When a power transformer is re-energized after along time of deenergizing and when the primary side circuit breaker is switched on, Inrush current will be only in the primary side of power transformer.
This current may lead to relay operation and circuit breaker tripping .In fact, the inrush current is a current with a frequency equal to twice the normal frequency (second harmonic). This current is separated from the fundamental current and used to resist the operation of the differential relay. figure (42) shows a second harmonic blocking diagram .
IN= Rated current of power transformer.
IFN=Rated frequency current.
I100=Second harmonic current.
IFN=Rated frequency current.
I100=Second harmonic current.