Analysis of capacitor voltage ripple minimization in modular multilevel converter based on average model

This paper investigates reduction of submodule voltage ripple in modular multilevel converter (MMC). The reduction is achieved by injecting the optimum amount of circulating current across the phase leg of the converter. The magnitude and phase angle of arm currents and submodule voltage quantities are calculated via an average model derived for the MMC topology. Then based on the derived equations for submodule voltage components at different ac frequencies, an effort is done to calculate and inject the optimum magnitude of circulating current in order to minimize the voltage fluctuation across submodule capacitors. Also in this paper, the effect of natural and optimized circulating currents on converter losses and efficiency is investigated via calculating semiconductor losses.