[en] The Plant Protection System (PPS) is a major component in the safety critical Nuclear Power Plant (NPP). Digital Online Active Test Plant Protection System (DOAT-PPS) developed in this work is an advanced PPS that improves the existing analog-systems and is designed to apply to the domestic operating NPP. DOAT-PPS, which adopts the basic concept of DSS, is developed pursuing the replacement of DPPS. DOAT-PPS has structures and algorithms that are simpler than those of DPPS. Furthermore, it has an intelligent testability to diagnose all types of errors and status of system components. DOAT-PPS is an advanced PPS that improves availability and has easiness of maintenance