Arm-Current-Based Model Predictive Control for Modular Multilevel Converter Under Unbalanced Grid Conditions

Abstract

Model predictive control (MPC) methods have attracted growing attention in modular multilevel converter (MMC) systems due to their fast dynamic response, straightforward principle, and ability to control multiple objectives with appropriate cost functions. This article proposes an arm-current-based MPC strategy for MMCs to achieve comprehensive control of ac-side current, circulating current, and submodule (SM) capacitor voltages under unbalanced grid conditions. By directly obtaining the optimal arm voltage according to the discrete time model of arm current, the computational complexity is significantly reduced and is independent of SM numbers. Meanwhile, the tedious cost function evaluation and weighting factors optimization can be avoided. Furthermore, with the arm voltages regulated separately, the proposed method is able to generate <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2{N} + 1$ </tex-math></inline-formula> output levels, contributing to enhanced total harmonic distortion (THD) performance and circulating current suppression ability. In addition, the calculation method of the arm current reference is presented, especially for the dc-side current reference, the capacitor voltages can be maintained at the rated value by adjusting the dc current reference. Finally, the validity of the proposed strategy is demonstrated under balanced and unbalanced grid conditions by both simulation and experimental results.