Research On Flyback Photovoltaic Grid-connected Micro Inverter And Its Control Strategy

In recent years,the consumption of natural resources represented by fossil energy has been rising.Largescale use of it not only has aggravated global environmental pollution,but also made fossil energy in a state of exhaustion.The development of new clean energy represented by photovoltaic power generation has become an effective solution to relieve environmental pollution and energy crisis.Nowadays,photovoltaic power generation has been improving,and various photovoltaic grid-connected power modes coexist.Considering the fact that traditional grid-connected methods have the disadvantages of poor scalability,low reliability and local shadow,the micro-inverter grid-connected power technology is proposed and applied.Firstly,taken the factors about the application,production cost and control method of micro-inverter into account,the flyback converter and the full-bridge circuit are selected as the pre-stage and post-stage circuits of the micro-inverter respectively.Specifically,the flyback converter adopts an interleaved parallel topology,which can reduce the output current ripple and improve the output power of the circuit.In order to reduce the voltage stress of the switch tube at the moment of switching,the active clamp circuit is added in the flyback circuit to absorb the leakage inductance energy of the transformer.Meanwhile,in this paper,the control modes of the three working modes of the flyback converter are compared and analyzed,and the discontinuous conduction mode(DCM)is finally selected as the working mode of the inverter.Secondly,the simulation model is built is carried out based on the equivalent circuit of the photovoltaic cell and the obtained characteristic curve is analyzed.Meanwhile,the pros and cons of the traditional maximum power point tracking(MPPT)algorithms are analyzed.Consequently,the particle swarm optimization(PSO)algorithm is introduced as the MPPT control strategy to achieve global optimization.In order to verify the effectiveness of PSO algorithm,the simulation model is built to simulate the changing conditions of the environment.The experimental results show that the PSO algorithm can track the maximum power point more efficiently in the case of the system startup phase and the sudden change in environment.Thirdly,the small-signal model of the flyback micro-inverter is built,and its stability is analyzed.Meanwhile,a current closed-loop controller based on quasi-PR control is designed.The current sharing control is designed to balance the two currents for the interleaved parallel topology.The non-detection zone(NDZ)of the grid-connected inverter based on the traditional active frequency drift(AFD)algorithm is analyzed in detail.Additionally,the simulation model is established based on improved AFD algorithm and the experimental results prove that the improved algorithm can detect the island state more quickly and reduce the THD of grid-connected current when the inverter is working properly.Finally,the parameter of the relevant components is calculated,and the hardware circuit and control software of the inverter are designed.Furthermore,an experimental platform with 300 W is built,and the feasibility of the design is verified based on the experimental platform.