Transformer

go to the link n read.

http://www.iea.lth.se/publications/Theses/LTH-IEA-1055.pdf

Differential Protection is given for critical transformers of size 5 MVA and above, to protect the transformer against any internal fault. Two current inputs are given to the relay coil from opposing directions - one each from the HV Side CTs & from the LV Side CTs. When there is a thro-fault (i.e.) fault outside the transformer, both the HV side & the LV side CTs will sense this current and the relay coil will see equal and opposite currents which would cancel out. Thus the relay would not operate for external faults. Had there been an internal fault, then only one side (probably the HV side CT) CT would sense this current and the other side CT would not sense this current. So, the realy coil would now receive only one current and no cancelling current. Thus the relay would operate for internal faults.