A Voltage-Level-Based Model Predictive Control of Modular Multilevel Converter

In this paper, an improved model predictive control (MPC) of the modular multilevel converter (MMC) with reduced computational burden is proposed. A mathematical model of the MMC system based on the sum and difference of arm voltages are derived. Instead of determining the switching state of individual submodule (SM), the voltage levels of MMC are considered as control options based on the assumption that the SM capacitor voltages are well balanced. The further reduction of calculation effort is realized by using the tolerance band of capacitor voltages. The proposed MPC has a hierarchical structure. The cost function taking into account the ac-side current control, circulating current elimination and arm energy balancing is presented. The optimal voltage level, selected by the cost function, provides the voltage reference for the pulse width modulation modulator. The SM capacitor voltage balancing is done using a separate control loop. The proposed control strategy is investigated using an MMC high-voltage direct current system with 200 SMs in each arm in real-time simulation and hardware-in-the-loop tests. The performance of proposed method is verified by both steady-state and transient-state operations.