Multi String Grid-Connected PV System with LLC Resonant DC/DC Converter

This paper presents a grid-connected multi string photovoltaic (PV) system with a three level voltage source converter using double closed loop control strategy. The outer DC voltage control loop regulates the DC bus voltage while the inner current control loop synchronizes the output current with the grid voltage, thus ensuring unity power factor. In the proposed configuration, LLC resonant DC/DC converter is used to extract the maximum power and to boost the PV array voltage. It is intrinsically isolated by a high frequency transformer, so that the parasitic capacitance of the PV panels to ground could not be of concern; furthermore, because of soft switching technique, LLC converter 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 with PI controller is developed on LLC resonant converter to extract maximum power. Simulation studies confirm that the control design approaches taken are robust and the dynamic performance of the proposed system during fast solar irradiation changes is acceptable.