Asymmetrical DC-DC Modular Multilevel Converter using Switching Cycle Control

Modular Multilevel Converters (MMC) have emerged as an excellent option for medium/high voltage DC-AC or AC-DC applications due to their unmatched modularity and scalability. Despite this, the DC-DC operation of MMC isn't very well studied. Even when DC-DC MMC operation is achieved, the submodule capacitor sizing challenge is enormous. Recently proposed switching-cycle-balancing control (SCC) solves this challenge of high submodule capacitance by balancing the submodule capacitor voltages every switching cycle instead of line cycle. Balancing within a switching cycle essentially decouples the MMC operation from line frequency, providing opportunity for a fundamentally new DC-DC converter. Despite its advantages for DC-DC operation, SCC has scope for improvement, especially if converter operating input/output voltage ratio is pre-determined. This paper develops a fundamentally new MMC topology for DC-DC operation while also proposing two SCC-derived control techniques. The advantages and limitations of the two control techniques are discussed extensively and compared to conventional SCC DC-DC operation. The analysis is verified using simulations and representative converter behavior is verified using 6kV test-bench utilising 10kV SiC MOSFETs.