Study On Pulse Width Amplitude Modulation Of Three Phase Quasi-Z-Source Inverter And Its Grid-Connected Behaviors

Compared with the conventional inverter, quasi-Z-source inverter achieves the buck-boost function because of its unique impedance network. It allows inverter bridge arms shoot-through or short, and it also overcomes the deficiencies of Z-source inverter such as large starting inrush current, discontinuous input current etc. Quasi-Z-source inverter is more suitable for application in photovoltaic, wind power, new energy vehicles etc which the input voltage changes large. It is of theoretical value and practical significance to further study of quasi-Z-source inverter.Pulse width amplitude modulation(PWAM) and the structure of LCL passive filter in quasi-Z-source inverter are studied in the thesis to simplify the DC bus voltage control method, and to reduce the parameters of passive components in quasi-Z-source network and power losses.The thesis describes the limitations of conventional inverter and the characteristics of quasi-Z-source inverter first. It introduces the research status of quasi-Z-source inverter in the following four aspects:topological structure, modeling, control strategy and applications then. Secondly, the principle of quasi-Z-source inverter is introduced. The state space model and small signal model of quasi-Z-source network are established by the state space average method. The transfer function from the straight duty cycle to the capacitor voltage in the quasi-Z-source impedance network is derived. The impact of the parameters change in quasi-Z-source network and the straight duty cycle change are analyzed combining with the pole-zero distribution. It provides a theoretical guidance for selecting capacitance and inductance parameters.The principles of PWAM and the maximum straight zero vector control are presented. The two are combined because PWAM needs the voltage fluctuation in the inverter DC bus and maximum straight zero vector control can generate it. An input voltage feed-forward and the output voltage feedback control is proposed and the closed-loop controller is designed. The validity of the control method is proved by the simulation results when the input voltage and the load change.The mathematical model of LCL quasi-Z-source grid connected inverter is established. The split capacitor passive damping control is proposed which divides the filter capacitor into two small ones and connects a damping resistor in series with one of them. The stability and damping resistor power losses at different capacitance division and damping resistor selection are analyzed and the inverter-side current decoupling control is designed. The working performances under the condition of steady-state, grid current mutation, input voltage transient and grid voltage mutation are analyzed in the simulation.Finally, a three-phase quasi-Z-source inverter is built. The design process of the prototype is introduced including the selection of parameters in quasi-Z-source impedance network, the design of LC filter, the selection of the power devices and the design of hardware and software in the control circuit. The experimental results verify the correctness of the usage of maximum straight zero vector control in the impedance network and PWAM in the inverter bridge.