Transformer Differential Protection
A Differential System can be arranged to cover a complete Transformer. The underlying principle of such a protection scheme is shown in the figure below.
The General Idea behind the Differential Protection is that the CT’s on the primary and Secondary side must transform the respective Line currents to the same value. For the Bias Coils in the relay to function without any damage, this transformed current may lie between 1A and 5A. Hence the function of the CT’s is to transform the line currents to the same magnitude and phase under normal operation of the Transformer.Should some imbalance occur within the Transformer, such as Interturn Faults within the Windings of the Transformer or Faults on the incoming or outgoing feeders to the transformer, the Line Currents are no longer at the balanced value. Thus, the transformed current at the relay coils is no longer the same at both ends, but different. This causes an imbalance within the differential relay, and as a result, some protection mechanism will be operated, so as to isolate the Transformer and protect it.
See detail : Transformer Differential Protection
Source : http://www.eng.uwi.tt/depts/elec/staff/alvin/ee35t/notes/Trans-Diff-Protect.html
