Differential relay reliability enhancement using fourth harmonic for a large power transformer

Abstract

A differential relay that is very sensitive relayoperating even at its limits may be used for protecting apower transformer. However, this characteristic may leadto unnecessary tripping due to transient currents such as aninrush and over excitation current. In order to avoid thismal-operation of the relay, a second and fifth harmonicblocking technique has been used; however this techniqueis not reliable if a second harmonic magnitude is weak. Inthis paper, a new approach is proposed using even har-monics (second and fourth). The test results show that thisproposed approach is a good blocking technique associatedwith the differential relay even for large modern powertransformer which has small second harmonic as well; itprovides a good discrimination between the transient cur-rents and the internal fault currents during internal fault.