Impedance-based stability studies of hybrid-synchronization-controlled grid-forming inverters

To improve the transient stability of the grid-forming (GFM) inverters, the hybrid power-and voltage-based synchronization control has attracted widely attentions in recent years. To further recognize influences of the hybrid synchronization contro l (HSC) on the small-signal stability of GFM inverters, impedance models of the hybrid-synchronization-controlled grid-forming (HSC-GFM) inverters are built firstly according to the harmonic linearization method. Then, considering the inductive power grid and series capacitor compensation power grid, instability mechanisms and stability improvement methods of the HSC-GFM inverters resulting from the added voltage-based synchronization are studied. The studies show that when the proportional coefficient of the voltage-based synchronization control loop is large, low-frequency oscillations may be induced. The low-frequency stability of the HSC-GFM inverters could be improved by the virtual resistor. The theoretical conclusions are corroborated by simulations in PLECS.