SYSTEM SEPARATION RELAY UNIT

PURPOSE:To facilitate protective coordination by separating the lower system when outputs are provided from both means for detecting flow-out of fault current from upper system to lower system and a detecting means responsive to an impedance. CONSTITUTION:When a fault occurs in a transmission line 215A and a circuit breaker 214A is disabled during operation of three transformers 113A, 113B, 113C, a bus tie circuit breaker 210 is interrupted and two transformers 113A, 113B are connected with the faulty line. Consequently, direction detecting elements 121A, 121B operate and a distance element 123 operates with an impedance represented by the ratio between the sum of fault currents introduced from current transformers 116A, 116B, 116C and the primary voltage of the transformer. Upon function of time limit operation timers 126A, 126B, primary and secondary circuit breakers 112A, 112B and 114A, 114B of the transformers 113A, 113B are interrupted thus preventing the fault from spreading to the upper system.