A Neutral Point Clamped Non-isolated PV Inverter With A Freewheeling Switch

Non-isolated PV inverter has the advantages of small volume, low cost and high conversion efficiency.In order to meet the requirement for common-mode leakage current in photovoltaic(PV) generation systems, a neutral point clamped non-isolated PV inverter with a freewheeling switch is proposed. A freewheeling switch is added in AC side of the full bridge inverter in this topology. That will improve the non-isolated PV inverter conversion efficiency because the freewheeling switch makes the output of the solar panels off the grid. On the other hand, common-mode voltage remains constant on power processing and freewheeling stage owing to the neutral point clamped circuit. The operational principle and controlling strategy were analyzed in this paper. It was proved that common-mode performance of the proposed topology was good by Saber simulation software.System uses a voltage and current double closed-loop controlling method, which use the difference between the reference voltage and the sampling voltage from inverter as the input voltage of PI controller of voltage loop, and error signal after PI control as a reference voltage of PI controller of current loop. Inductor current sampling signal go by I/V conversion, and Subtract with the reference signal of the inner current, and the error signal turns into the current error signal after the current loop, which as the modulation wave intersects the carrier to get the control signals of all the switch tubes. This control method is simple in hardware circuit, and it has a good dynamic response.The proposed topology and strategy has been verified in a prototype rated at 300 W.