  Unlike conventional converters, modular multilevel converter (MMC) has a
higher switching frequency -- which has direct implication on important
parameters like converter loss and reliability -- mainly due to increased
number of switching components. However, conventional switching techniques,
where submodule sorting is just based on capacitor voltage balancing, are not
able to achieve switching frequency reduction objective. A novel modulation
algorithm for modular multilevel converters (MMCs) is proposed in this paper to
reduce the switching frequency of MMC operation by defining a constrained
multi-objective optimization model. The optimized switching algorithm
incorporates all control objectives required for the proper operation of MMC
and adds new constraints to limit the number of submodule switching events at
each time step. Variation of severity of the constraints leads to a desired
level of controllability in MMC switching algorithm to trade-off between
capacitor voltage regulation and switching frequency reduction. Finally,
performance of the proposed algorithm is validated against a seven-level
back-to-back MMC-HVDC system under various operating conditions.
