Research Of Three-level PWM Rectifier Based On Direct Power Control

Diode-clamped three-level PWM rectifier has many advantages like small THD of current, ability of two-way energy flowing, arbitrary AC power factor, etc. The structure characteristics of diode-clamped three-level PWM rectifier ensure the power switch tube is worth only a half maximum voltage of DC bus in the steady state, which makes it has a good prospect in high-voltage transmission and distribution, frequency control for high voltage and power motor, etc.First, the topology structure of diode-clamped three-level PWM rectifier is analyzed in detail. According to the kirchhoffs law, the mathematical model of the diode-clamped three-level PWM rectifier is given in static abc coordinates. In order to further study its control characteristics, the definition of space vector is given. The distribution of the 27 space voltage vectors and the influence of different vector on neutral voltage are analyzed. Then a simple and feasible SVPWM modulation algorithm based on volt-second balance principle is given. The mathematical model of the rectifier in rotating dq coordinates is given, providing the theoretical basis for the design and implementation of direct power control.Second, the mathematical model of the rectifier based on instantaneous power theory is given. According to this model, the direct power control methods based on hysteresis comparison and grid-side voltage orientation are studied. The method of making vector-table of direct power control method based on hysteresis comparison is given. Because the direct power control method based on hysteresis comparison has many disadvantages, combining with SVPWM modulation method, the direct power control method based on grid-side voltage orientation is given, in which the vector-table is replaced by PI decoupling controller. The control effects of the two methods are compared by using simulation software.Third, the importance of phase-locked loop (PLL) in the direct power control method based on grid-side voltage orientation is analyzed. The influence on grid-side current by phase lag of PLL is analyzed, and two software algorithms based on three grid-side voltages and phase voltage are given. The feasibility and accuracies of them are shown using simulation software.Finally, the performance of the direct power control method based on grid-side voltage orientation is checked by experimental platform based on TMS320F28335. The design of reactor, hardware control circuit, software program and principal computer is given. According to the experimental results, it can be seen that the method is stable and reliable, and has a good control effect.