Modelling and Current-Mode Control of a Modular Multilevel DC-DC Converter

The visions of multi-terminal direct-current (MTDC) grids, DC distribution systems for densely populated urban areas, and DC microgrids for more straightforward integration of distributed energy resources (including renewable energies, electric vehicles, and energy storage devices) have sparked a great deal of research and development in the recent past. An enabling technology towards the fulfilment of these visions is efficient, highly-controllable, and fault-tolerant AC-DC and DC-DC electronic power converters capable of interfacing networks that operate at different voltage levels. This thesis thus presents the results of an in-depth investigation into the operation and control of a particular class of DC-DC converters. The DC-DC converter studied in this thesis is based upon the so-called modular multi-level converter (MMC) configuration, employing halfbridge submodules and with no galvanic isolation. The thesis first presents the governing dynamic and steady-state equations for the converter. Then, based on the developed mathematical model, it identifies suitable variables, strategies, and feedback loops for the regulation of the submodule DC voltages as well as converter power throughput. In particular, two current-control loops are proposed that, in coordination with one another, not only enable the control of the power flow within the converter, but also promise protection against overloads and terminal shorts. The validity of the mathematical model and effectiveness of the proposed control are verified through off-line simulation of a detailed circuit model as well as experiments conducted on a 1-kW experimental setup. The results of this exercise motivate the extension of the proposed control method to more compact designs with galvanic isolation and enhanced power handing capabilities.