Is Cable Differential Protection of a transformer different in working from a Differential Protection of a transformer?

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Naveed, first a caveat: it is essential that the protection system first determine the direction of current flow!

In general, differential-protection for Power-cable and Transformer are similar when the Power-cable is relatively short (say a few hundred mts/yds)! Then, in this case, CT secondary-currents circulate continuously thru a Auxiliary-circuits! Thus, as the Power-cable length increases, so must the size of the Aux-circuit conductors!

The problem above is eliminated for longer Power-cables (say km/miles) if the current in the Aux-circuit remains small, hence the introduction of a Pilot-circuit! Although connected to current and/or voltage measuring instruments at each end of the cable, the Pilot-circuit can be effectively carried by virtually any means of communication-type circuit in existence today, even fiber-optics!

A related subject
Recently, a Differential Protection Scheme using fiber-optic technology was introduced to this forum. It is: http://www.control.com/thread/1428603860

Regards, Phil Corso