[pt] Após a incidência de inúmeros colapsos de tensão em sistemas elétricos de potência por todo o mundo, a segurança de tensão tornou-se um assunto de extrema importância. O fenômeno de estabilidade de tensão deve-se à operação do sistema com carregamento elevado e se manifesta pela existência de uma máxima transferência de potência pela rede e ações de controle de tensão tendo efeito oposto ao usual. Com o aumento da inserção de geração distribuída, usualmente conectada à rede de distribuição existente e em tensões mais baixas, a possibilidade de ocorrência do fenômeno precisa ser investigada. É demonstrada a possibilidade de ocorrência em barras com geradores ou compensadores síncronos, onde a máxima potência gerada poderia estar limitada e o controle de tensão poderia ter o efeito oposto ao esperado. É apresentado o ferramental utilizado na avaliação das condições de estabilidade de tensão composto de índices com significado físico e matemático. Estabeleceu-se um procedimento para realizar simulações nos sistemas-teste IEEE 34 Barras e 70 Barras. É analisado o impacto do gerador nas condições de estabilidade de tensão, no perfil de tensão em regime permanente e nas perdas de potência ativa. Concluiu-se que, nos testes realizados, a injeção de potência ativa pelo gerador foi limitada por tensões fora da faixa permitida e não por problemas de estabilidade de tensão. / [en] After several incidence of voltage collapse in electric power systems around the world, the voltage security has become a matter of utmost importance. The phenomenon of voltage stability is due to the operation of the system with high loading and is manifested by the existence of a maximum power transfer over the network and efforts to control voltage having the opposite effect than usual. With increased integration of distributed generation, usually connected to the existing distribution network at lower voltages, the possibility of occurrence of the phenomenon should be investigated. It is demonstrated the possibility of occurrence in buses of synchronous generators or synchronous condensers, where the maximum generated power could be limited and the voltage control could have the opposite effect. It presented the tools used in assessing the conditions of voltage stability composite indexes with physical and mathematical meaning. I was established a procedure for performing simulations on IEEE 34 bus and 70 bus test systems. It analyzed the impact of the generator under the conditions of voltage stability, voltage profile in the steady state and the power losses. It was concluded that, in tests, the injection of active power from the generator was limited by voltage out of range allowed and not by voltage stability problems.

Voltage instability is a major concern in operation of power systems and it is well known that voltage instability and collapse have led to blackout or abnormally low voltages in a significant part of the power system. Consequently, tracking the proximity of the power system to an insecure voltage condition has become an important element of any protection and control scheme. The expected time until instability is a critical aspect. There are a few energy management systems including voltage stability analysis function in the real-time environment of control centres, these are based on assumptions (such as off-line models of the system loads) that may lead the system to an insecure operation and/or poor utilization of the resources. Voltage instability is driven by the load dynamics, and investigations have shown that load restoration due to the on-load tap changer (OLTC) action is the main cause of the voltage instability. However, the aggregate loads seen from bulk power delivery transformers are still the most uncertain power system components, due to the uncertainty of the participation of individual loads and shortcomings of the present approaches in the load modeling. In order to develop and implement a true on-line voltage stability analysis method, the on-line accurate modeling of the higher voltage (supply system) and the lower voltage level (aggregate load) based on the local measurements is required. In this research, using the changes in the load bus measured voltage and current, novel methods are developed to estimate the supply system equivalent and to identify load parameters. Random changes in the load voltage and current are processed to estimate the supply system Thevenin impedance and the composite load components are identified in a peeling process using the load bus data changes during a large disturbance in the system. The results are then used to anticipate a possible long-term voltage instability caused by the on-load tap changer operation following the disturbance. Work on the standard test system is provided to validate the proposed methods. The findings in this research are expected to provide a better understanding of the load dynamics role in the voltage stability, and improve the reliability and economy of the system operation by making it possible to decrease uncertainty in security margins and determine accurately the transfer limits.

Gotland har länge präglats av mycket speciella lösningar och legat i framkant vad gäller ny teknik. Under ca 20 år har Gotland haft en VSC-HVDC-station som har stabiliserat spänningen i det gotländska nätet. HVDC Light har löst en del av de tekniska begränsningarna som hindrat utvecklingen av vindkraftverk. Anläggningen börjar lida mot slutet av sin tekniska livslängd och examensarbetet är ett första steg i utredningen för en eventuell ersättning. Med hjälp av elkvalitetsmätare kartlagdes prestanda och eventuella förbättringsområden. Den svenska stamnätsoperatören Svenska kraftnät har sedan 2016 haft en VSC-HVDC-station som använts för att utväxla energi till asynkrona systemet i Östeuropa. Svenska kraftnät har på senare tid velat utnyttja spänningsregleringsmöjligheterna och utreda mättekniska metoder. Elkvalitetsmätare placerades ut på lämpliga mätpunkter för att utreda anläggningens beteenden. ELSPEC G4500 elkvalitetsmätare installerades på Nordbalt och Gotlands HVDC Light för att mäta under sensommar till hösten 2017. Skillnader mellan CVT och IVT samt Rogowskispole och CT mättes. Valet av ELSPEC lämpade sig bra eftersom att inga triggningsvillkor behövdes som tillåter att man kan upptäcka små men viktiga avvikelser. Gotlands HVDC Light stabiliserar nätet främst mot spänningsdippar efter kortslutningar i 10 kV-elnätet och flimmer ifrån vindkraftsparkerna i ände 2. Märkeffekten för en uppdaterad anläggning kommer baseras på den kortslutningseffekt i 10 kV-nätet som kan utvecklas under anläggningens livstid. Spänningsregleringen ska baseras på en PI-regulator och ska kunna reglera fullt på 40 ms. Komponentspänningar kan användas för att ge reaktiv effekt på de faser som behöver det. Behovet mot flimmer ska baseras på en mätning i närmaste konsumtionscentrum, två mil från ände 2. Om behovet finns, ska en separat loop för flimmerkontroll som motverkar 1,5 Hz-komponenter implementeras utifrån en punkt ca 1,5 mil ifrån ände 2. Teknikvalet står mellan two-level generation 3 eller MMC, beroende på uppgradering eller ersättning. Många olika framtida scenarion påverkar HVDC Lights roll och oavsett, kommer mycket resurser behövas för Gotlands och HVDC Lights framtid. Det har observerats två beteenden på Nordbalt varav ena är långsam och det andra beteendet är snabbt. Det snabba beteendet uppstår när lågohmiga fel sker som också synkronmaskinerna tar hand om. Nordbalt kan hjälpa till för mindre spänningsvariationer om den varit snabbare likt beteendet vid lågohmiga fel. En stabilitetsbedömning behövs dock eftersom att snabbare beteende ökar risken för instabilitet. Eftersom att data mellan CVT och IVT skiljde sig mycket, upplystes mättekniska problem. Några lösningar diskuteras varav RCVT och PQSensor gås igenom grundligt. Alternativa lösningar som MoW och mobila enheter presenteras också vagt. Samtliga lösningar visade sig ha praktiska hinder, vilket försvårar genomförandet. Det uppmanas därför att man bör testa teknikerna i laborationsmiljö eftersom begränsat med studier gjorts på dessa samt att konkurrerande tillverkare uppger olika uppgifter. / Gotland has long been known for various special solutions and been on the leading edge regarding new technology. For the past 20 years Gotland has had a VSC-HVDC station that has stabilized the voltage in the Gotlandic grid. HVDC Light has solved some of the technical limitations that has hindered the growth of wind turbines. The station has almost reached the end of its technical lifespan and the thesis is a first step in the investigation for an eventual replacement. With power quality analyzers performance and improvements were investigated. The Swedish transmission system operator Svenska kraftnät have since 2016 had a VSCHVDC station that have been used to exchange energy to the asynchronous system in Eastern Europe. Svenska kraftnät has of lately wanted to take advantage of the voltage control capabilities and investigate measurement technologies. Power quality analyzers were installed on suitable connection points to investigate the behavior of the station. ELSPEC G4500 power quality analyzers were installed on Nordbalt’s and Gotland’s HVDC Light to measure during the late summer to fall of 2017. The differences between CVT and IVT, and Rogowski coil and CT were captured. The choice of ELSPEC suited well since no trigger conditions are needed which allows small but important errors to be discovered. Gotland’s HVDC Light stabilizes the grid mostly against voltage sags for faults in the 10 kVgrid and flicker from the wind farms in node 2. The rated power for an upgrade should be based on the 10 kV-grid short-circuit power to be developed during the station’s life span. The voltage control should be based on a PI-controller and should be able to transmit full power in 40 ms. Component voltages can be used to produce reactive power on the phases in need. The need against flicker should be based on measurements in the closest center of consumption, 2 miles from node 2. If it’s needed, a separate loop for flicker-control that prevents 1.5 Hz-components based on a point 1.5 mile from node 2 should be implemented. Depending on an upgrade or replacement, the topology can either be “two-level generation 3” or MMC. Many different future scenarios affect HVDC Light’s role and more resources will be required for Gotland’s and HVDC Light’s future. Two behaviors have been observed on Nordbalt where one is slow and the other behavior is fast. The fast behavior was only observed when low impedance faults occurs, that the synchronous generators also handles. Nordbalt can contribute to small voltage variations if it was faster, like the behavior for low impedance faults. A consideration in stability is needed since a faster behavior can lead to stability problems. Since the data between CVT and IVT differed a lot, metrology problems were discovered. Some solutions were discussed, of which RCVT and PQSensor was thoroughly reviewed. Alternative solutions like MoW and mobile units were also presented vaguely. All solutions showed practical difficulties, which complicates the implementation. It is therefore encouraged to test the technologies in a laboratory environment since few studies have been made on these and competing manufactures state different information.

This work presents new analytical and computational methods for operation security assessment of electric power systems by considering Angle and Voltage Stability Limits. In the context of Angle Stability, it was developed a method for estimating transient security margins based on equivalent network reduction techniques and coherent generators. The angle speed deviation was the criterion used to identify generators that swing together. The reduced order was accomplished by replacing two clusters of coherent generators by an One-Machine Infinite Bus equivalent system. With this equivalent, critical fault clearing times and security margins are calculated with the aid of the Equal Area Criterion. The results were in a good agreement when compared with others methods, especially those based on Transient Energy Function used as a Lyapunov function. The method was also applied for estimating security margins and critical fault clearing times of power systems in the presence of wind power generation. It was demonstrated that the proposed method can be used to select critical contingencies, where detailed power system models are needed such that it can reproduce the actual behavior of the system. With respect to Voltage Stability, it was developed an analytical and computational method for steady state voltage stability analysis on a P-Q plane. First of all, it was applied on a simple two-bus power system, and the analytical and computational results were compared. Then, a Voltage Stability Index was derived, in order to obtain the security margins of each bus for any operational state of an n-bus power system. It was carried out by using a power system reduction technique. With the Voltage Stability Index, it is possible to identify critical buses and the regions that are prone to voltage collapse. The voltage stability limits of a distribution power system was also analyzed by means of the P-Q curves, by considering different operation scenarios of wind power generation. It was demonstrated that the wind power can contribute to improve the voltage security margins. Finally, the method was applied to a real power system of Companhia Estadual de Distribuição de Energia Elétrica, in the presence of wind power generation. The methods are computationally efficient and suitable for planning, operation and real-time operation of electric power systems. / Este trabalho apresenta novos métodos analíticos e computacionais para a avaliação da segurança da operação de sistemas elétricos de potência considerando os Limites de Estabilidade Angular e de Tensão. No tema Estabilidade Angular, desenvolveu-se um método para a estimativa de margens de segurança transitória baseado em técnicas de redução de redes e geradores coerentes. O desvio de velocidade angular foi o critério usado para identificar geradores que oscilam juntos. A redução foi realizada substituindo-se dois grupos de geradores coerentes por um sistema Equivalente Máquina-Barra Infinita. Com este equivalente, os tempos críticos de abertura de falta e as margens de segurança são calculados com auxílio do Critério das Áreas Iguais. Os resultados encontrados foram muito satisfatórios quando comparados com aqueles obtidos por outros métodos, especialmente os que utilizam a função energia como função de Lyapunov. O método também foi aplicado para a estimativa de margens de segurança e tempos críticos de abertura de falta, em sistemas de potência na presença de geração eólica. Foi demonstrado que o método proposto é capaz de selecionar contingências críticas que precisam ser estudadas com modelos completos de modo a reproduzir o comportamento real do sistema elétrico. Com relação à Estabilidade Tensão, foi desenvolvido um método analítico e computacional para análise de estabilidade estática de tensão no plano P-Q. Primeiramente, o método foi aplicado em um sistema de potência simples com duas barras, e os resultados analíticos e computacionais foram comparados. Então, um Índice de Estabilidade de Tensão foi deduzido, para determinar a margem de segurança de cada barra para qualquer estado de operação de um sistema de potência com n-barras. Com o Índice de Estabilidade de Tensão, é possível identificar barras críticas e regiões com tendência ao colapso de tensão. Os limites de estabilidade de tensão de um sistema de distribuição foram analisados através das curvas P-Q , no qual foram considerados diferentes cenários de operação da geração eólica. Deste modo, foi demonstrado que a geração eólica pode contribuir para melhorar as margens de segurança de tensão. Finalmente, o método foi aplicado em um sistema de potência real pertencente à Companhia Estadual de Distribuição de Energia Elétrica. Os métodos desenvolvidos são computacionalmente eficientes e adequados para o planejamento da expansão e operação, bem como na operação em tempo real dos sistemas elétricos de potência.