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Utilization of power electronics based controllable systems (or devices) in transmission systems has opened new opportunities for the power industry to optimize utilization of the existing transmission systems, and at the same time to keep high system reliability and security. As a member of these controllable systems, Voltage Source Converters-based High Voltage direct current (VSC-HVdc) systems have the ability to rapidly control the transmitted active power, and also to independently exchange reactive power with transmissions systems. Therefore, VSC-HVdcs with a suitable control scheme can offer an alternative means to enhance transient stability, to improve power oscillations damping, and to provide voltage support. An interesting application of this system is the analysis of a power system when a VSC-HVdc is connected in parallel with ac transmission lines. This thesis presents the derivation of control strategies to damp power oscillations, to enhance the transient stability and to provide voltage support for a VSC-HVdc. The thesis also formulates a multichoice control strategy and its application when the VSC-HVdc is connected in a synchronous system. The control strategy for enhancing transient stability is based on the theory of Control Lyapunov Function. The control strategy for increasing the damping is based on Linear Analysis. A very effective well known way to increase damping in the system is modulating the active power through the HVdc. However, besides the control of active power, the thesis explores an alternative way to mitigate power oscillations by controlling the reactive power. This condition might be very useful when the dc link in the VSC-HVdc system is out of service, but the converter stations are in operating conditions. A simple model of VSC-HVdc is considered in order to test the control strategy. The model represents the VSC-HVdc as an element in the power system that provides adequate interaction with other systems elements. The model is intended for analysis of power flows and electromechanical transients. It is then sufficient to consider the power frequency components of voltages and currents represented by phasors that vary with time during transients. The model is valid for symmetrical conditions, i.e. positive sequence phasors are used for the representation of the electrical state. / QC 20101117

Renewable energy is playing an increasingly significant role in power system operation and stability assessment with its numerous penetration expansion. This is not only brought by its uncertain power output and inverter-based equipment structures but also its operation characteristics like Low Voltage Ride Through (LVRT). It is thus necessary to take these characteristics into consideration and further to find more adaptive schemes to implement them for more effective analysis and safer power system operation. All the aforementioned is based on the accurate identification of the system fundamental information. In this dissertation, a systematic approach is proposed to find the valid system model by estimating the transmission line parameters in the system with PMU measurements. The system transient stability assessment is conducted based on this validated model. The constrained stability region is estimated with Lyapunov functions family based method in the center of angles reference frame considering renewables LVRT as operation limits. In order to integrate the LVRT constraints, a polytopic inner approximation mechanism is introduced to linearize and organize the transformed constraints in state space, which brings much scalability to the whole process. From the voltage stability perspective, an approach to adaptively adjust LVRT settings of the renewable energy sources in the system is formulated to guarantee the system load margin and thus the voltage security. A voltage prediction method is introduced for critical renewable energy sources identification. Estimation methods based on interpolation and sensitivities are developed and conducted for saving computation effort brought by continuation power flows. Multiple test cases are studied utilizing the proposed approaches and results are demonstrated. / Doctor of Philosophy / Renewable energy utilization is continuously rising nowadays. They are clean but highly dependent on natural resources, which causes their uncertainty and intermittence in electric power output. The power system, on the other hand, is designed for schedulable and controllable power generators, which make the traditional methods for system operation and analysis of the system stability much less effective facing the trend of renewables integration. In this dissertation, a series of systematic approaches are proposed firstly identify the system parameters for more accurate system modeling through PMU measurements, then to assess the system transient stability considering the renewable energy sources operation limits, and finally to adaptively adjust these operation limit for improving the system voltage security. The operation limits are transferred into the form in terms of system states. Linearization and approximation methods are also introduced to enhance the scalability of the processes. Multiple test cases are studied with the proposed approaches and the results demonstrate their effectiveness and efficiency.

O presente trabalho tem como objetivo investigar a presença de diferentes escalas de tempo nos modelos matemáticos que descrevem a dinâmica dos sistemas elétricos de potência (SEPs), em particular a existência de duas escalas de tempo distintas: lenta e rápida, e explorá-las no estudo de estabilidade transitória destes sistemas através da utilização de métodos diretos (funções energia). Em particular, o método do Ponto de Equilíbrio Instável de Controle (CUEP) para modelos com duas escalas de tempo será estudado e aplicado na análise de estabilidade transitória de SEPs. As bases teóricas para a análise de estabilidade transitória, de sistemas com duas escalas de tempo, serão apresentadas, assim como funções energia e novos algoritmos numéricos para o cálculo do CUEP nestes sistemas, a fim de evidenciar as melhorias e possíveis limitações deste novo método CUEP em duas escalas de tempo quando comparado ao método CUEP tradicional. Explorando as escalas de tempo lenta e rápida na análise de estabilidade transitória, espera-se que novos algoritmos numéricos mais robustos para o cálculo do CUEP sejam obtidos, assim como a diminuição do conservadorismo dos resultados. / The main objective of this work is to investigate the existence of several time-scales in the mathematical models of electric power systems, in particular the existence of two-time scales: slow and fast, and exploit these features in the direct transient stability assessment. In particular, the Controlling Unstable Equilibrium Point (CUEP) method is studied for two-time scale models of power systems and applied to transient stability analysis. In order to accomplish this aim, a sound theoretical basis for two-time scale transient stability analysis of electric power system models will be provided, as well as energy functions and new numerical algorithms for proper two-time scale CUEP calculations, with the purpose of investigating improvements and possible limitations of this method when compared with the traditional CUEP method. Exploiting the two-time scale features of power system models, it is intended to obtain new robust numerical algorithms for transient stability analysis, as well as to diminish the conservativeness of the results.

O presente trabalho tem como objetivo investigar a presença de diferentes escalas de tempo nos modelos matemáticos que descrevem a dinâmica dos sistemas elétricos de potência (SEPs), em particular a existência de duas escalas de tempo distintas: lenta e rápida, e explorá-las no estudo de estabilidade transitória destes sistemas através da utilização de métodos diretos (funções energia). Em particular, o método do Ponto de Equilíbrio Instável de Controle (CUEP) para modelos com duas escalas de tempo será estudado e aplicado na análise de estabilidade transitória de SEPs. As bases teóricas para a análise de estabilidade transitória, de sistemas com duas escalas de tempo, serão apresentadas, assim como funções energia e novos algoritmos numéricos para o cálculo do CUEP nestes sistemas, a fim de evidenciar as melhorias e possíveis limitações deste novo método CUEP em duas escalas de tempo quando comparado ao método CUEP tradicional. Explorando as escalas de tempo lenta e rápida na análise de estabilidade transitória, espera-se que novos algoritmos numéricos mais robustos para o cálculo do CUEP sejam obtidos, assim como a diminuição do conservadorismo dos resultados. / The main objective of this work is to investigate the existence of several time-scales in the mathematical models of electric power systems, in particular the existence of two-time scales: slow and fast, and exploit these features in the direct transient stability assessment. In particular, the Controlling Unstable Equilibrium Point (CUEP) method is studied for two-time scale models of power systems and applied to transient stability analysis. In order to accomplish this aim, a sound theoretical basis for two-time scale transient stability analysis of electric power system models will be provided, as well as energy functions and new numerical algorithms for proper two-time scale CUEP calculations, with the purpose of investigating improvements and possible limitations of this method when compared with the traditional CUEP method. Exploiting the two-time scale features of power system models, it is intended to obtain new robust numerical algorithms for transient stability analysis, as well as to diminish the conservativeness of the results.

Diante da crescente penetração da energia elétrica gerada por unidades eólicas no sistema elétrico de inúmeros países, a presente dissertação de mestrado investiga os problemas de estabilidade transitória, para geradores síncronos, de estabilidade de tensão, para geradores síncronos e aerogeradores e de estabilidade de velocidade, para aerogeradores, em sistemas de transmissão de energia elétrica, frente a grandes perturbações. São estabelecidas comparações entre os diferentes comportamentos dinâmicos de cada tipo de unidade eólica, referentes aos tipos A, B, C e D. Também, são verificados os efeitos de contingências aplicadas a sistemas teste, na presença de sistemas de geração convencionais dotados de geradores síncronos e parques eólicos compostos pelos tipos de unidades eólicas citadas, a fim de verificar a relação entre os diferentes sistemas de geração. As simulações realizadas contemplam aspectos dinâmicos de modelos genéricos de aerogeradores. São estudadas as partes constituintes de aerogeradores, realizadas as modelagens e discutidos os métodos de controle usuais. As principais características dos modelos genéricos de unidades eólicas, assim como os respectivos diagramas de bloco para cada modelo são apresentados. As simulações são realizadas no programa computacional PSS/E, cujos modelos, já validados, representam aerogeradores reais de fabricantes distintos. É observado que as diferentes características dos modelos de unidades eólicas e as estratégias de controle empregadas para atenuar os efeitos negativos de contingências impostas ao sistema, influenciam significativamente o perfil da tensão e o fluxo de potência na rede elétrica. Este fato é refletido em diferentes respostas dos sistemas de geração. As unidades eólicas mostram-se resistentes à perderem a estabilidade e apresentam-se capazes de estabilizar um sistema elétrico de potência, devido aos mecanismos de controle de velocidade e potência dos mesmos. A violação da curva LVRT mostra-se a principal causa da desconexão de aerogeradores da rede elétrica. Verifica-se neste trabalho que os aerogeradores somente tornam-se instáveis quando os geradores síncronos perdem o sincronismo, ou quando o controle do torque aerodinâmico é desconsiderado. / Considering the growing penetration of electrical energy generated by wind turbines in the power system of numerous countries, the present masters dissertation investigates the problems of transient stability, for synchronous generators, voltage stability, for synchronous generators and aerogenerators, and velocity stability, for aerogenerators, in transmission systems, before large disturbances. Comparisons are made between the different dynamic behaviors of each type of wind energy conversion system, namely types A, B, C and D. The effects of contingencies in some test systems in the presence of wind farms with different types of aerogenerators and conventional synchronous generators are verified, whose aim of the simulations is to analyze the relation between the different generation systems. The simulations performed contemplate dynamic aspects of generic models of wind turbines. The components of wind turbines are studied, the modeling of these components is elaborated and the usual control methods are discussed. The main characteristics of generic wind models and the diagram blocks are presented. The simulations were performed in software PSS/E, whose models already validated, represent real machines. The different characteristics of aerogenerator models and the control strategies employed to mitigate the negative consequences, from several contingencies, significantly influence the voltage profile and the power flow network. This fact implies in different responses of generation systems. It is verified that wind generators are very robust to perturbations and contribute to the stabilization of synchronous generators in a power system, increasing the average critical clearing times. Unstable modes related to the acceleration of the wind generator rotor are hardly ever observed due to the existing mechanisms of control of speed and generated power. It is observed that violation of the LVRT curve is the main cause of disconnection of wind turbines from the grid and it consists of the main cause of power system collapse triggered by problems in this type of generators.

Diante da crescente penetração da energia elétrica gerada por unidades eólicas no sistema elétrico de inúmeros países, a presente dissertação de mestrado investiga os problemas de estabilidade transitória, para geradores síncronos, de estabilidade de tensão, para geradores síncronos e aerogeradores e de estabilidade de velocidade, para aerogeradores, em sistemas de transmissão de energia elétrica, frente a grandes perturbações. São estabelecidas comparações entre os diferentes comportamentos dinâmicos de cada tipo de unidade eólica, referentes aos tipos A, B, C e D. Também, são verificados os efeitos de contingências aplicadas a sistemas teste, na presença de sistemas de geração convencionais dotados de geradores síncronos e parques eólicos compostos pelos tipos de unidades eólicas citadas, a fim de verificar a relação entre os diferentes sistemas de geração. As simulações realizadas contemplam aspectos dinâmicos de modelos genéricos de aerogeradores. São estudadas as partes constituintes de aerogeradores, realizadas as modelagens e discutidos os métodos de controle usuais. As principais características dos modelos genéricos de unidades eólicas, assim como os respectivos diagramas de bloco para cada modelo são apresentados. As simulações são realizadas no programa computacional PSS/E, cujos modelos, já validados, representam aerogeradores reais de fabricantes distintos. É observado que as diferentes características dos modelos de unidades eólicas e as estratégias de controle empregadas para atenuar os efeitos negativos de contingências impostas ao sistema, influenciam significativamente o perfil da tensão e o fluxo de potência na rede elétrica. Este fato é refletido em diferentes respostas dos sistemas de geração. As unidades eólicas mostram-se resistentes à perderem a estabilidade e apresentam-se capazes de estabilizar um sistema elétrico de potência, devido aos mecanismos de controle de velocidade e potência dos mesmos. A violação da curva LVRT mostra-se a principal causa da desconexão de aerogeradores da rede elétrica. Verifica-se neste trabalho que os aerogeradores somente tornam-se instáveis quando os geradores síncronos perdem o sincronismo, ou quando o controle do torque aerodinâmico é desconsiderado. / Considering the growing penetration of electrical energy generated by wind turbines in the power system of numerous countries, the present masters dissertation investigates the problems of transient stability, for synchronous generators, voltage stability, for synchronous generators and aerogenerators, and velocity stability, for aerogenerators, in transmission systems, before large disturbances. Comparisons are made between the different dynamic behaviors of each type of wind energy conversion system, namely types A, B, C and D. The effects of contingencies in some test systems in the presence of wind farms with different types of aerogenerators and conventional synchronous generators are verified, whose aim of the simulations is to analyze the relation between the different generation systems. The simulations performed contemplate dynamic aspects of generic models of wind turbines. The components of wind turbines are studied, the modeling of these components is elaborated and the usual control methods are discussed. The main characteristics of generic wind models and the diagram blocks are presented. The simulations were performed in software PSS/E, whose models already validated, represent real machines. The different characteristics of aerogenerator models and the control strategies employed to mitigate the negative consequences, from several contingencies, significantly influence the voltage profile and the power flow network. This fact implies in different responses of generation systems. It is verified that wind generators are very robust to perturbations and contribute to the stabilization of synchronous generators in a power system, increasing the average critical clearing times. Unstable modes related to the acceleration of the wind generator rotor are hardly ever observed due to the existing mechanisms of control of speed and generated power. It is observed that violation of the LVRT curve is the main cause of disconnection of wind turbines from the grid and it consists of the main cause of power system collapse triggered by problems in this type of generators.

[pt] Os sistemas de energia elétrica estão operando atualmente próximos dos limites de estabilidade, comprometendo a segurança. Este fato tem sido evidenciado por diversos blackouts no mundo inteiro. A avaliação da segurança dinâmica torna-se, então, fundamental. O objetivo é a busca de um método rápido e, sobretudo, confiável, para analisar o comportamento dinâmico de um sistema de energia elétrica. Esta dissertação trata, portanto, do problema da avaliação da segurança dinâmica de sistemas de energia elétrica. A avaliação é realizada através da determinação das margens de estabilidade, utilizando os resultados de simulações no domínio do tempo, que fornece informações qualitativas a respeito da estabilidade na primeira oscilação. O grupo de geradores severamente perturbados é determinado e a margem de estabilidade de cada um deles é calculada. O gerador que apresentar a menor margem determina a margem de estabilidade do sistema. Quando a margem de estabilidade assume valor nulo, o tempo crítico de eliminação da falta é obtido. Estes resultados são comparados com aqueles determinados pelo método de tentativa e erro, utilizando um programa convencional de estabilidade transitória. Desta forma, as contingências são classificadas em função dos tempos críticos de eliminação de falta, de acordo com o nível de severidade. Essa classificação permite reduzir o conjunto de contingências a ser estudado. A contribuição deste trabalho é mostrar que o critério baseado na aceleração imediatamente após a eliminação da falta é mais eficaz que aquele baseado na aceleração imediatamente após a ocorrência da falta para a indicação do grupo de geradores severamente perturbados. / [en] Power systems have been operating nowadays near to the stability limits putting security under risk. This is one of the reasons why the dynamic security assessment is a fundamental tool to avoid the occurrence of blackouts in the whole world. The goal is a reliable and fast way to evaluate the dynamic behavior of a power system. This dissertation deals with the problem of dynamic security assessment of power systems. The evaluation is performed based on stability margins calculated from time domain simulation results, providing qualitative information about the first swing stability. The group of severely disturbed machines is defined and the stability margins are determined. The machine with the lowest margin determines the stability margin of the system. When the system margin approaches the zero value, the critical clearing time is obtained. These outcomes are compared with that ones determined by trial and error method using a conventional transient stability program. Having done that, a contingency ranking is defined according to the critical clearing time. The ranking minimizes the number of contingencies that have to be studied. This dissertation shows that the criterion to define the group of severely disturbed machines based on the machine accelerations at the instant immediately after the fault clearing time is more efficient than that one based on the machine accelerations at the instant immediately after the fault occurrence.

<p>In this thesis, controllers for power oscillation damping, transient stability improvement and power flow control by means of a Controlled Series Compensator (CSC) and and a Dynamic Power Flow Controller (DPFC) are proposed. These devices belong to the group of power system components referred to as Flexible AC Transmission System (FACTS) devices. The developed controllers use only quantities measured locally at the FACTS device as inputs, thereby avoiding the risk of interrupted communications associated with the use of remote signals for control.</p><p>For power systems with one dominating, poorly damped inter-area power oscillation mode, it is shown that a simple generic system model can be used as a basis for damping- and power flow control design. The model for control of CSC includes two synchronous machine models representing the two grid areas participating in the oscillation and three reactance variables, representing the interconnecting transmission lines and the FACTS device. The model for control of DPFC is of the same type but it also includes the phase shift of the internal phase-shifting transformer of the DPFC.</p><p>The key parameters of the generic grid models are adaptively set during the controller operation by estimation from the step responses in the FACTS line power to the changes in the line series reactance inserted by the FACTS device. The power oscillation damping controller is based on a time-discrete, non-linear approach which aims to damp the power oscillations and set the desired power flow on the FACTS line by means of two step changes in the line reactance separated in time by half an oscillation cycle.</p><p>A verification of the proposed controllers was done by means of digital simulations using power system models of different complexities. The CSC and DPFC controllers were shown to significantly improve the small-signal- and transient stability in one four-machine system of a type commonly used to study inter-area oscillations. The CSC controller was also tested for 18 different contingencies in a 23-machine system, resulting in an improvement in both the system transient stability and the damping of the critical oscillation mode. </p>

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. / <p>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</p>