An Isolated Soft-Switched DC-DC Converter For Interconnection Of Medium- And Low-Voltage DC Grids

As the electric power grid increasingly hosts energy storage devices, renewable energy resources, plug-in hybrid and electric vehicles, and data centers, it is expected to benefit in the future from a multi-layer DC structure meshed within its legacy AC architecture. As such a multi-layer grid structure evolves, interconnection of DC grids with different voltage levels will become necessary. For such interconnections and for power-flow control, efficient isolated DC-DC converters are a key enabling technology. This thesis thus presents the results of an in-depth investigation into the operation, modulation, control, and performance assessment of a particular DC-DC converter configuration. The proposed DC-DC converter, which is based upon a hybrid combination of the conventional dual-active-bridge topology and the modular multi-level converter (MMC) configuration, is a potential candidate topology for interconnection of medium- and low-voltage DC grids. The thesis first introduces the circuit topology and presents the basics of operation and governing steady-state equations for the converter. Then, based on the developed mathematical model, it identifies a suitable modulation strategy for the converter bridges and submodules, as well as strategies for the regulation of the MMC submodule capacitor voltages and soft switching of the constituent semiconductor devices. The proposed converter topology offers significant benefits including galvanic isolation, utilization of the transformer’s leakage inductance, soft switching for high-frequency operation, and bidirectional power flow capability. The validity of the mathematical model, effectiveness of the proposed modulation and control strategies, and the realization of soft switching are verified through off-line simulation of a detailed circuit model as well as experiments conducted on a 1-kW experimental setup.