Modified Increased-Level Model Predictive Control Methods With Reduced Computation Load for Modular Multilevel Converter

Abstract

It is considerable to reduce the computation load while keeping a good total harmonic distortion (THD) performance for the modular multilevel converter (MMC) with model predictive control (MPC). In this paper, the discrete-time mathematical model of the MMC is derived and the output voltage ripples of the cascaded submodules (SMs) are analyzed in detail. Accordingly, a modified increased-level MPC method A (IL-MPC-A) is proposed with 2 N + 1 ac-side output voltage levels. The computation load is significantly reduced without losing the harmonic circulating currents suppression and capacitor voltage ripples compensation capabilities. In this method, the cost function minimization step and capacitor voltage sorting algorithm are combined to determine the optimal control option. In addition, to avoid the selection of weighting factors and further reduce the computation load as the increasing of the SM number, another modified IL-MPC-B is proposed by directly calculating the optimal ac-side output voltage level and arm summation voltage according to the discrete-time mathematical model. As a result, the ac-side output current and circulating current controls are decoupled. Furthermore, the capacitor voltages are balanced with a proposed control scheme by adjusting the circulating current reference. The effectiveness of the proposed MPC methods is verified by simulation and experimental results.