A Modular Multilevel DC/DC Converter With Fault Blocking Capability for HVDC Interconnects

This paper introduces a modular multilevel dc/dc converter, termed the DC-MMC, that can be deployed to interconnect HVDC networks of different or similar voltage levels. Its key features include: 1) bidirectional power flow; 2) step-up and step-down operation; and 3) bidirectional fault blocking similar to a dc circuit breaker. The kernel of the DC-MMC is a new class of bidirectional single-stage dc/dc converters utilizing interleaved strings of cascaded submodules. The DC-MMC operation is analyzed and an open loop voltage control strategy that ensures power balance of each submodule capacitor via circulating ac currents is proposed. Simulations performed in PLECS validate the DC-MMC´s principle of operation and the proposed control strategy. Experimental results for a 4-kW laboratory prototype illustrate the single-stage dc/dc conversion process for both step-down and step-up operating modes.