  This paper proposes a novel switching algorithm for modular multilevel
converters (MMCs) that significantly reduces the switching frequency while
fulfilling all control objectives required for their proper operation. Unlike
in the conventional capacitor voltage-balancing strategies, in addition to
submodule (SM) capacitor voltages, the proposed algorithm considers previous
switching statuses during sorting. The algorithm is applied to a seven-level
back-to-back MMC-HVDC system and tested under various operating conditions.
Significant reduction in the switching frequency with trivial impacts on
submodule capacitor voltages are observed.
