A grid-connected PV system with LLC resonant DC-DC converter

This paper presents a grid-connected photovoltaic (PV) system with a three level voltage source converter (VSC) using double closed loop control strategy. The outer DC voltage control loop is to regulate the DC bus voltage, and the inner current control loop is to synchronize the output current with the grid voltage, thus ensuring unity power factor. A LLC resonant DC-DC converter is used in the proposed system to step-up the voltage of the PV array and to extract maximum power. It is intrinsically isolated by a high frequency transformer, then the parasitic capacitance of the PV panels to ground could not be of concern; furthermore, because of soft switching technique, it operates at high frequency with low switching losses. Size and cost of the magnetic components and DC-link capacitor are decreased compared with traditional boost converters. An incremental conductance method integrated within PI controller was used to extract maximum power by PV. Simulation results based on MATLAB/Simulink verify the validity and dynamic performance of the proposed system during fast solar irradiation changes and ensure DC bus voltage stability.