An interconnected observer for modular multilevel converter

Abstract

Modular multilevel converter (MMC) is an attractive topology for transformer-less high voltage applications due to its capability to reach high voltage levels by the use of power semiconductors. Each submodule can be considered as a controlled voltage source where capacitor's voltage should be maintained at a certain level for proper operation of the MMC and for fault detection and protection purposes. Besides, the minimization of the circulating current, which does not flow to the load, is crucial to achieve stable and efficient operation of the MMC. Thus, this paper presents a novel Interconnected Observer (IO) based Model Predictive Control (MPC) for a single-phase MMC. The newly designed observer is used to estimate the capacitors' voltages using the circulating and load currents. This approach introduces the capacitance value of the cell capacitors as a parameter uncertainty toward making the system performance robust with anonymous constant parameters. The estimated capacitors' voltages are then used by the proposed MPC algorithm to achieve stable and balanced voltage and for current control with reduced circulating current in various operating conditions. Simulation studies are performed in Matlab/Simulink environment to verify the proposed design.