A Modular DC to Three-Phase AC Converter Topology with Minimized Intermediate Energy Storage Requirements

Modular Multilevel Converter (MMC) is an attractive candidate in high power conversion due to its modularity and scalability. The energy storage element, namely the module capacitance in the MMC module circuit, is typically large and requires very bulky module capacitors. In addition to deteriorating the power density, the conventional design necessitates electrolytic capacitors, further affecting the overall converter efficiency and reliability. This work presents an MMC module topology and the accompanying modulation approach that significantly reduces the module capacitance requirements (few micro-farads versus milli-farads). The proposed topology eliminates the single-phase AC processing power requirement, a characteristic of the conventional design, by presenting an integrated module design based on a DC-to-three-phase AC bridge concept. This three-phase integrated design results in zero AC processing power for the module capacitors per switching cycle. As a trade-off, the arm inductances are carefully distributed to use unidirectional voltage blocking devices in the modules. The work is verified using circuit simulations using PLECS and a laboratory-scale experimental prototype. The simulation and experimental results verify the proposed analytical approach permitting highly power dense modular converters. In contrast to bulky electrolytic capacitors, tiny film capacitors are utilized for the module's energy storage requirements.