Three-level Pwm Rectifier Control Method Study

The three-level neutral-point-clamped PWM rectifier has been widely studied and applied in industry because they can achieve a large-capacity high-voltage power conversion, and have a good sinusoidal input current, DC voltage and the grid side of the power factor can be adjusted, fast dynamic response, and energy can flow in both directions. In this paper, three-level neutral point clamped PWM rectifier as the research object, analyzes the main circuit topology, deduces three-level PWM rectifier model and studies the three-level PWM rectifier control strategy. This paper mainly includes the following:Three-level neutral point clamped PWM rectifier model is the basis of controller design. This paper clarifies the three-level rectifier voltage and current double closed loop control theory by establishing a mathematical model, achieves a current of the decoupling control, and carried out the current regulator and voltage regulator design parameters.High-performance control system determines the three-level PWM rectifier performance. To improve the dynamic performance of the rectifier, this paper presents three-level PWM rectifier direct power control strategy. After analyzing the three-level PWM rectifier in active and reactive power and the relationship, for the direct power control weakness that switching frequency are not fixed, a sinusoidal pulse width modulation based on a fixed switching frequency of the three-level PWM rectifier direct power control strategy is adopted.The imbalanced three-phase grid voltage will seriously affect the three-level PWM rectifier performance. By modeling the system under the unbalanced input voltages, the instantaneous active and reactive power in the PWM rectifier are calculated, a dual current control scheme which using the positive and negative sequence current as references is adopted. When adopting the dual current controller, the odd and even-order harmonics in the input currents and in output voltage can be reduced effectively to the level similar to the case under balanced input, the input average reactive power of the rectifier is zero and the operation of the unity power factor is achieved.Finally, the experimental system of three-level NPC PWM rectifier is built to verify the above research results. By designing the control circuit based on DSP and CPLD as well as programming the control software correspondingly, the digital control of three-level PWM rectifier is achieved.