The main objective of this thesis was to develop appropriate system protection schemes against two important causes of failure in power systems, namely, long-term voltage instability and cascade tripping of overloaded transmission lines, mainly due to overloading. To this purpose a distributed undervoltage load shedding scheme against voltage instability, and a centralized protection meant to alleviate line overload are proposed. The former, through the chosen system protection scheme characteristics, has the ability to adjust its actions to the disturbance location and severity. This behavior is achieved without resorting to a dedicated communication network. The distributed controllers do not exchange information, but are rather informed of their respective actions through voltage measurements. Neither do the controllers require a model of the system. This and the absence of communication makes the protection scheme simple and reliable. The other protection scheme, inspired of model predictive control, is aimed at bringing the currents in the overloaded lines below their limits in the time interval left by protections, while accounting for constraints on control changes. Its closed-loop nature allows to compensate for model uncertainties and measurement noise. In order to tune the proposed system protection schemes parameters and validate their performance it was preferred to detect plausible cascading event scenarios. To this purpose, an algorithm meant to identify such complex sequences has been developed. It encompasses hidden failures and the resulting system response. The tests performed on small systems as well as on a real-life one confirm not only that proposed protection schemes appropriately deal with the problems for which they were designed, but also that they cooperate satisfactorily for combined voltage and thermal problems that are beyond their individual capabilities.

[pt] O objetivo desta dissertação é realizar uma introdução aos estudos e às aplicações dos Esquemas de Proteção de Sistemas -EPSs- usados para manter a segurança dos Sistemas de Energia Elétrica. Um pequeno conjunto de definições e conceitos das áreas de proteção, confiabilidade e análise de desempenho de sistemas de potência é apresentado com o objetivo de distinguir os EPSs das proteções convencionais de componentes do sistema. Isto também ajuda a entender as funções dos EPSs com mais clareza. Os EPSs são usados para manter a integridade do sistema quando este está submetido a fenômenos que provocam instabilidade ou desligamentos em cascata. A classificação e a descrição destes fenômenos, bem como a aplicação dos EPSs estão mostradas nesta dissertação. A estrutura geral dos EPSs, as formas de classificá-los e as diferentes ações usadas pelos EPSs para controlar as perturbações são objeto de análise do trabalho. Para exemplificar, é dada uma visão de como se situam os EPSs no Sistema Interligado Brasileiro (SIB) e também pelo restante do mundo. Alguns EPSs usados no Brasil mereceram destaque especial.As análises dos principais blecautes ocorridos no Brasil e na América do Norte são apresentadas, de forma resumida, no apêndice I. Dessas análises são tiradas muitas lições para implementar medidas que minimizam o risco de interrupções no suprimento de energia. Também é feita uma comparação do grau de severidade dos grandes blecautes ocorridos no Brasil e, para os dois maiores, é realizada uma estimativa dos seus respectivos custos de interrupção. Procurou-se também, no apêndice II, estabelecer os requisitos básicos para a implantação de EPSs que usem tecnologia baseada em microprocessadores.Com base na experiência vivida nos estudos desenvolvidos pelo grupo que analisou a segurança do SIB após o blecaute de 11-MAR-99, foram estabelecidos critérios e procedimentos para analisar a segurança do sistema e realizar estudos de implantação de EPSs. E, como desfecho, é realizado um breve estudo do SIB, onde é analisado o desempenho dos EPSs para controlar graves emergências que podem ocorrer no presente momento. / [en] The objective of this dissertation is to accomplish an introduction to the studies and the applications of the System Protection Schemes -SPSs- used to maintain the Electric Energy Systems safety. A small group of definitions and concepts from the protection,reliability and power systems performance analysis areas is presented with the objective of distinguishing SPSs from the conventional system component protections. It also helps in understanding the functions of the SPSs with more clarity. The SPSs are used to maintain the integrity of the system when it is submitted to phenomena that provoke instability or cascade circuit outages. The classification and the description of these phenomena, as well as the SPSs application are shown in this dissertation. The general structure of the SPSs, the ways of classifying them and the different actions used by SPSs to control the disturbances are the objectives of the analysis of this work. In order to exemplify, a vision is given of how the SPSs are located in the Brazilian Interconnected System -BIS- and also in the rest of the world. Some SPSs used in Brazil deserved special attention.The analyses of the main blackouts that occurred in Brazil and in North America are presented, in a summarized term, in appendix I. Considerable knowledge is obtained from these analyses, leading to implementation measures that minimize the risk of interruptions in the energy supply. A comparison is also made of the degree of severity of the great blackouts that happened in Brazil and, for the two largest, an estimate of their respective interruption costs is made. In appendix II, an attempt was also made to establish the basic requirements for the SPSs implementation using microprocessing technology. Based on the experience lived during the studies developed by the group that analyzed the safety of the BIS after the blackout on March 11/1999, criteria and procedures to analyze the safety of the system and to accomplish studies of SPSs implantation were established. Finally, a brief study of the BIS is done, where the SPSs performances are analyzed to verify the ability to control serious emergencies that can happen in the present moment.