Research On Soft-Switching Technology Of Photovoltaic Grid-connected Three Phase Micro-inverter

Micro-inverter has lots of advantage, such as Enhanced energy production, high reliability, easy to installation and management. The relatively high cost of the Micro-inverter is the biggest barrier to its large scale deployment. Increasing the switching frequency may be the best way to reduce cost by shrinking the size of reactive components and heat sink. However, this approach could cause conversion efficiency to drop dramatically. Problem could be solved if a proper soft-switching technique is employed.This paper took the two-stage three-phase grid-connected inverter as research object, and analyzed its mathematical model and control mechanism in-depth, discussed the resonant full-bridge LLC resonant converter-type three-phase grid-connected inverters and optimized design method, and then proposed a control strategy of variable-frequency variable-duty-cycle based on direct power.The thesis from compares and analyze the existing DC-DC and DC-AC converter topologies in terms of the control complexity, the conversion efficiency and cost, topologies of pre-and full-bridge LLC resonant converter stage using resonant pole inverter two is adopted. The working principle of achieving zero-voltage switching of these two topologies is analyzed. Then FHA model of LLC resonant converter and mathematical model of resonant pole inverter are established. Parameters of full bridge LLC resonant converter been designed. Parameters of the three-phase grid-connected micro-inverter are analyzed using the models established and obtain conclusion the effects of parameters of the three-phase grid-connected micro-inverter conversion efficiency on conversion efficiency.The traditional incremental conductance MPPT algorithm improved since in full bridge LLC resonant converter controller changes the frequency rather than the duty cycle. Variable frequency variable duty cycle modulation was taken into the direct power control. This strategy make the MOSFET of resonant pole inverter zero-voltage turn on. The relevant test results of simulation and experimental prototype verified the effectiveness of the control strategy in this paper.