Impedance Shaping of LCL -Type Grid-Connected Inverter to Improve Its Adaptability to Weak Grid

As the penetration of distributed power generation systems goes high, the power grid exhibits more like a weak grid which features a large set of grid impedance values and varieties of background voltage harmonics. The grid impedance can change the loop gain of the grid-connected inverter and thus challenge the control performance or even lead to system instability. The background voltage harmonics will distort the grid current and result in a poor power quality. Therefore, the grid-connected inverter should be designed with strong stability robustness and high harmonic-rejection ability, both of which correlate closely with the inverter output impedance. To shape the inverter output impedance, an impedance shaping method is proposed in this chapter, which introduces a virtual parallel impedance and a virtual series inductor. The virtual parallel impedance is used to maximize the magnitude of output impedance, and meanwhile, a virtual series inductor is used to boost its phase. Thus, the grid-connected inverter exhibits strong rejection ability of grid voltage harmonics and achieves strong stability robustness against the variations of the typical inductive-resistive grid impedance. Experimental results of a 6-kW single-phase grid-connected inverter with LCL filter confirm the effectiveness of the proposed method.