A New Techniques of Differential Relaying the Differential Relay

Standard percentage differential relaying with through current restraint is insensitive to high impedance (i.e. low fault current) faults when applied to apparatus which experience substantial, near 1.0 pu, prefault through currents. A new technique of differential protection is proposed which considers only fault contribution currents through the use of microcomputer cancellation of the prefault through currents. This new form of differential relaying maintains the external fault selectivity of standard percentage differential relaying, has a large buffer zone between the tripping and restraint zones of relay operation, and is substantially more sensitive to high impedance faults than the standard arrangement. This paper presents the theory for this relay, a simplified analysis of the effects of CT errors on the relay operation, and the experimental results for a real time laboratory version of the relay using a 16-bit single- board microcomputer.