A Modified Selective Harmonic Elimination method for balancing capacitor voltage in modular multilevel converter

The modular multilevel converter (MMC) has proven itself as an exceptionally effective converter in High Voltage DC (HVDC) applications, due to its modularity, reliability and robustness. Yet, it requires unique modulation techniques to accomplish the troublesome capacitor voltage balancing. In this paper, a Modified Selective Harmonic Elimination (M-SHE) is introduced where the switching of the submodules is done in a certain preconfigured arrangement which accomplishes the required capacitor balancing. The proposed modulation technique is then compared with Third Harmonic Injection — Phase Shift — Sinusoidal Pulse Width Modulation (THI-PS-SPWM) and with the Classical Selective Harmonic Elimination (SHE). Performance of the proposed M-SHE, classical SHE and THI-PS-SPWM for a five-level MMC unit is evaluated based on time-domain simulation studies in the MATLAB/SIMULINK software. The reported study results demonstrate that the M-SHE satisfies capacitor balancing and has lower Total Harmonic Distortion (THD) than the classical SHE and THI-PS-SPWM.

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