Patent ID: 8861153

Claim:
A current differential relay apparatus comprising: an operation-level calculating unit that calculates an operation level Id which is a vector sum of a local-terminal current and a received opposing-terminal current; a suppression-level calculating unit that calculates a suppression level Ir which is a scalar sum of the local-terminal current and the opposing-terminal current; and a ratio-differential-characteristic determining unit that determines an operation in a ratio differential characteristic based on the operation level Id and the suppression level Ir, wherein with relationships a 1 ≦a 2 and b 1 ≧b 2 being satisfied where a 1 , a 2 , b 1 and b 2 are real numbers, the current differential relay apparatus further comprises: a first range determining unit which determines that a first range is activated when Id >a 1 Ir +b 1 is satisfied; a second range determining unit which determines that a second range is activated when Id >a 2 Ir+b 2 is satisfied; and operation determining means for determining that an operation is made in the ratio differential characteristic when both the first range and second range are activated and for continuing the operation until an operation in the first range is reset.