Medium Voltage (MV) power conversion systems are essential in high power applications to address the increasing demand of energy and the increasing penetration of renewable energy sources. MV power electronics converters are the key elements for power conversion in MV systems and are the focus of this study. Multilevel converter topologies are promising topologies in MV applications because of their reduced voltage stress on devices, excellent output quality, reduced semiconductor losses, lower common mode voltage among other advantages. However, they may suffer from the large number of switching devices and capacitors, as well as the need to regulate capacitor voltages. SiC MOSFETs can achieve higher switching speeds, higher switching frequencies, higher voltage ratings, higher operation temperatures compared to traditional Si devices. They have shown promise to increase the efficiency and power density of the converters, but may suffer from higher voltage overshoots, increased Electromagnetic Interference (EMI) emission and so on. In SiC-based multilevel converters, the features of multilevel topologies, and the features of SiC MOSFETs are coupled together. The benefits, challenges, and solutions of using SiC MOSFETs in multilevel converters are studied explicitly in this work. With the high switching speeds and high switching frequencies of SiC MOSFETs, and the large number of switches and capacitors in multilevel topologies, SiC-based multilevel converters need to be studied while considering high-frequency voltage and current behaviors and the interactions among them at different locations. Firstly, the use of SiC-based multilevel converter in the high-speed motor drive application is explored. A three-phase inverter is designed and built employing five-level Stacked Multicell Converter topology and SiC MOSFETs. The benefits and challenges of using multilevel converter topology and using SiC MOSFETs for this application are explored. A fitting topology is selected, and a prototype is designed, both with attentions paid to deal with the high switching speeds of SiC MOSFETs. The inverter is verified through experiments to meet all specifications with a high efficiency. Then a unique type of converter, converters with Integrated Capacitor Blocked Transistor (ICBT) cells are studied. Unlike the traditional methods, there are no fast-developing voltage unbalances, or high cell capacitor voltage ripples in ICBT-based converters. The ideal operation principle is analyzed and verified by the simulation results. Then the impacts of non-idealities on the operation are analyzed, and a control method is proposed for this type of converter. The operation and control of ICBT-based converters are verified by experimental results to achieve low cell capacitor voltage ripples and excellent voltage balance in Medium Voltage high power applications. Lastly, the conducted EMI emission in SiC-based multilevel converters are studied. Four SiC-based multilevel converters are studied, with the focus on the power circuit in one converter and the auxiliary circuits in the other three converters. The complexity of noise generation and propagation in multilevel converters is presented. The conducted EMI disturbances are experimentally evaluated, analyzed, and effectively mitigated in all four cases. / Doctor of Philosophy / Medium Voltage (MV) power conversion systems are essential in high power applications to address the increasing demand of energy and the increasing penetration of renewable energy sources. MV power electronics converters are the key elements for power conversion in MV systems and are the focus of this study. Multilevel converter topologies are promising topologies in MV applications because of their reduced voltage stress on devices, excellent output quality, reduced semiconductor losses, lower common mode voltage among other advantages. However, they may suffer from the large number of switching devices and capacitors, as well as the need to regulate capacitor voltages. SiC MOSFETs can achieve higher switching speeds, higher switching frequencies, higher voltage ratings, higher operation temperatures compared to traditional Si devices. They have shown promise to increase the efficiency and power density of the converters, but may suffer from higher voltage overshoots, increased Electromagnetic Interference (EMI) emission and so on. In SiC-based multilevel converters, the features of multilevel topologies, and the features of SiC MOSFETs are coupled together. The benefits, challenges, and solutions of using SiC MOSFETs in multilevel converters are studied explicitly in this work. With the high switching speeds and high switching frequencies of SiC MOSFETs, and the large number of switches and capacitors in multilevel topologies, SiC-based multilevel converters need to be studied while considering high-frequency voltage and current behaviors and the interactions among them at different locations. Firstly, the use of SiC-based multilevel converter in the high-speed motor drive application is explored. A three-phase inverter is designed and built employing five-level Stacked Multicell Converter topology and SiC MOSFETs. The benefits and challenges of using multilevel converter topology and using SiC MOSFETs for this application are explored. A fitting topology is selected, and a prototype is designed, both with attentions paid to deal with the high switching speeds of SiC MOSFETs. The inverter is verified through experiments to meet all specifications with a high efficiency. Then a unique type of converter, converters with Integrated Capacitor Blocked Transistor (ICBT) cells are studied. Unlike the traditional methods, there are no fast-developing voltage unbalances, or high cell capacitor voltage ripples in ICBT-based converters. The ideal operation principle is analyzed and verified by the simulation results. Then the impacts of non-idealities on the operation are analyzed, and a control method is proposed for this type of converter. The operation and control of ICBT-based converters are verified by experimental results to achieve low cell capacitor voltage ripples and excellent voltage balance in Medium Voltage high power applications. Lastly, the conducted EMI emission in SiC-based multilevel converters are studied. Four SiC-based multilevel converters are studied, with the focus on the power circuit in one converter and the auxiliary circuits in the other three converters. The complexity of noise generation and propagation in multilevel converters is presented. The conducted EMI disturbances are experimentally evaluated, analyzed, and effectively mitigated in all four cases.

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.