Three-phase Unbalanced Control Method of MMC System Based on Improved One-cycle-control

In order to deal with the operation of modular multilevel converter (MMC) system under the condition of unbalanced three-phase condition, this paper proposes an improved one-cycle-control method, which uses the negative grid-side voltage to compensate the current feedback of the MMC grid-side based on basic one-cycle-control, to avoid the positive and negative sequence transformation calculation of three-phase current. Moreover, virtual loop mapping scheme is added to balance sub-module capacitor voltages. Compared with the existing MMC unbalanced control strategy, control process is simplified and parameter design is relatively easy. The simulation results of three-phase voltages dip show that the proposed method can not only realizes one-cycle-control of MMC under three-phase unbalance condition, reduces the twice fundamental-frequency harmonics in the DC voltage, and makes the output active power of the grid more stable, but also it is able to make the submodule capacitor voltages automatically reach the balance, which reduces device loss of the system.