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Ester liquids including natural ester and synthetic ester are considered as potential substitutes for mineral oil, due to their good biodegradability and high fire points. Although these liquids have been widely used in distribution and traction transformers, research efforts are required for the purposes of design and manufacture of high voltage and large power transformers which are filled by esters. Indeed, it would be risky to apply esters in large power transformers without thorough understandings of their behaviours in large gaps and/or when combined with pressboard insulation. Therefore, investigations of creepage discharges along the surface of pressboard in esters are vitally important and their behaviours should be compared with those of mineral oils. This thesis is aimed to investigate the creepage discharges along pressboard in esters and mineral oil under ac divergent electric field. Apparent charges, current signals and images of streamer channels were obtained synchronously to identify whether and how the introduction of pressboard surface would influence the inception and propagation of discharges as compared to tests in open gap. When over-stressed by higher voltages, the surface tracking along the pressboard-ester interface, triggered by sustaining creepage discharges, was studied and the evolutions of accompanying creepage discharge patterns were investigated. In these experiments, both esters and mineral oil impregnated pressboards were comparatively studied. The test results indicated that at the inception stage, the presence of pressboard or any other solid types in different liquids under test do not influence the PD inception voltages; in the propagation stage, solid surface tends to promote the development of discharges, especially those occurring in negative half cycles, and shifts more discharges towards the zero-crossing phase angles. This discharge promotion effect is much more evident in esters than in mineral oil, probably because of higher discharge intensity in esters and higher viscosity of esters. The space charge effect and the residual low density channel effect are proved as the mechanisms best explaining the influences of solids on creepage discharges. Under higher voltages, it was found that the impregnated pressboard is susceptible to discharge erosion characterized by “white and carbonized tree-shaped marks”, due to intense discharges occurring on or near the pressboard surface. The “white mark” appears at a lower voltage and propagates more easily on ester impregnated pressboard. The gaseous “white mark” channels will attract the subsequent discharges to follow the same discharge routes; the accumulative energy dissipation in these channels will then result in the carbonization of the channels. Once formed, the surface tree-shaped mark can continue to grow even under reduced voltage levels until it bridges the gap and causes the final flashover.

Dans le cadre du projet d’avion plus électrique entamé depuis quelques années, l’objectif visé est le remplacement des énergies hydraulique et pneumatique par l’énergie électrique. Cette transition technologique drastique nécessite une augmentation de puissance électrique à bord des avions, qui se traduit par une élévation des niveaux de tension du réseau embarqué (+/- 270 VDC et 230 VAC). Dans ces conditions, les phénomènes de décharges électriques doivent être considérés, en particulier pour les systèmes présents dans les zones non pressurisées des appareils. Les travaux de thèse ont porté sur l’étude des décharges partielles dans des connecteurs aéronautiques aujourd’hui exploités et susceptibles d’être mis en oeuvre dans les nouvelles conditions de tension. Une modélisation électrostatique du connecteur a été effectuée dans le but de mettre en évidence les zones où règne un fort champ électrique pouvant être à l’origine de l’apparition de décharges partielles. L’influence de paramètres, tels que la géométrie des conducteurs, la pression et la température, a été examinée d’un point de vue théorique par adaptation de la courbe de Paschen. Des mesures expérimentales, effectuées dans différentes conditions d’alimentation électrique, de pression, de température et d’état de vieillissement des connecteurs, ont permis de déterminer les tensions seuil d’apparition des décharges partielles. Ces mesures ont également permis de localiser les décharges au sein du connecteur, et ainsi d’identifier les points faibles de la structure. Des solutions visant à repousser les seuils d’apparition de décharges partielles sont alors proposées. / The objective of the more electrical aircraft project is the replacement certain of the hydraulic and pneumatic systems by electrical ones. This drastic technological transition will require the increase of the onboard electrical power, which will result in the increase of voltage levels of the embedded networks. Current systems of 28 VDC and 115 VAC (400 Hz) are to be replaced by +/- 270 VDC and 230 VAC (360 to 1000 Hz). These new conditions of voltage levels will have a significant impact on the appearance of partial discharges that were almost absent up until present. Aeronautical grade electrical connectors being one of the major elements of embedded networks, the aim of this thesis is to study the conditions of occurrence of partial discharges in them. In this work, electrostatic modeling is carried out in order to identify the zones within the connectors where there is a strong electric field facilitating the appearance of partial discharges. Also presented are the results of experiments under both DC and AC voltage in which the ignition voltages of partial discharges and their location within the structure of the connector were studied.

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[pt] O Monitoramento do Sistema de Isolamento em Máquinas Elétricas de Alta Tensão representa um avanço no que diz respeito ao planejamento operacional de uma indústria. Com posse de dados on-line referentes às condições apresentadas pelos isolamentos, as devidas precauções podem ser tomadas, evitando danos maiores. O trabalho apresentado tem como objetivo principal analisar um sistema de monitoramento desenvolvido pela CEA (Canadian Electrical Association), com suporte técnico da Ontario Hydro. O sistema chamado PDA (Partial Discharge Analyzer) mede, através de acopladores permanentes ou portáteis (capacitores), as Descargas Parciais nos enrolamentos estatóricos de grande máquinas de alta tensão. A quantificação exata das Descargas Parciais são indicadas das reais condições do isolamento. A análise do aparelho foi realizada em conjunção aos dados obtidos, tendo como referência a Companhia Siderúrgica Nacional (CSN). A partir de tais dados, é possível demostrar em que estágio atual de aproveitamento estamos em relação à utilização de tal aparelho e quais são os passos futuros para melhor compreensão das medidas. / [en] The Monitoring of te Isolation System in High Tension Electrical Machines represents na advance in what concerns the operational planning of na industry. Knowing the on- line data referring to the conditions presented by the isolations, the necessary cautions can be taken, avoiding bigger damages. This work has as the main aim to analyze a monitoring system developed by the CEA (Canadian Electrical Association). The system is called PDA ( Partial Discharge Analyzer) and measures, through permanent or portable couplers (capacitors), the Partial Discharges is indicator of the isolation real conditions. The appliance analyses was made with the obtained data, having as reference the CSN. With this data, it is possible to show in what profiting stage of appliance and which are the future steps for a better comprehension of the measures.

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Medium-voltage silicon carbide power devices, due to their higher operational temperature, higher blocking voltage, and faster switching speed, promise transformative possibilities for power electronics in grid-tied applications, thereby fostering a more sustainable, resilient, and reliable electric grid. The pursuit of increasing power density, however, escalates the blocking voltage and shrinks the module size, consequently posing unique insulation challenges for the medium voltage power module packaging. The state-of-the-art solutions, such as altering the geometry of the insulated-metal-substrates or thickening or stacking them, exhibit limited efficacy, inflate manufacturing costs, raise reliability concerns, and increase thermal resistance. This dissertation explores a material-based approach that utilizes a nonlinear resistive polymer nanocomposite field-grading coating to enhance insulation performance without compromising thermal performance for medium-voltage power modules. The studied polymer nanocomposite is a mutual effort of this research and NBE Technologies. Instead of using field-grading materials as encapsulation, a thin film coating (about 20 μm) can be achieved by painting the polymer nanocomposite solution to the critical regions to grade the electric field and extend the range of the applicability of the bulk encapsulation. A polymer nanocomposite's electrical properties were characterized and found theoretically and experimentally to be effective in improving the insulation performance or increasing the partial discharge inception voltage, of direct-bonded-copper substrates for medium-voltage power modules. By applying the polymer nanocomposite coating on the direct-bonded- copper triple-point edges, the partial discharge inception voltages of a wide range of direct-bonded-coppers increased by 50-100%. To assure its effectiveness for heated power modules during operation, this field-grading effect was then evaluated at elevated temperatures up to 200°C and found almost unchanged. The nanocomposite's long-term efficacy was further corroborated by voltage endurance tests. Building on these promising characterizations, functional power modules were designed, fabricated, and tested, employing the latest packaging techniques, including double-sided cooling and silver-sintering. Prototypes of 10-kV and 20-kV silicon carbide diode modules confirmed the practicality and efficacy of the polymer nanocomposite. The insulation enhancements observed at the module level mirrored those at the substrate level. Moreover, the polymer nanocomposite coating enabled modules to use insulated-metal-substrates with at least 100% thinner ceramic, resulting in a reduction of at least 30% in the junction-to-case thermal resistance of the module. Subsequently, to test the nanocomposite's performance during fast-switching transients (> 300 V/ns), 15-kV silicon carbide MOSFET modules were designed, fabricated, and evaluated. These more complex modules passed blocking tests, partial discharge tests, and double-pulse tests, further validating the feasibility of the nonlinear resistive polymer nanocomposite field-grading for medium-voltage power modules. In summary, this dissertation presents a comprehensive evaluation of a nonlinear resistive polymer nanocomposite field-grading coating for medium-voltage power modules. The insights and demonstrations provided in this work bring the widespread adoption of this packaging concept for medium-voltage power modules significantly closer to realization. / Doctor of Philosophy / This dissertation delves into a novel approach to improving the resilience and reliability of our electric grid by employing medium-voltage silicon carbide power devices. These power devices, due to their superior performance at higher temperatures and faster switching speeds, can revolutionize grid-tied power electronics. However, the challenge lies in safely packaging these devices, given their high blocking voltage and compact size. To address this, the study explores an innovative solution that uses a material called a nonlinear resistive polymer nanocomposite. This nanocomposite can improve insulation and endure high temperatures, promising a significant boost in performance for these power devices. The study reveals that applying this nanocomposite coating to the edges of direct- bonded-copper, a component of the power module, can enhance the insulation performance by 50-100%. Building on these findings, we designed, made, and tested functional power modules, using cutting-edge packaging techniques that we developed. The tests confirmed the practicality and effectiveness of the polymer nanocomposite, leading to insulation improvements on both the substrate and module levels. Importantly, this coating also reduced the thermal resistance of the module by at least 30%, signifying a more efficient operation. Then we evaluated the nanocomposite's performance during fast-switching transients in more complex silicon carbide modules. The modules passed multiple tests, further validating the feasibility of the nanocomposite coating for medium-voltage power modules. In essence, this dissertation uncovers a promising approach to more efficient and resilient power module packaging, paving the way for potential widespread adoption in the power electronics industry.

The development and implementation of silicon carbine (SiC) devices is steadily increasing facilitating the electrification of aircrafts. In this thesis, a printed circuit board (PCB) based heavy copper busbar design and verification are introduced for a SiC based 250 kW multiphase drive system operated at 40,000 ft. Finite-element analysis (FEA) simulation studies of the PCB busbar are conducted to optimize the electric field intensity. Busbar modeling technic is also discussed to derive the current distribution and extract the loss. The measured partial discharge inception voltage (PDIV), switching transients and converter-level validations are provided for insulation, thermal and commutation loop verifications. As the part of the inverter system, the integrated gate driver is designed with SPI communication to drive the wide bandgap SiC power modules. With feature of drain-to-source current sensing feature, the gate driver could also provide over-current protection to fast-switching SiC power modules. The converter level verification is performed under single, dual, and quadruple three-phase inverter system for aviation motor drive to evaluate the overall performance of the powertrain converter. The outcomes of this research contribute to the advancement of electric aircraft technology by leveraging the benefits of SiC devices and optimizing busbar design, providing valuable insights and guidelines for engineers and researchers involved in the development and optimization of power electronic systems for all-electric aircraft applications. / Master of Science / This research focuses on the design and verification of PCB (Printed Circuit Board) busbars for a multiphase SiC-based all-electric aircraft powertrain converter. Silicon Carbide (SiC) devices, known for their high efficiency and fast-switching capabilities, are used in the converter to enhance its performance. The goal is to develop an optimized busbar design that ensures efficient power distribution and minimizes energy losses in this advanced aviation powertrain system. The study explores different aspects of busbar insulation design and analyzes busbar current distribution and loss extraction using simulation and modeling techniques. Additionally, gate driver design and communication network are investigated to drive and protect the wide bandgap SiC devices and to ensure the overall performance of the powertrain converter. The converter level verification is also performed under single, dual, and quadruple three-phase inverter system for aviation motor drive. The findings of this research contribute to the advancement of electric aircraft technology, utilizing SiC devices and optimized busbar design, and provide valuable insights for engineers and researchers working on power electronic systems for electric aircraft applications.

In comparison with Si-based converters, the emerging Medium Voltage (MV) SiC-based converters can achieve higher blocking voltage capability, lower on-resistance, faster switching speed with less switching related losses and run under higher temperature. Thus, theoretically, it can achieve much higher power density, which becomes very promising for future power transmission and distribution. However, in order to achieve the desired high power density, insulation system of the MV SiC-based converter must be compact. Therefore, challenges for the insulation system gradually appeared, as the insulation size becomes smaller and the Electric field (E-field) intensity significantly increases. Under such high E-field intensity, it is necessary and important to eliminate Partial Discharge (PD) for such power converters, since the converter system is vulnerable to PDs. Developing an insulation design, assessment and online monitoring method to help reach a compact and PD free insulation system for MV SiC-based converters is a goal of this work. General insulation design and assessment guidelines based on experimental PD investigation and physics-based model –Experimental PD investigation is completed for internal void discharge, surface discharge, and point discharge representative coupons under square excitations. Based on the data and the existing knowledge about PD mechanisms, widely accepted PD models are selected. Using these physics-based models, simulation results can demonstrate the major features observed in the experiments. With the experimental data and valid PD models, several general insulation design and assessment guidelines are proposed, which could be further applied during converter prototypes development. Partial Discharge elimination methodology and design examples – By using the laminated bus as the design example, internal void evaluation and analysis method is demonstrated. Then, targeting the internal PD-free design with reasonable insulation thickness, several insulation improvement methods are applied and experimentally verified by using representative coupons. After understanding the possible ways for evaluating and eliminating internal voids, a PCB-based planar bus is designed and fabricated, which shows great insulation improvement after experimental verification. In order to eliminate PDs in the air and shrink the insulation distance, three ways for managing E-field distribution in air are demonstrated by three examples. First, by using the interconnections among the power modules, Rogowski-based current-sensing board, and the laminated bus as an example, E-field distribution can be estimated by Finite Element Analysis (FEA) and its management can be achieved by geometrical modifications. Second, for the one-turn inductor, a methodology is demonstrated that builds a coaxial insulation structure with proper termination technology in order to squeeze air out of the insulation system. Finally, E-field shielding technology is applied along the heatsink edges in order to make the E-field distribution uniform and to shrink the insulation distance between the heatsink and the cooling system. After improving the insulation, this work shrinks the converter unit size by around 50% while maintaining its PD-free status under normal operation conditions. Besides the significant increase in power density and weight reduction, the entire converter system has less ringing and better current-sharing performance due to reductions of the parasitic inductance. Partial Discharge online monitoring via acoustic and photon detection methods –Targeting the online monitoring and even localization of surface discharge for power converter applications, two novel types of sensors have been proposed and fabricated. In order to verify the concepts, one example with experimental results has been given for each type of sensor. The experimental data demonstrates that such sensors can be placed inside the converter and online monitoring can be realized for surface or corona discharges by capturing either the acoustic signal or the photons that are generated by discharge events. / Doctor of Philosophy / A unproper designed insulation system can take more than 50% volume of Medium Voltage (MV) SiC-based converters and have significant internal or external Partial Discharge (PD), which can not only accelerate the insulation aging but also risk to multiple aspects of the converter system. Therefore, developing an insulation design, assessment and online monitoring method to help reach a compact and PD free insulation system for MV SiC-based converters is a goal of this work. Experimental PD investigation is completed for internal void discharge, surface discharge, and point discharge representative coupons under square excitations. Several general insulation design and assessment guidelines are proposed based on the experimental PD investigation and physics-based explanations, which are further applied during converter prototypes development. Then, PD elimination methodology is developed and demonstrated by design examples. By using the laminated bus as an example, internal void evaluation and analysis method is demonstrated. Then, targeting the internal PD-free design with reasonable insulation thickness, several insulation improvement methods are applied and experimentally verified by using representative coupons. In order to eliminate PDs in air and shrink the insulation distance, three ways for managing E-field distribution in air are demonstrated by three examples. First, by using the interconnections among the power modules, Rogowski-based current-sensing board, and the laminated bus as an example, E-field distribution can be estimated by Finite Element Analysis (FEA) and its management can be achieved by geometrical modifications. Second, for the one-turn inductor, a coaxial insulation structure with proper termination technology in order to squeeze air out of the insulation system is demonstrated. Finally, E-field shielding technology is applied along the heatsink edges in order to make the E-field distribution uniform and to shrink the insulation distance between the heatsink and the cooling system. After improving the insulation, this work shrinks the converter unit size by around 50% while maintaining its PD-free status under normal operation conditions. Besides the significant increase in power density and weight reduction, the entire converter system has less ringing and better current-sharing performance due to reductions of the parasitic inductance. Targeting the PD online monitoring for power converter applications, two novel types of sensors have been proposed and fabricated. In order to verify the concepts, one example with experimental results has been given for each type of sensor. The experimental data demonstrates that such sensors can be placed inside the converter and online monitoring can be realized for surface or corona discharges by capturing either the acoustic signal or the photons that are generated by discharge events.