For an Electrical Vehicle, the power is delivered from the battery pack to the electric motor through the use of power converter. Many research projects have been conducted in improving the efficiency of traction inverters. Inverter topologies are categorized based on the number of voltage levels of the inverter output phase voltage. The conventional 2-level Voltage Source Inverter (VSI) is commonly used as a traction inverter due to its simple structure. Due to the recent trend in utilizing higher DClink voltage in traction motor drives to achieve a higher power rating, multi-level inverters are gaining attention to replace the conventional VSI in EV powertrain. Multi-level voltage source inverters (MLVSI) have been widely adopted in high-power converters and medium-voltage drives. There are four major categories of MLVSI: the Flying Capacitor (FC), Neutral Point Clamped (NPC), Cascaded and Hybrid. The power rating of the MLVSI increases with the increase of inverter levels, but the size, number of switching devices, cost and control difficulty also increases. Due to the above reasons, 3-level NPC can be a good solution for traction inverters. Due to the structure and control limitation, Diode Clamp NPC suffers from uneven loss distribution and neutral point voltage balancing issues. This issue can be resolved with Active Clamped NPC (ANPC). In this thesis, the design, simulation, prototyping and testing of a 70kw 3-level ANPC traction is introduced. / Thesis / Master of Science (MSc)

For decades, the Silicon-based semiconductors have been the solution for power electronics applications. However, these semiconductors have approached their limits of operation in blocking voltage, working temperature and switching frequency. Due to material superiority, the relatively-new wide-bandgap semiconductors such as Silicon-Carbide (SiC) MOSFETs enable higher voltages, switching frequencies and operating temperatures when compared to Silicon technology, resulting in improved converter specifications. The current study tries to investigate the impact of emerging medium-voltage SiC MOSFETs on industrial motor drive application, where over a quarter of the total electricity in the world is being consumed. Firstly, non-commercial SiC MOSFETs at 3.3 kV and 400 A rating are characterized to enable converter design and simulation based on them. In order to feature the best performance out of the devices under test, an intelligent high-performance gate driver is designed embedding required functionalities and protections. Secondly, total of three converters are targeted for industrial motor drive application at medium-voltage and high-power range. For this purpose the cascaded H-bridge, the modular multilevel converter and the 5-L active neutral point clamped converters are designed at 4.16-, 6.9- and 13.8 kV voltage ratings and 3- and 5 MVA power ratings. Selection of different voltage and power levels is done to elucidate variation of different parameters within the converters versus operating point. Later, comparisons are done between the surveyed topologies designed at different operating points based on Si IGBTs and SiC MOSFETs. The comparison includes different aspects such as efficiency, power density, semiconductor utilization, energy stored in converter structure, fault containment, low-speed operation capability and parts count (for a measure of reliability). Having the comparisons done based on simulation data, an H-bridge cell is implemented using 3.3 kV 400 A SiC MOSFETs to evaluate validity of the conducted simulations. Finally, a novel method is proposed for series-connecting individual SiC MOSFETs to reach higher voltage devices. Considering the fact that currently the SiC MOSFETs are not commercially available at voltages higher above 1.7 kV, this will enable implementation of converters using medium-voltage SiC MOSFETs that are achieved by stacking commercially-available 1.7 kV MOSFETs. The proposed method is specifically developed for SiC MOSFETs with high dv/dt rates, while majority of the existing solutions could only work merely with slow Si-based semiconductors. / Ph. D. / Despite their mature technology and low manufacturing cost, the traditional Si-based power semiconductors had reached their limitations in operation from different points of view. The SiC MOSFETs which are the new generation of power semiconductors however seem to be able to shift the existing boundaries of operation for the Si-based semiconductors, resulting in significant improvement in design and operation of power electronics converters. This dissertation focuses on investigating the impact of emerging medium-voltage SiC MOSFETs on industrial motor drives, which consume over 28% of the total electricity used in the world. Firstly, the state-of-the-art non-commercial 3.3 kV SiC MOSFETs are characterized. Characterization of the devices is done to extract their key features such as switching and conduction losses, to enable loss calculation and performance evaluation in any target application. Since the mentioned devices are not commercial yet, the gate driving circuitry that can feature the best performance out of them are not commercially available either. Thus, the characterization process starts with design of an intelligent high-performance gate driver for the devices under test. Secondly, total of three topologies that are targeted for the study are discussed and their basics of operation is investigated. For this purpose the cascaded H-bridge, the modular multilevel converter and the 5-L active neutral point clamped converters are designed at three different voltage levels (4.16-, 6.9- and 13.8 kV) and two power levels (3- and 5 MVA). Selection of different voltage and power levels is done to enable comparison from different aspects as the operating point changes. Later, comparisons are done between the surveyed topologies designed at different operating points using different semiconductor technologies. The performed comparisons provide an unbiased input for the manufacturers and customers of these converters for selection of the target topology in motor drive application. Also to verify validity of the conducted simulations and calculations, a full-bridge converter cell is experimentally implemented using 3.3 kV 400 A SiC MOSFETs. Finally, a novel method is proposed for series-connecting lower-voltage SiC MOSFETs to reach higher-voltage devices. As of late 2017, the medium-voltage SiC MOSFETs are not commercially available. Also it is expected that upon commercialization, their price will be multiple times of that of low-voltage SiC MOSFETs. Thus, connecting lower-voltage SiC MOSFETs in series is an effective way of achieving higher-voltage devices and take advantage of superior properties if the SiC MOSFETs, while the availability and high cost problems are taken care of.