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In the era of ever-increasing electrification, power-electronic converters play the crucial role of transforming electrical energy from one form to another. However, converters today face multiple challenges in meeting ever-growing demands for higher power density, broader operation ranges, and lower costs. The cost-benefits of economies of scale further emphasize the need for modular and scalable converters. While no single converter for high-power applications satisfies all criteria, the modular multilevel converter (MMC) emerges as the clear frontrunner. MMC is extremely modular, being developed using multiple smaller units or building blocks called power electronic building blocks (PEBBs) or submodules (SMs). The SMs are themselves developed using fast-switching low-voltage (LV) semiconductors meaningfully combined with energy storage components like capacitors or batteries. MMCs are highly modular and scalable and have a very broad operation range, making them a key solution already used today for a wide range of high-voltage applications like high-voltage direct-current (HVDC) transmission. However, the use of voluminous and heavy capacitors in SMs also makes MMCs much lower in power density compared to other similar voltage source converters (VSCs). Employing at least twice the number of devices compared to a conventional two-level VSC for the same ratings also increases the converter costs. These challenges have hindered MMC applications in medium-voltage (MV) and more power-dense HVDC systems. This research aims to overcome these limitations by enhancing MMCs in terms of power density, efficiency, and cost-effectiveness. These modifications would expand MMC's applications to much broader HV and MV markets. Three fundamental aspects are targeted to achieve such improvements: Topology, Components, and Controls. The first modification focuses on changing the topology by replacing some fast-switching LV-switch-based SMs with fewer low-frequency HV/MV switches. This greatly reduces the total number of components and, when combined with intelligent control, decreases costs and losses. The second modification focuses on components, proposing the replacement of fully controlled MV switches with more efficient and cost-effective but partially controlled ones like thyristors. Despite thyristors' historical controllability challenges, incorporating SMs can help resolve control challenges, creating a modular, scalable converter with a wide operation range, high power density, and lower costs. The third avenue explores advanced control strategies while maintaining the traditional MMC topology. By accelerating and precisely controlling the capacitor current, the SM capacitor energy, SM capacitor size can be significantly reduced. Although these control methods are complex, they offer potential improvements across all five criteria: modularity, scalability, power density, cost, and operational range. These innovations extend MMCs' applicability to emerging fields such as energy storage systems, electric vehicle charging stations, motor drives, and data centers. Moreover, these modifications enhance MMCs for traditional high-voltage direct-current transmission applications. The research emphasizes the advantages and addresses each modification's limitations, paving the way for a more efficient and versatile power electronics technology. / Doctor of Philosophy / In our electrically powered world, the unsung workhorse is the power(-electronic) converter. Power converters play the crucial role of transforming electrical energy from one form to another using switches that can turn on and off to accurately control the flow of electrical energy. Power converters are critical to integrating systems at different voltage, current, and power ratings. For instance, power converters enable low-power systems like cellphone chargers and high-power industrial drives to be integrated into the same interconnected power grid. However, these converters face challenges in adapting to the evolving demands of our modern world. The expectations from power converters are high – they need to be affordable, lightweight, and capable of processing large amounts of power in a compact size. Additionally, modularity and scalability are desired qualities to enable economies of scale and bring the total cost down. Yet, finding a converter that fulfills all these criteria remains a challenge. The modular multilevel converter (MMC) is a promising power converter developed to address most of these considerations. Currently employed in high-power, high-voltage applications such as transmitting energy over vast distances or linking power grids between countries, the MMC is constructed using smaller power units or building blocks called power electronic building blocks (PEBBs) or submodules (SMs). These SMs utilize fast-switching low-voltage switches along with energy storage components like capacitors or batteries. Despite its versatility, the MMC faces many limitations. The main challenge for MMCs is the inability to process more power in lower volume, commonly referred to as power density. The MMC power density is low due to the use of large capacitors or batteries. Additionally, it utilizes twice the number of switches compared to traditional non-modular power converters for the same rating, leading to higher costs. These challenges restrict its application in medium-voltage and power-dense high-voltage high-power systems. This research aims to address these challenges, focusing on enhancing the power density and cost-effectiveness of MMCs. Three key areas of MMC are targeted for improvement: topology, components, and controls. Firstly, MMC's structure is reimagined, replacing many low-voltage switches with fewer medium- or high-voltage, fully-controlled switches. Such a system is referred to as a hybrid MMC, and this reduces the converter volume and costs. This adjustment has the added benefit of making the converter more efficient. Secondly, the focus is also on the components used to develop hybrid MMCs. Instead of fully controlled medium- or high-voltage switches, partially controlled switches like thyristors provide advantages like lower losses and higher power ratings. However, these partially controlled switches have traditionally been very difficult to control. Despite historical controllability challenges, incorporating these partially controlled switches in conjunction with smart control of SMs addresses control issues, creating a modular, scalable converter with high power density and lower costs. The third enhancement involves fundamental improvements to MMC controls. By managing the energy flow to the capacitor at a much faster rate and precision than conventional methods, the size of a critical component can be significantly reduced, opening avenues for overall improvements. Furthermore, such fast control introduces additional challenges like active control in the face of non-idealities and higher losses. This dissertation further meaningfully addresses these challenges to develop a much more power-dense MMC. These improvements transform the MMC and its variants into a versatile power converter family that can extend much beyond traditional MMC applications of high-voltage transmission applications. With these modifications, the MMC can be further positioned as an excellent candidate to contribute towards energy storage systems, electric vehicle charging stations, industrial-level motor drives, dc microgrids, and data centers, meeting the diverse needs of our equally diverse and ever-more electrified world.

Outdoor insulators are one of the most important parts of a power system. The reliability of a power system depends also on the reliability of the insulators. The main functions of an insulator used for outdoor applications are to give the necessary insulation, provide the necessary mechanical support to the transmission line conductor and also to resist the various environmental stresses like pollution, ultra violet rays etc. Traditionally porcelain and glass insulators have been used for outdoor insulator applications. They are good insulators under normal conditions and the cap and pin arrangement allows them to take up the mechanical load of the line. But owing to their large weight and brittle nature they are susceptible to vandalism and also they have increased cost of installation and commissioning. But the main problem of porcelain and glass insulators is its performance under polluted environmental condition. Under wet and polluted conditions, the porcelain insulators allow the formation of a conducting layer on the surface which results in setting up of leakage current, dry band arcing and power loss. This problem is further augmented under dc voltages where the stress is unidirectional and the contaminant deposition is higher as compared to ac. Polymeric insulators are a good alternative for porcelain and ceramic insulators for use especially under dc voltages because of their good pollution performance. The property of surface hydrophobicity resists the setting up of leakage currents and hence polymeric insulators help in reducing power loss. They are also light in weight and vandalism resistant and hence easier to install. But being polymeric, they form conductive tracks and erode when exposed to high temperatures which occur at the surface during dry band arcs and when exposed to corona discharges. The surface hydrophobicity is also temporarily lost when exposed to different electrical stresses. Silicone rubber is the most popular among the various choices of polymers for outdoor insulator applications. They have good surface hydrophobicity and tracking performance. But polymers in their pure form cannot be used as insulators because of their poor mechanical strength. Adding inorganic fillers into the polymer matrix not only improves its mechanical properties but also its erosion resistance. Micron sized Alumina Trihydrate (ATH) is used traditionally to improve the tracking and erosion resistance of polymeric insulators. A very high loading (up to 60%) is used. Adding such a high filler loading to the base polymer hampers its flexibility and the material processing. With the advent of nanotechnology, nano fillers have come into vogue. Studies conducted on nano filled polymers showed exciting results. A small amount of nano fillers in the polymer matrix showed significant improvement in the mechanical strength without hampering its flexibility. The electrical properties like tracking and erosion also improved with filler loading. Hence the use of nano filled silicone rubber is a good alternative for use as a high voltage insulator especially under dc voltages. Reports suggest that adding nano fillers into the silicone rubber matrix improves the tracking and erosion resistance and the corona degradation as compared to the unfilled samples under ac voltages. The literature on the dc performance of silicone rubber nano composites is scarce. So the present study aims to evaluate the performance of silicone rubber nano composites for tracking and erosion resistance and corona degradation under dc voltages. The tracking and erosion resistance under dc voltages was measured using the Inclined Plane Tracking and Erosion Resistance set up as per ASTM D2303 which was modified for dc voltage studies. The performance of nano Alumina and nano Silica fillers were evaluated under negative dc and the performance was compared with micron sized Alumina Trihydrate filled samples. The effect of filler loading was also studied. It was seen that the performance of the silicone rubber improved with filler loading. A small loading percentage of nano fillers were enough to give performance similar to silicone rubber filled with micron sized ATH filler. The silicone rubber performed better under negative dc as compared to ac and positive dc. The positive dc tests showed a migration of ions from the electrodes onto the sample surface. The increased surface conductivity resulted in very heavy erosion in the case of positive dc tested samples. The corona aging studies were also conducted on silicone rubber nano composites. Nano silica was used as filler in this case. Different filler loadings were employed to understand the effect of filler loading. The corona was generated using a needle plane electrode and samples were exposed to both positive and negative dc corona. The samples were exposed to corona for different time intervals – 25 and 50 hours to study the effect of exposure time. The hydrophobicity, crack width and surface roughness were measured after the tests. Adding nano fillers into the polymer matrix improved the corona performance. With filler loading, the performance improved. The samples exposed to positive dc corona performed better than those under negative dc corona. The loss of hydrophobicity, surface cracks and the surface roughness was less in the case of positive dc corona tested samples. With exposure time, the performance of silicone rubber became poorer for positive dc corona tested samples. For the negative dc corona tested samples, the performance seemed to improve with exposure time. The tracking and erosion resistance and the corona aging studies conducted showed that the performance of silicone rubber is improved by adding nano fillers into the polymer matrix. A small amount of nano filler loading was enough to perform similar to a heavily loaded micron filled sample. Hence nano fillers can be used as a good functional material to improve the performance of silicone rubber insulators especially under wet and polluted conditions.

Thesis submitted in fulfillment of the requirements for the degree Master of Technology: Electrical Engineering in the Faculty of Engineering at the Cape Peninsula University of Technology 2013 / The development of power electronics did remove most of technical limitations that high voltage direct current (HVDC) used to have. HVDC, now, is mostly used for the transmission of bulk power over long distances and for the interconnection of asynchronous grid. Along with the development of the HVDC, the growth of power demand also increased beyond the utilities capacities. Besides the on-going increasing of power demand, the reforms in electricity market have led to the liberalization and the incorporation of Independent power providers in power system operation. Regulations and rules have been established by regulating authority for grid integration of Independent power providers. With the expected increase of penetration level of those new independent power providers, result of economic reason and actual green energy trend, best method of integration of those new power plants are required. In this research HVDC technology, namely VSC-HVDC is used as interface for connecting independent power providers units to the grid. VSC-HVDC has various advantages such as short-circuit contribution and independent control of active and reactive power. VSC-HVDC advantages are used for a safe integration of IPPs and make them participate to grid stabilization. MATLAB/Simulink simulations of different grid connected, through VSC-HVDC system, IPPs technologies models are performed. For each IPP technology model, system model performances are studied and dynamics responses during the disturbance are analyzed in MATLAB/ Simulink program. The simulation results show that the model satisfy the standard imposed by the regulating authority in terms of power quality and grid support. Also the results show the effect of the VSC-HVDC in preventing faults propagation from grid to integrated IPPs units.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-02-11T10:37:14Z No. of bitstreams: 1 leandroalmeidavasconcelos.pdf: 2921811 bytes, checksum: acf68048e9da96cbcc9355d4ebc70813 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2016-02-26T11:53:39Z (GMT) No. of bitstreams: 1 leandroalmeidavasconcelos.pdf: 2921811 bytes, checksum: acf68048e9da96cbcc9355d4ebc70813 (MD5) / Made available in DSpace on 2016-02-26T11:53:39Z (GMT). No. of bitstreams: 1 leandroalmeidavasconcelos.pdf: 2921811 bytes, checksum: acf68048e9da96cbcc9355d4ebc70813 (MD5) Previous issue date: 2014-10-23 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A tecnologia HVDC (High Voltage Direct Current) possui características que a tornam especialmente atrativa para determinadas aplicações em transmissão de energia elétrica. Além disso, pode-se verificar a partir do estudo de utilização desse tipo de tecnologia no mundo que existe uma tendência e perspectiva de utilização crescente nos Sistemas Elétricos de Potência. Desta forma, torna-se cada vez mais importante dispor de técnicas que possibilitem a inclusão dos modelos destes equipamentos em programas de análise de redes de forma eficiente, principalmente no fluxo de potência, com a finalidade de permitir a correta modelagem da rede como um todo nos estudos de planejamento da expansão e operação. A transmissão em corrente contínua vem se tornando amplamente reconhecida no que tange as suas vantagens no transporte de grandes blocos de energia a grandes distâncias, no transporte de potência entre parques eólicos offshore para terra, na interconexão de sistemas com frequências não compatíveis, em travessias subaquáticas, dentre outras questões que a tornam técnica e economicamente viável em algumas situações. Nesse contexto, este trabalho tem por principal objetivo desenvolver e implementar uma metodologia genérica para a representação de Sistemas de Transmissão HVDC Multiterminais no problema de fluxo de potência. Neste sentido, tal metodologia é baseada na solução simultânea de um sistema de equações não lineares composto pelas representações em regime permanente das redes C.C. e C.A., utilizando-se o método de Newton-Raphson para sua solução. A partir deste contexto, são apresentadas as equações que representam a resposta de regime permanente dos conversores, da rede C.C. e das estratégias de controle aplicáveis a esses sistemas. Além disso, são apresentadas as principais configurações existentes de conversores HVDC, suas características e como é feita sua modelagem em regime permanente e no problema de Fluxo de Potência. A metodologia proposta é validada através do estudo de sistemas tutoriais e sistemas teste encontrados como referência na literatura especializada. Os resultados apresentados demonstram que a metodologia proposta é capaz de representar de forma satisfatória os modelos de sistemas HVDC Multiterminais nos estudos de regime permanente em Sistemas Elétricos de Potência. / High Voltage Direct Current (HVDC) technology has characteristics that make it especially attractive for certain transmission applications. Furthermore, it is possible to notice that there is a trend and prospect of increased use of this technology in Electric Power Systems around the world. In this context, it has been increasingly important to have techniques that efficiently include these equipment models in network analysis programs, especially in power flow, in order to allow a correct modeling of the network in studies of expansion planning and operation. The direct current transmission is becoming widely recognized by their advantages in transporting large blocks of power over long distances, to transport power from offshore wind farms to land, in asynchronous interconnection of systems, in underwater crossings, and other issues that make it technically and economically feasible in some situations. In this context, this thesis has the objective to develop and implement a generic methodology for the representation of HVDC Multi-Terminal Systems in the power flow problem. In this sense, this methodology is based on the simultaneous solution of a system of nonlinear equations that represent, in steady state studies, the DC and AC networks, using the Newton-Raphson method to solve the problem. Equations that represent the steady state response of the converters, the DC network and control strategies are presented. In addition, it will be presented the main settings of HVDC converters, their characteristics and how their modelling are set forth in the Power Flow problem. The proposed methodology is validated by studying tutorial and test systems found in the literature. The results show that the proposed methodology is able to represent satisfactorily models of HVDC Multi-Terminal Systems in studies of steady state in Electric Power Systems.