Spatial repetitive controller for minimizing circulating harmonic currents in modular multilevel converters for variable frequency applications

In a modular multilevel converter (MMC), ripple component in the capacitor voltages of the submodules(SMs) results in second- and other even-order harmonics in the circulating currents. When, MMC is used for variable frequency applications like a front end converter in variable speed wind generator or as an inverter in a motor drive, the circulating current harmonic frequencies varies over a wide range because of the change in fundamental operating frequency. Traditional controllers like proportional resonant (PR) controller and time-domain based repetitive controllers have limited harmonic rejection capability at variable frequencies. This paper presents a control structure with spatial repetitive controller (SRC) to alleviate the circulating harmonic currents in MMC, when used for variable frequency applications. SRC has a good steady-state harmonic suppression capability and is independent of frequency. Submodule capacitor voltage balancing control strategy is also presented. Analytical equations governing the operation of a 3-phase MMC are derived and presented. MATLAB/Simulink model of a 3-phase, 9-level (Ln-Ln) MMC is developed to demonstrate the effectiveness of the developed submodule capacitor balancing and circulating harmonic current suppressing controller.