This paper presents a detailed nonlinear state-space model of a grid-connected Renewable Power Plant (RPP) comprising multiple grid-following Power Generation Units (PGUs) interconnected through an internal collector network. The proposed modelling approach preserves the hierarchical PGU--String--Group--RPP structure while retaining the underlying electrical, control, and network states, thereby providing a physically meaningful representation of plant dynamics. A modular aggregation approach is used to represent parallel PGUs and Strings, enabling the model to be easily scaled to different plant configurations. The accuracy of the proposed complete nonlinear model is validated against a corresponding PSCAD electromagnetic transient (EMT) model by varying active and reactive power references. The results demonstrate close agreement in active power, reactive power, voltage magnitude, and voltage angle responses, demonstrating the model's ability to accurately capture the relevant dynamic behaviour of RPPs. The developed state-space model provides a unified foundation for small-signal stability analysis, eigenvalue and participation-factor analysis, controller design and tuning, model reduction, and the derivation of impedance/admittance representations for investigating converter–network interactions.