Systematic modeling and control of indirect modular multilevel converter (MMC) with grid unbalance estimation

Modular multilevel converter (MMC) is considered as an idea and promising candidate for multilevel technology. By connecting two MMCs in back-to-back configuration with a DC transmission cable, a so-called indirect MMC can be constructed for high-voltage direct current transmission application. This paper presents the dq model of indirect MMC by generalized state-space representation. Correspondingly, a complete LQR-based state feedback control theory is applied to regulate the multiple currents in indirect MMC. The resulted control architecture is unified and flexible to include other control criterion while guaranteeing an optimized control performance. The grid-side unbalanced situation of indirect MMC is analyzed by mathematical modeling and an estimator is proposed to reconstruct the unbalanced disturbances. Simulation results are presented to illustrate the effectiveness of estimator-based LQR controller under both balanced and unbalanced conditions.