Impedance Reshaping for Grid‐Connected Inverters via Coordinated Design of Voltage Feedforward and PLL in Weak Grids

ABSTRACT Grid‐connected inverters (GCIs) typically employ a phase‐locked loop (PLL) to achieve synchronization with the grid, whereas voltage feedforward is used to mitigate grid voltage disturbances and improve current quality. In weak grids, however, the interaction between these control loops may degrade stability. To this end, an output impedance model of the GCI that incorporates both the PLL and voltage feedforward is established, and their impact on system stability is systematically analyzed. Accordingly, a voltage feedforward structure with phase‐lead compensation is proposed. Through appropriate parameter design, the system phase characteristics are improved while harmonic suppression capability is maintained. Nevertheless, as the grid impedance continues to increase, using only the proposed voltage feedforward provides limited improvement in stability. Therefore, a reconfigured PLL with an all‐pass filter (RPLL‐APF) is introduced to further enhance system stability. Experimental results demonstrate that the coordinated design of the PLL and voltage feedforward enables stable operation and maintains satisfactory current quality over a wide range of grid impedance variations.