Percentage biased differential relay for transformer protection

       Percentage biased differential relay for                                 transformer protection  

~ The circuit diagram of this scheme for star-delta transformer is showing.

~ the primary winding of the transformer is connected to the star and the side CT is connected to the delta.

~ the transformer in enclosed in a secondary delta and the side CT is enclosed in a star.

~ doing so dose not result in an out of  balance cut in a healthy condition due to phase displacement between the primary and secondary.

~ A restraing coil is attached to the pilot wire.

~ the operating coil is connected between the head of the restraing coil and the neutral pilot wire.

~ the current increases when the transformer feeds the fault outside the protective zone.

~ if the characteristic of CT is not the same then different cut is obtained.

~ but at the same time the average restraing current also increase.

~ hence the relay can not be operated.

~ but when there is an internal fault in the transformer the transformer the differential cut increases and the relay operates.