[Objecttives] A comprehensive solution is proposed to address the issues of insufficient accuracy in nuclear power plant (NPP) models for large-scale power grid stability simulation-leading to distorted dynamic characteristics—and the lack of coordination mechanisms between grid-related protections and the grid. [Methods] First

a high-precision nonlinear model of the NPP power system is developed

covering the reactor core

primary circuit

and secondary circuit. Key algorithms are introduced

including steady-state equilibrium adjustment and variable initialization under off rated conditions

a power interface method based on working fluid enthalpy drop

and a multi rate hybrid numerical integration scheme. Furthermore

a model order reduction method combining balanced truncation and singular perturbation theory is proposed

which significantly improves the computational efficiency of electro-mechanical transient simulation for large scale grids. Based on the developed model

the interaction mechanism between nuclear power plants and the grid is elucidated. For typical fault scenarios such as over frequency

under frequency

and under voltage

coordinated control strategies between grid related protections and grid safety automatic devices are presented. [Results] Simulation results verify the accuracy of the model and the effectiveness of the strategies

providing a powerful tool and research reference for studying the dynamic characteristics of NPPs and their interaction and coordinated operation with the power grid. [Conclusions] The research results can provide powerful simulation tools and research reference for safety and stability analysis and control decision-making of nuclear power units connected to large power grids.