Critical-Mode-Aware Robust Two-Stage Safety Projection for Constraint-Aware Operation of Multi-Inverter PV Plants Under Weak Grids

Multi-inverter photovoltaic plants connected to weak grids can satisfy point-of-common-coupling requirements while individual converters approach internal limits. This paper develops a critical-mode-aware two-stage safety projection for constraint-aware operation. A two-port sequence model and a critical-frequency-band (CFB) criterion locate the interaction band in which the return-difference margin is most sensitive to active-power commands. The first stage computes the curtailment required to satisfy predicted current-reference, modulation, and dc-link constraints. The second stage uses any remaining dispatch freedom to improve the return-difference margin with minimum weighted curtailment. A 40-case calibration selects a 0.05 contraction of the normalized device-risk target; an independent 60-case test with parameter variations of up to ±20% produces no post-20 ms current-reference violations. In five OP5700 real-time model-execution cases, three meet the strict unit current-reference limit, while two finish slightly above it at 1.03 and 1.04. The isolated MATLAB/Simulink R2024b projection kernel has mean, 99th-percentile, and maximum execution times of 4.57, 7.20, and 213.4 microseconds over 20,000 calls. Nine-inverter nonlinear studies, a three-inverter PSCAD V5.0 switching simulation, and OP5700 real-time model execution support constraint recovery over the tested operating set, but not universal strict feasibility.