Abstract

Solid state transformer (SST) has attracted wide attention due to its advantages of active and reactive power control, harmonic cancelation, and power factor correction. The input stage of SST based on modular multilevel converter (MMC) topology can be applied to the transmission systems in the high voltage range. Under the positive and negative sequences system, this paper proposes an unbalanced-grid-fault ride-through nonlinear control method based on the input stage and output stage of MMC-SST. The control strategy of input stage aims at minimizing the fluctuation of DC voltage on the input side and reducing the negative input stage's sequence current. The control strategy of output stage focuses on controlling the active power oscillation of output stage. To further reduce the impact of unbalanced grid fault, a PI-based phase-shift control of the isolation stage during the fault is proposed. To verify the proposed control strategy, a three-stage MMC-SST model during the fault is built in Matlab/Simulink. The simulation results show that the control strategy proposed in this paper has the advantages of eliminating transient power quality problems in both grids.

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