Study On Three-phase Cascaded Quasi-Z-source AC-AC Converter

With the rapid development of technology and economy,the requirements for power quality in industrial production,daily life are becoming more stringent.Improving power conversion efficiency and power supply quality has become a hot issue.The AC-AC chopper has the characteristics of simple structure,fast response,good filtering effect and so on.As a voltage regulator,it has a broad development prospect in power grid voltage stabilization,large-scale voltage regulation.The two kinds of three-phase cascade Quasi-Z-source AC-AC converters proposed in this paper have very good effects in voltage gain ability,reducing voltage stress,avoiding a large number of high-order harmonics,and have good research value in the need of large-scale voltage regulation and grid connected application.First,this thesis analyzes the working principle of the converter topology,conducts a detailed steady-state analysis of the two converter topologies,and compares the performance of the two converters in terms of voltage gain capability,capacitor voltage stress,and whether the input current is continuous.It can be seen that converter topology I has the advantages of continuous input current and less capacitor voltage stress,while converter topology II is more advantageous in terms of boost capability.Secondly,the dq transformation method and the state space averaging method are used to establish the small-signal model of the three-phase cascade Quasi-Z-source AC-AC converter.The transfer function is derived through the mathematical model of the converter and its dynamic characteristics are analyzed.The steady-state characteristic analysis and dynamic characteristic analysis verify the theoretical reliability and correctness of the converter topology.Then,the overall design of the system is carried out,mainly the demonstration of the system control method and the design of the closed-loop control system.In this thesis,PWM pulse conduction mode is used to control the complementary conduction of switching devices.The conduction strategy is simple and reliable.The voltage output can be adjusted by adjusting the duty cycle(D)of switching devices.In this thesis,the voltage and current double closed-loop instantaneous value feedback control is adopted,and the stability of the system is greatly improved by the closed-loop control.The parameters of cascade Quasi-Z-source structure,output filter circuit and other parts of the system are designed to make each structure complete and optimize the overall stability of the system.Finally,the theoretical analysis of the system is verified.Firstly,the simulation model of the system is built by using MATLAB/Simulink simulation software.By analyzing the input and output voltage parameters of the simulation model,the correctness of the theoretical analysis of the two three-phase cascade Quasi-Z-source AC-AC converters is verified.And the physical prototype is built,and the correctness of theoretical analysis and simulation model is verified again through experiments.