Modulation Index Optimization and Fault Blocking Capability of Transformer-Less Modular Multilevel DC–DC Converters

High-power DC-DC Converters are crucial for interconnecting HVDC systems with different voltage levels. This paper presents the optimal design and operation of transformer-less hybrid modular multilevel dc-dc converters (MMDCs) that consist of both the half bridge submodules (HBSMs) and the full bridge submodules (FBSMs). Since the arm internal dc component voltage and ac component voltage can be freely selected, the optimal modulation index that minimizes the kVA of switches is derived for different voltage conversion ratios. Overmodulation is also considered to minimize the cost of the converter. For system security and reliability reasons, the dc fault-blocking capability is a preferred feature of dc-dc converters for HVDC applications. Benefitting from using the hybrid SMs, the dc fault-blocking capability of hybrid MMDCs is also investigated. The performance of the hybrid MMDC is validated by real-time simulation and a lab-scale test bench.