String inverter-based photovoltaic systems differ substantially from centralized photovoltaic systems in terms of topology and control-loop time scales

and it is necessary to further investigate whether their grid-connected stability characteristics differ from those of centralized photovoltaic systems. To analyze the stability characteristics of string inverter-based PV plants

this paper establishes an equivalent model and the corresponding state-space equations based on the typical topology of string PV stations. The subsynchronous oscillation problem of grid-connected string inverter-based photovoltaic systems is investigated through eigenmode analysis and detailed electromagnetic transient simulations. The results reveal that

although the theoretically designed bandwidth of string PV converter current control loops far exceeds the sub/super-synchronous frequency range

under low-short-circuit-ratio grid-connected conditions

the eigenmode dominated by the current control loop is located in the subsynchronous frequency band and presents oscillation risks. Eigenvalue sensitivity analysis demonstrates that the system short-circuit ratio and current-loop control parameters significantly affect the stability of this mode. Eigenvalue trajectory analysis reveals how operational conditions and control parameters affect the stability of this subsynchronous oscillation mode. Electromagnetic transient simulations validate the accuracy of the eigenmode analysis results. Frequency-domain analysis shows that a decrease in the short-circuit ratio of the grid-connected system rapidly reduces the bandwidth of the current control loop in string photovoltaic systems to the sub-/super-synchronous oscillation frequency range

indicating that string photovoltaic systems may have sub-/super- synchronous oscillation risks similar to those of centralized grid-connected photovoltaic systems.