FACTS atuando como um capacitor série. Em se tendo controle de módulo e da fase da tensão inserida em série com a linha pode-se fazê-la comportar-se como uma queda em uma reatância série capacitiva ou indutiva. O controle dessa reatância série (aumentando/diminuindo) permitirá a aplicação do conceito de compensação série em qualquer ponto do sistema de distribuição, provendo benefícios de um controle contínuo da tensão e também do controle do fluxo de carga no sistema independente da corrente. O segundo aspecto refere-se ao uso dos dispositivos na conexão de alimentadores controlando a potência ativa entre eles. Para esta operação outro dispositivo UPFC, com conceito similar ao descrito acima, entretanto atuando na fase da tensão entre 2 barras, comporta-se como um transformador defasador com variação contínua de ?taps?, podendo controlar a potência ativa entre os alimentadores. A aplicação destas tecnologias propiciarão vários benefícios para a expansão da distribuição tais como, flexibilização do uso da rede, interligação de alimentadores permitindo manobras de blocos de energia sem ?pisca?, ajuste contínuo do suporte de reativos durante a operação, controle dinâmico do fluxo de potência. O objetivo deste trabalho é estudar a aplicabilidade da tecnologia FACTS e estender este conceito para aplicação em sistemas de distribuição e conduzir simulações digitais em redes de distribuição (15kV) identificando o desempenho e os benefícios atingidos. O programa de simulação utilizado é o ATP (Alternative Transients Program).
The new FACTS technologies applied to the transmission system, based on power electronics, can also be useful to the distribution. For that, it is necessary to drive a procedure to consolidate the use and the performance for their application without risks. In this work two aspects will be approached. The first refers to the application of a FACTS device acting as series compensator. This device will be able to control the voltage in module and phase in order to act as a voltage drop in a serie reactance with capacitive or inductive features. The control of this series reactance (increasing/ decreasing) will allow the application of series compensation concept to any point of the distribution system, providing the benefits of continuous control of the voltage added to the load flow control in the system independent of the current. The second aspect refers to its use in the connection of two feeders controlling the active power between them. For this operation other device, UPFC, with similar concept as described previously, acts mainly in the phase of the injected voltage in the line, performing as a phase-shift with continuous taps variation and is able to control the active power flow between feeders. The application of this technology will provide several benefits for the distribution expansion, such as, a greater flexibility in the use of the network, connection of feeders without load flow interruption, continuous adjust of reactive power during the operation and dynamic control of power flow. The purpose of this work is to study the applicability of the FACTS technology, to extend this concept for the application in the distribution system by using digital simulations in distribution network up to 15kV identifying the performance and the reached benefits.

The main tasks of power electronics in power transmission and distribution systems is to process and control the flow of electric energy by supplying voltages and currents in a form that is suitable for user loads. In recent years, the field of power electronics has experienced a large growth. Electric utilities expected that by the year 2000 over half of the electrical load may be supplied through power electronic systems. In order to take advantage of this highly developed technology a number of detailed modelling procedures and simulation facilities are needed. The work in this thesis is concentrated on modelling, control and design of various power electronic based models for use within transmission and distribution systems. The overall objective is to provide effective methods and tools for assessing the impact of the latest technology based on power electronic devices in the reinforcement of power system networks. The thesis clarifies modelling and control of various variable speed drive models, such as the six-step, PWM and vector control and gives a detailed account of the systematic derivation of equations that are necessary for the dynamic and transient analysis of a multi-machine multi-node power system with associated adjustable speed drives. Simulation of Flexible AC Transmission Systems (FACTS) models has also been developed for a number of devices including: the SVC (Static Var Compensator), the STATCON (Static Condenser) and the UPFC (Unified Power Flow Controller). The methodologies for development of the models are described and a number of case studies are included in order to give a broad overview of the applications and to prove the usefulness of the results. The last part of the thesis includes simulation, control and design of Custom Power Devices for use within distribution system architectures. It starts with a complete control system strategy for the modelling of a solid-state switch and continues with the modelling of a Dynamic Voltage Restorer model, using an innovative control system. The creation of the power electronics models library provides several opportunities for future developments, which are discussed in the concluding sections of the thesis.

This thesis is devoted to the development of new models for a recently-implemented FACTS (flexible alternating current transmission system) device, the unified power flow controller (UPFC), and the control coordination of power systems with FACTS devices in steady-state operating mode. The key objectives of the research reported in the thesis are, through online control coordination based on the models of power systems having FACTS devices, those of maximising the network operational benefit and restoring system static security following a disturbance or contingency. Based on the novel concept of interpreting the updated voltage solutions at each iteration in the Newton-Raphson (NR) power-flow analysis as dynamic variables, the thesis first develops a procedure for representing the unified power flow controllers (UPFCs) in the steady-state evaluation. Both the shunt converter and series converter control systems of a UPFC are modeled in their dynamical form with the discrete time variable replaced by the NR iterative step in the power-flow analysis. The key advantage of the model developed is that of facilitating the process of UPFC constraint resolution during the NR solution sequence. Any relative priority in control functions pre-set in the UPFC controllers is automatically represented in the power-flow formulation. Although the developed UPFC model based on the dynamic simulation of series and shunt converter controllers is flexible and general, the number of NR iterations required for convergence can be large. Therefore, the model is suitable mainly for power system planning and design studies. For online control coordination, the thesis develops the second UPFC model based on nodal voltages. The model retains all of the flexibility and generality of the dynamic simulation-based approach while the number of iterations required for solution convergence is independent of the UPFC controller dynamic responses. Drawing on the constrained optimisation based on Newton’s method together with the new UPFC model expressed in terms of nodal voltages, a systematic and general method for determining optimal reference inputs to UPFCs in steady-state operation is developed. The method is directly applicable to UPFCs operation with a high-level line optimisation control (LOC) for maximising the network operational benefit. By using a new continuation technique with adaptive parameter, the algorithm for solving the constrained optimisation problem extends substantially the region of convergence achieved with the conventional Newton’s method. Having established the foundation provided by the comprehensive models developed for representing power systems with FACTS devices including the UPFC, the research, in the second part, focuses on real-time control coordination of power system controllers, with the main purpose of restoring power system static security following a disturbance or contingency. At present, as the cost of phasor measurement units (PMUs) and wide-area communication network is on the decrease, the research proposes and develops a new secondary voltage control where voltages at all of the load nodes are directly controlled, using measured voltages.

Les systèmes FACTS (Flexible AC Transmission system) sont pressentis pour l'amélioration des performances des réseaux de transport et d'interconnexion. De nombreuses études ont été faites récemment sur ces systèmes concernant l'augmentation de la vitesse de contrôle des paramètres des lignes (tension, impédance et déphasage). Les compensations shunt et série utilisant des systèmes d'électronique de puissance sont des concepts FACTS et permettent aux réseaux d'être plus flexibles. La compensation shunt réalise de préférence le support de la tension alors que la compensation série est employée pour réduire l'impédance des lignes et donc pour augmenter la capacité de transfert de puissance ainsi qu'améliorer la répartition des transits de puissance dans le réseau, aussi bien que les stabilités statiques et dynamiques. Plusieurs systèmes ont été développés ces dernières années. Ce rapport décrit une étude comparative concernant le comportement statique et dynamique de trois systèmes FACTS dans les réseaux de transport : le SVC (static var compensator), le Statcon (static condenser) et le TCSC (thyristors controlled series compensator). Ce travail est axé sur la stabilité de tension et les capacités de transfert de puissance et inclu l'étude de l'action des LTC (load tap changers). Les effets des PSS (power system stabilizers) sont également analysés dans le but de les comparer ultérieurement aux systèmes FACTS. Les résultats ont été obtenus à l'aide de différents réseaux tests (réseau 14 nœuds IEEE, réseau UHV français simplifié, réseau new-england 39 nœuds) et ont montré l'impact de chaque système FACTS sur les réseaux de puissance
FACTS Systems (Flexible AC Transmission System) are approached to improve performance transport networks and interconnection. Many studies have been done recently on these systems for increasing the speed of parameter control lines (voltage, impedance and phase shift). The shunt and series compensation using systems power electronics are FACTS concepts and enable networks to be more flexible. Shunt compensation is preferably carried out the tension carrier while the series compensation is used to reduce the impedance of the lines and thus to increase the transfer capability power and improve the distribution of power flows in the network, as well as Static and dynamic stability. Several systems have been presented in recent years. This report presents a comparative study of the static and dynamic behavior of three FACTS systems in transport networks: the SVC (Var Compensator Statie), the STATCON (ST condense ATIC) and the TCSC (Thyristor Controlled Series Compensator). This work focuses on voltage stability and power transfer capacity and undue study of the action of L TC (Load Tap Changers). The effects of PSS (Power System Stabilizers) are also analyzed in the purpose of later compare them with FACTS systems. The results were obtained using different tests networks (network 14 noeus IEEE, Simplified French UHV network, New England network 39 knots) and showed the impact of each FACTS system on power networks

It is specifically important to focus on voltage stability analysis of the power system to avoid worst case scenarios such as voltage collapse. The purpose of this thesis is to identify methods for enhancing the steady-state voltage stability using FACTS devices and determining their impact on real and reactive power losses, improvement of bus voltage magnitude, and transmission line loadability. To achieve this, FACTS devices such as Static VAR Compensator (SVC), Static Synchronous Compensator (STATCOM), and Thyristor Controlled Series Capacitor (TCSC) are used in the test system as three separate test cases. The results obtained assist in drawing conclusions on the effectiveness of each FACTS devices at generator, load and swing buses, on lines between two load buses, and between a load bus and a generator bus, in terms of metrics such as voltage magnitude profile, PV curves, and active and reactive power losses.

The electric power supply industry has evolved into one of the largest industries. Even though secure and reliable operation of the electric power system is fundamental to economy, social security and quality of modern life, the complicated power grid is now facing severe challenges to meet the high-level secure and reliable operation requirements. New technologies will play a major role in helping today's electric power industry to meet the above challenges. This dissertation has focused on some key technologies among them, including the emerging technologies of energy storage, controlled power electronics and wide area measurement technologies. Those technologies offer an opportunity to develop the appropriate objectives for power system control. The use of power electronics based devices with energy storage system integrated into them, such as FACTS/ESS, can provide valuable added benefits to improve stability, power quality, and reliability of power systems. The study in this dissertation has provided several guidelines for the implementation of FACTS/ESS in bulk power systems. The interest of this study lies in a wide range of FACTS/ESS technology applications in bulk power system to solve some special problems that were not solved well without the application of FACTS/ESS. The special problems we select to solve by using FACTS/ESS technology in this study include power quality problem solution by active power compensation, electrical arc furnace (EAF) induced problems solution, inter-area mode low frequency oscillation suppression, coordination of under frequency load shedding (UFLS) and under frequency governor control (UFGC), wide area voltage control. From this study, the author of this dissertation reveals the unique role that FACTS/ESS technology can play in the bulk power system stability control and power quality enhancement in power system. In this dissertation, almost all the studies are based on the real system problems, which means that the study results are special valuable to certain utilities that have those problems. The study in this dissertation can assist power industry choose the right FACTS/ESS technology for their intended functions, which will improve the survivability, minimize blackouts, and reduce interruption costs through the use of energy storage systems.
Ph. D.

Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Ronald G. Harley; Committee Member: David G. Taylor; Committee Member: Deepakraj M. Divan; Committee Member: Ganesh Kumar Venayagamoorthy; Committee Member: Thomas G. Habetler. Part of the SMARTech Electronic Thesis and Dissertation Collection.

A single-phase, buck-boost based, dual-output AC-DC converter is studied in this thesis. The converter has two DC outputs with opposite polarities, which share the same ground with the input power line. The power stage performance, including the input filter, is studied and procedure to select power components is given. The circuit model is analyzed to develop appropriate control. Zerocrossing distortion of the source input current is addressed and a solution is proposed. Experimental results are satisfactory in that a high power factor line current results for steady-state operation.

Nowadays, power systems are dealing with some new challenges raisedby the major changes that have been taken place since 80’s, e.g., deregu-lation in electricity markets, significant increase of electricity demands andmore recently large-scale integration of renewable energy resources such aswind power. Therefore, system operators must make some adjustments toaccommodate these changes into the future of power systems.One of the main challenges is maintaining the system stability since theextra stress caused by the above changes reduces the stability margin, andmay lead to rise of many undesirable phenomena. The other important chal-lenge is to cope with uncertainty and variability of renewable energy sourceswhich make power systems to become more stochastic in nature, and lesscontrollable.Flexible AC Transmission Systems (FACTS) have emerged as a solutionto help power systems with these new challenges. This thesis aims to ap-propriately utilize such devices in order to increase the transmission capacityand flexibility, improve the dynamic behavior of power systems and integratemore renewable energy into the system. To this end, the most appropriatelocations and settings of these controllable devices need to be determined.This thesis mainly looks at (i) rotor angle stability, i.e., small signal andtransient stability (ii) system operation under wind uncertainty. In the firstpart of this thesis, trajectory sensitivity analysis is used to determine themost suitable placement of FACTS devices for improving rotor angle sta-bility, while in the second part, optimal settings of such devices are foundto maximize the level of wind power integration. As a general conclusion,it was demonstrated that FACTS devices, installed in proper locations andtuned appropriately, are effective means to enhance the system stability andto handle wind uncertainty.The last objective of this thesis work is to propose an efficient solutionapproach based on Benders’ decomposition to solve a network-constrained acunit commitment problem in a wind-integrated power system. The numericalresults show validity, accuracy and efficiency of the proposed approach.

The Doctoral Degrees issued upon completion of the programme are issued by Comillas Pontifical University, Delft University of Technology and KTH Royal Institute of Technology. The invested degrees are official in Spain, the Netherlands and Sweden, respectively.QC 20141028

In this thesis, design and implementation of a distribution-type, voltage source converter (VSC) based static synchronous compensator (D-STATCOM) having the simplest converter and coupling transformer topologies have been carried out. The VSC STATCOM is composed of a +/- 750 kVAr full-bridge VSC employing selective harmonic elimination technique, a low-pass input filter, and a &
#8710
/Y connected coupling transformer for connection to medium voltage bus. The power stage of VSC based STATCOM is composed of water-cooled high voltage IGBT modules switched at 850 Hz for the elimination of 5th, 7th, 11th, 13th, 17th, 19th, 23rd,and 25th voltage harmonics. Special care has been taken in the laminated busbar design to minimize stray inductances between power semiconductors and dc link capacitor. Reactive power control is achieved by applying the phase angle control technique. The effect of input filter on total demand distortion has been investigated theoretically by mathematical derivations. The proposed VSC STATCOM has been implemented for reactive power compensation of Coal Preparation System in Kemerkö
y Thermal Power Plant. The field test results have shown the success of the implemented system in view of fast response in reactive power compensation, and minimum input current harmonic content, and compliance with the IEEE Std. 519-1992 even for the weakest power systems. The application of selective harmonic elimination technique and phase angle control to VSC STATCOM has led to optimum switching frequency and device utilization for high voltage IGBTs at the expense of slower response as compared to other PWM techniques.

As electric utilities move into more competitive generation supply regimes, with limited scope to expand transmission facilities, the optimisation of existing transmission corridors for power transfer becomes of paramount importance. In this scenario, Flexible AC Transmission System (FACTS) technology, which aims at increasing system operation flexibility, appear as an attractive alternative. Many of the ideas upon which the foundations of FACTS rest were conceived some time ago. Nevertheless, FACTS as a single coherent integrated philosophy is a newly developed concept in electrical power systems which has received the backing of the major manufacturers of electrical equipment and utilities around the world. It is looking at ways of capitalising on the new developments taking place in the area of high-voltage and highcurrent power electronics in order to increase the control of the power flows in the high voltage side of the network during both steady state and transient conditions, so as to make the network electronically controllable. In order to examine the applicability and functional specifications of FACTS devices, it is necessary to develop accurate and flexible digital models of these controllers and to upgrade most of the software tools used by planners and operators of electric power systems. The aim of this work is to develop general steady-state models FACTS devices, suitable for the analysis of positive sequence power flows in, large-scale real life electric power systems.Generalised nodal admittance models are developed for the Advance Series Compensator (ASC), Phase Shifter (PS), Static Var Compensator (SVC), Load Tap Changer (LTC) and Unified Power Flow Controller (UPFC). In the case of the ASC, two models are presented, the Variable Series Compensator (VSC) and the Thyristor Controlled Series CapacitorFiring Angle (TCSC-F A). An alternative UPFC model based on the concept of Synchronous Voltage Source (SVS) is also developed. The Interphase Power Controller (IPC) is modelled by combining PSs and VSCs nodal admittance models. The combined solution of the power flow equations pertaining to the FACTS devices models and the power network is described in this thesis. The set of non-linear equations is solved through a Newton-Rapshon technique. In this unified iterative environment, the FACTS device state variables are adjusted automatically together with the nodal network state variables so as to satisfy a specified nodal voltage magnitudes and specified power flows. Guidelines and methods for implementing FACTS devices and their adjustments within the Newton-Rapshon algorithm are described. It is shown that large increments in the adjustments of FACTS devices and nodal network state variables during the backward substitution may dent the algorithm's quadratic convergence. Suitable strategies are given which avoid large changes in these variables and retain the Newton-RapshRapshon method's quadratic convergence. The influence of initial conditions of FACTS devices state variables on the iterative process is investigated. Suitable initialisation guidelines are recommended. Where appropriate, analytical equations are given to assure good initial conditions.

Thesis (M.S.)--West Virginia University, 2000.
Title from document title page. Document formatted into pages; contains viii, 78 p. : ill. Includes abstract. Includes bibliographical references (p. 61-63).

This thesis examines how Flexible AC Transmission Systems (FACTS) devices can improve the secure-economic operation of a power system. More specifically, the benefits of FACTS devices in a network are evaluated in terms of four areas of power system study: system security, economic dispatch operation, maximum network loadability and electric industry deregulation. Simulations of a simple network are made to evaluate how a FACTS device can be used to increase the security region of a network. Based on this analysis, simulations are performed using the 24-bus IEEE reliability test network to assess the possible savings in generation costs, the increase in maximum network loadability and the improvements in flexibility of exchanges resulting from the use of a FACTS device in this network. The results demonstrate that FACTS devices can be used effectively to increase the security region of a network thereby allowing for a better optimum operating point in any optimization problem performed over such a region.

Orientador: Vivaldo Fernando da Costa
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação
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Resumo: Essa dissertação de mestrado apresenta um estudo que avalia o desempenho dos controladores FACTS (Flexible AC Transmission Systems) para a melhoria da estabilidade de ângulo a pequenas perturbações de sistemas elétricos de potência. O potencial do Modelo de Sensibilidade de Potência (MSP), uma alternativa ao clássico modelo Heffron-Phillips (MHP) para o estudo e análise do problema de oscilações eletromecânicas de baixa frequência fracamente amortecidas, é explorado. A análise da estabilidade e o projeto de estabilizadores POD (Power Oscillation Damping) para controladores FACTS são baseados em análise modal, bifurcações de Hopf, gráficos do lugar das raízes, e técnicas de resposta em frequência e no tempo. O desempenho de diferentes sinais de entrada para estabilizadores POD é investigado. Os resultados das simulações revelam que tanto os controladores FACTS série quanto os controladores em derivação possuem um grande potencial para a manutenção da estabilidade angular do sistema
Abstract: This master¿s dissertation presents an assessment of Flexible AC Transmission Systems (FACTS) controllers performance on power system small-signal angle stability improvement. The potential of the Power Sensitivity Model (PSM), an alternative approach to the classical Heffron-Phillips model (HPM) for study and analysis of poorly damped low frequency electromechanical oscillations problem, is explored. The stability analysis and design of FACTS Power Oscillation Damping (POD) controllers are based on modal analysis, Hopf bifurcations, root locus plots, and time and frequency response techniques. The performance of different input signals to the POD controllers is investigated. Simulation results reveal that both shunt FACTS controllers and series ones are very effective on keeping system angle stability
Mestrado
Energia Eletrica
Mestre em Engenharia Elétrica

Thesis (Ph. D.)--University of Missouri--Rolla, 2007.
Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed November 29, 2007) Includes bibliographical references.

With increased penetration of renewable energy sources, maintaining equilibrium between production and consumption in the world’s electrical power systems (EPS) becomes more and more challenging. One way to increase stability and efficiency in an EPS is to use flexible alternating current transmission systems (FACTS). However, an EPS containing multiple FACTS-devices with overlapping areas of influence can lead to negative effects if the reference values they operate around are not updated with sufficient temporal resolution. The reference values are usually set manually by a system operator. The work in this master thesis has investigated how three different reinforcement learning (RL) algorithms can be used to set reference values automatically with higher temporal resolution than a system operator with the aim of increased voltage stability. The three RL algorithms – Q-learning, Deep Q-learning (DQN), and Twindelayed deep deterministic policy gradient (TD3) – were implemented in Python together with a 2-bus EPS test network acting as environment. The 2-bus EPS test network contain two FACTS devices: one for shunt compensation and one for series compensation. The results show that – with respect to reward – DQN was able to perform equally or better than non-RL cases 98.3 % of the time on the simulation test set, while corresponding values for TD3 and Q-learning were 87.3 % and 78.5 % respectively. DQN was able to achieve increased voltage stability on the test network while TD3 showed similar results except during lower loading levels. Q-learning decreased voltage stability on a substantial portion of the test set, even compared to a case without FACTS devices. To help with continued research and possible future real life implementation, a list of suggestions for future work has been established.

Thesis (M.S.)--University of Missouri--Rolla, 2007.
Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed February 14, 2008) Includes bibliographical references (p. 37-40).

Thesis (MTech (Electrical Engineering))--Cape Peninsula University of Technology, 2009
The industrial and population growth of a nation can cause power delivery problems to localized areas of a national grid through their increased demand for electrical energy. One reason for these power shortages is the insufficient current carrying capacity of existing high voltage alternating current, (HVAC), transmission lines supplying the area. High voltage direct current (HVDC) transmission lines are a possible solution as they provide better power delivery than HVAC lines. New or upgraded HVAC lines, or HVDC lines or combinations of HVAC and HVDC lines are possible solutions to improve power delivery. This research investigates the various line possibilities using theory. and cutting edge frequency and time domain software tools. The challenge is how to approach this problem. What methodology or structure should be used? Thus one of the contributions of this work is the development of a strategy (flow chart), for solving power delivery problems to localized areas of a national grid through individual or combinations (e.g. parallel operation) of HVAC and/or HVDC transmission lines. The main contribution is the evaluation of a HVDC system as a solution to overcoming power delivery shortages to a localized area of a national grid. Three different software packages (two industrial and one academic) namely, PSCAD/EMTDC (time domain), DlgSILENT PowerFactory (frequency domain) and MathCAD software are evaluated for their capability to perform the simulation studies necessary to prove the possible solutions given in the developed flow chart. The PSCAD/EMTDC software package is evaluated for integrated HVAC/HVDC load flow analyses, DlgSILENT for individual and parallel combinations of HVAC lines and MathCAD to prove hand calculations to software results. Five case studies are conducted. The first case study demonstrates a healthy system with no delivery shortcomings, the second case study portrays the delivery shortcoming due to increased localized area demand, and the remaining three case studies explore possible solutions to solve the problem. The first possible solution is to construct an identical HVAC line in parallel to the existing line.

Orientador: Prof. Dr. Claudionor Francisco do Nascimento
Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Engenharia Elétrica, 2014.
Os sistemas de distribuição de energia elétrica, de um modo geral, são radiais. Estes sistemas podem possuir problemas de qualidade da energia elétrica em suas barras, tais como os desequilíbrios de tensão e de corrente. Ademais, as redes de distribuição podem conter uma grande quantidade de cargas monofásicas e trifásicas desequilibradas, indutivas e não lineares, o que pode resultar nos seguintes problemas: baixo fator de potência, desequilíbrio de tensão e distorção harmônica. Nesse sentido, devido à dinâmica destas cargas e a ocorrência de fenômenos, como afundamento de tensão por partidas de grandes motores e curtos-circuitos, há a necessidade de solução destas anormalidades, com respostas rápidas de estabilização. De maneira a atender a esta necessidade, os resultados das recentes pesquisas baseadas na eletrônica de potência têm proporcionado a utilização de equipamentos capazes de trabalhar em sistemas de potência de alta tensão. Esse avanço permite aplicações de dispositivos FACTS (Flexible Alternating Current Transmission Systems) nas operações de flexibilidade e mitigação de problemas comuns às redes de energia elétrica de transmissão e de distribuição, melhorando, assim, a qualidade da energia elétrica, o que aumenta a confiabilidade destes sistemas. Com este propósito, o presente trabalho tem como objetivo principal o estudo e a modelagem de um DSTATCOM (Distribution Static Compensator) em um sistema de distribuição de energia elétrica com variações de tensão, avaliando os possíveis benefícios proporcionados a este sistema. Os resultados de simulação foram obtidos com o auxílio do software MatLab/Simuling.
Distribution systems of electric power, in general, are radial. These systems have power quality problems in their power bars, such as the voltage variations. Moreover, the distribution networks contain a large amount of single-phase and three-phase unbalanced loads, inductive and non-linear, which may result in the following problems: low power factor, voltage unbalance, and harmonic distortion. Accordingly, due to the dynamic loads and the occurrence of these phenomena, such as voltage sag matches by large motors and short circuits, there is a need for solution of these abnormalities, with responsive stabilization. In order to meet this need, the results of recent research based on power electronics have provided the use of equipment to work on systems of high voltage power. This advancement allows applications FACTS devices (Flexible Alternating Current Transmission Systems) flexibility in operations and mitigation of problems common to the networks of electricity transmission and distribution, thus improving the quality of electric power, which increases the reliability of these systems. For this purpose, the present work has as main objective the study and modeling of a DSTATCOM (Distribution Static Compensator) in a system of electricity distribution, evaluating the possible benefits provided to the electric power system. The simulation results were obtained with the aid of software Matlab/Simuling. The effectiveness of the work has been proven since the DSTATCOM installed, the distribution network operated within established standards.

Orientador : Vivaldo Fernando da Costa
Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de Computação
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Resumo: Este trabalho aborda o problema das oscilações eletromecânicas de baixa freqüência de modo interárea em sistemas de energia elétrica através de duas metodologias: primeiramente, através do método convencional da análise modal linear e, posteriormente, através da aplicação do método da forma normal dos campos vetoriais como ferramenta para viabilizar o acréscimo dos termos não lineares de segunda ordem resultantes da expansão em série de Taylor. Neste caso, o método da forma normal é aplicado a um Modelo de Sensibilidade de Potência com representação de segunda ordem, para investigar os efeitos das interações não lineares entre os modos naturais de oscilação de sistemas de energia elétrica. São consideradas, em ambas as análises, a inclusão de dispositivos FACTS e da modelagem dinâmica das cargas. As simulações são realizadas para um sistema simétrico de duas áreas e para o Sistema Equivalente Sul-Sudeste Brasileiro. A metodologia de análise proposta mostra-se bastante satisfatória como alternativa à simulação não linear no domínio do tempo e à análise modal convencional
Abstract: In this work, the analysis of power systems inter-area mode oscillations is performed by the application of two different methodologies: first, the linear modal analysis, and then the analysis including second order nonlinear terms from a Taylor series expansion, with the application of the method of normal forms of vector fields. In this case, the method of normal forms is applied to a Power Sensitivity Model including second order nonlinear terms, in order to investigate the effects of nonlinear interactions between system modes. Both methodologies consider the inclusion of FACTS devices and dynamic load model. Simulations are performed for a symmetric two-area test power system and for the Equivalent South-Southeast Brazilian system. The results obtained show that the methodology proposed is very effective as an alternative to linear modal analysis and timedomain simulation in the performance of inter-area mode oscillations analysis
Doutorado
Energia Eletrica
Doutor em Engenharia Elétrica

With limited enhancement or expansion of the transmission infrastructure, the contemporary power systems are operating under more stressed conditions. It becomes important to fully utilize the existing transmission system to supply load demand as much as possible, thus eliminating or reducing the need for new transmission investment. Flexible AC Transmission System (FACTS) technology provides an alternative to fully utilize the existing transmission lines as well as new and upgraded lines, by controlling power and also enhancing the power transfer capability of transmission lines. However, the implementation of FACTS controllers in the transmission system has introduced new power system dynamics that must be addressed in the area of power system protection, such as rapid changes in line impedance, power angle, line currents, transients introduced by the occurrence of fault and associated control action of the FACTS controller. Therefore, the performance of the protection system must be carefully analyzed in the presence of FACTS controllers. The thesis aims at evaluating the performance of distance relays when different types of FACTS controllers, in particular Voltage Source Converter (VSC) based FACTS controllers, are incorporated at the midpoint of the transmission system to achieve voltage profile improvement and power transfer capability. The detailed models of these controllers and their control strategies are described. The presence of FACTS controllers in the loop affects both steady state and transient components of voltage and current signals. The rapid response of FACTS controllers to different power system configurations significantly affects the apparent impedance seen by distance relays. The apparent impedance seen by distance relays would be different from that of the system without FACTS controller. Due to this, the distance relay may malfunction, resulting in unreliable operation of the power system during faults. Furthermore, the effect of FACTS controllers on distance relay operation depends on the type of FACTS controller used, the application for which it has been installed and its location in the power system. The distance relay is evaluated for different loading conditions and for various fault conditions. Simulation studies are carried out using PSCAD/EMTDC based transient simulation package.

Small signal oscillation has been always a major concern in the operation of power systems. In a generator, the electromechanical coupling between the rotor and the rest of the system causes it to behave in a manner similar to a spring mass damper system. Following any disturbance, such as sudden change in loads, actuations in the output of turbine and faults etc. it exhibits an oscillatory behaviour around the equilibrium state. The use of fast acting high gain AVRs and evolution of large interconnected power systems with transfer of bulk power across weak transmission links have further aggravated the problem of these low frequency oscillations. Small oscillations in the range of about 0.1Hz to 3.5Hz can persist for long periods, limiting the power transfer capability of the transmission lines. These oscillations can be reduced by incorporating auxiliary controllers on generator excitation system. Power System Stabilizers (PSSs) were developed to produce additional damping by modulating the generator excitation voltage. Designing effective PSS for all operating conditions especially in large interconnected power systems still remains a difficult and challenging task. More and more power electronic based controllers have been and will be used in power systems. Many of these controllers such as Static Var Compensators (SVCs), Static Synchronous Compensators (STATCOMs) and Unified Power Flow Controllers (UPFCs) etc., are incorporated in power transmission networks to improve its operational capability. In addition, some of the energy storage systems such as Battery Energy Storage systems (BESS), Super conducting Magnetic Energy Storage System (SMES) as well large non-conventional energy sources are also increasingly being integrated with the power grid. With large integration of these devices, there is a significant impact on system stability, more importantly on small signal oscillatory instability of the power system. This thesis primarily focuses on impact of such devices on small signal oscillatory stability of the power systems. More specifically in this thesis small signal stability analysis of a Single Machine Infinite Bus (SMIB) system with a grid connected wind powered Permanent Magnet Synchronous Generator (PMSG) has been presented. A SMIB system has been purposely chosen so that general conclusions can be obtained on the behaviour of the embedded STATCOM/Energy Source (ES) system on system stability. With a better understanding of the impact of such a system it would be probably possible to analyze more complicated multimachine power system and their impact on system stability. Small signal model of the complete system which comprises the generator, transmission network, inter connecting STATCOM, the wind power generator and all associated controllers has been developed. The performances of the system following a small disturbance at various operating conditions have been analyzed. To obtain quantitative estimates of the damping and synchronizing torques generated in the system, expressions for damping and synchronizing torque clients have been developed. With these analyses, the relative impact of the STATCOM and STATCOM with ES on system performance have been assessed. It is shown that with active and reactive power modulation capabilities effective and efficient control of small signal oscillations in power systems can be achieved.

Programa doutoral em Engenharia Electrónica e de Computadores
Railway systems have progressively been developed since James Watt presented a technique of converting steam power into a circular movement back in 1763. With the novelty of steam engines at that time and the increasing of railway networks, railway industry quickly became an economic catalyst throughout the world due to the advantages of passenger and freight transport. In 1879, Siemens & Halske company introduced the world’s first electric train in the city of Berlin, consisting of a locomotive and three wagons, and supplied via an insulated third rail with 150 V direct current (DC). From that time, the world has begun to recognize the important transition from steam power to electric power, and the potential of the electrified railway as a mode of mass transport. Due to the plenty of fuel in the last century, Diesel trains were not only common, but they also dominated the railway sector for a few decades. Consequently, the development in the infrastructures of electric trains decelerated, and the path to having fully electrified trains was long enough. In this context, electric trains have introduced progressively, in which Diesel and electric power have been combined to create hybrid locomotives. However, and with the increased demand for transportation and the higher fuel prices in the last decades, electric trains can substantially offer lower operating costs and lower emissions compared with the Diesel-powered trains. Nowadays, most of the high-speed electric trains use alternating current (AC) power supply for their traction power systems, which provide better performance under long-distance power transmission than DC power supply. However, as the need for railway transportation increases due to more passengers and higher mobility requirements, more flexible and efficient traction systems are always needed. In Europe, AC traction power systems are mainly classified according to the voltage and frequency parameters (15 kV, 16.7 Hz) or (1×25 kV or 2×25 kV, 50 Hz). In all cases, railway operators have an absolute interest to run the electrified trains with the lowest possible operation and maintenance costs. In this context, power quality improvement at the three-phase power grid, associated with the AC electrified railway has drawn more attention in the last decades, especially after the evolution in the Power Electronics field. Subsequently, various solutions based on Power Electronics converters have been proposed to improve power quality in the electrified railway, e.g., the flexible AC transmission systems (FACTS). The rail power conditioner (RPC) is one of the FACTS devices that can be used to improve power quality by compensating harmonic contents, reactive power and negative sequence components of currents generated by the railway system. Among the other possible multilevel power converters, the modular multilevel converter (MMC) is an attractive solution for medium-voltage applications due to harmonics reduction, lower switching losses, and higher flexibility, scalability and reliability. Therefore, the MMC has been enhanced to be combined with the FACTS family. Taking into consideration the existing opportunities in the railway industry, not only in the development of the electric train itself, but also on the power quality improvement in the electrified railway, there is a strong investment in technological development for electrified railway systems. Therefore, this work presents a new topology of Power Electronics converter (RPC based on MMC) that compensates power quality problems associated with traction power systems, thus, reducing the operating costs of the electrified trains and increasing the power capacity of the electric traction grid. The main innovations of the RPC based on MMC are the integration of the MMC topology to operate as a railway power quality conditioner, benefiting from the advantages of the MMC in the traction power supply system. In this context, the research work proposed and developed in this Ph.D. thesis aimed to design, develop and validate a reduced-scale laboratory prototype of the RPC based on MMC, including all the necessary control algorithms and simulation models that are important to support the correct operation of the proposed system. Under simulation conditions, this work developed control algorithms for different RPC topologies, (full-bridge, half-bridge, three-wire, etc.) for demonstrating the general capabilities of the RPC system, and also for two different transformers connections (V/V and Scott). The most favorable RPC based on MMC topology (based on half-bridge MMC) was deeply and extensively simulated, namely employing predictive control approach. The experimental results obtained from a developed reduced-scale prototype confirm the validity of the presented control theory, as well as the power quality improvement capability of the proposed solution.
Os sistemas ferrovias foram progressivamente desenvolvidos desde que James Watt apresentou uma técnica de conversão da energia a vapor para um movimento circular em 1763. Com a novidade dos motores a vapor e a sua implementação nas redes ferroviárias, a indústria ferroviária rapidamente se tornou um catalisador econômico em todo o mundo devido às vantagens no transporte de passageiros e mercadorias. Em 1879, a empresa Siemens & Halske introduziu o primeiro comboio elétrico do mundo na cidade de Berlim, consistindo numa locomotiva com três vagões, alimentado por um terceiro trilho isolado alimentado com corrente contínua em 150 V (CC). A partir desse momento, o mundo começou a reconhecer a importante transição da energia a vapor para a energia elétrica e o potencial na ferrovia eletrificada como um meio de transporte de massa. Devido à abundância de combustível fóssil no século passado, os comboios a Diesel não eram apenas comuns, mas também dominaram o setor ferroviário. Consequentemente, o desenvolvimento das infraestruturas dos comboios elétricos desacelerou, e o caminho para haver comboios totalmente eletrificados tornou-se bastante longo. Nesse contexto, os comboios elétricos começaram a impor-se progressivamente, inicialmente pela combinação do motor Diesel e do motor elétrico, resultando numa locomotiva híbrida. No entanto, com o aumento da demanda pelo transporte, e com o aumento do preço dos combustíveis nas últimas décadas, os comboios elétricos afirmaram-se por poderem oferecer custos operacionais mais baixos, assim como melhor desempenho ambiental. Atualmente, a maioria dos comboios elétricos de alta velocidade utilizam sistema de tração em corrente alternada (CA), que oferece melhor desempenho na transmissão de energia a longa distância do que sistema de tração em corrente contínua CC. No entanto, o aumento do transporte ferroviário requer a melhoria da eficiência energética devido a haver mais passageiros e maiores requisitos de mobilidade. Na Europa, os sistemas de tração elétrica são classificados principalmente de acordo com os parâmetros de tensão e frequência (15 kV, 16,7 Hz) ou (1×25 kV ou 2×25 kV, 50 Hz). Em ambos os casos, os operadores ferroviários têm interesse absoluto em otimizar os custos. Nesse contexto, a melhoria da qualidade de energia elétrica na ferrovia suscitou mais atenção nas últimas décadas, principalmente pela introdução da eletrônica de potência. Posteriormente, várias soluções baseadas em conversores de eletrônica de potência foram propostas para melhorar a qualidade de energia elétrica na ferrovia, como por exemplo, os sistemas flexíveis de transmissão CA (FACTS – Flexible AC Transmission Systems). O condicionador ativo de potência ferroviário (RPC – Rail Power Conditioner) é um dos dispositivos FACTS que pode ser usado para melhorar a qualidade da energia elétrica, compensando o conteúdo harmônico, a potência reativa e os componentes de sequência negativa das correntes. Por outro lado, o conversor multinível modular (MMC – Modular Multilevel Converter) é uma solução atraente para aplicações de média tensão, devido à redução dos harmónicos e das perdas de comutação, e ao aumento da flexibilidade, confiabilidade e escalabilidade. Deste modo, o MMC foi aprimorado para ser integrado na família FACTS. Levando em consideração as oportunidades existentes no setor ferroviário, não apenas no desenvolvimento do próprio comboio elétrico, mas também na melhoria da qualidade de energia elétrica na ferrovia, existe um forte investimento no desenvolvimento tecnológico para os sistemas ferroviários eletrificados. Assim sendo, este trabalho apresenta uma nova topologia de conversor de eletrônica de potência (RPC baseado em MMC) que compensa os problemas de qualidade de energia elétrica associados aos sistemas de tração, reduzindo os custos operacionais dos comboios elétricos e otimizando a qualidade de energia da rede elétrica. As inovações principais do RPC baseado em MMC são a integração da topologia do MMC para operar como condicionador de qualidade de energia elétrica na ferrovia, beneficiando das vantagens do MMC. Neste contexto, o trabalho de investigação proposto e desenvolvido nesta tese apontou como objetivo projetar, desenvolver e validar um protótipo laboratorial em escala reduzida do RPC baseado em MMC, incluindo todos os algoritmos de controlo necessários e os modelos de simulação que são importantes para suportar a operação correta do sistema. Sob condições de simulação, este trabalho desenvolveu algoritmos de controlo para diferentes topologias do RPC (ponte completa, meia ponte, três fios, etc.), para demonstrar as capacidades gerais do sistema do RPC, e também para dois transformadores diferentes (V/V e Scott). O RPC mais favorável baseado na topologia do MMC (baseado no MMC de meia ponte) foi profunda e extensivamente simulado, nomeadamente utilizando uma abordagem de controlo preditivo. As simulações e os resultados experimentais confirmam a validade da teoria de controlo apresentada, bem como a capacidade de melhoria da qualidade de energia elétrica na solução proposta.
Fundação para a Ciência e a Tecnologia – FCT), which allowed me to continue my studies with the PD/BD/127815/2016 Ph.D. scholarship under the Innovation in Railway Systems and Technologies Doctoral Program – iRail.

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.

Η παρούσα διπλωματική εργασία έχει ως αντικείμενο τη διερεύνηση της λειτουργίας και το σχεδιασμό συστήματος ελέγχου του δυναμικού αποκαταστάτη τάσης (DVR - Dynamic Voltage Restorer, όπως αναφέρεται στη διεθνή βιβλιογραφία) που χρησιμοποιείται στα δίκτυα διανομής. Η συγκεκριμένη συσκευή ανήκει στην κατηγορία των FACTS (Flexible ac Transmission Systems), παρέχει εν σειρά αντιστάθμιση, και ο σκοπός λειτουργίας της είναι η βελτίωση της ποιότητας της παρεχόμενης ισχύος και η αύξηση της αξιοπιστίας του συστήματος. Εξειδικεύεται στις βυθίσεις τάσεως. Πιο συγκεκριμένα, ο δυναμικός αποκαταστάτης τάσης έχει στόχο, όπως μαρτυρά και το όνομά του, να διατηρεί την τάση ενός φορτίου κατά το δυνατόν σταθερή στην τιμή που αυτή είχε πριν συμβεί η όποια βύθιση. Έτσι, θα καταβληθεί προσπάθεια προκειμένου να προσομοιωθεί η λειτουργία μιας τέτοιας συσκευής σε ένα απλό δίκτυο με τη βοήθεια του λογισμικού PSCAD. Θα κατασκευαστεί ο DVR καθώς και το σύστημα ελέγχου του κι αφού συνδεθεί σε ένα φορτίο, θα δημιουργήσουμε διάφορα είδη σφαλμάτων και θα μελετήσουμε την απόκρισή του και την ικανότητά του να αποκαθιστά την τάση.
This diploma thesis refers to Dynamic Voltage Restorer (DVR), a series compensator used in transmission systems. It is a device that belongs to FACTS and its main function is the mitigation of volatge sags and swells.

Analytical modeling of HVDC systems has been a difficult task without a to-date reported model convenient for serious analysis of practically reported HVDC stability problems. In order to cover the frequency range f<100Hz, and to cater for different model requirements, three different HVDC-HVAC models are developed in this Thesis: Detailed linear-continuous model, simplified linear continuous model and linear discrete model. Detailed HVDC-HVAC system model is intended for small signal analysis of HVDC-HVAC interactions and resulting stability problems. It demonstrates good response matching against PSCAD/EMTDC simulation, where the CIGRE HVDC Benchmark model is used as the test system. All model variables (states) and parameters have physical meaning, and the model consists of modules, which reflect actual physical subsystems. Simplified HVDC-HVAC system dynamic model is developed as a fourth order dynamic model, which is less accurate but more convenient for the analysis, than the detailed model. The model proves to be reliable for controller design for mitigation of composite resonance and for the study of non-linear effects in HVDC systems. The developed linear discrete model is primarily intended for the system analysis at frequencies close to 100Hz on DC side of HVDC system. A new approach in modeling of TCR/TCSC, based on the same principles for HVDC modeling, is presented in this Thesis. The model development is far less difficult than the similar models presented in literature. PSCAD/EMTDC simulation confirms the model validity. The simplified, linear-continuous model is used for the analysis of dominant non-linear effects in HVDC systems. The analysis of non-linear mode transformation between constant beta and constant gamma operation, shows that limit-cycle oscillations are not expected to develop, for normal operating conditions. The analysis of converter firing angle modulation shows that converter behaves as a non-linear element only for some unlikely operating conditions. In this Thesis, it is attempted to counteract the composite resonance phenomenon by modifying the resonant condition on DC system impedance profile. This is accomplished by designing a supplementary HVDC controller that acts on HVDC firing angle on both line ends. PSCAD/EMTDC simulation results show that significant reduction in DC side first harmonic component (in some cases to 1/4 of the original value) is possible with the newly designed controller. Chapter 5 studies 100Hz oscillations on DC side of HVDC system. A methodology for designing a new controller to counteract negative affects of these oscillations is presented. Linear simulation of the detailed controller design confirms noticeable reduction in second harmonic on DC side. The eigenvalue decomposition and singular value decomposition is used for small-signal analysis of HVDC-HVAC interactions. The analysis of sensitivity of the dominant system eigenvalues with respect to the AC system parameters, shows the frequency range for the possible oscillatory instabilities at rectifier and at inverter side. The rectifier side of the system is most likely to experience instability at higher frequencies, whereas at inverter side the instabilities can be expected at lower frequencies. Further analysis shows that reduction in the AC system strength i will predominantly affect the eigenvalues at lower frequencies, where the SCR reduction at inverter side much more affects the system stability. The analysis of interactions between AC and DC systems through the influence of inherent feedback loops gives recommendations for the possible control of interaction variables with the aim of system stability improvement. The root locus technique is used for the stability analysis of HVDC control loops, where all conventional and some alternative control methods suggested in the literature are investigated. It is found that DC feedback, at rectifier side, significantly improves the system stability at lower frequencies, however, at frequencies close to the first harmonic this feedback control degrades the system stability, actually accelerating the development of composite resonance. At inverter side, most of the feedback loops improve the system stability in a certain frequency range, whereas at other frequencies they noticeably deteriorate stability. Among all reported inverter control methods, reactive current feedback is found to be the best option. The last Chapter develops a new controller for HVDC system operation with very weak inverter AC systems. The selection of feedback signal for this controller, is based on the analysis of positioning of zeros for candidate feedback signals. It is found that AC current angle is the best inverter feedback signal. This feedback signal can move the unstable complex eigenvalues left, into the stable region, without significantly affecting remaining eigenvalues. For the additional improvement in the system performance, a second order filter, designed using control theory, is placed into the feedback loop. The main design objective is the system robustness with respect to the AC system parameters changes. The controller designed in this Thesis, tolerates very wide changes in system strength, 1.7