A Transformerless Hybrid Modular Multilevel DC-DC Converter With DC Fault Ride-Through Capability

This paper proposes a hybrid modular multilevel dc-dc converter with dc fault ride-through capability. In each phase, the lower arm and shunt arm are composed of half-bridge submodules and full-bridge submodules (FBSMs) respectively, and the upper arm is composed of both types of submodules. The shunt arm eliminates the large ac filter required at the output of dc side. Moreover, it provides the bidirectional fault ride-through capability together with the FBSMs in the upper arm. Required kVA rating of the switches in the proposed topology is analyzed and compared to other topologies. With the optimal number of FBSMs in the upper and shunt arms designed by overmodulation, the proposed topology has a reduced kVA rating of switches, especially for the voltage conversion ratio above 0.54. A control strategy is presented to control the ac circulating current and balance the energy between the three arms. The performance of the proposed topology is validated by real-time simulation and a lab-scale test bench.