  This letter determines the nonlinear stability boundary of a wind power plant
(WPP) connected to an AC power grid via a long HVAC cable. The analysis focuses
on the slow Phase-Locked Loop (PLL) dynamics, with an assumption that the fast
current control dynamics can be neglected. To begin, we propose an aggregated
reduced-order wind turbine model. This aggregation can be applied up to a
limited frequency, e.g. 400Hz, which aligns with our assumption regarding
low-frequency dynamics. The WPP collector and transmission network model is
established using impedance/frequency scan approximated around $\pm$5 Hz of the
PLL nominal frequency, accounting for the hard saturation limits. The stability
boundary of the reduced-order system is determined by reverse time trajectory,
offering valuable insights into the WPP's overall stability. The work presents
a routine from modelling to nonlinear stability assessment for offshore wind
farm applications.
