Impact of Circulating Currents on Capacitor Voltage Ripple and Losses in MMCs

This paper characterizes the impact of second-harmonic circulating currents on the capacitor voltage ripple and losses in a modular multilevel converter. A control method is then developed to minimize the capacitor voltage ripple or losses of the converter by manipulating the arm voltage references so that judicious amounts of circulating currents that achieve these objectives are created. The paper shows that both losses and submodule capacitor voltage ripple can be further reduced below the point where the commonly used circulating current suppression controls are deployed. Detailed PSCAD/EMTDC simulations are used for validation.