Common-Mode-Voltage Regulation of Modular Multilevel Converters Through Model Predictive Control

Modular multilevel converters (MMCs) have emerged as an attractive converter topology for renewable energy integration systems. A smooth operation of an MMC depends on the proper regulation of multiple control variables, e.g., phase current, circulating current, capacitor voltage, etc. The model predictive control (MPC) is an excellent candidate for such a scenario since different control objectives can be simultaneously handled in a unified cost function. Although extensive research has been conducted in the literature, a computationally efficient MPC implementation capable of regulating the common-mode voltage (CMV) of the ac-side neutral point of an MMC as a control target has not been properly investigated. This article proposes such an MPC strategy that can flexibly regulate the CMV by incorporating the CMV tracking error into the cost function. The biggest challenge is the dramatically increased number of available control actions and the consequent computational burden as the three phases are controlled as a whole, which are addressed employing a quadratic programming technique, leading to only 64 cost-function evaluations per control period regardless of the number of submodules of the MMC. Several experiments and simulations are carried out considering the regulation of the CMV in different scenarios, and the results validate the effectiveness of the proposed method.