Research And Implementation Of Control Strategy For Three-Phase Photovoltaic Grid-Connected Inverter Based On Quasi-Z-Source

With the increasing use of distributed renewable energy in power systems,grid-connected inverter,as the interface equipment between distributed generation and distribution network,will gradually occupy a dominant position in the future power system.Its performance directly affects the utilization efficiency of distributed energy such as photovoltaic and wind power.Compared with the traditional two-stage inverter,the quasi-Z-source inverter can realize the function of boosting and inverting simultaneously in a single conversion structure because it does not need to insert dead time and only needs to control its unique shoot-through switch state,which is conducive to reducing the cost of distributed generation and improving the efficiency of power transmission.Therefore,this thesis mainly focuses on quasi-Z-source inverter and its application in photovoltaic grid-connected system.(1)Based on the analysis of the working principle of the quasi-Z-source inverter,the small-signal interference model of the quasi-Z-source network is established by using the state space averaging method.The steady and transient performance of the system is analyzed by drawing the zero pole distribution diagram of the system with MATLAB,which provides a theoretical basis for the design of system controller and the component parameters tuning of quasi-Z-source network.(2)The traditional continuous three-bridge arm straight-through,single-bridge arm straight-through and discontinuous modulation strategy of quasi-Z-source inverter are studied.In view of the shortcomings of the traditional quasi-Z-source inverter,such as the excessive number of modulation signals,the complex generation of the drive signals,and the excessive number of commutations in a single fundamental cycle,an improved noncontinuous single arm through SVM modulation method is proposed.The modulation strategy greatly simplifies the generation of driving signal and reduces the conduction loss of the switch by simplifying the generation of reference signal and discontinuous switching.(3)The control strategy applied to the quasi-Z-source grid-connected inverter is studied,and the application of model predictive control(MPC)in quasi-Z-source inverter is analyzed in detail.In view of the specific application background of photovoltaic power generation and considering the reactive power compensation characteristics of distributed generation when connected to the large grid,a decoupling active/reactive power grid-connected predictive control strategy is proposed.This control method can effectively simplify the PWM modulation generation,improve the dynamic response speed and reference tracking performance of the system,and reduce the design cost and regulation complexity of the controller.(4)A 10kW three-phase photovoltaic grid-connected inverter experimental platform based on quasi-Z-source is designed.The dual core cpu control structure based on DSP and FPGA is adopted.The selection of device parameters such as inductance,capacitance,and power switch tube in the main circuit is introduced.Based on the MPPT control of PV side and the realization logic of the predictive control algorithm proposed in this paper,the related hardware circuit and software program are designed.The experimental results show that the quasi-Z-source inverter can realize the tracking of PV maximum power point and accurate reactive power compensation for distribution network and local load,with fast dynamic response and reference tracking performance.