Patent ID: 6937453

Claim:
A directional comparison distance relay system configured to control a circuit breaker installed in a parallel three-phase AC electric power transmission system, the relay system comprising: a fault detection unit including a directional distance relay element configured to decide whether a fault is in forward or reverse directions relative to a location of a set of transformers which detect current and voltage in the power transmission system; a short-circuit distance relay element configured to decide whether a fault is within a protection zone by distance calculation based on the current and voltage; a transmission unit configured to send signals to and receiving signals from a neighboring relay system at an far end of the protection zone; an evolving fault detecting unit configured to decide an evolving fault when a forward earth fault directional relay in a single phase and a reverse directional relay in at least one phase are in operation; and a block unit configured to block trip of a circuit breaker at a near end due to the short-circuit distance relay, and configured to block permissive trip signal to a circuit breaker at the far end, based on an operation of the evolving fault detecting unit.