A new hybrid modular multilevel converter with increased output voltage levels

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Abstract
The modular multilevel converter (MMC) has become the most promising converter technology for high power applications. However, to get more number of levels in the output voltage, a conventional MMC requires large number of submodules. The main technical challenges of an MMC, with a large number of submodules, are the requirement of large number of semiconductor switches, submodule capacitors and measurement of submodules capacitor voltages. This makes MMC bulky and increases control complexity. This paper presents a new hybrid MMC, which uses half bridge submodules (HBSMs) and auxiliary full bridge submodules (FBSMs) in each arm of the converter. Compared with the conventional MMC, the proposed topology uses fewer submodules to get the same number of output voltage levels. This reduces the footprint and control complexity in the proposed MMC. A modified selecting and sorting algorithm for ensuring capacitor voltage balancing of HBSMs and auxiliary FBSMs is developed. The performance of the proposed converter and capacitor voltage control technique for standalone model is evaluated by using time domain simulation studies in PSCAD/EMTDC. The simulation results for standalone operation and HVDC applications demonstrate feasibility and validity of the proposed hybrid MMC. Experimental studies are also presented to further demonstrate the effectiveness of the proposed converter and control techniques.

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