This start-up transition in the transformer effects so that the flux in the rst half cycle after energization
shall be up to 2 times of the nominal flux value. The transformer core generally is saturated just above
the steady state value of the flux and because of this the transformer core will be decreasingly
saturated during this transition time. During this saturation time transformer primary draws very high
current with heavy amount of even harmonics which of highest is 2
nd
. This current is generally called
“(magnetizing) inrush current in transformer”. Inrush current in transformer may be up to 10 times
higher than nominal rated current of transformer. Energizing characteristics of transformer are related
on the ratings of the transformer as well as design of the transformer (transformer limbs constructions
etc.)
Basically this energization current shall be seen in the differential relay as a differential current since it
will flow through the primary side winding only. For this purpose the 2
nd
harmonic component
generated by the saturation of the transformer core can be used to block the biased sensitive
differential stage during energization.
AQ-T257
Instruction manual
Version: 2.00
167
© Arcteq Relays Ltd