Distance measuring relay for line faults

Claim 1

In a short-circuit distance relay for protecting a single-channel power transmission system with a transmission line of length L and local and remote power supplies installed at the respective two ends of the transmission line, the improvement comprising: first and second deriving means for respectively deriving values of a positive-phase voltage (V) and a positive-phase current (IL) at the location of said relay at the local end of the transmission line during a normal state of the system; third, fourth and fifth deriving means for respectively deriving values of a positive-phase voltage (VF), a positive-phase current (IF) and a negative-phase current (I2) at the location of said relay during a fault state of the system; a first memory circuit with selector switch means operated in response to detection of a fault for storing the derived value of the positive phase voltage (V) during a normal state of the system and for storing the derived value of the positive phase voltage (VF) during a fault state of the system; a second memory circuit with selector switch means operated in response to detection of a fault for storing the derived value of a positive phase current (IL) during a normal state of the system and for storing the derived value of a positive phase current (IF) during a fault state of the system; and arithmetic and computation means for obtaining the fault impedance (XZ1) between the location of the relay and the point of fault in accordance with the following formula: ##EQU14## where: (ZP+ZQ) is a sum of predetermined positive phase impedance of the system, LZ2 is a predetermined negative phase impedance of the power transmission line, and ZQ2 is a predetermined negative phase impedance behind the remote end.