Research On Topology And Control Strategy Of A Half-Controlled Three-Level PWM Rectifier

At present, the three-level Neutral-Point Clamped (NPC)-converter is highly concerned in application research of the medium and high voltage conversion technology. And improving power factor, reducing the input and output current harmonics and balancing neutral-point potential are the key problems .This paper proposes a new three-level rectifier topology and provides the control strategies that can raise the power factor, reduce input and output current harmonics and balance the neutral-point potential.The half-controlled three-level rectifier improves the main circuit topology of the traditional three-level NPC-rectifier and reduces switching devices on rectifier side by half. This paper compares the improved topology with 12-pulse rectifier and the traditional NPC-rectifier and then establishes the mathematical model and equivalent circuit of the new topology.The one-cycle control strategy is proposed here to improve power factor, reduce the input current harmonics and stabilize the output voltage of the rectifier, taking into account of the simplicity and reliability. Then the improved unipolar one-cycle control strategy against the bipolar one-cycle control is proposed and the comparative analysis between the two control strategies is provided in terms of their stability. Simulation results show that the half-controlled three-level rectifier topology and its control strategy can realize power conversion with high power factor, low input and output current harmonics under medium and high voltage.Finally, an integrated method combines the one-cycle control and the Neutral-point balancing (NPB) control is proposed to better solve the problem of NPB, avoiding the compromising of the converter performance caused by NPB on the inverter side.