As the integration of wind power accelerates, wind power plants (WPPs) are expected to play a crucial role in ensuring stability in future power grids. This paper examines the nonlinear stability boundary of a multi-converter system in a wind power plant (WPP) connected to an AC power grid via a long HVAC cable. Traditionally, for nonlinear analysis of WPPs, a simplification is adopted wherein the WPP is treated as an aggregation of individual wind turbines (WTs), with a simplified portrayal of the collector network. However, in the presence of different technologies, such as STATCOM, that are placed away from the WTs, the model aggregation will not hold. This paper presents a unified methodology to model and investigate the high-dimensional stability boundary of a WPP with a STATCOM. The stability region of the system, i.e. the region of attraction (RoA), is determined by the reverse time (backwards) trajectory technique. Furthermore, the estimated stability boundary is verified using time-domain simulation studies in PSCAD.