Research On Flyback Micro-Inverter For Grid-connected Photovoltaic System

With the exhaustion of fossil energy reserves and the aggravation of environmental pollution problems,efficient and clean photovoltaic power generation technology has gradually become a research hot spot.As the connection between photovoltaic cells and power grid,inverter realizes DC to AC inverter process.The micro-inverter has many advantages,such as small size,flexible installation,simple expansion,etc.It can be connected one-to-one with the photovoltaic cells,so that each cell can work at the maximum power point,which improves the conversion efficiency of the entire photovoltaic system.In this thesis,the research on micro-inverter is carried out.The MPPT,power decoupling and inverter grid-connected function of photovoltaic system are studied by using flyback micro-inverter in low power application.The inverter has a simple topology and flexible control,and can simultaneously implement boosting and electrical isolation.According to the conversion efficiency,service life and grid-connected quality of the system,the thesis studies the system model,topology and control strategy.The specific research contents are as follows:(1)To solve the problem that the flyback inverter in discontinuous mode cannot find the maximum power point by controlling the input voltage.To solve the problem that the flyback inverter in intermittent mode can not find the maximum power point by controlling the photovoltaic voltage,an adaptive step-size MPPT method based on current is improved based on the conductance increment method.When the tracking point is closer to the maximum power point,the current disturbance step size is a smaller value,which reduces the oscillation phenomenon of the tracking point.When the tracking point is far from the maximum power point,the current disturbance step size is a fixed larger value,which improves the tracking speed of the tracking point.In the PSIM software,a common variable step size conductance increment method and the MPPT method proposed in this thesis are simulated and compared.It can be verified that the MPPT method proposed in this thesis has faster response speed,smaller steady-state oscillation and higher conversion efficiency.(2)For the secondary power disturbance phenomenon at the input of the flyback inverter,the conventional filtering method has parallel electrolytic capacitors in the front stage of the inverter,and the decoupling performance of the electrolytic capacitor is limited by the capacitance value and the life limit.This thesis proposestwo new decoupling circuits.Through the design of the circuit,the decoupling circuit increases the voltage across the decoupling capacitor,so that the film capacitor with small capacitance and long life can be used to replace the electrolytic capacitor with large capacitance and short life.The new decoupling circuit extends the life of the system while filtering out power disturbances.In the PSIM software,the connection control method is realized by the current peak control method.By comparing the grid-connected waveforms of the two new decoupling circuits,it can be seen that the second decoupling circuit proposed in this thesis has a smaller grid current distortion and better decoupling performance.(3)Based on the hardware and software design of the photovoltaic system,a100 W prototype is built to test grid-connected quality and efficiency of the flyback photovoltaic system with the second decoupling circuit in parallel.