Research And Design Of Three-phase PWM Rectifier In Modular Power Supply

Power supplies have been used in various fields of our society and the rectifiers are the important part of the power supplies.However,the traditional uncontrolled and phase-controlled rectifiers bring a lot of harmonic pollution to the power grid.The three-phase PWM rectifier has the advantage of low current harmonics on the AC side and working under the unit power factor,which will realize high power quality and green conversion.With the demand for higher power system capacity and reliability,modular power systems based on parallel connection of multiple devices have been the research focus.Because of the advantages of large capacity,flexibility and high reliability,modular power systems have broad application prospects in aerospace,new energy vehicles,and finance.Therefore,the three-phase PWM rectifier and its parallel system have been studied in this thesis.Firstly,the thesis firstly analyzes the topology structure and working principle of three-phase PWM rectifier system.The mathematical models of the system in three-phase stationary coordinate system and two-phase rotating coordinate system are established.The methods on how to select apposite parameters of the inductors and capacitors are designed.The control strategy of double loops with PI controllers was analyzed.According to the theory of automatic control,the parameter selection of PI controller is designed,and the three-phase PWM rectifier system is verified by Matlab/Simulink software.Secondly,the definition and calculation of the instantaneous power of the PWM rectifier are described and the direct power control technology is studied.By analyzing the principle of hysteresis-based direct power control and SVPWM-based direct power control,the control block diagrams are designed and the disadvantages of the traditional direct power control technology are discussed.A direct power control strategy based on predictive algorithm is proposed in this thesis and the model of the prediction algorithm under the condition of low switching frequency is improved.The simulation software verifies the improvement effect of the direct power control technology strategy based on prediction.Thirdly,the basic principle of space vector pulse width modulation is expounded.The causes of the dead zone and the commonly strategies which reduce the dead zone effect are studied.A modulation strategy combined the traditional SVPWM with the single switch control SVPWM is proposed in this thesis.By detecting the polarity of the current,it is ensured that only one switch tube in the upper and lower arms has the drive signal.The problem of polarity detection when the current crosses zero spot is studied.A threevector synthesis method is designed for the situation of the single-phase current zero-crossing.The traditional SVPWM modulation strategy is adopted for the situation of single-phase current zero-crossing and multiphase current zero-crossing.The simulation verifies the correctness of the hybrid modulation strategy.Finally,the parallel system of two three-phase PWM rectifiers is studied.The topology of the parallel system and the causes of the zero-sequence circulation are analyzed.The commonly used parallel current sharing control strategies are analyzed.Then,a parallel system control strategy based on common power control and zero-sequence current control is proposed in this thesis.A common external power loop control is applied to keep the reference values of the inner loops of the modules equal.The zero-sequence current is suppressed by adjusting the zero vector proportion by using the PI regulator.The simulation verifies the feasibility of the parallel control strategy.