Cost-Effective DC Current Suppression for Single-Phase Grid-Connected PV Inverter

Due to the disparity of power modules, asymmetry of driving pulses and measurement errors of sensors, dc currents may be injected to grid-connected photovoltaic (PV) inverters. The dc current injection may cause magnetic saturation of the power transformers. To solve this issue, this article thus proposes an effective current control strategy and compensation method, which does not require any extra sensor and hardware circuit. First, the root-cause of dc current injection is comprehensively analyzed. Subsequently, a proportional-integral-resonant (PIR) controller is proposed to eliminate the dc component caused by disparity of power modules, asymmetry of driving pulses and measurement errors of grid voltage. The injected dc current caused by grid current measurement error is estimated from the line-frequency ripple of the dc-link voltage, and then it is suppressed by a feedback compensation controller. In addition, the dc current rejection capability is evaluated, and the proposed method is benchmarked with the virtual capacitor-based method. Finally, experimental tests are performed on a 1.2-kW single-phase PV inverter to verify the effectiveness of the proposal.