PURPOSE: To obtain a digital protective relay which can employ data prepared from different signals sampled at different stations as it is by determining the phase difference of electrical amounts which can be decided as a common data based on the connecting state of conducting systems and employing thus determined phase difference in the operation of protective control.

CONSTITUTION: Sampling signal SPc from other signal generating source 73 is made asynchronous with a sampling signal SP1 of one signal generating source 83. A processing unit 86 performs following processing based on one station data 82, other station data and system conditions 87. Since bus voltage is equal to the line voltage when a circuit breaker is closed on the system side, digitized data received at a bus voltage transformer 5 from other station is same as the data at a line voltage transformer 6 digitized based on the sampling signal at one station. A phase difference operating means 88 utilizes the fact to determine the difference of sampling time of data from other station(corresponds to phase difference) with reference to the sampling signal at one station and then protective control operation is carried out based on thus determined phase difference.