A Study On Three-phase PWM Rectifier Based On Hybrid-clamped Five-level Topology

Due to the particular advantages, multilevel PWM rectifier has been a research hotspot in the field of high-voltage and high-power active AC-DC conversion. However, for the diode-clamped (Neutral-Point-Clamped, NPC) topology, the problem of DC-link capacitor voltages imbalance has been the difficult point. For the three-level topology, mature solutions have been put forward, and wide applications can be seen in the industrial field. Unfortunately, there are no practical methods solving this problem in the topology of higher than three-level, consequently, the research of PWM rectifier has been mostly limited to three-level.This paper adopts a hybrid-clamped five-level topology applying in PWM rectifier, which can balance the dc-link capacitors voltages naturally, so the control algorithm can be simplified greatly, and the mature control method for two-level or three-level PWM rectifier can be applied in the five-level PWM rectifier.The paper firstly analyzes the operating principle of hybrid-clamped five-level PWM rectifier and the circuit working modes when it is working in the condition of unit power factor. In allusion to the selection of ac-side input inductance, which is an important parameter for voltage-source rectifier (VSR), the paper deduces the theoretical equation based on the particular alternative hybrid PWM (AHPWM) modulation method. Meanwhile, due to the existence of the switched-capacitor circuit, different capacitance of the dc-link will be presented in a three-phase system. The paper deduces the function equation between the dc-link equivalent capacitance and the modulation index.Based on the hybrid-clamped five-level topology, the paper analyzes the mathematical model of PWM rectifier, and deduces the control algorithm of direct current control. Then the control principle of two-loop control is expounded, and the design procedure of the control system is presented, moreover, the strategy of input voltage feedforward control for limiting start inrush current is analyzed.Furthermore, the paper analyzes the commutation modes of the hybrid-clamped five-level PWM rectifier in nonideal conditions in detail. All the commutation modes under the condition of unit power factor control are divided into five types, and the reason of voltage and current overshoots on the devices caused by commutation is analyzed based on the experimental data. Then the paper summarizes all the commutation modes and analyzes the factors influencing reliable operation of the system.A prototype of the three-phase hybrid-clamped five-level PWM rectifier was built up in the lab, and the experiments were carried out under the resister load and inverter load (back-to-back converter). The experimental results show that the five-level PWM rectifier designed in this paper can guarantee the dc-link capacitor voltages balanced and stable, restrain the distortion of the ac-side input current effectively and realize power factor correction.