Second Harmonic Current Reduction in Front-End DC−DC Converter for Two-Stage Single-Phase Photovoltaic Grid-Connected Inverter

The instantaneous output power of the two-stage single-phase grid-connected photovoltaic (PV) inverter pulsates at twice the line frequency (2f_o), generating second harmonic current (SHC) in the front-end dc-dc converter and PV panel, which will affect the maximum power point tracking operation and deteriorate the overall conversion efficiency. The generating mechanism of the SHC is analyzed in this paper and it is pointed that in order to eliminate the SHC in the front-end dc-dc converter and PV panel, the voltage loop gain of the front-end dc-dc converter should be high enough at 2f_o. Since there is a -180° phase abrupt at the resonant frequency of the input side filter capacitor and the inductor, the system may be unstable. To cope with this problem, the inductor current feedback active-damping scheme (ADS) is adopted. For further improving the loop gain at 2f_o, proportional- integral-resonant regulator with ADS (PIR + ADS) is proposed in this paper. Besides, a step-by-step closed-loop parameters design method is presented. Finally, a 3-kW two-stage single-phase grid-connected PV inverter has been fabricated and tested, and the experimental results verify the feasibility of the proposed control schemes.