Medium-voltage (MV) drive systems are applied in a wide variety of applications with high to very high power ratings. Nowadays they are absolutely essential for industry (water, oil and gas, mining, metals, paper, etc.), traction, and marine engineering.The most prominent topology for such drives is the three-level Neutral Point Clamped Voltage Source Converter (NPC VSC). Beside its many outstanding advantages the topology suffers from the significant drawback of an inherently unequal loss distribution between the semiconductors. This uneven utilization of the main switches limits the switching frequency and the output power of the converter.The objective of this work is to overcome this drawback and to optimize the threelevel NPC VSC for medium-voltage applications. A completely new approach is introduced: the Active NPC (ANPC) principle. It combines all benefits of the conventional NPC VSC as its simple structure and a low part count, and requires only little additional effort and cost. Nevertheless, it promises to considerably extend the output power range of the three-level NPC topology.For typical industrial MV drive converters featuring IGBT and IGCT devices a significant output power increase by up to 20% or a switching frequency increase by more than 100% is achieved by the ANPC principle. Moreover, it pushes the limits of single three-level converter units from 10MVA to up to 16MVA. Where two converters connected in parallel are required today, a single-converter solution can yield a rewarding reduction of cost and installation space.Throughout this work the focus is directed towards industrial MV drives. However, the ANPC VSC can be applied in traction, railway interties, new Flexible AC Transmission Systems (FACTS) solutions as universal power flow controllers (UPFC) or STATCOMs, and high-voltage dc transmission.Beside the introduction of the ANPC principle the work includes the following topics:. design issues of commercial three-level medium-voltage converters, . modulation schemes for three-level converters (discontinuous modulation, schemes for small modulation depths, losses and harmonic distortion, NP control), . junction temperature measurement using infrared imaging.