Two-Layer Grid-Following/Grid-Forming Operating-Mode Configuration for Photovoltaic–Battery Energy Storage Plants Considering Operating Power and Branch Impedance

As photovoltaic penetration increases, dynamic interactions among inverters pose growing stability challenges for renewable energy plants connected to weak grids. In hybrid power plants comprising photovoltaic (PV) generation and battery energy storage systems (BESSs), hereafter referred to as PV-BESS plants, operating selected BESS inverters in grid-forming (GFM) mode can improve stability under weak-grid conditions. However, when BESS units differ in operating power and branch impedance, allocations with the same number of GFM units can exhibit different stability boundaries. Mode configuration must therefore specify both the number of GFM units and which units operate in this mode. To address this issue, this paper proposes a two-layer operating-mode configuration method that incorporates unit-specific operating power and branch impedance. The method is built on a plant-level dq-domain impedance model that represents each inverter’s control mode, steady-state operating point, and branch impedance. In the first layer, a homogeneous reduced-order model is used to determine a candidate GFM unit number. In the second layer, the actual operating power and branch impedance of each unit are restored; the steady-state feasibility and small-signal stability of different mode allocations are jointly evaluated; and the feasible allocation with the largest relative short-circuit-ratio margin is selected. Theoretical analysis shows that the number of GFM units influences the plant’s aggregate grid-forming support. Differences in operating power and branch impedance result in allocation-dependent critical SCRs and stability margins, even when the GFM unit count is fixed. MATLAB/Simulink simulations support the choice of the candidate GFM unit count and confirm the predicted stability differences among mode allocations.