Design Of Small And Medium Power String Two-stage Non-isolated Grid-connected Inverter

This paper takes the small and medium power household string inverter as the research object,starting from the system’s basic principle,topology,control strategy,etc.,focusing on the inverter’s maximum power point tracking and grid-connected current control technology.The specific content of this article includes:Firstly,the overall design scheme of the inverter is determined.The front-stage boost circuit adopts the Boost structure,and the latter-stage inverter circuit adopts the H6 topology structure for the defect that the traditional full-bridge inverter cannot suppress the commonmode leakage current.According to the technical indicators,the parameters of the main components in the circuit are calculated,and the H6 inverter circuit is simulated based on the calculated parameters to verify its inhibitory effect on the common mode current.Secondly,the control strategy of the front and rear stages of the two-stage inverter topology is analyzed and designed.Through the analysis of the output characteristics of the photovoltaic module model,combined with the step size selection problem existing in the traditional MPPT algorithm,a segmented adaptive inverter is proposed.The step-size conductance incremental method is used to simulate the good dynamic performance and steadystate performance of the algorithm.The transfer function model of the post-stage inverter circuit is established,and the mathematical model of the commonly used grid-connected control strategy is used for comparative analysis.The grid-connected control strategy of the resonant controller is verified,and its suppression effect on current harmonics and tracking effect on grid voltage are verified by simulation.Then,on the basis of simulation analysis,the software and hardware of the inverter are designed.The TMS320F28335 chip is used as the main control chip,and the main circuits such as AC and DC sampling circuits,front and rear drive circuits,and overcurrent protection circuits are designed.The software part is mainly analyzed.The main program,interrupt program,SPWM program and other parts are designed.Finally,the experimental platform of the inverter system was built to test the waveforms of the front and rear drive signals,DC bus voltage,grid-connected voltage and current,and the power tester was connected to analyze the power of the inverter.The effective value of the output voltage was 221.1V and the power was 4.94 KW.which basically meets the design requirements,the frequency and phase of the grid-connected current are basically the same as the grid voltage,the total harmonic ratio of the output current is less than 5%,and the quality of the grid-connected current is good,it verifies the effectiveness of the control strategy proposed in this paper in terms of voltage and current tracking,harmonic suppression,etc.