Integration of Solar Photovoltaic Systems in Medium-Voltage DC Networks Using Modular DC Converters

Solar photovoltaic (PV) energy systems have been most commonly constructed based on ac technologies. The recent visions of multi-terminal direct-current (MTdc) grids, dc distribution systems for densely populated urban areas, and dc microgrids for more convenient integration of distributed energy resources (including renewable energies, electric vehicles, and energy storage devices such as batteries) have motivated the shift towards solar PV power systems based on dc technologies. A key enabling technology required for such a change in the status quo are dc/dc power electronic converters that are capable of efficiently integrating low-voltage (LV) solar PV farms into medium-voltage (MV) dc networks. This dissertation, therefore, proposes the use of a modular dc transformer (DCT) based on the dual active bridge (DAB) converter and modular multilevel converter (MMC) to facilitate the integration of solar PV sources in MVdc networks. The proposed DCT employs a Voltage Matching Scheme (VMS) to address the issues of high current stress and reactive power losses that plague DAB-based converter topologies in the presence of unmatched transformer terminal voltages. The DCT is analyzed in detail to model various aspects of the converter such as the power throughput and zero-voltage switching regions, while also providing insight on more practical matters such as the sizing of passive components and their impact on converter operation. A novel modular multiport converter (MPC) is then derived based on the DCT to allow for the integration of both solar PV sources and battery energy storage through a single power conversion stage. It is shown that the MPC is capable of extracting power from the PV sources using maximum-power-point-tracking (MPPT) over a wide range of operating points. The MPC also employs the VMS to minimize current stress on the transformer. The developed models are all verified through simulation studies based on the Mat-lab & Simulink software environment, a 1.2-kW experimental DCT prototype, and a 550-W experimental MPC prototype. Both prototypes are shown to have efficiencies exceeding 97%.