A Control Method for Improving Power Quality of Parallel Three-Level Rectifiers Under Unbalanced Grid Voltage Conditions

In parallel three-level neutral point clamped (3L-NPC) rectifiers with the common dc and ac buses directly under unbalanced grid voltage operation conditions, the power quality issues such as neutral-point potential (NPP) unbalance, zero sequence circulating current (ZSCC), dc bus voltage fluctuations, grid current distortion, and inaccurate sharing of current must be considered to ensure high-quality operation of parallel 3L-NPC rectifiers. A controlmethod with carrier-based pulse width modulation (PWM) for improving power quality of parallel 3L-NPC rectifiers is proposed in the stationary reference frame. Themodel of parallel 3L-NPC rectifiers is established under unbalanced operation conditions. To simultaneously improve the power quality of dc bus voltage and grid-connected current, an improved control strategy with voltage outer-loop proportional integral multiresonant (PIMR) controller and current innerloop proportional feedback multiresonant (PFMR) controller is proposed. Meanwhile, a coordinated control method of ZSCC and NPPB with additional feedforward is proposed to overcome the coupling problembetween them. The dual control attribute and parameter design method of PFMR controller are analyzed. Finally, an experimental platform of parallel 3L-NPC rectifiers is established based on dSPACE 1007, the experimental results verify the effectiveness of the proposed controlmethod.