Le SF6 (Hexafluorure de soufre) a été utilisé depuis longtemps comme un gaz d'isolation très adapté pour les appareillages électriques. Cependant, son impact sur le réchauffement climatique, estimé 24000 fois plus actif que le CO2 sur 100 ans, pose le problème de son remplacement à moyen terme. L'objectif de cette thèse est d'étudier deux solutions de remplacement du SF6 : les liquides biodégradables et les mélanges de gaz CF3I/N2. Les deux phénomènes physiques conduisant à un claquage : la génération d'une décharge et sa propagation, ont été étudiés séparément. L'influence des différents paramètres tels que la pression, distance, distribution de champ, teneur en particules ou humidité relative…a été également mise en évidence. La fonction d'isolation est en fin validée dans un maquette disjoncteur moyenne tension qui représente un cas d'application réelle. / SF6 gaz (sulfur hexafluoride) has been widely used as a very suitable insulation material for HV apparatus. However, it is also known as a remarkable green house gas, which Global Warming Potential is 24000 times greater than CO2. Consequently, its utilisation in the electrical industry should be reduced. The aim of this work is to study two alternatives to SF6: biodegradable liquids and CF3I/N2 gas mixtures. The two physical phenomena leading to a breakdown: the generation of a discharge and its propagation have been studied separately. The influence of various parameters such as pressure, distance, field distribution ,particle content and relative humidity ... was also highlighted. The insulation function is then validated in a MV circuit breaker, which represent a real application case.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This Ph.D. Thesis proposes a comparison of modulation techniques, semiconductors devices technologies and topologies for multilevel inverters, in order to point out the multilevel drive system that presents the best performance for a given application. Initially, a comparison methodology is proposed which is based on selecting the switching frequency where all systems present 99% efficiency. The performance indeces included in this analysis are: semiconductor devices power losses, heat-sink volume, THD, DF1, DF2, common mode voltage and harmonic spectrum. For each topology of multilevel inverter it is made a comparison among the modulation techniques and semiconductor devices to obtain the set (modulation + semiconductor) that presents better overall performance. After, the comparisons for each topology, a comparison among the sets that present better results is made, leading to the choice of the best system for a given specific application. The modulation techniques that present the best performance for each multilevel inverter are implemented on an FPGA. Experimental results, such as inverters efficiency and output voltage waveform, are presented for a reduced scale prototype, with the intention of validating the models employed in this Phd Thesis. / Esta Tese de Doutorado propõe a comparação de técnicas de modulação, tecnologias de dispositivos semicondutores e topologias de inversores multiníveis, para realizar o apontamento do sistema de acionamento de média tensão que apresenta melhor desempenho para uma dada aplicação. Inicialmente é proposta uma metodologia de comparação, que tem como objetivo encontrar a frequência de comutação na qual cada sistema apresenta 99% de rendimento. Os índices de desempenho propostos para análise são: perdas nos dispositivos semicondutores, volume do dissipador, THD, DF1, DF2, tensão de modo comum e espectro harmônico. Para cada topologia de inversor multinível é efetuada uma comparação entre as técnicas de modulação e os dispositivos semicondutores selecionados, para determinar o conjunto (modulação + semicondutor) que apresentam os melhores resultados nos índices de desemplenho supracitados. Após as comparações para cada topologia, é realizada uma comparação entre os conjuntos que apresentaram melhor resultado, levando ao apontamento do sistema mais adequado para uma dada aplicação. As técnicas de modulação que apresentaram melhor desempenho para cada inversor multinível são implementadas em FPGA. Os resultados experimentais como rendimento dos inversores e formas de onda das tensões de saída são apresentados para protótipos de escala reduzida, objetivando validar alguns dos modelos empregados na Tese.

This thesis focuses on the issue of heating of medium-voltage switchgears. The paper contains descriptions of the individual devices which the switchgear consists of. Furthermore, the paper examine the theoretical relations which describe the heating of the conductor. It also present the normative regulations regarding the heading of MV switchgears as well as specific impacts which influence the final temperature-rise. This is followed by chapters that describe the refining of a 3D model of specific switchgear for purposes of simulating temperature-rise, further followed by chapters that look at the actual simulation environment. The thesis further focuses on the actual simulation of MV switchgear temperature-rise and it will compare the results of the simulation with measurements taken under real conditions. The thesis also includes results of simulated temperature-rise while taking into consideration the forced cooling of the switchgear. In the last part, it focuses on the creation of an excel file, which enables the prediction of final results for the temperature-rise of disconnector based on its contact resistance.

Overhead lines are using in the vast majority for distribution of electricity in medium voltage level in the Czech republic. This Master’s Thesis deals with theoretical and practical knowledges to design and construction of these lines. Particularly, the mechanical properties of overheads lines and solving their calculation in accordance with applicable standards. The main task is to acquaint potentional designers in this sphere with this issue, which is recently ignored. An important benefit is the practical calculation of the actual overhead 22kV.

Load models play an essential role in many studies, including calculating voltage drops and technical losses in distribution systems, for distributed generator (DG) integration planning, and in tariff analysis and design models. The Herman-Beta transform used in the low voltage network modelling studies in South Africa is based on loads modelled as Beta probability density functions. Recently, the transform was extended to make it useful also for probabilistic load flow modelling in medium voltage (MV) networks with non-unity power factor loads and DGs. The electricity supply industry in South Africa has transformed and saw an increased penetration of Independent Power Producers as a result of the government encouraged the renewable independent power procurement programme (REIPPP). There has also been a steady decrease in the costs of procuring power from renewable energy sources, mainly from photovoltaic (PV) systems. South Africa also saw significant tariff increases in the recent past. These have resulted in both new load patterns and uncertainties in the power systems inputs required for network planning and tariff development. Other factors affecting loads and renewable energy output include weather, location and economic factors. Load models are essential for technical and tariff studies. Long term and short term planning models in both technical and tariff modelling require information about the usage behaviour of customers. Planning cannot be separated from the financial impact and tariffs in general. The literature review indicated that planning has the objective of designing a network for optimal usage, thus minimising the costs and deferring investment where possible. Load patterns have been recognised to represent the usage behaviours of customers better and these behaviours influence the planning parameters. There have been studies by numerous researchers to extract parameters from the load profiles for load flow modelling and simulation purposes. The same challenge exists for South Africa, where there has been progress made on the development of LV models, and the same is not replicated in the MV network space. The derivation of load models primarily involves the classification of loads, identifying and estimating the parameters of loads, and assigning load profiles to different loads for studies. Customer measurements are an essential input in load model development and load estimation. Identification of parameters is one of the areas where research is ongoing since there is no global consensus on which attributes best describe customer load profiles. In this study, a proposition on how the parameters for technical and tariff analysis models should be defined was made. The use of 24-hour load profiles to classify calendar days into typical days was also suggested. The availability of measurements data made it possible to develop load models for MV and conduct a study on actual customer data. The customers' measurements data, made it possible to identify the parameters and develop load models that could be used for technical and tariff analysis and conduct a pilot study to evaluate the load models. This study proposes a load model that can be used to model typical days and to model customer loads. The load models proposed here uses the k-means clustering algorithm as the basis for classification. The load models enable the classification of loads and assignment of load profiles accordingly. The results of this study indicated that load parameter models could be extracted from the customer measurements, for technical and tariff studies in distribution networks. It has also been possible to identify and determine the parameters from the load profiles and proposed a process for developing a load model for technical, economic and tariff analysis. The results also indicate that of the five identified parameters, the most significant parameters that affected the clustering results were the load factor, average power and the normalised peak usage parameter when the results of each of the factors were compared on an individual basis. The study also revealed improvements to the clustering results when all the parameters identified in this study were combined and a PCAbased clustering algorithm was used. Finally, the results indicate that the loads in the different economic activitybased classifications do not necessarily have similar shapes although they belong to the same cluster. The modelling process developed in this study may be implemented by utilities for determining load parameter models for MV feeders when measurements are available. The process may also be used to guide future data collection.

Medium-Voltage Direct-Current (MVDC) power system has been considered as the trending technology for future All-Electric Ships (AES) to produce, convert and distribute electrical power. With the wide employment of highrequency power electronics converters and motor drives in DC system, accurate and fast assessment of system dynamic behaviors , as well as the optimization of system transient performance have become serious concerns for system-level studies, high-level control designs and power management algorithm development. The proposed technique presents a coordinated and automated approach to determine the system adjustment strategy for naval power systems to improve the transient performance and prevent potential instability following a system contingency. In contrast with the conventional design schemes that heavily rely on the human operators and pre-specified rules/set points, we focus on the development of the capability to automatically and efficiently detect and react to system state changes following disturbances and or damages by incooperating different system components to formulate an overall system-level solution. To achieve this objective, we propose a generic model-based predictive management framework that can be applied to a variety of Shipboard Power System (SPS) applications to meet the stringent performance requirements under different operating conditions. The proposed technique is proven to effectively prevent the system from instability caused by known and unknown disturbances with little or none human intervention under a variety of operation conditions. The management framework proposed in this dissertation is designed based on the concept of Model Predictive Control (MPC) techniques. A numerical approximation of the actual system is used to predict future system behaviors based on the current states and the candidate control input sequences. Based on the predictions the optimal control solution is chosen and applied as the current control input. The effectiveness and efficiency of the proposed framework can be evaluated conveniently based on a series of performance criteria such as fitness, robustness and computational overhead. An automatic system modeling, analysis and synthesis software environment is also introduced in this dissertation to facilitate the rapid implementation of the proposed performance management framework according to various testing scenarios.

Shipboard Power System (SPS) is known as an independent controlled small electric network powered by the distributed onboard generation system. Since many electric components are tightly coupled in a small space and the system is not supported with a relatively stronger grid, SPS is more susceptible to unexpected disturbances and physical damages compared to conventional terrestrial power systems. Among different distribution configurations, power-electronic based DC distribution is considered the trending technology for the next-generation U.S. Navy fleet design to replace the conventional AC-based distribution. This research presents appropriate control management frameworks to improve the Medium-Voltage DC (MVDC) shipboard power system performance. Model Predictive Control (MPC) is an advanced model-based approach which uses the system model to predict the future output states and generates an optimal control sequence over the prediction horizon. In this research, at first, a centralized MPC is developed for a nonlinear MVDC SPS when a high-power pulsed load exists in the system. The closed-loop stability analysis is considered in the MPC optimization problem. A comparison is presented for different cases of load prediction for MPC, namely, no prediction, perfect prediction, and Autoregressive Integrated Moving Average (ARIMA) prediction. Another centralized MPC controller is also designed to address the reconfiguration problem of the MVDC system in abnormal conditions. The reconfiguration goal is to maximize the power delivered to the loads with respect to power balance, generation limits and load priorities. Moreover, a distributed control structure is proposed for a nonlinear MVDC SPS to develop a scalable power management architecture. In this framework, each subsystem is controlled by a local MPC using its state variables, parameters and interaction variables from other subsystems communicated through a coordinator. The Goal Coordination principle is used to manage interactions between subsystems. The developed distributed control structure brings out several significant advantages including less computational overhead, higher flexibility and a good error tolerance behavior as well as a good overall system performance. To demonstrate the efficiency of the proposed approach, a performance analysis is accomplished by comparing centralized and distributed control of global and partitioned MVDC models for two cases of continuous and discretized control inputs.

The Medium Voltage DC (MVDC) architecture of shipboard power system (SPS) with higher power density and enhanced power control is seen as a future prospect in warships by US Navy. Optimal operation of SPS is essential to enable efficient power and energy usage in the tightly coupled and power limited systems. It helps in obtaining adequate and reliable power supply by rescheduling generator output and energy storage devices for different operating scenarios and can also ensure power supply to critical loads during fault conditions. The self-commutated Voltage Source Converters (VSCs) with high dynamic performance and independent control over the real and reactive powers are best suited in the MVDC architecture. Therefore, an optimal operation tool is developed for the multi-terminal VSC based MVDC SPS which minimizes the system operating costs by optimally coordinating generators and energy storage, and will also implement preventive and corrective actions for managing credible contingencies.

Cascaded H-bridge (CHB) multilevel inverters have significant popularity with motor drives applications due to their modularity, scalability, and reliability. Typical CHB inverters employ diode rectifiers that allow unidirectional power flow from the grid to the load. To capture and utilize the regenerated energy in regenerative applications, regenerative CHB drives were introduced with two-level voltage source converters in the front end to allow bidirectional energy flow. This solution is accompanied by challenges of high number of switches and control circuits, high switching power losses, and massive dimensions. Recently, developing more economic versions of regenerative cascaded H-bridge drives has become one of the hottest topics in power electronics research. In this thesis work, two solutions are proposed for more energy efficient and economic regenerative CHB drives. The first solution is a proposed power cell configuration that reduces the number of switches per cell by two. Additionally, phase alternation connection method and carrier phase-shifting techniques are introduced to address the challenges of the presented configuration. The switch-count reduction reduces the system’s complexity, switches’ cost, and footprint. The second proposed solution is a new controller to operate the front-end converters as fundamental frequency ends (FFEs). The proposed controller is employed in both the conventional regenerative cascaded H-bridge and the proposed reduced switch-count configuration. This solution minimizes the switching power losses, and results in more compact and economic design, with higher DC-link utilization. Theoretical analysis and simulation studies of both proposed solutions show promising performance and capability to be applied as energy-efficient and cost effective regenerative CHB motor drives. Experimental validation of the proposed reduced switch-count configuration is presented for STATCOM operation of a scaled-down 7-Level regenerative CHB drive system. The future work of this thesis includes experimental validation of the proposed FFE controller, and operation of the system with regenerative motor load. / Thesis / Master of Applied Science (MASc)

High power converters are widely used in many industries. At power levels in the range of Mega Watt (MW), power conversion at medium voltage (MV) is preferred due to better efficiency and lower cost. For medium voltages applications, multilevel converters are widely adopted due to the features they offer with respect to two-level converters. Cascaded H-bridge topology is a widely adopted multilevel topology because of its modularity, scalability, and reliability. The conventional cascaded H-bridge topology allows two-quadrant operation. In order to allow fourquadrant operation, an active front end version of the cascaded H-bridge topology has been proposed in literature and recently commercialized. In the field, power converters operates under harsh loading and environmental conditions. The resulting stresses imposed on converter components cause their gradual degradation. In cascaded H-bridge converters, typically power cell components such as power modules, DC-bus capacitors, and control PCBs are v highly stressed. Under these stresses power cell components degrade and require replacement in the field, otherwise unexpected failures may occur. The thesis aim is to address power cell components reliability through proposing novel regenerative cascaded H-bridge converter control schemes to reduce components stresses and failure probability without increasing size, cost, or complexity. First, a novel PWM active front end control scheme has been proposed to reduce the inherent ripple current stresses on the DC-bus capacitors. Second, the thesis proposes a novel grid or near grid switching frequency front end control scheme to reduce stresses on power modules and the power cell cooling requirements. Third, novel cascaded H-bridge front end control schemes are proposed to reduce the sensor count, thereby decreasing failure rate and cutting down cost. The proposed work has been thoroughly validated through detailed 9- cell regenerative cascaded H-bridge system simulation and experimentation. / Thesis / Doctor of Philosophy (PhD)