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.

There is always a need for more compact designs of power transformers free of partial discharges, in order to save cost on the construction and required material resources. The physical geometric constrictions inside the transformer tank would demand field-grading techniques to homogenise the field strength distribution on oil-solid interfaces, when required. Standard filler materials such as carbon black or silicon carbide (SiC) have a too high electrical conductivity yielding an appropriate grading field strength values for air-related applications. Because insulation oil has a higher electrical breakdown strength, the electrical conductivity must be engineered to lower values in order to reach a higher effective grading field strength. This paper presents the investigation of a new material system based on a phenolic resin Lerg FL-500 and the electrically functionalized ceramic filler particles Merck Iriotec®7550 that enable a resistive electrical field grading in insulation oil. In order to verify the principle functionality of the proposed field grading system, a layer is applied on a substrate surface representing possible oil-solid-interface inside oil-filled power transformers. First, the manuscript describes the methods of specimen preparation and the measurement of the nonlinear current-time behaviour under AC voltage stress for different filler contents. Second, a concurring optical and electrical determination of the partial discharge inception and extinction voltage of a modified Toepler arrangement allows the indirect determination of the electrical field strength distribution along the functionalized layer without the need of direct measurement. To do so, the radius of the circular functional layer is varied and with it the specific grading length. In analogy to state of the art SiC-filled systems, a linear dependency between the effective grading length and the PD inception voltage is observed. The quotient of voltage drop over a varied radius yields the effective graded electric field strength.

The trend towards more and all-electric apparatuses and more electrification will lead to higher electrical demand. Increases in electrical power demand can be provided by either higher currents or higher voltages. Due to "weight" and "voltage" drop, a raise in the current is not preferred; so, "higher voltages" are being considered. Another trend is to reduce the size and weight of apparatuses. Combined, these two trends result in the high voltage, high power density concept. It is expected that by 2030, 80% of all electric power will flow through "power electronics systems". In regards to the high voltage, high power density concept described above, "wide bandgap (WBG) power modules" made from materials such as "SiC and GaN (and, soon, Ga2O3 and diamond)", which can endure "higher voltages" and "currents" rather than "Si-based modules", are considered to be the most promising solution to reducing the size and weight of "power conversion systems". In addition to the trend towards higher "blocking voltage", volume reduction has been targeted for WBG devices. The blocking voltage is the breakdown voltage capability of the device, and volume reduction translates into power density increase. This leads to extremely high electric field stress, E, of extremely nonuniform type within the module, leading to a higher possibility of "partial discharge (PD)" and, in turn, insulation degradation and, eventually, breakdown of the module. Unless the discussed high E issue is satisfactorily addressed and solved, realizing next-generation high power density WBG power modules that can properly operate will not be possible. Contributions and innovations of this Ph.D. work are as follows. i) Novel electric field grading techniques including (a) various geometrical techniques, (b) applying "nonlinear field-dependent conductivity (FDC) materials" to high E regions, and (c) combination of (a) and (b), are developed; ii) A criterion for the electric stress intensity based upon accurate dimensions of a power device package and its "PD measurement" is presented; iii) Guidelines for the electrical insulation design of next-generation high voltage (up to 30 kV), high power density "WBG power modules" as both the "one-minute insulation" and PD tests according to the standard IEC 61287-1 are introduced; iv) Influence of temperature up to 250°C and frequency up to 1 MHz on E distribution and electric field grading methods mentioned in i) is studied; and v) A coupled thermal and electrical (electrothermal) model is developed to obtain thermal distribution within the module precisely. All models and simulations are developed and carried out in COMSOL Multiphysics. / Doctor of Philosophy / In power engineering, power conversion term means converting electric energy from one form to another such as converting between AC and DC, changing the magnitude or frequency of AC or DC voltage or current, or some combination of these. The main components of a power electronic conversion system are power semiconductor devices acted as switches. A power module provides the physical containment and package for several power semiconductor devices. There is a trend towards the manufacturing of electrification apparatuses with higher power density, which means handling higher power per unit volume, leading to less weight and size of apparatuses for a given power. This is the case for power modules as well. Conventional "silicon (Si)-based semiconductor technology" cannot handle the power levels and switching frequencies required by "next-generation" utility applications. In this regard, "wide bandgap (WBG) semiconductor materials", such as "silicon carbide (SiC)"," gallium nitride (GaN)", and, soon, "gallium oxide" and "diamond" are capable of higher switching frequencies and higher voltages, while providing for lower switching losses, better thermal conductivities, and the ability to withstand higher operating temperatures. Regarding the high power density concept mentioned above, the challenge here, now and in the future, is to design compact WBG-based modules. To this end, the extremely nonuniform high electric field stress within the power module caused by the aforementioned trend and emerging WBG semiconductor switches should be graded and mitigated to prevent partial discharges that can eventually lead to breakdown of the module. In this Ph.D. work, new electric field grading methods including various geometrical techniques combined with applying nonlinear field-dependent conductivity (FDC) materials to high field regions are introduced and developed through simulation results obtained from the models developed in this thesis.