Sensitivity improvement of the digital differential relay for internal ground fault protection in the power transformer with tap changer

A new method for improving the sensitivity of the digital differential relays that used to detect the internal ground fault in the (/spl Delta/-Y) power transformer with on-load tap changer is presented in this paper. The suggested method can be applied to the digital relays and/or may be applied to the conventional differential relays. Digital differential relay is sensitive to internal faults, except the ground fault, due to the high current ratio required for transformer with tap changer. This is true especially for percentage differential relay. The adopted method deals with this case to increase the number of protected winding against ground faults from 35% to about 100%.