On energy variations in modular multilevel converters with full-bridge submodules for Ac-Dc and Ac-Ac applications

The modular multilevel converter is a promising topology for high-voltage high-power applications. By the series-connection of submodules it can generate high-quality voltage waveforms with low harmonic distortion at low switching frequencies. The submodules are low-voltage converters with capacitive energy storages. These capacitive energy storages are a driving factor of the size, weight, and cost of the converter. For this reason it is important to ensure that the stored energy in the converter is distributed as evenly among the submodules as possible. In this paper it is found that during nominal operating conditions, a large amount of energy is moved back and forth between the upper and lower arms in the converter. These energy oscillations can, however, be reduced or even eliminated if an appropriate modulation index or voltage ratio is used. It is also found that an appropriate modulation index or voltage ratio can reduce the power rating of the semiconductors as well. The theoretical findings are validated by simulating two systems with different voltage ratios for ac-ac conversion.