Closed-Loop Performance Improvement for MMCs Based on Optimal Reference Governor

Model predictive control (MPC) has emerged as a promising approach to govern modular multilevel converters (MMCs) due to its flexibility to include multiple control objectives and its simple design process. However, this control scheme presents relevant issues, such as high computational complexity and the proper references design requirement. This work proposes a phase-shifted model predictive control (PS-MPC) for MMCs with an optimal circulating currents reference governor based on a quadratic programming MPC (QP-MPC). The PS-MPC is formulated to fully exploit a phase-shifted pulsewidth modulation technique by obtaining independent optimal modulating signals for each carrier sequentially, with reduced computational effort independent of the number of submodules. Moreover, the proposed QP-MPC reference governor improves the overall dynamic performance of the MMC and brings flexibility for prioritizing the capacitor voltage ripples reduction or the MMC efficiency by changing the weighting factors of its cost function. Simulation results are provided to verify the proposed control scheme's effectiveness when governing a three-phase MMC with four half-bridges per arm.