Abstract: High-penetration wind integration causes insufficient system inertia and scattered frequency regulation. Coupling between PLL stability and wind turbine parameter perturbations distorts frequency curves. This paper presents a transient frequency security assessment method: construct a time-varying equivalent inertia model with PLL instability coupling; adaptively infer frequency drop rate via trajectory curvature to decouple wind fluctuation and inertia loss; establish a margin criterion covering unit tripping probability and non-convex security regions for fast multi-timescale state identification. Tests capture time-varying inertia and low-frequency inflection jumps, fixing traditional defects to support grid early warning.