Application Of Neutral Point Voltage Balance Control Of Three-level Power Converter In Active Rectifier

At present, three-level converter has been more and more widely studied and applied in the field of application of different voltage levels because of large output capacity and low harmonic content. This paper focused on the key technologies of diode neutral-point-clamped three-level PWM rectifier.According to the physical model of three-level PWM rectifier and its basic working principle, this paper established its mathematical model in the three-phase static coordinate system. Through the coordinate transformation and the feed-forward decoupling, the mathematical model in two-phase synchronous rotating coordinate system was obtained, providing the theoretical basis for instantaneous control of converter. At the same time, the space vector PWM modulation algorithm of three-level converter was analyzed, and the sector judgment, the synthesis time calculation and switching state distribution were introduced to analyze control strategy formation of SVPWM converter.The balance of three-level neutral point potential is the basis for the stable operation of three-level PWM rectifier. In view of inherent imbalance problem of neutral point potential for three-level converters, this paper analyzed neutral point potential unbalance reasons mainly from switching state and the neutral point current, and make in-depth study of the impact of modulation index and power factor on neutral point current. At the same time, research and analysis of the three kinds of control strategy for three-level neutral point potential balance based on small vector control were made. Through comparative analysis of the simulation results, the correctness of the theory was verified.According to the requirements of the harmonic current of three-phase PWM rectifier when connected to the power grid, grid-side LCL filter model was established, the main factors that influenced the grid-side current harmonic was analyzed, and the constraint conditions of filter design was obtained based on the suppression of harmonic current and the decrease of the resonance effect. Through the research on two-level and three-level converter ripple peak current, comparative analysis of the requirements of filter inductance was obtained, and draw the conclusion that under the same parameters, three-level converter can achieve higher power density.Finally, the feed-forward decoupling current and voltage double closed-loop control scheme based on the combination of neutral point voltage balance control strategy was studied. Finally, through the Simulink module in MATLAB, a series of full digital discrete simulation PWM rectifier was analyzed in the inverter and the rectifier mode at different operating points, verifying the relationship between three-level neutral point and the modulation index and power factor, providing the basis for the design of three-level PWM rectifier at the optimal operating point.