Small Signal Model and Modified Control Strategy for Three-phase Four-wire Three-level Rectifier

Based on the analysis of small signal AC model of three-phase four-wire three-level diode neutral-point clamped (NPC) rectifier, a current control method of PI+resonant controller based on complex vector decoupling is proposed, aiming at the sinusoidal current of power grid and the unit power factor. In a dq0 coordinate system, the dq components of the grid currents are cross-decoupled, and PI controllers of current loops are designed by the zero-pole cancellation method. In order to suppress negative sequence and third harmonic currents, which are caused by device parameter dispersion and unbalanced grid-side voltage, the resonant controller with two times fundamental frequency is connected in parallel with PI controllers. On the other hand, in order to solve the problem of unbalanced voltages in DC buses of rectifier caused by unbalanced loads, the voltage difference between the positive and negative buses is used as feedback to establish a voltage sharing control loop based on the total voltage control. At last, a 10 kW experimental platform of inverter fed by three-phase four-wire NPC rectifier is established. The experimental results show that currents on the grid side are superior in quality,and the voltages of both positive and negative DC buses are stable and balanced by the use of the proposed strategy. © 2021, High Voltage Engineering Editorial Department of CEPRI. All right reserved.