ABSTRACT Voltage instability has become a major concern in power systems. Many blackouts have been reported where the main cause is voltage instability. This thesis deals with two specific areas of voltage stability in on-line power system security assessments: small-disturbance (long-term) and large-disturbance (short-term) voltage stability assessment. For each category of voltage stability, both voltage stability analysis and controls are studied. The overall objective is to use the learning capabilities of computational intelligence technology to build up the comprehensive on-line power system security assessment and control strategy as well as to enhance the speed and efficiency of the process with minimal human intervention. The voltage stability problems are quantified by voltage stability indices which measure the system for the closeness of current operating point to voltage instability. The indices are different for small-disturbance and large-disturbance voltage stability assessment. Conventional approaches, such as continuation power flow or time-domain simulation, can be used to obtain voltage stability indices. However, these conventional approaches are limited by computation time that is significant for on-line computation. The Artificial Neural Network (ANN) approach is proposed to compute voltage stability indices as an alternative to the conventional approaches. The proposed ANN algorithm is used to estimate voltage stability indices under both normal and contingency operating conditions. The input variables of ANN are obtained in real-time by an on-line measurement system, i.e. Phasor Measurement Units (PMU). This thesis will propose a suboptimal approach for seeking the best locations for PMUs from a voltage stability viewpoint. The ANN-based method is not limited to compute voltage stability indices but can also be extended to determine suitable control actions. Load shedding is one of the most effective approaches against short-term voltage instability under large disturbances. The basic requirement of load shedding for recovering voltage stability is to seek an optimal solution for when, where, and how much load should be shed. Two simulation based approaches, particle swarm optimization (PSO) algorithm and sensitivity based algorithm, are proposed for load shedding to prevent voltage instability or collapse. Both approaches are based on time-domain simulation.

ABSTRACT Voltage instability has become a major concern in power systems. Many blackouts have been reported where the main cause is voltage instability. This thesis deals with two specific areas of voltage stability in on-line power system security assessments: small-disturbance (long-term) and large-disturbance (short-term) voltage stability assessment. For each category of voltage stability, both voltage stability analysis and controls are studied. The overall objective is to use the learning capabilities of computational intelligence technology to build up the comprehensive on-line power system security assessment and control strategy as well as to enhance the speed and efficiency of the process with minimal human intervention. The voltage stability problems are quantified by voltage stability indices which measure the system for the closeness of current operating point to voltage instability. The indices are different for small-disturbance and large-disturbance voltage stability assessment. Conventional approaches, such as continuation power flow or time-domain simulation, can be used to obtain voltage stability indices. However, these conventional approaches are limited by computation time that is significant for on-line computation. The Artificial Neural Network (ANN) approach is proposed to compute voltage stability indices as an alternative to the conventional approaches. The proposed ANN algorithm is used to estimate voltage stability indices under both normal and contingency operating conditions. The input variables of ANN are obtained in real-time by an on-line measurement system, i.e. Phasor Measurement Units (PMU). This thesis will propose a suboptimal approach for seeking the best locations for PMUs from a voltage stability viewpoint. The ANN-based method is not limited to compute voltage stability indices but can also be extended to determine suitable control actions. Load shedding is one of the most effective approaches against short-term voltage instability under large disturbances. The basic requirement of load shedding for recovering voltage stability is to seek an optimal solution for when, where, and how much load should be shed. Two simulation based approaches, particle swarm optimization (PSO) algorithm and sensitivity based algorithm, are proposed for load shedding to prevent voltage instability or collapse. Both approaches are based on time-domain simulation.

[pt] Atualmente os sistemas elétricos altamente carregados estão enfrentando um crescente risco de estabilidade de tensão na medida que restrições ambientais e incertezas econômicas limitam o investimento em novas linhas de transmissão. Uma revisão dos problemas ocorridos em vários países indicam claramente que o problema de estabilidade de tensão é real e que o pior ainda está por vir [1]. Portanto existe a necessidade do desenvolvimento de ferramentas analíticas capazes de prever o colapso de tensão em redes complexas. Estas ferramentas devem ser capazes de quantificar com precisão as margens de estabilidade e os limites de transferência de potência, identificando os pontos fracos e as áreas susceptíveis á estabilidade de tensão. Um método para determinação das condições de estabilidade local de tensão é mostrado. A eficiência computacional desta ferramenta analítica é enfatizada em estudos de análise de cotingência. Estudos comparativos mostram a influência do modelo da carga estática, dos equivalentes de redes e da representação dos limites de geração ou absorção de potência reativa a partir da curva de capabilidade da máquina síncrona, no índice que avalia a estabilidade local de tensão. Um estudo sobre o determinante da matriz que serve como índice que avalia a estabilidade local de tensão. Neste trabalho são propostos novos estudos para dar continuidade aos avanços obtidos até o momento. / [en] Nowadays, heavy loaded eletric power systems are facing voltage stability problems due to environmental restrictions and budget cuts. Power systems incidents involving voltage stability clearly show that it is real problem and worse conditions are expected [1]. Therefore, there is a need of developing analytical tools to detect voltage collapse in complex networks. These tools must show, with good accuracy, stability ranges and power transfer limits, identifying weak nodes and susceptible areas to voltage stability. It is show a method to determine local voltage stability conditions and its computational efficiency is emphasized through contingency analysis studies. Comparative studies show the influence of static load model, network equivalent and reactive power generation/absorption limits representation using the synchronous machine capability curve, over the index which evaluates local voltage stability. It is show a study about the determinant which plays as an index to evaluate local voltage stability. Suggestions are made for future research.

[pt] Muito tem sido feito para que se possa entender os mecanismos que levam o sistema a uma situação de instabilidade de tensão e até mesmo ao colapso, assim como também para se suprir a necessidade de ferramentas analíticas capazes de prever e avaliar a proximidade do colapso de tensão em sistemas complexos, identificando as áreas susceptíveis à instabilidade de tensão. Uma vez detectada a inconveniente proximidade a uma condição de operação susceptível ao colapso de tensão, é necessária a identificação de grandezas-chave e sensibilidades, que forneçam entendimento suficiente, para ajudar no desenvolvimento de metodologias para reforçar as condições de estabilidade de tensão. Tendo isto como objetivo, este trabalho trata do problema de reforço da estabilidade de tensão através da mudança do perfil de tensão, utilizando como ferramenta computacional um algoritmo de otimização. Há no sistema variáveis de controle que interferem diretamente no perfil de tensão, tais como, tensão base nos geradores e compensadores, taps de transformadores, e capacitadores e reatores usados para compensação de potência reativa. Entretanto, quando as condições de estabilidade já se encontram deterioradas, as ações de controle do perfil de tensão podem levar a resultados insatisfatórios, podendo mesmo agravar o problema. Neste trabalho é apresentada uma metodologia, baseada em algoritmo de otimização, que fornece a lista de variáveis de controle mais eficientes para o reforço da estabilidade de tensão, e a direção em que os controles devem ser acionados. O interesse é obter um ponto de operação viável mais robusto, do ponto de vista de estabilidade de tensão, e não necessariamente um ponto de operação ótimo. / [en] Much hás been done for the understanding of the voltage instabilitu mechanisms and to supply the necessity of analytical tools, which should have the ability tom predict and evaluate the proximity of voltage collapse in complex systems, identifying the critical áreas susceptible to voltage istability. Once the inconvenient proximity to an operating condition susceptible to voltage collapse is identified, it is necessary to look for the key factors and sensitivities that provide the reinforcement of the voltage stability conditions. Having it as a goal, this work treats the problem of reinforcing the voltage stability conditions, through changes of the voltage profile, using an optimal algorithm as a computational tool. There are control variables in the system which directly interfere on the voltage profile, such as, voltage set point on generators and compensators, transformer taps, capacitors and reactors. However, when the voltage stability conditions are already deteriorated, the voltage profile control actions may lead to unsatisfactory results, and may even aggravate the problem. It is presented a methodology based on an optimal, that provides the list of the most efficient control variables for the reinforcement of voltage stability, and the corresponding direction of movement. The search is for a feasible operating point, more robust in relation to voltage stability, and not necessarily for an optimal operating point.

As modern power systems operate closer to the limits due to load growth and financial imperatives, voltage stability becomes a more important issue and there have been more incidents caused by voltage collapse. For example, there have been 11 outages affecting more than 4000MW between 1984 and 2000 in North America [1]. In power systems, load voltages decrease as the supplied loads increase until the maximum power transfer point is reached. The voltage will collapse if the load is increased above this limit. Therefore, it is important to monitor the loadability of a system to avoid voltage collapse. The loadability of a system can be calculated when the Thevenin impedance is available as the maximum power transfer occurs when the Thevenin impedance and the load impedance are the same in magnitude. This thesis suggests a method to estimate the Thevenin impedance of a system. ABB corporation suggests the Voltage Stability Predictor (VIP) method to estimate the Thevenin impedance, but there are problems with this method and it is not gaining popularity in industry. In this thesis, a method is suggested to estimate the Thevenin impedance by taking advantage of the existance of negative sequence components in the system. The concept of this method has been proved mathematically. Simulations were performed on simple systems and on the modified IEEE 13 bus power flow test case to verify the feasibility of the method and the results are promising. Then, the method was verified with field measurements for a 25kV substation. The voltages and currents were analyzed to estimate the Thevenin equivalent impedance of the power system and the results were compared with the design Thevenin equivalent impedance. The result confirms the viability of the method as the estimated Thevenin impedance matched the design value. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Shipboard MVDC power systems need to support pulsed loads, which have destabilizing ef-fects on the MVDC power transmission bus voltage. Despite the reference shipboard MVDC architecture having energy storage to buffer the large power swings of pulsed loads, a large constant power still needs to be delivered to maintain the energy storage state of charge. This recharging constant power itself introduces small signal instability to the MVDC bus voltage. This thesis investigates the advantages of adding a dynamically tuneable virtual capacitor and resistor in parallel to the pulsed load for maintaining small signal stability. The stabi-lizer is implemented in a negative load configuration in the existing reference architecture hardware, where the stabilizer negatively impacts the power quality of the downstream load. To address this, a dual use is added to existing hardware by having the energy storage also cancel out the newly introduced noise. A controller was designed to control a MVDC power converter module for providing these stability services. In addition, the controller manages its internal energy storage and stabilizes its internal DC bus that powers its downstream pulsed load. / Future ships will have a special shipboard power grid and power converters to power future electronics. Most of these power converters will have an internal battery device that provides power when the generators do not provide enough power. Generators are very slow to change their power output. Some shipboard electronics may consume very large amounts of power at very quickly changing rates, causing instability to the power system. The batteries can accomodate the instability caused by these electronics. However, the batteries need to be quickly recharged, which is also unstable to the special power grid. This thesis modifies the recharging behavior so that it does not cause this instability. Also, it is preferable that the batteries will only draw power from the power grid in one direction and send power to the power consuming electronics. This setup is called negative load. This setup is preferable, because sending power back to the power grid will require extra hardware. Ships can only carry so much equipment due to constraints in weight or room, so additonal hardware is undesireable. There already exists similar research to provide this stabilizing service, but they are not designed for a shipboard power grid supporting these quick high power electronics. This thesis also makes a controls system that manages the battery and other requirements of the power system.

To reinforce its Inez network, which was operated close to its stability limits, American Electric Power (AEP) undertook two major developments, one being the installation of a Static Var Compensator (SVC) in November, 1980 and the other one being the installation of the world's first Unified Power Flow Controller (UPFC) in 1998. The controllers in the system include the Automatic Voltage Regulators (AVRs) of the generators, the controllers of the SVC, and UPFC. To coordinate the control actions of these controllers and prevent voltage instability resulting from their fighting against each other, a two level hierarchical control scheme using fuzzy logic has been developed and its performance was assessed via simulations. The second level of the hierarchy determines the set points of the local controllers of the AVRs, SVC, and UPFC and defines the switching sequences of the capacitor banks, the goal being to maximize the reactive reserve margins of the Inez subsystem. Numerous simulations were carried out on this system to determine the actions of the fuzzy controller required to prevent the occurrence of voltage collapse under double contingency. Simulations have revealed the occurrence of nonlinear interactions between the machines resulting in stable limit cycles, nonlinear oscillations undergoing period doubling leading to chaos and possible voltage collapse. The proposed fuzzy scheme provides a fast, simple and effective way to stretch the stability limit of the system for double contingency conditions, up to 175 MW in some cases. This is a significant increase in the system capacity. / Master of Science

Recent decades have witnessed a significant increment in power contribution from wind generators. This increment in penetration requires power engineers to tackle multi-fold challenges concerning operational and stability aspects. There exists a significant attention among the researchers in analyzing the impact of wind generation on various system aspects. This thesis focuses on steady state voltage stability aspects with penetration of Variable speed wind generators. Traditionally, ancillary services are supplied by large conventional generators. However, with the huge penetration of wind generators as a result of the growing interest in satisfying energy requirements, and considering the benefits that they can bring along to the electrical system and to the environment, it appears reasonable to assume that ancillary services could also be provided by wind generators in an economical and efficient way. Certain types of wind generators can support reactive power for the Power Market. Fixed and Semi-Variable speed wind turbine generators were predominantly employed during the early installations of wind generators. These units require reactive power support from the grid and are usually equipped with capacitor banks to provide the necessary reactive power. Further, STATic synchronous COMpensator (STATCOM) and Static Var Compensator (SVC) with various configurations were proposed to enhance the system operations under normal and disturbed conditions. On the other hand, Variable speed wind turbine generators provide flexibility in control and hence are becoming increasingly popular. Popular among this class of wind turbine generating units are Doubly Fed Induction Generator(DFIG) and induction/synchronous Generator with Front End Converter(GFEC). Contrary to Fixed and Semi-Variable speed wind machines, Variable speed wind turbine generators are capable of providing reactive power to the grid. The converter and control schemes associated with these machines permits controlling the active and reactive power output to desired level. It is possible to control the reactive power output of these machines independently of the active power. Researchers in the past have investigated the impact of reactive power output of variable speed wind machines on system stability. In literature, approaches are proposed to utilize the flexibility in reactive power generation of DFIG to reduce system losses, improve reliability in static and dynamic system operation. Approaches in literature investigate the impact on voltage stability of system by considering the flexibility of reactive power output of wind machine in isolation. However, significant improvement in static voltage stability, voltage profile, system power losses etc. can be attained if the Reactive Power output of Variable Speed wind machines can be properly coordinated with other reactive power controllers. The prime objective of the thesis is to propose algorithms to coordinate the reactive power output of Variable Speed Wind Generators with other reactive power controllers for Enhancement in voltage stability margin, system losses and voltage profiles.

Made available in DSpace on 2014-06-11T19:30:50Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-02-18Bitstream added on 2014-06-13T20:07:53Z : No. of bitstreams: 1 matarucco_rr_dr_ilha.pdf: 2669428 bytes, checksum: c42a359efd2169b25baa0b6c3a967339 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho apresenta dois métodos para a análise estática de contingências em Sistemas Elétricos de Potência utilizando o Método da Continuação. No primeiro método a margem de carregamento pós-contingência é obtida a partir do ponto de máximo carregamento do caso base. A magnitude de tensão de uma barra qualquer é usada como parâmetro na etapa de parametrização do fluxo de carga continuado. O ramo selecionado para avaliação da contingência é parametrizado por um fator de escalonamento que possibilita a remoção gradual do ramo e assegura a convergência nos casos em que o método diverge para a retirada total da linha de transmissão. Em geral, para a maioria das contingências analisadas são necessárias poucas iterações para a determinação do ponto de máximo carregamento pós-contingência. Mostra-se que o método pode ser usado como uma técnica alternativa para a averiguação e até mesmo para a obtenção da lista de contingências críticas fornecida pela função de análise de segurança de sistemas elétricos. No outro método, o qual obtém o ponto de máximo carregamento de pós-contingência a partir do caso base, as variáveis ângulo de fase e magnitude de tensão de uma barra k qualquer, e a perda total de potência ativa, são propostas como parâmetros para a etapa de parametrização do fluxo de carga continuado utilizado na averiguação da lista de contingências críticas fornecida pela função de análise de segurança de sistemas elétricos. Nos casos em que há divergência do fluxo de carga, o método proposto possibilita confirmar se esta ocorre devido à deficiência numérica do método em si ou a inexistência de um ponto de operação factível de pós-contingência. O uso da perda total de potência ativa como parâmetro traz como vantagem a possibilidade da determinação de pontos além do ponto de singularidade sem a necessidade... / This work presents two methods for static contingency analysis of electric power systems by using Continuation Methods. In the first the post-contingency loading margin is obtained from the base case maximum loading point. The voltage magnitude of any bus can be used as a parameter in the parameterization step of the proposed continuation power flow. The branch selected for contingency evaluation is parameterized by a scaling factor which allows its the gradual removal and assures the continuation power flow convergence for the cases where the method would diverge for the complete transmission line removal. In general, for most of the analyzed contingencies little iterations are necessary for the determination of the post-contingency maximum loading point. It is shown that the method can be used as an alternative technique to verify and even to obtain the list of critical contingencies supplied by the electric power systems security analysis function. In the other method, which obtains the maximum loading point from the base case, new parameters, namely the voltage magnitudes, phase angles and the total power losses, for evaluating the effects of branch outages. The approach can be used as a verification tool after a list of critical contingencies had been ranked according to their severities by the contingency selection functions. It is then possible to find whether the non-convergence of a power flow is due to a numerical problem or to an infeasible operating situation. The mains advantage of using the total real power losses as a parameter is that it is not necessary to change parameters during the solutions tracing until beyond the simple limit point, where the original Jacobian is singular. The proposed methods facilitate the development and the implementation of continuation methods for contingencies analysis

abstract: The holomorphic embedding method (HEM) applied to the power-flow problem (HEPF) has been used in the past to obtain the voltages and flows for power systems. The incentives for using this method over the traditional Newton-Raphson based nu-merical methods lie in the claim that the method is theoretically guaranteed to converge to the operable solution, if one exists. In this report, HEPF will be used for two power system analysis purposes: a. Estimating the saddle-node bifurcation point (SNBP) of a system b. Developing reduced-order network equivalents for distribution systems. Typically, the continuation power flow (CPF) is used to estimate the SNBP of a system, which involves solving multiple power-flow problems. One of the advantages of HEPF is that the solution is obtained as an analytical expression of the embedding parameter, and using this property, three of the proposed HEPF-based methods can es-timate the SNBP of a given power system without solving multiple power-flow prob-lems (if generator VAr limits are ignored). If VAr limits are considered, the mathemat-ical representation of the power-flow problem changes and thus an iterative process would have to be performed in order to estimate the SNBP of the system. This would typically still require fewer power-flow problems to be solved than CPF in order to estimate the SNBP. Another proposed application is to develop reduced order network equivalents for radial distribution networks that retain the nonlinearities of the eliminated portion of the network and hence remain more accurate than traditional Ward-type reductions (which linearize about the given operating point) when the operating condition changes. Different ways of accelerating the convergence of the power series obtained as a part of HEPF, are explored and it is shown that the eta method is the most efficient of all methods tested. The local-measurement-based methods of estimating the SNBP are studied. Non-linear Thévenin-like networks as well as multi-bus networks are built using model data to estimate the SNBP and it is shown that the structure of these networks can be made arbitrary by appropriately modifying the nonlinear current injections, which can sim-plify the process of building such networks from measurements. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017