Stability analysis and seamless mode transition for a hybrid battery formation and grading testing system

The battery formation and grading testing system (BFGTS) is core equipment for battery activation, performance testing, and grading process. The grid-forming (GFM) inverters emerge as the promising solution for BFGTS to address instability issues induced by grid-following (GFL) inverters under weak grid conditions. Nevertheless, the stability of hybrid battery formation and grading testing system integrating both GFM and GFL inverters has become increasingly complex, presenting two critical challenges that demand urgent solutions: assessing the capacity ratio of GFM inverters for oscillation suppression and developing a seamless transition method between GFL and GFM mode under different current dynamics. Hence, a broadband sequence impedance considering the frequency coupling effect of the hybrid system is established and verified in this study. The influences of control parameters, capacity ratio and grid strength are analyzed in detail to reveal the stability mechanism. As the capacity ratio of GFM inverters increases, their low output impedance dominate the system, which effectively shrinks the capacitive negative damping region and thereby suppresses oscillations. However, an excessively high GFM ratio further increases the system’s stability risk under weak grid conditions. Additionally, a novel mode switching method integrating adaptive virtual impedance and feedforward control is proposed. Its advantage is that it can effectively suppress current surges and achieve seamless mode switching, even with different current loop structures and parameters between GFM and GFL control loops. Finally, simulation results of a BFGTS with six inverters validate the correctness of the stability analysis and the effectiveness of the proposed mode switching method.