Cellular networks in Africa have grown exponentially over the past 10 years and their data centres (DCs) on average consume 3 MW of electrical power. They require a reliable electrical power supply and can have a downtime loss of over a million dollars per hour. Power quality, reliability and availability have emerged as key issues for the successful operation of a data centre. Investigations are carried out into emerging technologies and their application in data centre power distribution systems for cellular networks in Africa. Best practices are applied to develop a power distribution system (PDS) with the objective of achieving optimal reliability and availability. Analytical techniques are applied to determine and compare the reliability and availability of various power systems. Minimal cut set simulations identify system weak points and confirm component selection. Components’ inherent characteristics (CIC) and system connectivity topology (SCT) are key factors in the improvement of data centre availability. The analysis practices can be used by engineers and managers as a basis for informed decision making in determining power system reliability and the availability of an existing or a new data centre design. Weak points in the PDS of a data centre causing downtime are identified through analysis, and accurate solutions can be determined to prevent or minimise downtime. System connectivity topology (SCT) techniques were identified that could increase the reliability and availability of data centres for cellular networks in Africa. These techniques include multiple incomers from the utility company, redundancy levels of critical equipment and parallel distribution paths. Two case studies were carried out on data centres for a cellular network, one in Nigeria and one in Cameroon. The reliability and availability of both data centres was improved, with substantial reduction in downtime per year. The outcome of the case studies shows the importance of designing and implementing the power distribution system with sufficient levels of redundancy for critical equipment, and parallel distribution paths. / MSc (Engineering Sciences in Nuclear Engineering), North-West University, Potchefstroom Campus, 2014

Over the past decade, underground power distribution has become increasingly popular due to its reliability, safety, aesthetic characteristics, as well as the ever increasing focus on the environmental impacts of the various stages of power generation and distribution. With the technological advances in this area, the process of running these cables have become more economical and efficient. This thesis explores the practice of grouping multiple three phase cables in a common conduit, using the duct bank process, and analyzes the thermal and ampacity consequences on the individual lines. This analysis is done in an effort to better define and understand the various limitations of the practice and explore future possibilities in its expansion.

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

An Integrated Power System (IPS) with a DC Zonal Electrical Distribution System (DC ZEDS) is a strong candidate for the next generation submarine and surface ship. To study the implementation of an IPS with DC ZEDS, members of the Energy Sources Analysis Consortium (ESAC) are currently constructing a reduced-scale laboratory. One fundamental component of DC ZEDS is the Ships Service Converter Module (SSCM), commonly known as a buck DC-DC converter. This thesis documents the design, simulation, construction and testing of a 500V/400V, 8kW resonant soft-switched DC-DC converter. In theory, resonant converters will operate more efficiently and generate less Electromagnetic Interference (EMI) when compared to a standard hard-switched converter. In this thesis, the resonant converter is tested and compared to a hard-switched DC-DC converter that was designed for ESAC's reduced-scaled IPS. The results verify that the resonant DC-DC converter realizes significant efficiency and EMI generation improvements over the hard-switched converter at the cost of a more complex control system and power section. / US Navy (USN) author

Abstract With an ever increasing emphasis on reliability of supply, improvement in the lightning performance of distribution lines is required. The arc quenching properties of wooden distribution line poles during lightning strikes are an important factor in the reduction of switchgear operation, hence outages. Measurements were conducted on a 22 kV distribution line and it was suspected, in some cases, that direct lightning strikes to the line did not cause switchgear operation. Distribution lines predominantly use wooden poles with a specific configuration which incorporates a 'wooden' spark gap. This paper provides background to the basic configuration of a typical distribution line and the processes which govern the electric arc. A simulation using a dynamic arc model shows that field measured lightning overvoltages on a distribution line are reproducible through system modelling. The simplistic dynamic arc model developed is sufficiently accurate to describe a set of arcs in a larger system such as a distribution line.

A reconfiguração de sistemas de distribuição de energia elétrica consiste em alterar a topologia das redes através da abertura ou fechamento das chaves de interconexão existentes nos alimentadores de distribuição primários, de forma a otimizar uma determinada função objetivo. Normalmente os objetivos são a minimização de perdas ativas, o isolamento de faltas, o balanceamento de cargas entre alimentadores e/ou a melhoria dos níveis de tensão. Neste trabalho considera-se a minimização da perda ativa total. As dificuldades do problema de reconfiguração de redes de distribuição resultam do tamanho dos sistemas reais, aos quais correspondem um número elevado de variáveis binárias que representam as chaves, e também da relação quadrática existente entre a perda elétrica e a corrente que flui nos elementos da rede. Este trabalho desenvolve algumas novas formulações para o problema de reconfiguração de redes de distribuição, utilizando Programação Não Linear Inteira Mista. Além disso, demonstra-se que a parte contínua de todas as formulações é convexa, o que garante a unicidade da solução ótima para um dado estado das chaves na rede. Esta propriedade permitiu a utilização do Método de Newton na resolução do problema contínuo, com as seguintes vantagens: impossibilidade de o método identificar mínimos locais em vez do mínimo global procurado, e convergência em apenas uma iteração, proporcionada pela natureza quadrática das formulações. As formulações desenvolvidas foram implementadas na forma de programas computacionais. O desempenho das formulações é descrito e analisado através de diversos casos de estudo. / The reconfiguration of electricity distribution systems is concerned with finding the state of switching and protective devices so as to optimize a given objective function, which is usually defined as minimization of total loss, fault isolation, load balancing among feeders, or improvement of voltage profile. In this work, the objective function is defined as the minimization of total active loss. The main difficulties associated with this problem arise from the high number of binary variables that represent the switching and protective devices, as well as the quadratic relationship between electric loss and currents flowing through the network branches. This work develops some new formulations for the distribution system reconfiguration problem, which are then solved through mixed-integer nonlinear programming. In addition, it is shown that the continuous part in all formulations is convex, which guarantees the uniqueness of the optimal solution for a given switch profile. This property allows using the Standard Newton Method for solving the continuous part of the problem, with the following advantages: impossibility of the Newton Method identifying a local minimum instead of the desired global minimum, and convergence in just one iteration owing to the quadratic nature of all formulations. The proposed formulations were implemented as computational programs and their performance was evaluated through various study cases.

Energy security concerns and the impact of traditional sources of power generation on the climate have prompted a rise in renewable energy expansion around the world. Power transmission from remote generation sites to consumers over long distance is most efficient using High-Voltage Direct Current (HVDC) transmission lines. Consequently, HVDC and the integration of renewable resources are considered as key perspectives in the improvement of sustainable energy systems capable of secure and stable electric power supply. With the intention of huge energy demand in the future, the multi-terminal DC grid concept is proposed based on various converter topologies like Line Commutated Converter (LCC), Voltage Sourced Converter (VSC), and Modular Multilevel Converter (MMC) HVDC technologies. These converters play a vital role in integrating remotely-located renewable generation and reinforcing existing power systems. The MMC has become increasingly popular in HVDC transmission compared to conventional line commutated converters, two-level and multilevel voltage source converters. Low generation of harmonics, a low switching frequency of semiconductors, sine formed AC voltages and currents, black start capability and higher overall efficiency are a few of the unique features of MMC. The MMC is characterised by a modular arm structure, formed by a cascade connection of a vast number of simple cells with floating DC capacitors. These cells are called Sub-Modules (SMs) and can be easily assembled into a converter for high voltage power conversion systems. Compared with traditional VSCs, the analytical modelling of MMC is more challenging. This is because of technical issues such as higher order system, the discontinuous and non-linear nature of signal transfer through converters, the complexity of the interaction equations between the AC and DC variables, and harmonic frequency conversion through AC side and DC side of the converter. This work intends to resolve these challenges by developing a detailed non-linear model using fundamental switching Selective Harmonic Elimination (SHE) modulation technique, an average MMC model in DQ0 frame and an analytical dynamic MMC model, which can be suitable for small-signal stability studies, and control design. Firstly, the detailed model of MMC using fundamental switching SHE modulation scheme has been developed using PSCAD/EMTDC (Power systems computer aided design Electromagnetic transients for DC) software. The basic terms and equations of the MMC have been presented along control loops. The significance of the switching frequency on the performance of the MMC has been studied as well as the relation between the switching frequency, the Total Harmonic Distortion (THD) and the number of output voltage levels. Detailed representation of MMC systems in PSCAD/EMTDC programs incorporates the modelling of Insulated-Gate Bipolar Transistor (IGBT) valves and should typically utilise small integration time-steps to represent fast switching events precisely. Computational burden introduced by such detailed models make the study of steady-state and transient events more complex, highlighting the need to implement more efficient models that provide comparative behaviour and dynamic response. Secondly, average DQ0 models has been implemented to accurately replicate the steady-state, dynamic and transient behaviour of MMC in PSCAD/EMTDC programs. These simplified models represent the average response of switching devices and converters by using averaging techniques involving controlled sources and switching functions. Developing the MMC average model in DQ0 frame was a challenging task because of the multiplication terms in the MMC average model in ABC frame. The proposed approach to overcome this challenge is considering generic form for the product variables and multiplying them in ABC frame and then transferring only the DC and fundamental frequency components of the results to DQ0 frame. The comparisons between detailed model and the average model validated the effectiveness of the average model in representing the dynamics of MMC. It is at least one hundred times faster than the detailed model for the same simulation time step. Finally, a dynamic analytical MMC model and associated controls have been proposed. To enable the model application to a broad range of system configurations and various dynamic studies, the model is built on a modular modelling approach using four sub-systems; an AC system, Phase Locked Loop (PLL) system, MMC system and a DC arrangement. The developed MMC system model has been linearized and implemented in state-space form. To select the best open-loop controller gains, eigenvalue analysis is performed for each particular test system. The rationality and correctness of the proposed model are verified against non-linear PSCAD/EMTDC simulations, and good accuracy is obtained in the time domain analysis. Further, the model is also verified in the frequency domain, and it is concluded that the developed model can be employed for dynamic analysis below 300 Hz.

Enhancing power grids' performance and resilience has been one of the greatest challenges in engineering and science over the past decade. A recent report by the National Academies of Sciences, Engineering, and Medicine along with other studies emphasizes the necessity of deploying new ideas and mathematical tools to address the challenges facing the power grids now and in the future. To full this necessity, numerous grid modernization programs have been initiated in recent years. This thesis focuses on one of the most critical challenges facing power grids which is their vulnerability against failures and attacks. Our approach bridges concepts in power engineering and computer science to improve power grids resilience and security. We analyze the vulnerability of power grids to cyber and physical attacks and failures, design efficient monitoring schemes for robust state estimation, develop algorithms to control the grid under tension, and introduce methods to generate realistic power grid test cases. Our contributions can be divided into four major parts: Power Grid State Prediction: Large scale power outages in Australia (2016), Ukraine (2015), Turkey (2015), India (2013), and the U.S. (2011, 2003) have demonstrated the vulnerability of power grids to cyber and physical attacks and failures. Power grid outages have devastating effects on almost every aspect of modern life as well as on interdependent systems. Despite their inevitability, the effects of failures on power grids' performance can be limited if the system operator can predict and understand the consequences of an initial failure and can immediately detect the problematic failures. To enable these capabilities, we study failures in power grids using computational and analytical tools based on the DC power flow model. We introduce new metrics to efficiently evaluate the severity of an initial failure and develop efficient algorithms to predict its consequences. We further identify power grids' vulnerabilities using these metrics and algorithms. Power Grid State Estimation: In order to obtain an accurate prediction of the subsequent effects of an initial failure on the performance of the grid, the system operator needs to exactly know when and where the initial failure has happened. However, due to lack of enough measurement devices or a cyber attack on the grid, such information may not be available directly to the grid operator via measurements. To address this problem, we develop efficient methods to estimate the state of the grid and detect failures (if any) from partial available information. Power Grid Control: Once an initial failure is detected, prediction methods can be used to predict the subsequent effects of that failure. If the initial failure is causing a cascade of failures in the grid, a control mechanism needs to be applied in order to mitigate its further effects. Power Grid Islanding is an effective method to mitigate cascading failures. The challenge is to partition the network into smaller connected components, called islands, so that each island can operate independently for a short period of time. This is to prevent the system to be separated into unbalanced parts due to cascading failures. To address this problem, we introduce and study the Doubly Balanced Connected graph Partitioning (DBCP) problem and provide an efficient algorithm to partition the power grid into two operating islands. Power Grid Test Cases for Evaluation: In order to evaluate algorithms that are developed for enhancing power grids resilience, one needs to study their performance on the real grid data. However, due to security reasons, such data sets are not publicly available and are very hard to obtain. Therefore, we study the structural properties of the U.S. Western Interconnection grid (WI), and based on the results we present the Network Imitating Method Based on LEarning (NIMBLE) for generating synthetic spatially embedded networks with similar properties to a given grid. We apply NIMBLE to the WI and show that the generated network has similar structural and spatial properties as well as the same level of robustness to cascading failures. Overall, the results provided in this thesis advance power grids' resilience and security by providing a better understanding of the system and by developing efficient algorithms to protect it at the time of failure.

A reconfiguração de sistemas de distribuição de energia elétrica consiste em alterar a topologia das redes através da abertura ou fechamento das chaves de interconexão existentes nos alimentadores de distribuição primários, de forma a otimizar uma determinada função objetivo. Normalmente os objetivos são a minimização de perdas ativas, o isolamento de faltas, o balanceamento de cargas entre alimentadores e/ou a melhoria dos níveis de tensão. Neste trabalho considera-se a minimização da perda ativa total. As dificuldades do problema de reconfiguração de redes de distribuição resultam do tamanho dos sistemas reais, aos quais correspondem um número elevado de variáveis binárias que representam as chaves, e também da relação quadrática existente entre a perda elétrica e a corrente que flui nos elementos da rede. Este trabalho desenvolve algumas novas formulações para o problema de reconfiguração de redes de distribuição, utilizando Programação Não Linear Inteira Mista. Além disso, demonstra-se que a parte contínua de todas as formulações é convexa, o que garante a unicidade da solução ótima para um dado estado das chaves na rede. Esta propriedade permitiu a utilização do Método de Newton na resolução do problema contínuo, com as seguintes vantagens: impossibilidade de o método identificar mínimos locais em vez do mínimo global procurado, e convergência em apenas uma iteração, proporcionada pela natureza quadrática das formulações. As formulações desenvolvidas foram implementadas na forma de programas computacionais. O desempenho das formulações é descrito e analisado através de diversos casos de estudo. / The reconfiguration of electricity distribution systems is concerned with finding the state of switching and protective devices so as to optimize a given objective function, which is usually defined as minimization of total loss, fault isolation, load balancing among feeders, or improvement of voltage profile. In this work, the objective function is defined as the minimization of total active loss. The main difficulties associated with this problem arise from the high number of binary variables that represent the switching and protective devices, as well as the quadratic relationship between electric loss and currents flowing through the network branches. This work develops some new formulations for the distribution system reconfiguration problem, which are then solved through mixed-integer nonlinear programming. In addition, it is shown that the continuous part in all formulations is convex, which guarantees the uniqueness of the optimal solution for a given switch profile. This property allows using the Standard Newton Method for solving the continuous part of the problem, with the following advantages: impossibility of the Newton Method identifying a local minimum instead of the desired global minimum, and convergence in just one iteration owing to the quadratic nature of all formulations. The proposed formulations were implemented as computational programs and their performance was evaluated through various study cases.

Orientador: Rubén Augusto Romero Lázaro / Banca: Antonio Padilha Feltrin / Banca: Sergio Azevedo de Oliveira / Banca: Antonio César Baleeiro Alves / Banca: Luis Gustavo Wesz da Silva / Resumo: Neste trabalho é realizada uma análise teórica e a implementação computacional de um algoritmo de Busca Dispersa especializado para resolver o problema de planejamento da expansão de sistemas de transmissão (PPEST) de energia elétrica estático. O problema de planejamento consiste em determinar entre um conjunto de circuitos candidatos, aqueles que quando forem incorporados ao sistema apresentarem o menor custo de investimento possível. Este problema é considerado complexo e difícil de ser resolvido por ser um problema não linear inteiro misto, altamente ilhado, envolvendo "explosões" combinatórias. A meta-heurística de busca dispersa é um algoritmo evolutivo que se propõe a combinar soluções de qualidade e de diversidade do espaço de busca. O algoritmo de busca dispersa desenvolvido apresentou-se altamente eficiente para encontrar soluções de ótima qualidade para todos os problemas testados comparados com a literatura consultada, especializada na área. A garantia da diversidade oferecida pelo algoritmo é adicionada intencionalmente como forma de evitar, ou mesmo avançar por ótimos locais. Outro fato importante é que o método opera sobre um conjunto reduzido de soluções do espaço de busca criteriosamente gerado, que faz reduzir significativamente o número de combinações que são realizadas. Rotinas geradas com a utilização de algoritmos heurísticos construtivos gulosos de Garver e Villasana-Garver-Salon para modelos como o de Transportes e o modelo DC no PPEST apresentaram alto desempenho neste trabalho. Uma perturbação controlada nos custos de instalação das linhas de transmissão foi decisiva para que o processo de geração de soluções (factíveis, diversas e/ou com qualidade) fosse altamente eficiente, sendo possível encontrar o ótimo global em alguns problemas ainda nesta etapa de geração de soluções. Mecanismos de melhoramento local aplicados ... / Abstract: This work presents a theoretical analysis and computational implementation of a specialized Scatter Search algorithm to solve the static transmission network expansion planning (TNEP) problem of electric power systems. The objective of such planning problems is to determine a set of circuits among the candidates in which not only satisfy the demands but also the minimum investment cost is at hand. This problem is considered as a complex mixed integer nonlinear programming (MINLP) problem that has a lot of local optimum problem. The scatter search is an evolutionary method with the objective of maintaining a set of diverse and high-quality candidate solutions. The proposed scatter search algorithmhas been applied in engineering optimization problems especially in electric power system problems and has presented high quality solutions. The diversity sets ensure to avoid getting trapped in a local optimum. Another important factor is that the proposed methodology reduces the search space and consequently the number of combinations is reduced. In this work, a high quality solution of TNEP is obtained using the greedy constructive heuristic algorithms such as Garver, and Villasana-Garver-Salon that work based on Transport model and DC model respectively. In this work, in order to generate the initial solutions, a controlled disturbance has been added in the costs of the transmission lines in order to obtain diverse and high quality solutions that lead to find the global optimum for some problems even in the initial generation step. Moreover, the proposed scatter search algorithm presents a local improvement phase during the implementation. In order to show the effectiveness of the proposed algorithm, 5 case studies are conducted such as Garver 6-bars and 15 branches , IEEE 24-bars and 41 branches , South Brazilian 46-bars and 79 branches, Colombian 93-bars and 155 branches, and the North-Northeast 87-bars ... / Doutor