Start-up control with constant precharge current for the modular multilevel converter

One specific issue associated with the modular multilevel converter (MMC) is that the capacitors of the submodules (SMs) need to be precharged before MMC begins its normal operating. Otherwise, a large inrush current will occur if these SM capacitors were not charged to the rated value. This paper proposes a novel start-up control scheme for MMC, in which the closed-loop control is adopted to regulate a constant rms precharge current with a unity power factor. As such, the SM capacitors will be linearly precharged to the rated value without any inrush current and much shortened charging time. Moreover, this scheme requires no extra power supply and is very flexible as it can be easily implemented into any MMC application regardless of the power rating and the number of SMs. Finally, a three-phase MMC prototype has been built in the laboratory to verify the effectiveness of the proposed start-up control scheme.