In light of the most widely used pressurized water reactor（PWR）nuclear power units（NPUs）in China，the interaction and mutual influence between grid-related generator protection of nuclear power plants and automatic security devices of power grids are analyzed，and the coordination and control principle between them is proposed.Based on the existing models for PWR NPUs in PSD-FDS，the dynamic performance of NPUs and power grids due to voltage and frequency disturbance are studied.Then the action performance of grid-related generator protection（such as the over-speed protection，over-excitation limitation and protection，abnormal frequency protection，and primary pump protection）and automatic security devices（such as generator tripping at a high frequency and load shedding at a low frequency）are analyzed，as well as the mutual influence and coordination between them.Finally，the coordination principle is validated with case studies based on an actual power grid.The principle can be used as a reference for the study of coordination control and parameter optimization for nuclear power plants and the power grid connected.