Topology and Modulation of Fractional-Level MMC for DC Distribution Grids

For the modular multilevel converter (MMC) with a small quantity of submodules, neither nearest level modulation (NLM) nor carrier phase-shifted pulse width modulation (CPS-PWM) can achieve high voltage quality and low operation loss simultaneously. In this paper, a novel fractional-level MMC (FLMMC) topology is proposed, in which fractional-level submodules are introduced to multiply the output levels. To balance the capacitor voltages between conventional submodules and fractional-level submodules, this paper develops a fractional-level modulation (FLM) method suitable for FLMMC. Considering control complexity and device quantity, the FLMMC with two half-level or one-third-level submodules in each arm is investigated in this paper. Simulation and experiment tests are carried out to validate the effectiveness and superiority of FLMMC under FLM. FLMMC can improve the voltage quality without increasing control complexity, circulating current and switching loss, so as to comprehensively adapt to the power quality requirements of dc distribution grids.