Common-Mode Voltage Minimization for Grid-Tied Modular Multilevel Converter

Abstract

Transformerless modular multilevel converter (MMC)-based drive system usually installs a grid-tied MMC to interface with medium-voltage ac utility. Since no front-end transformer is installed, the grid-tied MMC is expected to directly defense the common-mode voltage (CMV) from asymmetrical grid fault and switching operation of converters for maximally protecting the medium-voltage motor. This paper presents a control strategy to minimize the CMV of the grid-tied MMC. The CMV due to asymmetrical grid fault is canceled by MMC counterpart voltage, while the switching ripples caused by MMC switching operation are reduced by arranging the arm-voltage pulses end-to-end. Moreover, the influence of MMC counterpart voltage on phase-leg power deviation is analyzed, and a solution based on feed-forward control is developed accordingly. Under the control of the proposed strategy, the maximal CMV of the grid-tied system is reduced from $1/3$ per unit (p.u.) to ( $1/3{\boldsymbol{N}}$ ) p.u. under serious asymmetrical grid condition, where ${\boldsymbol{N}}$ is the per-arm submodule number. Meanwhile, the united power factor, constant dc-bus voltage, and balanced phase-leg power are also realized. Simulations and experiments confirm the validation of the proposal.