Weighted-Feedforward Scheme of Grid Voltages for the Three-Phase LCL -Type Grid-Connected Inverters Under Weak Grid Condition

The full-feedforward scheme of grid voltages for three-phase LCL-type grid-connected inverter has been introduced in Chap. 7, and the injected grid current harmonics and imbalance caused by grid voltages can be effectively suppressed. However, during the derivation of the full-feedforward scheme, the grid impedance is assumed to be zero. For weak grid condition, as the grid impedance becomes large, the full-feedforward scheme might cause the instability of the grid-connected inverter. In this chapter, the stationary α-β frame-controlled three-phase LCL-type grid-connected inverter with the full-feedforward scheme is taken as the example to analyze the system stability under weak grid condition, and the weighted-feedforward scheme is proposed to achieve a trade-off between the stability and the harmonic suppression. With the weighted-feedforward scheme, the extracted and weighted grid voltages are fed forward, and the injected grid current harmonics can be effectively suppressed while the stability of grid-connected inverter under weak grid condition is also guaranteed. Finally, the experimental results verify the analysis in this chapter.