A novel method for power quality multiple disturbance decomposition based on Independent Component Analysis

In this paper, a novel method for power quality signal decomposition is proposed based on Independent Component Analysis (ICA). This method aims to decompose the power system signal (voltage or current) into components that can provide more specific information about the different disturbances which are occurring simultaneously during a multiple disturbance situation. The ICA is originally a multichannel technique. However, the method proposes its use to blindly separate out disturbances existing in a single measured signal (single channel). Therefore, a preprocessing step for the ICA is proposed using a filter bank. The proposed method was applied to synthetic data, simulated data, as well as actual power system signals, showing a very good performance. A comparison with the decomposition provided by the Discrete Wavelet Transform shows that the proposed method presented better decoupling for the analyzed data. (C) 2012 Elsevier Ltd. All rights reserved.

Investigation of the Impacts of Large-Scale Wind Power Penetration on the Angle and Voltage Stability of Power Systems

The complexity of power systems has increased in recent years due to the operation of existing transmission lines closer to their limits, using flexible AC transmission system (FACTS) devices, and also due to the increased penetration of new types of generators that have more intermittent characteristics and lower inertial response, such as wind generators. This changing nature of a power system has considerable effect on its dynamic behaviors resulting in power swings, dynamic interactions between different power system devices, and less synchronized coupling. This paper presents some analyses of this changing nature of power systems and their dynamic behaviors to identify critical issues that limit the large-scale integration of wind generators and FACTS devices. In addition, this paper addresses some general concerns toward high compensations in different grid topologies. The studies in this paper are conducted on the New England and New York power system model under both small and large disturbances. From the analyses, it can be concluded that high compensation can reduce the security limits under certain operating conditions, and the modes related to operating slip and shaft stiffness are critical as they may limit the large-scale integration of wind generation.

Tecniche di progettazione tollerante alle variazioni per circuiti digitali in tecnologie nanometriche

The digital electronic market development is founded on the continuous reduction of the transistors size, to reduce area, power, cost and increase the computational performance of integrated circuits. This trend, known as technology scaling, is approaching the nanometer size. The lithographic process in the manufacturing stage is increasing its uncertainty with the scaling down of the transistors size, resulting in a larger parameter variation in future technology generations. Furthermore, the exponential relationship between the leakage current and the threshold voltage, is limiting the threshold and supply voltages scaling, increasing the power density and creating local thermal issues, such as hot spots, thermal runaway and thermal cycles. In addiction, the introduction of new materials and the smaller devices dimension are reducing transistors robustness, that combined with high temperature and frequently thermal cycles, are speeding up wear out processes. Those effects are no longer addressable only at the process level. Consequently the deep sub-micron devices will require solutions which will imply several design levels, as system and logic, and new approaches called Design For Manufacturability (DFM) and Design For Reliability. The purpose of the above approaches is to bring in the early design stages the awareness of the device reliability and manufacturability, in order to introduce logic and system able to cope with the yield and reliability loss. The ITRS roadmap suggests the following research steps to integrate the design for manufacturability and reliability in the standard CAD automated design flow: i) The implementation of new analysis algorithms able to predict the system thermal behavior with the impact to the power and speed performances. ii) High level wear out models able to predict the mean time to failure of the system (MTTF). iii) Statistical performance analysis able to predict the impact of the process variation, both random and systematic. The new analysis tools have to be developed beside new logic and system strategies to cope with the future challenges, as for instance: i) Thermal management strategy that increase the reliability and life time of the devices acting to some tunable parameter,such as supply voltage or body bias. ii) Error detection logic able to interact with compensation techniques as Adaptive Supply Voltage ASV, Adaptive Body Bias ABB and error recovering, in order to increase yield and reliability. iii) architectures that are fundamentally resistant to variability, including locally asynchronous designs, redundancy, and error correcting signal encodings (ECC). The literature already features works addressing the prediction of the MTTF, papers focusing on thermal management in the general purpose chip, and publications on statistical performance analysis. In my Phd research activity, I investigated the need for thermal management in future embedded low-power Network On Chip (NoC) devices.I developed a thermal analysis library, that has been integrated in a NoC cycle accurate simulator and in a FPGA based NoC simulator. The results have shown that an accurate layout distribution can avoid the onset of hot-spot in a NoC chip. Furthermore the application of thermal management can reduce temperature and number of thermal cycles, increasing the systemreliability. Therefore the thesis advocates the need to integrate a thermal analysis in the first design stages for embedded NoC design. Later on, I focused my research in the development of statistical process variation analysis tool that is able to address both random and systematic variations. The tool was used to analyze the impact of self-timed asynchronous logic stages in an embedded microprocessor. As results we confirmed the capability of self-timed logic to increase the manufacturability and reliability. Furthermore we used the tool to investigate the suitability of low-swing techniques in the NoC system communication under process variations. In this case We discovered the superior robustness to systematic process variation of low-swing links, which shows a good response to compensation technique as ASV and ABB. Hence low-swing is a good alternative to the standard CMOS communication for power, speed, reliability and manufacturability. In summary my work proves the advantage of integrating a statistical process variation analysis tool in the first stages of the design flow.

Modelling and analysis of networked control strategies in smart power distribution grids: development of co-simulation tools

This thesis is focused on Smart Grid applications in medium voltage distribution networks. For the development of new applications it appears useful the availability of simulation tools able to model dynamic behavior of both the power system and the communication network. Such a co-simulation environment would allow the assessment of the feasibility of using a given network technology to support communication-based Smart Grid control schemes on an existing segment of the electrical grid and to determine the range of control schemes that different communications technologies can support. For this reason, is presented a co-simulation platform that has been built by linking the Electromagnetic Transients Program Simulator (EMTP v3.0) with a Telecommunication Network Simulator (OPNET-Riverbed v18.0). The simulator is used to design and analyze a coordinate use of Distributed Energy Resources (DERs) for the voltage/var control (VVC) in distribution network. This thesis is focused control structure based on the use of phase measurement units (PMUs). In order to limit the required reinforcements of the communication infrastructures currently adopted by Distribution Network Operators (DNOs), the study is focused on leader-less MAS schemes that do not assign special coordinating rules to specific agents. Leader-less MAS are expected to produce more uniform communication traffic than centralized approaches that include a moderator agent. Moreover, leader-less MAS are expected to be less affected by limitations and constraint of some communication links. The developed co-simulator has allowed the definition of specific countermeasures against the limitations of the communication network, with particular reference to the latency and loss and information, for both the case of wired and wireless communication networks. Moreover, the co-simulation platform has bee also coupled with a mobility simulator in order to study specific countermeasures against the negative effects on the medium voltage/current distribution network caused by the concurrent connection of electric vehicles.

A probabilistic approach to classify the levee reliability

This work aims to evaluate the reliability of these levee systems, calculating the probability of “failure” of determined levee stretches under different loads, using probabilistic methods that take into account the fragility curves obtained through the Monte Carlo Method. For this study overtopping and piping are considered as failure mechanisms (since these are the most frequent) and the major levee system of the Po River with a primary focus on the section between Piacenza and Cremona, in the lower-middle area of the Padana Plain, is analysed. The novelty of this approach is to check the reliability of individual embankment stretches, not just a single section, while taking into account the variability of the levee system geometry from one stretch to another. This work takes also into consideration, for each levee stretch analysed, a probability distribution of the load variables involved in the definition of the fragility curves, where it is influenced by the differences in the topography and morphology of the riverbed along the sectional depth analysed as it pertains to the levee system in its entirety. A type of classification is proposed, for both failure mechanisms, to give an indication of the reliability of the levee system based of the information obtained by the fragility curve analysis. To accomplish this work, an hydraulic model has been developed where a 500-year flood is modelled to determinate the residual hazard value of failure for each stretch of levee near the corresponding water depth, then comparing the results with the obtained classifications. This work has the additional the aim of acting as an interface between the world of Applied Geology and Environmental Hydraulic Engineering where a strong collaboration is needed between the two professions to resolve and improve the estimation of hydraulic risk.

Thermal Energy Storage for Solar Power Production

Solar energy is the most abundant persistent energy resource. It is also an intermittent one available for only a fraction of each day while the demand for electric power never ceases. To produce a significant amount of power at the utility scale, electricity generated from solar energy must be dispatchable and able to be supplied in response to variations in demand. This requires energy storage that serves to decouple the intermittent solar resource from the load and enables around-the-clock power production from solar energy. Practically, solar energy storage technologies must be efficient as any energy loss results in an increase in the amount of required collection hardware, the largest cost in a solar electric power system. Storing solar energy as heat has been shown to be an efficient, scalable, and relatively low-cost approach to providing dispatchable solar electricity. Concentrating solar power systems that include thermal energy storage (TES) use mirrors to focus sunlight onto a heat exchanger where it is converted to thermal energy that is carried away by a heat transfer fluid and used to drive a conventional thermal power cycle (e.g., steam power plant), or stored for later use. Several approaches to TES have been developed and can generally be categorized as either thermophysical (wherein energy is stored in a hot fluid or solid medium or by causing a phase change that can later be reversed to release heat) or thermochemical (in which energy is stored in chemical bonds requiring two or more reversible chemical reactions).

Parameter estimation for transformer modeling

Large Power transformers, an aging and vulnerable part of our energy infrastructure, are at choke points in the grid and are key to reliability and security. Damage or destruction due to vandalism, misoperation, or other unexpected events is of great concern, given replacement costs upward of $2M and lead time of 12 months. Transient overvoltages can cause great damage and there is much interest in improving computer simulation models to correctly predict and avoid the consequences. EMTP (the Electromagnetic Transients Program) has been developed for computer simulation of power system transients. Component models for most equipment have been developed and benchmarked. Power transformers would appear to be simple. However, due to their nonlinear and frequency-dependent behaviors, they can be one of the most complex system components to model. It is imperative that the applied models be appropriate for the range of frequencies and excitation levels that the system experiences. Thus, transformer modeling is not a mature field and newer improved models must be made available. In this work, improved topologically-correct duality-based models are developed for three-phase autotransformers having five-legged, three-legged, and shell-form cores. The main problem in the implementation of detailed models is the lack of complete and reliable data, as no international standard suggests how to measure and calculate parameters. Therefore, parameter estimation methods are developed here to determine the parameters of a given model in cases where available information is incomplete. The transformer nameplate data is required and relative physical dimensions of the core are estimated. The models include a separate representation of each segment of the core, including hysteresis of the core, λ-i saturation characteristic, capacitive effects, and frequency dependency of winding resistance and core loss. Steady-state excitation, and de-energization and re-energization transients are simulated and compared with an earlier-developed BCTRAN-based model. Black start energization cases are also simulated as a means of model evaluation and compared with actual event records. The simulated results using the model developed here are reasonable and more correct than those of the BCTRAN-based model. Simulation accuracy is dependent on the accuracy of the equipment model and its parameters. This work is significant in that it advances existing parameter estimation methods in cases where the available data and measurements are incomplete. The accuracy of EMTP simulation for power systems including three-phase autotransformers is thus enhanced. Theoretical results obtained from this work provide a sound foundation for development of transformer parameter estimation methods using engineering optimization. In addition, it should be possible to refine which information and measurement data are necessary for complete duality-based transformer models. To further refine and develop the models and transformer parameter estimation methods developed here, iterative full-scale laboratory tests using high-voltage and high-power three-phase transformer would be helpful.

Development of system separation strategy using synchrophasor data

In the current market system, power systems are operated at higher loads for economic reasons. Power system stability becomes a genuine concern in such operating conditions. In case of failure of any larger component, the system may become stressed. These events may start cascading failures, which may lead to blackouts. One of the main reasons of the major recorded blackout events has been the unavailability of system-wide information. Synchrophasor technology has the capability to provide system-wide real time information. Phasor Measurement Units (PMUs) are the basic building block of this technology, which provide the Global Positioning System (GPS) time-stamped voltage and current phasor values along with the frequency. It is being assumed that synchrophasor data of all the buses is available and thus the whole system is fully observable. This information can be used to initiate islanding or system separation to avoid blackouts. A system separation strategy using synchrophasor data has been developed to answer the three main aspects of system separation: (1) When to separate: One class support machines (OC-SVM) is primarily used for the anomaly detection. Here OC-SVM was used to detect wide area instability. OC-SVM has been tested on different stable and unstable cases and it is found that OC-SVM has the capability to detect the wide area instability and thus is capable to answer the question of “when the system should be separated”. (2) Where to separate: The agglomerative clustering technique was used to find the groups of coherent buses. The lines connecting different groups of coherent buses form the separation surface. The rate of change of the bus voltage phase angles has been used as the input to this technique. This technique has the potential to exactly identify the lines to be tripped for the system separation. (3) What to do after separation: Load shedding was performed approximately equal to the sum of power flows along the candidate system separation lines should be initiated before tripping these lines. Therefore it is recommended that load shedding should be initiated before tripping the lines for system separation.

Wind plant interaction with series-compensated power systems

Wind power based generation has been rapidly growing world-wide during the recent past. In order to transmit large amounts of wind power over long distances, system planners may often add series compensation to existing transmission lines owing to several benefits such as improved steady-state power transfer limit, improved transient stability, and efficient utilization of transmission infrastructure. Application of series capacitors has posed resonant interaction concerns such as through subsynchronous resonance (SSR) with conventional turbine-generators. Wind turbine-generators may also be susceptible to such resonant interactions. However, not much information is available in literature and even engineering standards are yet to address these issues. The motivation problem for this research is based on an actual system switching event that resulted in undamped oscillations in a 345-kV series-compensated, typical ring-bus power system configuration. Based on time-domain ATP (Alternative Transients Program) modeling, simulations and analysis of system event records, the occurrence of subsynchronous interactions within the existing 345-kV series-compensated power system has been investigated. Effects of various small-signal and large-signal power system disturbances with both identical and non-identical wind turbine parameters (such as with a statistical-spread) has been evaluated. Effect of parameter variations on subsynchronous oscillations has been quantified using 3D-DFT plots and the oscillations have been identified as due to electrical self-excitation effects, rather than torsional interaction. Further, the generator no-load reactance and the rotor-side converter inner-loop controller gains have been identified as bearing maximum sensitivity to either damping or exacerbating the self-excited oscillations. A higher-order spectral analysis method based on modified Prony estimation has been successfully applied to the field records identifying dominant 9.79 Hz subsynchronous oscillations. Recommendations have been made for exploring countermeasures.

Optimal game theoretic distributed control methodologies in small scale power systems

This dissertation presents the competitive control methodologies for small-scale power system (SSPS). A SSPS is a collection of sources and loads that shares a common network which can be isolated during terrestrial disturbances. Micro-grids, naval ship electric power systems (NSEPS), aircraft power systems and telecommunication system power systems are typical examples of SSPS. The analysis and development of control systems for small-scale power systems (SSPS) lacks a defined slack bus. In addition, a change of a load or source will influence the real time system parameters of the system. Therefore, the control system should provide the required flexibility, to ensure operation as a single aggregated system. In most of the cases of a SSPS the sources and loads must be equipped with power electronic interfaces which can be modeled as a dynamic controllable quantity. The mathematical formulation of the micro-grid is carried out with the help of game theory, optimal control and fundamental theory of electrical power systems. Then the micro-grid can be viewed as a dynamical multi-objective optimization problem with nonlinear objectives and variables. Basically detailed analysis was done with optimal solutions with regards to start up transient modeling, bus selection modeling and level of communication within the micro-grids. In each approach a detail mathematical model is formed to observe the system response. The differential game theoretic approach was also used for modeling and optimization of startup transients. The startup transient controller was implemented with open loop, PI and feedback control methodologies. Then the hardware implementation was carried out to validate the theoretical results. The proposed game theoretic controller shows higher performances over traditional the PI controller during startup. In addition, the optimal transient surface is necessary while implementing the feedback controller for startup transient. Further, the experimental results are in agreement with the theoretical simulation. The bus selection and team communication was modeled with discrete and continuous game theory models. Although players have multiple choices, this controller is capable of choosing the optimum bus. Next the team communication structures are able to optimize the players’ Nash equilibrium point. All mathematical models are based on the local information of the load or source. As a result, these models are the keys to developing accurate distributed controllers.

Transformer modeling ATP : internal faults & high-frequency discretization

Power transformers are key components of the power grid and are also one of the most subjected to a variety of power system transients. The failure of a large transformer can cause severe monetary losses to a utility, thus adequate protection schemes are of great importance to avoid transformer damage and maximize the continuity of service. Computer modeling can be used as an efficient tool to improve the reliability of a transformer protective relay application. Unfortunately, transformer models presently available in commercial software lack completeness in the representation of several aspects such as internal winding faults, which is a common cause of transformer failure. It is also important to adequately represent the transformer at frequencies higher than the power frequency for a more accurate simulation of switching transients since these are a well known cause for the unwanted tripping of protective relays. This work develops new capabilities for the Hybrid Transformer Model (XFMR) implemented in ATPDraw to allow the representation of internal winding faults and slow-front transients up to 10 kHz. The new model can be developed using any of two sources of information: 1) test report data and 2) design data. When only test-report data is available, a higher-order leakage inductance matrix is created from standard measurements. If design information is available, a Finite Element Model is created to calculate the leakage parameters for the higher-order model. An analytical model is also implemented as an alternative to FEM modeling. Measurements on 15-kVA 240?/208Y V and 500-kVA 11430Y/235Y V distribution transformers were performed to validate the model. A transformer model that is valid for simulations for frequencies above the power frequency was developed after continuing the division of windings into multiple sections and including a higher-order capacitance matrix. Frequency-scan laboratory measurements were used to benchmark the simulations. Finally, a stability analysis of the higher-order model was made by analyzing the trapezoidal rule for numerical integration as used in ATP. Numerical damping was also added to suppress oscillations locally when discontinuities occurred in the solution. A maximum error magnitude of 7.84% was encountered in the simulated currents for different turn-to-ground and turn-to-turn faults. The FEM approach provided the most accurate means to determine the leakage parameters for the ATP model. The higher-order model was found to reproduce the short-circuit impedance acceptably up to about 10 kHz and the behavior at the first anti-resonant frequency was better matched with the measurements.

Validation Study of the Use of Matlab/Simulink Synchronous-Machine Block for Accurate Power-Plant Stability Studies

This paper presents results of the validity study of the use of MATLAB/Simulink synchronous-machine block for power-system stability studies. Firstly, the waveforms of the theoretical synchronous-generator short-circuit currents are described. Thereafter, the comparison between the currents obtained through the simulation model in the sudden short-circuit test, are compared to the theoretical ones. Finally, the factory tests of two commercial generating units are compared to the response of the synchronous generator simulation block during sudden short-circuit, set with the same real data, with satisfactory results. This results show the validity of the use of this generator block for power plant simulation.

Frequency control support of a wind-solar isolated system by a hydropower plant with long tail-race tunnel

Pumped storage hydro plants (PSHP) can provide adequate energy storage and frequency regulation capacities in isolated power systems having significant renewable energy resources. Due to its high wind and solar potential, several plans have been developed for La Palma Island in the Canary archipelago, aimed at increasing the penetration of these energy sources. In this paper, the performance of the frequency control of La Palma power system is assessed, when the demand is supplied by the available wind and solar generation with the support of a PSHP which has been predesigned for this purpose. The frequency regulation is provided exclusively by the PSHP. Due to topographic and environmental constraints, this plant has a long tail-race tunnel without a surge tank. In this configuration, the effects of pressure waves cannot be neglected and, therefore, usual recommendations for PID governor tuning provide poor performance. A PI governor tuning criterion is proposed for the hydro plant and compared with other criteria according to several performance indices. Several scenarios considering solar and wind energy penetration have been simulated to check the plant response using the proposed criterion. This tuning of the PI governor maintains La Palma system frequency within grid code requirements.

Evaluación estocástica de huecos de tensión en sistemas eléctricos: estudio de sensibilidad, estimación de índices y localización óptima de medidores

I. GENERALIDADES 1.1. Introducción Entre los diversos tipos de perturbaciones eléctricas, los huecos de tensión son considerados el problema de calidad de suministro más frecuente en los sistemas eléctricos. Este fenómeno es originado por un aumento extremo de la corriente en el sistema, causado principalmente por cortocircuitos o maniobras inadecuadas en la red. Este tipo de perturbación eléctrica está caracterizado básicamente por dos parámetros: tensión residual y duración. Típicamente, se considera que el hueco se produce cuando la tensión residual alcanza en alguna de las fases un valor entre 0.01 a 0.9 pu y tiene una duración de hasta 60 segundos. Para un usuario final, el efecto más relevante de un hueco de tensión es la interrupción o alteración de la operación de sus equipos, siendo los dispositivos de naturaleza electrónica los principalmente afectados (p. ej. ordenador, variador de velocidad, autómata programable, relé, etc.). Debido al auge tecnológico de las últimas décadas y a la búsqueda constante de automatización de los procesos productivos, el uso de componentes electrónicos resulta indispensable en la actualidad. Este hecho, lleva a que los efectos de los huecos de tensión sean más evidentes para el usuario final, provocando que su nivel de exigencia de la calidad de energía suministrada sea cada vez mayor. De forma general, el estudio de los huecos de tensión suele ser abordado bajo dos enfoques: en la carga o en la red. Desde el punto de vista de la carga, se requiere conocer las características de sensibilidad de los equipos para modelar su respuesta ante variaciones súbitas de la tensión del suministro eléctrico. Desde la perspectiva de la red, se busca estimar u obtener información adecuada que permita caracterizar su comportamiento en términos de huecos de tensión. En esta tesis, el trabajo presentado se encuadra en el segundo aspecto, es decir, en el modelado y estimación de la respuesta de un sistema eléctrico de potencia ante los huecos de tensión. 1.2. Planteamiento del problema A pesar de que los huecos de tensión son el problema de calidad de suministro más frecuente en las redes, hasta la actualidad resulta complejo poder analizar de forma adecuada este tipo de perturbación para muchas compañías del sector eléctrico. Entre las razones más comunes se tienen: - El tiempo de monitorización puede llegar a ser de varios años para conseguir una muestra de registros de huecos estadísticamente válida. - La limitación de recursos económicos para la adquisición e instalación de equipos de monitorización de huecos. - El elevado coste operativo que implica el análisis de los datos de los medidores de huecos de tensión instalados. - La restricción que tienen los datos de calidad de energía de las compañías eléctricas. Es decir, ante la carencia de datos que permitan analizar con mayor detalle los huecos de tensión, es de interés de las compañías eléctricas y la academia poder crear métodos fiables que permitan profundizar en el estudio, estimación y supervisión de este fenómeno electromagnético. Los huecos de tensión, al ser principalmente originados por eventos fortuitos como los cortocircuitos, son el resultado de diversas variables exógenas como: (i) la ubicación de la falta, (ii) la impedancia del material de contacto, (iii) el tipo de fallo, (iv) la localización del fallo en la red, (v) la duración del evento, etc. Es decir, para plantear de forma adecuada cualquier modelo teórico sobre los huecos de tensión, se requeriría representar esta incertidumbre combinada de las variables para proveer métodos realistas y, por ende, fiables para los usuarios. 1.3. Objetivo La presente tesis ha tenido como objetivo el desarrollo diversos métodos estocásticos para el estudio, estimación y supervisión de los huecos de tensión en los sistemas eléctricos de potencia. De forma específica, se ha profundizado en los siguientes ámbitos: - En el modelado realista de las variables que influyen en la caracterización de los huecos. Esto es, en esta Tesis se ha propuesto un método que permite representar de forma verosímil su cuantificación y aleatoriedad en el tiempo empleando distribuciones de probabilidad paramétricas. A partir de ello, se ha creado una herramienta informática que permite estimar la severidad de los huecos de tensión en un sistema eléctrico genérico. - Se ha analizado la influencia la influencia de las variables de entrada en la estimación de los huecos de tensión. En este caso, el estudio se ha enfocado en las variables de mayor divergencia en su caracterización de las propuestas existentes. - Se ha desarrollado un método que permite estima el número de huecos de tensión de una zona sin monitorización a través de la información de un conjunto limitado de medidas de un sistema eléctrico. Para ello, se aplican los principios de la estadística Bayesiana, estimando el número de huecos de tensión más probable de un emplazamiento basándose en los registros de huecos de otros nudos de la red. - Plantear una estrategia para optimizar la monitorización de los huecos de tensión en un sistema eléctrico. Es decir, garantizar una supervisión del sistema a través de un número de medidores menor que el número de nudos de la red. II. ESTRUCTURA DE LA TESIS Para plantear las propuestas anteriormente indicadas, la presente Tesis se ha estructurado en seis capítulos. A continuación, se describen brevemente los mismos. A manera de capítulo introductorio, en el capítulo 1, se realiza una descripción del planteamiento y estructura de la presente tesis. Esto es, se da una visión amplia de la problemática a tratar, además de describir el alcance de cada capítulo de la misma. En el capítulo 2, se presenta una breve descripción de los fundamentos y conceptos generales de los huecos de tensión. Los mismos, buscan brindar al lector de una mejor comprensión de los términos e indicadores más empleados en el análisis de severidad de los huecos de tensión en las redes eléctricas. Asimismo, a manera de antecedente, se presenta un resumen de las principales características de las técnicas o métodos existentes aplicados en la predicción y monitorización óptima de los huecos de tensión. En el capítulo 3, se busca fundamentalmente conocer la importancia de las variables que determinen la frecuencia o severidad de los huecos de tensión. Para ello, se ha implementado una herramienta de estimación de huecos de tensión que, a través de un conjunto predeterminado de experimentos mediante la técnica denominada Diseño de experimentos, analiza la importancia de la parametrización de las variables de entrada del modelo. Su análisis, es realizado mediante la técnica de análisis de la varianza (ANOVA), la cual permite establecer con rigor matemático si la caracterización de una determinada variable afecta o no la respuesta del sistema en términos de los huecos de tensión. En el capítulo 4, se propone una metodología que permite predecir la severidad de los huecos de tensión de todo el sistema a partir de los registros de huecos de un conjunto reducido de nudos de dicha red. Para ello, se emplea el teorema de probabilidad condicional de Bayes, el cual calcula las medidas más probables de todo el sistema a partir de la información proporcionada por los medidores de huecos instalados. Asimismo, en este capítulo se revela una importante propiedad de los huecos de tensión, como es la correlación del número de eventos de huecos de tensión en diversas zonas de las redes eléctricas. En el capítulo 5, se desarrollan dos métodos de localización óptima de medidores de huecos de tensión. El primero, que es una evolución metodológica del criterio de observabilidad; aportando en el realismo de la pseudo-monitorización de los huecos de tensión con la que se calcula el conjunto óptimo de medidores y, por ende, en la fiabilidad del método. Como una propuesta alternativa, se emplea la propiedad de correlación de los eventos de huecos de tensión de una red para plantear un método que permita establecer la severidad de los huecos de todo el sistema a partir de una monitorización parcial de dicha red. Finalmente, en el capítulo 6, se realiza una breve descripción de las principales aportaciones de los estudios realizados en esta tesis. Adicionalmente, se describen diversos temas a desarrollar en futuros trabajos. III. RESULTADOS En base a las pruebas realizadas en las tres redes planteadas; dos redes de prueba IEEE de 24 y 118 nudos (IEEE-24 e IEEE-118), además del sistema eléctrico de la República del Ecuador de 357 nudos (EC-357), se describen los siguientes puntos como las observaciones más relevantes: A. Estimación de huecos de tensión en ausencia de medidas: Se implementa un método estocástico de estimación de huecos de tensión denominado PEHT, el cual representa con mayor realismo la simulación de los eventos de huecos de un sistema a largo plazo. Esta primera propuesta de la tesis, es considerada como un paso clave para el desarrollo de futuros métodos del presente trabajo, ya que permite emular de forma fiable los registros de huecos de tensión a largo plazo en una red genérica. Entre las novedades más relevantes del mencionado Programa de Estimación de Huecos de Tensión (PEHT) se tienen: - Considerar el efecto combinado de cinco variables aleatorias de entrada para simular los eventos de huecos de tensión en una pseudo-monitorización a largo plazo. Las variables de entrada modeladas en la caracterización de los huecos de tensión en el PEHT son: (i) coeficiente de fallo, (ii) impedancia de fallo, (iii) tipo de fallo, (iv) localización del fallo y (v) duración. - El modelado estocástico de las variables de entrada impedancia de fallo y duración en la caracterización de los eventos de huecos de tensión. Para la parametrización de las variables mencionadas, se realizó un estudio detallado del comportamiento real de las mismas en los sistemas eléctricos. Asimismo, se define la función estadística que mejor representa la naturaleza aleatoria de cada variable. - Considerar como variables de salida del PEHT a indicadores de severidad de huecos de uso común en las normativas, como es el caso de los índices: SARFI-X, SARFI-Curve, etc. B. Análisis de sensibilidad de los huecos de tensión: Se presenta un estudio causa-efecto (análisis de sensibilidad) de las variables de entrada de mayor divergencia en su parametrización entre las referencias relacionadas a la estimación de los huecos de tensión en redes eléctricas. De forma específica, se profundiza en el estudio de la influencia de la parametrización de las variables coeficiente de fallo e impedancia de fallo en la predicción de los huecos de tensión. A continuación un resumen de las conclusiones más destacables: - La precisión de la variable de entrada coeficiente de fallo se muestra como un parámetro no influyente en la estimación del número de huecos de tensión (SARFI-90 y SARFI-70) a largo plazo. Es decir, no se requiere de una alta precisión del dato tasa de fallo de los elementos del sistema para obtener una adecuada estimación de los huecos de tensión. - La parametrización de la variable impedancia de fallo se muestra como un factor muy sensible en la estimación de la severidad de los huecos de tensión. Por ejemplo, al aumentar el valor medio de esta variable aleatoria, se disminuye considerablemente la severidad reportada de los huecos en la red. Por otra parte, al evaluar el parámetro desviación típica de la impedancia de fallo, se observa una relación directamente proporcional de este parámetro con la severidad de los huecos de tensión de la red. Esto es, al aumentar la desviación típica de la impedancia de fallo, se evidencia un aumento de la media y de la variación interanual de los eventos SARFI-90 y SARFI-70. - En base al análisis de sensibilidad desarrollado en la variable impedancia de fallo, se considera muy cuestionable la fiabilidad de los métodos de estimación de huecos de tensión que omiten su efecto en el modelo planteado. C. Estimación de huecos de tensión en base a la información de una monitorización parcial de la red: Se desarrolla un método que emplea los registros de una red parcialmente monitorizada para determinar la severidad de los huecos de todo el sistema eléctrico. A partir de los casos de estudio realizados, se observa que el método implementado (PEHT+MP) posee las siguientes características: - La metodología propuesta en el PEHT+MP combina la teoría clásica de cortocircuitos con diversas técnicas estadísticas para estimar, a partir de los datos de los medidores de huecos instalados, las medidas de huecos de los nudos sin monitorización de una red genérica. - El proceso de estimación de los huecos de tensión de la zona no monitorizada de la red se fundamenta en la aplicación del teorema de probabilidad condicional de Bayes. Es decir, en base a los datos observados (los registros de los nudos monitorizados), el PEHT+MP calcula de forma probabilística la severidad de los huecos de los nudos sin monitorización del sistema. Entre las partes claves del procedimiento propuesto se tienen los siguientes puntos: (i) la creación de una base de datos realista de huecos de tensión a través del Programa de Estimación de Huecos de Tensión (PEHT) propuesto en el capítulo anterior; y, (ii) el criterio de máxima verosimilitud empleado para estimar las medidas de huecos de los nudos sin monitorización de la red evaluada. - Las predicciones de medidas de huecos de tensión del PEHT+MP se ven potenciadas por la propiedad de correlación de los huecos de tensión en diversas zonas de un sistema eléctrico. Esta característica intrínseca de las redes eléctricas limita de forma significativa la respuesta de las zonas fuertemente correlacionadas del sistema ante un eventual hueco de tensión. Como el PEHT+MP está basado en principios probabilísticos, la reducción del rango de las posibles medidas de huecos se ve reflejado en una mejor predicción de las medidas de huecos de la zona no monitorizada. - Con los datos de un conjunto de medidores relativamente pequeño del sistema, es posible obtener estimaciones precisas (error nulo) de la severidad de los huecos de la zona sin monitorizar en las tres redes estudiadas. - El PEHT+MP se puede aplicar a diversos tipos de indicadores de severidad de los huecos de tensión, como es el caso de los índices: SARFI-X, SARFI-Curve, SEI, etc. D. Localización óptima de medidores de huecos de tensión: Se plantean dos métodos para ubicar de forma estratégica al sistema de monitorización de huecos en una red genérica. La primera propuesta, que es una evolución metodológica de la localización óptima de medidores de huecos basada en el criterio de observabilidad (LOM+OBS); y, como segunda propuesta, un método que determina la localización de los medidores de huecos según el criterio del área de correlación (LOM+COR). Cada método de localización óptima de medidores propuesto tiene un objetivo concreto. En el caso del LOM+OBS, la finalidad del método es determinar el conjunto óptimo de medidores que permita registrar todos los fallos que originen huecos de tensión en la red. Por otro lado, en el método LOM+COR se persigue definir un sistema óptimo de medidores que, mediante la aplicación del PEHT+MP (implementado en el capítulo anterior), sea posible estimar de forma precisa las medidas de huecos de tensión de todo el sistema evaluado. A partir del desarrollo de los casos de estudio de los citados métodos de localización óptima de medidores en las tres redes planteadas, se describen a continuación las observaciones más relevantes: - Como la generación de pseudo-medidas de huecos de tensión de los métodos de localización óptima de medidores (LOM+OBS y LOM+COR) se obtienen mediante la aplicación del algoritmo PEHT, la formulación del criterio de optimización se realiza en base a una pseudo-monitorización realista, la cual considera la naturaleza aleatoria de los huecos de tensión a través de las cinco variables estocásticas modeladas en el PEHT. Esta característica de la base de datos de pseudo-medidas de huecos de los métodos LOM+OBS y LOM+COR brinda una mayor fiabilidad del conjunto óptimo de medidores calculado respecto a otros métodos similares en la bibliografía. - El conjunto óptimo de medidores se determina según la necesidad del operador de la red. Esto es, si el objetivo es registrar todos los fallos que originen huecos de tensión en el sistema, se emplea el criterio de observabilidad en la localización óptima de medidores de huecos. Por otra parte, si se plantea definir un sistema de monitorización que permita establecer la severidad de los huecos de tensión de todo el sistema en base a los datos de un conjunto reducido de medidores de huecos, el criterio de correlación resultaría el adecuado. De forma específica, en el caso del método LOM+OBS, basado en el criterio de observabilidad, se evidenciaron las siguientes propiedades en los casos de estudio realizados: - Al aumentar el tamaño de la red, se observa la tendencia de disminuir el porcentaje de nudos monitorizados de dicho sistema. Por ejemplo, para monitorizar los fallos que originan huecos en la red IEEE-24, se requiere monitorizar el 100\% de los nudos del sistema. En el caso de las redes IEEE-118 y EC-357, el método LOM+OBS determina que con la monitorización de un 89.5% y 65.3% del sistema, respectivamente, se cumpliría con el criterio de observabilidad del método. - El método LOM+OBS permite calcular la probabilidad de utilización del conjunto óptimo de medidores a largo plazo, estableciendo así un criterio de la relevancia que tiene cada medidor considerado como óptimo en la red. Con ello, se puede determinar el nivel de precisión u observabilidad (100%, 95%, etc.) con el cual se detectarían los fallos que generan huecos en la red estudiada. Esto es, al aumentar el nivel de precisión de detección de los fallos que originan huecos, se espera que aumente el número de medidores requeridos en el conjunto óptimo de medidores calculado. - El método LOM+OBS se evidencia como una técnica aplicable a todo tipo de sistema eléctrico (radial o mallado), el cual garantiza la detección de los fallos que originan huecos de tensión en un sistema según el nivel de observabilidad planteado. En el caso del método de localización óptima de medidores basado en el criterio del área de correlación (LOM+COR), las diversas pruebas realizadas evidenciaron las siguientes conclusiones: - El procedimiento del método LOM+COR combina los métodos de estimación de huecos de tensión de capítulos anteriores (PEHT y PEHT+MP) con técnicas de optimización lineal para definir la localización óptima de los medidores de huecos de tensión de una red. Esto es, se emplea el PEHT para generar los pseudo-registros de huecos de tensión, y, en base al criterio planteado de optimización (área de correlación), el LOM+COR formula y calcula analíticamente el conjunto óptimo de medidores de la red a largo plazo. A partir de la información registrada por este conjunto óptimo de medidores de huecos, se garantizaría una predicción precisa de la severidad de los huecos de tensión de todos los nudos del sistema con el PEHT+MP. - El método LOM+COR requiere un porcentaje relativamente reducido de nudos del sistema para cumplir con las condiciones de optimización establecidas en el criterio del área de correlación. Por ejemplo, en el caso del número total de huecos (SARFI-90) de las redes IEEE-24, IEEE-118 y EC-357, se calculó un conjunto óptimo de 9, 12 y 17 medidores de huecos, respectivamente. Es decir, solamente se requeriría monitorizar el 38\%, 10\% y 5\% de los sistemas indicados para supervisar los eventos SARFI-90 en toda la red. - El método LOM+COR se muestra como un procedimiento de optimización versátil, el cual permite reducir la dimensión del sistema de monitorización de huecos de redes eléctricas tanto radiales como malladas. Por sus características, este método de localización óptima permite emular una monitorización integral del sistema a través de los registros de un conjunto pequeño de monitores. Por ello, este nuevo método de optimización de medidores sería aplicable a operadores de redes que busquen disminuir los costes de instalación y operación del sistema de monitorización de los huecos de tensión. ABSTRACT I. GENERALITIES 1.1. Introduction Among the various types of electrical disturbances, voltage sags are considered the most common quality problem in power systems. This phenomenon is caused by an extreme increase of the current in the network, primarily caused by short-circuits or inadequate maneuvers in the system. This type of electrical disturbance is basically characterized by two parameters: residual voltage and duration. Typically, voltage sags occur when the residual voltage, in some phases, reaches a value between 0.01 to 0.9 pu and lasts up to 60 seconds. To an end user, the most important effect of a voltage sags is the interruption or alteration of their equipment operation, with electronic devices the most affected (e.g. computer, drive controller, PLC, relay, etc.). Due to the technology boom of recent decades and the constant search for automating production processes, the use of electronic components is essential today. This fact makes the effects of voltage sags more noticeable to the end user, causing the level of demand for a quality energy supply to be increased. In general, the study of voltage sags is usually approached from one of two aspects: the load or the network. From the point of view of the load, it is necessary to know the sensitivity characteristics of the equipment to model their response to sudden changes in power supply voltage. From the perspective of the network, the goal is to estimate or obtain adequate information to characterize the network behavior in terms of voltage sags. In this thesis, the work presented fits into the second aspect; that is, in the modeling and estimation of the response of a power system to voltage sag events. 1.2. Problem Statement Although voltage sags are the most frequent quality supply problem in electrical networks, thistype of disturbance remains complex and challenging to analyze properly. Among the most common reasons for this difficulty are: - The sag monitoring time, because it can take up to several years to get a statistically valid sample. - The limitation of funds for the acquisition and installation of sag monitoring equipment. - The high operating costs involved in the analysis of the voltage sag data from the installed monitors. - The restrictions that electrical companies have with the registered power quality data. That is, given the lack of data to further voltage sag analysis, it is of interest to electrical utilities and researchers to create reliable methods to deepen the study, estimation and monitoring of this electromagnetic phenomenon. Voltage sags, being mainly caused by random events such as short-circuits, are the result of various exogenous variables such as: (i) the number of faults of a system element, (ii) the impedance of the contact material, (iii) the fault type, (iv) the fault location, (v) the duration of the event, etc. That is, to properly raise any theoretical model of voltage sags, it is necessary to represent the combined uncertainty of variables to provide realistic methods that are reliable for users. 1.3. Objective This Thesis has been aimed at developing various stochastic methods for the study, estimation and monitoring of voltage sags in electrical power systems. Specifically, it has deepened the research in the following areas: - This research furthers knowledge in the realistic modeling of the variables that influence sag characterization. This thesis proposes a method to credibly represent the quantification and randomness of the sags in time by using parametric probability distributions. From this, a software tool was created to estimate the severity of voltage sags in a generic power system. - This research also analyzes the influence of the input variables in the estimation of voltage sags. In this case, the study has focused on the variables of greatest divergence in their characterization of the existing proposals. - A method was developed to estimate the number of voltage sags of an area without monitoring through the information of a limited set of sag monitors in an electrical system. To this end, the principles of Bayesian statistics are applied, estimating the number of sags most likely to happen in a system busbar based in records of other sag network busbars. - A strategy was developed to optimize the monitorization of voltage sags on a power system. Its purpose is to ensure the monitoring of the system through a number of monitors lower than the number of busbars of the network assessed. II. THESIS STRUCTURE To describe in detail the aforementioned proposals, this Thesis has been structured into six chapters. Below is are brief descriptions of them: As an introductory chapter, Chapter 1, provides a description of the approach and structure of this thesis. It presents a wide view of the problem to be treated, in addition to the description of the scope of each chapter. In Chapter 2, a brief description of the fundamental and general concepts of voltage sags is presented to provide to the reader a better understanding of the terms and indicators used in the severity analysis of voltage sags in power networks. Also, by way of background, a summary of the main features of existing techniques or methods used in the prediction and optimal monitoring of voltage sags is also presented. Chapter 3 essentially seeks to know the importance of the variables that determine the frequency or severity of voltage sags. To do this, a tool to estimate voltage sags is implemented that, through a predetermined set of experiments using the technique called Design of Experiments, discusses the importance of the parameters of the input variables of the model. Its analysis is interpreted by using the technique of analysis of variance (ANOVA), which provides mathematical rigor to establish whether the characterization of a particular variable affects the system response in terms of voltage sags or not. In Chapter 4, a methodology to predict the severity of voltage sags of an entire system through the sag logs of a reduced set of monitored busbars is proposed. For this, the Bayes conditional probability theorem is used, which calculates the most likely sag severity of the entire system from the information provided by the installed monitors. Also, in this chapter an important property of voltage sags is revealed, as is the correlation of the voltage sags events in several zones of a power system. In Chapter 5, two methods of optimal location of voltage sag monitors are developed. The first one is a methodological development of the observability criteria; it contributes to the realism of the sag pseudo-monitoring with which the optimal set of sag monitors is calculated and, therefore, to the reliability of the proposed method. As an alternative proposal, the correlation property of the sag events of a network is used to raise a method that establishes the sag severity of the entire system from a partial monitoring of the network. Finally, in Chapter 6, a brief description of the main contributions of the studies in this Thesis is detailed. Additionally, various themes to be developed in future works are described. III. RESULTS. Based on tests on the three networks presented, two IEEE test networks of 24 and 118 busbars (IEEE-24 and IEEE-118) and the electrical system of the Republic of Ecuador (EC-357), the following points present the most important observations: A. Estimation of voltage sags in the absence of measures: A stochastic estimation method of voltage sags, called PEHT, is implemented to represent with greater realism the long-term simulation of voltage sags events in a system. This first proposal of this thesis is considered a key step for the development of future methods of this work, as it emulates in a reliable manner the voltage sag long-term records in a generic network. Among the main innovations of this voltage sag estimation method are the following: - Consideration of the combined effect of five random input variables to simulate the events of voltage sags in long-term monitoring is included. The input variables modeled in the characterization of voltage sags on the PEHT are as follows: (i) fault coefficient, (ii) fault impedance, (iii) type of fault, (iv) location of the fault, and (v) fault duration. - Also included is the stochastic modeling of the input variables of fault impedance and duration in the characterization of the events of voltage sags. For the parameterization of these variables, a detailed study of the real behavior in power systems is developed. Also, the statistical function best suited to the random nature of each variable is defined. - Consideration of sag severity indicators used in standards as PEHT output variables, including such as indices as SARFI-X, SARFI-Curve, etc. B. Sensitivity analysis of voltage sags: A cause-effect study (sensitivity analysis) of the input variables of greatest divergence between reference parameterization related to the estimation of voltage sags in electrical networks is presented. Specifically, it delves into the study of the influence of the parameterization of the variables fault coefficient and fault impedance in the voltage sag estimation. Below is a summary of the most notable observations: - The accuracy of the input variable fault coefficient is shown as a non-influential parameter in the long-term estimation of the number of voltage sags (SARFI-90 and SARFI-70). That is, it does not require a high accuracy of the fault rate data of system elements for a proper voltage sag estimation. - The parameterization of the variable fault impedance is shown to be a very sensitive factor in the estimation of the voltage sag severity. For example, by increasing the average value of this random variable, the reported sag severity in the network significantly decreases. Moreover, in assessing the standard deviation of the fault impedance parameter, a direct relationship of this parameter with the voltage sag severity of the network is observed. That is, by increasing the fault impedance standard deviation, an increase of the average and the interannual variation of the SARFI-90 and SARFI-70 events is evidenced. - Based on the sensitivity analysis developed in the variable fault impedance, the omission of this variable in the voltage sag estimation would significantly call into question the reliability of the responses obtained. C. Voltage sag estimation from the information of a network partially monitored: A method that uses the voltage sag records of a partially monitored network for the sag estimation of all the power system is developed. From the case studies performed, it is observed that the method implemented (PEHT+MP) has the following characteristics: - The methodology proposed in the PEHT+MP combines the classical short-circuit theory with several statistical techniques to estimate, from data the of the installed sag meters, the sag measurements of unmonitored busbars of a generic power network. - The estimation process of voltage sags of the unmonitored zone of the network is based on the application of the conditional probability theorem of Bayes. That is, based on the observed data (monitored busbars records), the PEHT+MP calculates probabilistically the sag severity at unmonitored system busbars. Among the key parts of the proposed procedure are the following: (i) the creation of a realistic data base of voltage sags through of the sag estimation program (PEHT); and, (ii) the maximum likelihood criterion used to estimate the sag indices of system busbars without monitoring. - The voltage sag measurement estimations of PEHT+MP are potentiated by the correlation property of the sag events in power systems. This inherent characteristic of networks significantly limits the response of strongly correlated system zones to a possible voltage sag. As the PEHT+MP is based on probabilistic principles, a reduction of the range of possible sag measurements is reflected in a better sag estimation of the unmonitored area of the power system. - From the data of a set of monitors representing a relatively small portion of the system, to obtain accurate estimations (null error) of the sag severity zones without monitoring is feasible in the three networks studied. - The PEHT+MP can be applied to several types of sag indices, such as: SARFI-X, SARFI-Curve, SEI, etc. D. Optimal location of voltage sag monitors in power systems: Two methods for strategically locating the sag monitoring system are implemented for a generic network. The first proposal is a methodological development of the optimal location of sag monitors based on the observability criterion (LOM + OBS); the second proposal is a method that determines the sag monitor location according to the correlation area criterion (LOM+COR). Each proposed method of optimal location of sag monitors has a specific goal. In the case of LOM+OBS, the purpose of the method is to determine the optimal set of sag monitors to record all faults that originate voltage sags in the network. On the other hand, the LOM+COR method attempts to define the optimal location of sag monitors to estimate the sag indices in all the assessed network with the PEHT+MP application. From the development of the case studies of these methods of optimal location of sag monitors in the three networks raised, the most relevant observations are described below: - As the generation of voltage sag pseudo-measurements of the optimal location methods (LOM+OBS and LOM+COR) are obtained by applying the algorithm PEHT, the formulation of the optimization criterion is performed based on a realistic sag pseudo-monitoring, which considers the random nature of voltage sags through the five stochastic variables modeled in PEHT. This feature of the database of sag pseudo-measurements of the LOM+OBS and LOM+COR methods provides a greater reliability of the optimal set of monitors calculated when compared to similar methods in the bibliography. - The optimal set of sag monitors is determined by the network operator need. That is, if the goal is to record all faults that originate from voltage sags in the system, the observability criterion is used to determine the optimal location of sag monitors (LOM+OBS). Moreover, if the objective is to define a monitoring system that allows establishing the sag severity of the system from taken from information based on a limited set of sag monitors, the correlation area criterion would be appropriate (LOM+COR). Specifically, in the case of the LOM+OBS method (based on the observability criterion), the following properties were observed in the case studies: - By increasing the size of the network, there was observed a reduction in the percentage of monitored system busbars required. For example, to monitor all the faults which cause sags in the IEEE-24 network, then 100% of the system busbars are required for monitoring. In the case of the IEEE-118 and EC-357 networks, the method LOM+OBS determines that with monitoring 89.5 % and 65.3 % of the system, respectively, the observability criterion of the method would be fulfilled. - The LOM+OBS method calculates the probability of using the optimal set of sag monitors in the long term, establishing a relevance criterion of each sag monitor considered as optimal in the network. With this, the level of accuracy or observability (100%, 95%, etc.) can be determined, with which the faults that caused sags in the studied network are detected. That is, when the accuracy level for detecting faults that cause sags in the system is increased, a larger number of sag monitors is expected when calculating the optimal set of monitors. - The LOM + OBS method is demonstrated to be a technique applicable to any type of electrical system (radial or mesh), ensuring the detection of faults that cause voltage sags in a system according to the observability level raised. In the case of the optimal localization of sag monitors based on the criterion of correlation area (LOM+COR), several tests showed the following conclusions: - The procedure of LOM+COR method combines the implemented algorithms of voltage sag estimation (PEHT and PEHT+MP) with linear optimization techniques to define the optimal location of the sag monitors in a network. That is, the PEHT is used to generate the voltage sag pseudo-records, and, from the proposed optimization criterion (correlation area), the LOM+COR formulates and analytically calculates the optimal set of sag monitors of the network in the long term. From the information recorded by the optimal set of sag monitors, an accurate prediction of the voltage sag severity at all the busbars of the system is guaranteed with the PEHT+MP. - The LOM + COR method is shown to be a versatile optimization procedure, which reduces the size of the sag monitoring system both at radial as meshed grids. Due to its characteristics, this optimal location method allows emulation of complete system sag monitoring through the records of a small optimal set of sag monitors. Therefore, this new optimization method would be applicable to network operators that looks to reduce the installation and operation costs of the voltage sag monitoring system.

Hybrid wind-diesel power plant valuation and optimization using real option theory and dynamic programming

El sistema de energía eólica-diesel híbrido tiene un gran potencial en la prestación de suministro de energía a comunidades remotas. En comparación con los sistemas tradicionales de diesel, las plantas de energía híbridas ofrecen grandes ventajas tales como el suministro de capacidad de energía extra para "microgrids", reducción de los contaminantes y emisiones de gases de efecto invernadero, y la cobertura del riesgo de aumento inesperado del precio del combustible. El principal objetivo de la presente tesis es proporcionar nuevos conocimientos para la evaluación y optimización de los sistemas de energía híbrido eólico-diesel considerando las incertidumbres. Dado que la energía eólica es una variable estocástica, ésta no puede ser controlada ni predecirse con exactitud. La naturaleza incierta del viento como fuente de energía produce serios problemas tanto para la operación como para la evaluación del valor del sistema de energía eólica-diesel híbrido. Por un lado, la regulación de la potencia inyectada desde las turbinas de viento es una difícil tarea cuando opera el sistema híbrido. Por otro lado, el bene.cio económico de un sistema eólico-diesel híbrido se logra directamente a través de la energía entregada a la red de alimentación de la energía eólica. Consecuentemente, la incertidumbre de los recursos eólicos incrementa la dificultad de estimar los beneficios globales en la etapa de planificación. La principal preocupación del modelo tradicional determinista es no tener en cuenta la incertidumbre futura a la hora de tomar la decisión de operación. Con lo cual, no se prevé las acciones operativas flexibles en respuesta a los escenarios futuros. El análisis del rendimiento y simulación por ordenador en el Proyecto Eólico San Cristóbal demuestra que la incertidumbre sobre la energía eólica, las estrategias de control, almacenamiento de energía, y la curva de potencia de aerogeneradores tienen un impacto significativo sobre el rendimiento del sistema. En la presente tesis, se analiza la relación entre la teoría de valoración de opciones y el proceso de toma de decisiones. La opción real se desarrolla con un modelo y se presenta a través de ejemplos prácticos para evaluar el valor de los sistemas de energía eólica-diesel híbridos. Los resultados muestran que las opciones operacionales pueden aportar un valor adicional para el sistema de energía híbrida, cuando esta flexibilidad operativa se utiliza correctamente. Este marco se puede aplicar en la optimización de la operación a corto plazo teniendo en cuenta la naturaleza dependiente de la trayectoria de la política óptima de despacho, dadas las plausibles futuras realizaciones de la producción de energía eólica. En comparación con los métodos de valoración y optimización existentes, el resultado del caso de estudio numérico muestra que la política de operación resultante del modelo de optimización propuesto presenta una notable actuación en la reducción del con- sumo total de combustible del sistema eólico-diesel. Con el .n de tomar decisiones óptimas, los operadores de plantas de energía y los gestores de éstas no deben centrarse sólo en el resultado directo de cada acción operativa, tampoco deberían tomar decisiones deterministas. La forma correcta es gestionar dinámicamente el sistema de energía teniendo en cuenta el valor futuro condicionado en cada opción frente a la incertidumbre. ABSTRACT Hybrid wind-diesel power systems have a great potential in providing energy supply to remote communities. Compared with the traditional diesel systems, hybrid power plants are providing many advantages such as providing extra energy capacity to the micro-grid, reducing pollution and greenhouse-gas emissions, and hedging the risk of unexpected fuel price increases. This dissertation aims at providing novel insights for assessing and optimizing hybrid wind-diesel power systems considering the related uncertainties. Since wind power can neither be controlled nor accurately predicted, the energy harvested from a wind turbine may be considered a stochastic variable. This uncertain nature of wind energy source results in serious problems for both the operation and value assessment of the hybrid wind-diesel power system. On the one hand, regulating the uncertain power injected from wind turbines is a difficult task when operating the hybrid system. On the other hand, the economic profit of a hybrid wind-diesel system is achieved directly through the energy delivered to the power grid from the wind energy. Therefore, the uncertainty of wind resources has increased the difficulty in estimating the total benefits in the planning stage. The main concern of the traditional deterministic model is that it does not consider the future uncertainty when making the dispatch decision. Thus, it does not provide flexible operational actions in response to the uncertain future scenarios. Performance analysis and computer simulation on the San Cristobal Wind Project demonstrate that the wind power uncertainty, control strategies, energy storage, and the wind turbine power curve have a significant impact on the performance of the system. In this dissertation, the relationship between option pricing theory and decision making process is discussed. A real option model is developed and presented through practical examples for assessing the value of hybrid wind-diesel power systems. Results show that operational options can provide additional value to the hybrid power system when this operational flexibility is correctly utilized. This framework can be applied in optimizing short term dispatch decisions considering the path-dependent nature of the optimal dispatch policy, given the plausible future realizations of the wind power production. Comparing with the existing valuation and optimization methods, result from numerical example shows that the dispatch policy resulting from the proposed optimization model exhibits a remarkable performance in minimizing the total fuel consumption of the wind-diesel system. In order to make optimal decisions, power plant operators and managers should not just focus on the direct outcome of each operational action; neither should they make deterministic decisions. The correct way is to dynamically manage the power system by taking into consideration the conditional future value in each option in response to the uncertainty.