Supervisor:
Professor Remus Teodorescu

Co-Supervisor:
Associate Professor Tamas Kerekes

Assessment Committee:
Associate Professor Szymon M. Beczkowski, AAU Energy (Chair)
Professor Mariusz Malinowski, Warsaw University of Technology
Professor Massimo Bongiorno, Chalmers University of Technology

Moderator:
TBA

Abstract:

The fast development of the modern industry has been leading to a constant increase in the energy demand. At the same time, serious environmental issues are calling for a severe change in the world’s energy consumption paradigm moving from a fossil-fuel-based energy matrix to a green one. The high penetration of renewable-energy generation leads to an increased complexity of the control and operation of power systems. Power electronics is the technology that allows for the high flexibility and controllability of these modern electrical grids. However, until recently, the power ratings of the most advanced semiconductor devices limited the usage of power-electronic converters, especially if high-voltage applications were considered. The usage of stacks of several series-connected semiconductor devices operating at high frequencies was the only solution to build flexible high-voltage converters, even though these converters presented poor reliability and high switching losses. The invention of the modular multilevel converter (MMC) represented a breakthrough in the electrical engineering filed. The scalability and modularity of this converter topology allows obtaining high voltages by connecting in series several low-voltage sub blocks that can be built with semiconductor devices with voltage ratings currently available in the industry. Moreover, the MMC is a flexible converter solution that presents high reliability and that can synthesize high-power-quality multilevel voltages at its AC terminals. Due to its many important features, the MMC became the standard power-electronic solution for high-power applications, especially for the flexible voltage-source-converter high-voltage direct-current (VSC-HVDC) transmission systems. MMC solutions have also been adopted for applications such as static synchronous compensators (STATCOMs) and for high-power medium-voltage electrical-machine drives. Despite its many advantages, the MMC also presents some drawbacks such as its high structural and control complexity, its high number of components, which leads to high volume, weight and cost, and finally its poor performance at low frequencies because of the intolerably high submodule-capacitor voltage ripple in this operation condition. In high-power industrial motor drives for example, the high submodule-capacitor voltage ripple at low frequencies can be quite critical. Another potential machine-drive application of an MMC are the modern/future high-power wind turbines (WTs)with a medium-voltage structure aiming at avoiding technical issues with excessively high currents, and aiming at obtaining more compact solutions with reduced conductor and transformer requirements. In this case, the increased dimensions of the MMC would play a negative role in the general solution. In this Ph.D. thesis, novel converter solutions with a modular multilevel structure are proposed aiming at overcoming some of the drawbacks and limitations of the traditional MMC, and of other converters with a modular multilevel structure, in high-power medium-voltage machine drives. Moreover, the modular features of the converters with a modular multilevel structure are explored in a way as to integrate energy storage devices into the proposed topologies, aiming at obtaining flexible solutions that are capable to provide ancillary services to the electrical grid.