The purpose of this dissertation is to analyze dynamic behavior of a stressed power system and to correlate the dynamic responses to a near future system voltage abnormality. It is postulated that the dynamic response of a stressed power system in a short period of time-in seconds-contains sufficient information that will allow prediction of voltage abnormality in future time-in minutes. The PSSE dynamics simulator is used to study the dynamics of the IEEE 39 Bus equivalent test system. To correlate dynamic behavior to system voltage abnormality, this research utilizes two different pattern recognition methods one being algorithmic method known as Regularized Least Square Classification (RLSC) pattern recognition and the other being a statistical method known as Classification and Regression Tree (CART). Dynamics of a stressed test system is captured by introducing numerous contingencies, by driving the system to the point of abnormal operation, and by identifying those simulated contingencies that cause system voltage abnormality. Normal and abnormal voltage cases are simulated using the PSSE dynamics tool. The results of simulation from PSSE dynamics will be divided into two sets of training and testing set data. Each of the two sets of data includes both normal and abnormal voltage cases that are used for development and validation of a discriminator. This research uses stressed system simulation results to train two RLSC and CART pattern recognition models using the training set obtained from the dynamic simulation data. After the training phase, the trained pattern recognition algorithm will be validated using the remainder of data obtained from simulation of the stressed system. This process will determine the prominent features and parameters in the process of classification of normal and abnormal voltage cases from dynamic simulation data. Each of the algorithmic or statistical pattern recognition methods have their advantages and disadvantages and it is the intention of this dissertation to use them only to find correlations between the dynamic behavior of a stressed system in response to severe contingencies and the outcome of the system behavior in a few minutes into the future.

Installation of reactive compensators is widely used for improving power system voltage stability. Reactive compensation also improves the system loading margin resulting in more stable and reliable operation. The improvements in system performance are highly dependent on the location where the reactive compensation is placed in the system. This paper compares three load flow analysis methods - PV curve analysis, QV sensitivity analysis, and Continuation Load Flow - in identifying system weak buses for placing reactive compensation. The methods are applied to three IEEE test systems, including modified IEEE 14-bus system, IEEE 30-bus system, and IEEE 57-bus system. Locations of reactive compensation and corresponding improvements in loading margin and voltages in each test system obtained by the three methods are compared. The author also analyzes the test systems to locate the optimal placement of reactive compensation that yields the maximum loading margin. The results when compared with brute force placement of reactive compensation show the relationship between effectiveness of the three methods and topology of the test systems.

[pt] O crescente aumento da complexidade dos sistemas elétricos de potência gera a necessidade de desenvolvimento de ferramentas que melhorem as condições de análise. O objetivo deste trabalho é aprimorar a ferramenta computacional de avaliação das condições de segurança (ou estabilidade) de tensão. No que diz respeito às barras associadas ao controle remoto de tensão por geração de potência reativa, são propostos novos modelos que representam mais adequadamente as condições operativas no momento do cálculo dos índices de segurança de tensão. Em relação à barra associada ao controle local de tensão por geração de potência reativa é proposta nova modelagem, aplicável tanto no problema de fluxo de potência, utilizando o método de Newton- Raphson, quanto no cálculo dos índices de segurança de tensão. Este modelo,mais robusto e flexível, inclui o controle de tensão local da barra no problema geral de fluxo de potência, formando um sistema de equações de ordem (2*número de barras+número de barras controladas localmente). Para o tratamento de múltiplas barras swing, é proposto um novo modelo, de novo para representar mais adequadamente as condições operativas. É aplicável tanto no problema básico de fluxo de potência, como no cálculo dos índices de segurança de tensão. O modelo proposto considera que apenas o ângulo de uma barra swing é especificado, com os ângulos das demais barras swing livres para variar. Testes numéricos com sistemas-teste (5 e 6 barras) comprovam a aplicabilidade e adequação dos modelos propostos comparando-os aos modelos usados atualmente. / [en] The crescent increase of the complexity of the electric power systems generates the need of development of tools to improve the analysis conditions. The objective of this work is to improve the computational tool of voltage security (stability) conditions assessment. As regards to the buses associated to remote voltage control by reactive power generation, new models that represent more appropriately the operatives conditions at the moment of the calculations of the voltage security indexes, are proposed. As regards to the bus associated to local voltage control by reactive power generation, it is proposed a new modeling, applicable as much in the power flow problem, using the Newton-Raphson method, as in the calculation of the voltage security indexes. This model, more robust and flexible, includes the local voltage control of the bus in the general power flow problem, constituting an equations system of order (2*number of system buses + number of buses with local voltage control). As regard to the multiples swing buses, it is proposed a new model, again to represents more appropriately the operatives conditions. It is applicable as much in the basic power flow problem, as in the calculation of the voltage security indexes. The proposed model considers that just one swing bus has your voltage angle specified and the others swing buses of the power system have your voltage angles free to vary.

[pt] Com o uso extremo das linhas de transmissão surgiram os problemas de estabilidade, ou mais apropriadamente, de segurança de tensão. A avaliação das condições de segurança de tensão é realizada pelo programa computacional ESTABTEN. Como os índices calculados são baseados em um ponto de operação do sistema e em um modelo linearizado das equações de fluxo de carga, assim como a função Fluxo de Carga do pacote computacional ANAREDE também o é, é importante que os modelos matemáticos do sistema, de equipamentos, de controles e de limites sejam compatíveis nos dois programas. Assim como o programa de fluxo de carga é continuamente estendido, o programa ESTABTEN deve continuar a ter sua capacidade estendida para atender as necessidades dos estudos.Estuda-se neste trabalho a modelagem em regime permanente de alguns equipamentos especiais da rede de transmissão e sua incorporação à função de avaliação da segurança de tensão. Os equipamentos contemplados foram: elos de corrente contínua (elo CC), compensadores estáticos de potência reativa (CER), esquemas de HVDC/CCC e linhas com compensação série controlada a tiristores (CSC). São mostrados exemplos numéricos que ilustram a necessidade de uma modelagem realista, na medida do possível. / [en] With the extreme use of the transmission lines, the voltage stability problem, or more properly, the voltage security problem has appeared. The voltage security condition assessment is achieved using the computational program ESTABTEN. The calculated indexes are based on a system operation point and on a linear model of load flow equations, likewise the ANAREDE load flow function. It is important that the mathematical models of systems, equipments, control devices and limits are compatible in both programs. As the load flow program is continually extended, the ESTABTEN program is to have its capability enhanced in order to attend the study requirements. This work is concerned with the steady-state modelling of some special equipments of the transmission network and its incorporation to the voltage security assessment function. The equipments considered were: direct current link (DC link), static VAR compensators (SVC), HVDC/CCC and lines with series controlled compensation (TSSC).Numerical examples are presented to illustrate the necessity of realistic modelling.

The voltage stability problem in electric power systems is concerned with the analysis of events and mechanisms that can lead a system into inadmissible operating conditions from the voltage viewpoint. In the worst case, total collapse of the system may result, with disastrous consequences for both electricity utilities and customers. The analysis of this problem has become an important area of research over the past decade due to some instances of voltage collapse that have occurred in electric systems throughout the world. This work addresses the voltage stability problem within the framework of artificial neural networks. Although the field of neural networks was established during the late 1940s, only in the past few years has it experienced rapid development. The neural network approach offers some potential advantages to the solution of problems for which an analytical solution is difficult. Also, efficient and accurate computation may be achieved through neural networks. The first contribution of this work refers to the development of an artificial neural network capable of computing a static voltage stability index, which provides information on the stability of a given operating state in the power system. This analytical tool was implemented as a self-contained computational system which exhibited good accuracy and extremely low processing times when applied to some study cases. Dynamic characteristics of the electrical system in the voltage stability problem are very important. Therefore, in a second stage of the present work, the scope of the research was extended so as to take into account these new aspects. Another neural network-based computational system was developed and implemented with the purpose of providing some information on the behaviour of the electrical system in the immediate future. Examples and case studies are presented throughout the thesis in order to illustrate the most relevant aspects of both artificial neural networks and the computational models developed. A general discussion summarises the main contributions of the present work and topics for further research are outlined.

Interest in Distributed Generation (DG) in power system networks has been growing rapidly. This increase can be explained by factors such as environmental concerns, the restructuring of electricity businesses, and the development of technologies for small-scale power generation. DG units are typically connected so as to work in parallel with the utility grid; however, with the increased penetration level of these units and the advancements in unit’s control techniques, there is a great possibility for these units to be operated in an autonomous mode known as a microgrid. Integrating DG units into distribution systems can have an impact on different practices such as voltage profile, power flow, power quality, stability, reliability, and protection. The impact of the DG units on stability problem can be further classified into three issues: voltage stability, angle stability, and frequency stability. As both angle and frequency stability are not often seen in distribution systems, voltage stability is considered to be the most significant in such systems. In fact, the distribution system in its typical design doesn’t suffer from any stability problems, given that all its active and reactive supplies are guaranteed through the substation. However, the following facts alter this situation: • With the development of economy, load demands in distribution networks are sharply increasing. Hence, the distribution networks are operating more close to the voltage instability boundaries. • The integration of distributed generation in distribution system introduces possibility of encountering some active/reactive power mismatches resulting in some stability concerns at the distribution level. Motivated by these facts, the target of this thesis is to investigate, analyze and enhance the voltage stability of distribution systems with high penetration of distributed generation. This study is important for the utilities because it can be applied with Connection Impact Assessment (CIA ). The study can be added as a complement assessment to study the impacts of the installation of DG units on voltage stability. In order to accomplish this target, this study is divided into three perspectives: 1) utilize the DG units to improve the voltage stability margin and propose a method to allocate DG units for this purpose, 2) investigate the impact of the DG units on proximity to voltage stability 3) conduct harmonic resonance analysis to visualize the impacts of both parallel and series resonance on the system’s stability. These perspectives will be tackled in Chapter 3, Chapter 4, and Chapter 5, respectively. Chapter 3 tackles placing and sizing of the DG units to improve the voltage stability margin and consider the probabilistic nature of both the renewable energy resources and the load. In fact, placement and sizing of DG units with an objective of improving the voltage stability margin while considering renewable DG generation and load probability might be a complicated problem, due to the complexity of running continuous load flow and at the same time considering the probabilistic nature of the load and the DG unit’s resources. Therefore, this thesis proposes a modified voltage index method to place and size the DG units to improve the voltage stability margin, with conditions of both not exceeding the buses’ voltage, and staying within the feeder current limits. The probability of the load and DG units are modeled and included in the formulation of the sizing and placing of the DG units. Chapter 4 presents a model and analysis to study the impact of the DG units on proximity to voltage instability. Most of the modern DG units are equipped with power electronic converters at their terminals. The power electronic converter plays a vital role to match the characteristics of the DG units with the requirements of the grid connections, such as frequency, voltage, control of active and reactive power, and harmonic minimization. Due to the power electronics interfacing, these DG units have negligible inertia. Thus, they make the system potentially prone to oscillations resulting from the network disturbances. The main goal of this chapter is to model and analyze the impact of distributed generation DG units on the proximity of voltage instability, with high penetration level of DG units. Chapter 5 studies the harmonic resonance due to the integration of DG units in distribution systems. Normally, the harmonic resonance phenomenon is classified as a power quality problem, however, this phenomenon can affect the stability of the system due to the parallel and series resonance. Thus, the main goal of this chapter is to study and analyze the impact of the integration of distributed generation on harmonic resonance by modeling different types of DG units and applying impedance frequency scan method.

The thesis use capacitors, Static Synchronous Compensator (STATCOM) and wind generator to get optimal voltage stability for twenty-four-hour dynamic load by compensating real/reactive power. In the thesis, Modified Orthogonal Particle Swarm Optimizer (MOPSO) is proposed to find the sitting and sizing of capacitors, STATCOM and wind generator, and integrate Equivalent Current Injection (ECI) algorithm to solve Optimal Power Flow (OPF) to achieve optimal voltage stability. The algorithm uses MOPSO to renew STATCOM and wind turbine sizing Gbest with multiple choices and Taguchi orthogonal array, which improves Particle Swarm Optimizer (PSO) without falling into the local optimal solution and searches optimal voltage stability of power system by load balancing equation and inequality constraints. Average Voltage Variation (AVV) and Average Voltage Drop Variation (AVDV) are proposed as objective function to calculate whole system voltage variations, and convergence test of MOPSO. The IEEE 33 Bus distribution system and Miaoli-Houlong distribution system were used for simulation to test the voltage control during peak and off-peak periods of Taipower. Compensation of real/reactive power was used to get optimal system voltage stability for each simulated case.

This dissertation attempts to establish an analytical and comprehensive framework to deal with two critical challenges associated with voltage stability analysis: 1. To study the new competitive environment appropriately and give more incentive for reactive power supports, one has to evaluate the impacts of distributed market forces on voltage stability, which complicates the voltage stability analysis. 2. Accurately estimating voltage stability margin online is always the goal of the industry. Industry used to apply static analysis for its computation speed at the cost of losing accuracy. On the other hand, dynamic analysis can result in more accurate estimation, but generally has a huge computation cost. So a challenge is to estimate the voltage stability margin accurately and efficiently at a reasonable cost, especially for large system. Considering the first challenge, this dissertation applied eigenvalue based bifurcation analysis to allocate the contribution of voltage stability. We investigate how parameters of the system influence the bifurcations. Three bifurcations (singularity induced bifurcation, saddle-node and Hopf bifurcation) and their relationship to several commonly used controllers are analyzed. Their parameters’ impact on these bifurcations have been investigated, from which we found a way to allocate the contribution by analyzing the relative positions of the bifurcations. For the second challenge, a new fast numerical scheme is developed to estimate voltage stability margin by intelligently adjusting the load increase ratio. A criterion, named EMD (Equilibrium Manifold Deviation) criterion, is proposed to gauge the accuracy of the estimation. And based on this criterion, a new computation scheme is proposed. The validity of our new approach is proven based on the well-known Runge-Kutta-Fehlberg method, and can be extended to other explicit single-step methods easily. Numerical tests demonstrate that the new approach is very practical and has great potential for industrial applications. This dissertation extends our new numerical scheme to stiff systems. When a system is ill-conditioned, the implicit method would be applied to achieve numerical stability. We further demonstrate the validity to combine the intelligent load adjustment technique with the implicit method to save the computation cost without loss of accuracy. This dissertation also delves into the auto detection of stiffness of the power system, and extends our new numerical scheme to general sytems.

This thesis investigates an important power system phenomenon, voltage stability, by using continuation power flow method. Voltage collapse scenario is presented which can be a serious result of voltage instability and the parameters that affect voltage collapse are discussed. In analyzing power system voltage stability, continuation power flow method is utilized which consists of successive load flows. This method is applied to a sample test system and Turkish Power System and load-voltage curves for several buses are obtained.

Solar power, clean and abundant, is considered as a vital contributor in the effort of transforming world energy-mix to pollution-free and natural-regenerative sources.   The solar micro inverters have gained greater visibility during the past several years due to their higher efficiency, greater performances, longer life expectancy and many other benefits. But, integrating small scale [<15kW] renewable energy sources, especially the low frequency switching solar inverters to the low voltage distribution grid has its own challenges due to their inability to generate reactive power to maintain the static voltage stability of the grid. Higher level of solar penetration has identified as a potential cause of low voltage grid instability due to lack of reactive power feeding and their tendency to keep on increasing the voltage higher than grid at the point of common connection [PCC] in order to inject the current to the grid. The studies and experience in voltage stability issues has resulted in introducing many new grid regulations to manage the grid voltage stability throughout the world. The new regulation, VDE-AR-N-4105-2011 is a German grid regulation standard specifically focuses on the low voltage grid connected power generators. This regulation has addressed the reactive power requirements in terms of power factor and supply management to maintain the grid static voltage variation less than 3% at the PCC, when connecting any type of distributed power generators to the low voltage network. This report discuss about the voltage stability issues related to low frequency switching inverters and present a solution to comply with low voltage grid regulation - VDE-AR-N-4105-2011; a SmartVar Generator concept, theory, design and functionality.