Reduction of low-frequency harmonics in modular multilevel converters (MMCs) by harmonic function analysis

Modular multilevel converter (MMC) is troubled by the inherent second harmonic of single-phase ac power. By adding feedforward to the modulation signal, low-frequency harmonic voltages are significantly reduced thus overcoming an important weakness of MMC. Capacitors of sub-modules do not have to be over-sized. If desired for protection, transformers can be wye-connected with grounded neutral. The reduction method is based on harmonic function analysis. In addition to deriving algebraic formulae of harmonic voltage components of `open loop´ control, this study makes original contributions by deriving formulae which include feedforward control. Analytical insights from the formulae have shown the way to design feedforward methods to reduce low-frequency harmonics. Validation is by simulations using SIMULINK/MATLAB.