Grid-forming Control of Two-stage PV Systems Under Capacity Constraints

Grid-forming (GFM) controllers in photovoltaic (PV) systems encounter distinct challenges since ac power demand may exceed PV generation capacity. In this paper, we propose a multi-mode control architecture for two-stage grid-tied PV systems by introducing a limited grid-forming mode to sustain operation during capacity-limiting disturbances. The framework combines three operational modes, namely a pre-synchronization mode for startup, a mode with prototypical GFM dynamics, and the proposed limited GFM mode when power constraints are reached. The ability to ride through large disturbances requires only dc-dc controller adaptation while the dc-ac converter contiguously retains a unified control law. Accordingly, this approach minimizes implementation complexity. Tuning of major control gains involved in three modes is established based on symbolic and numerical analysis. Finally, simulations validate seamless ride-through during large disturbances.