Why REF required?????
REF protection applied on transformer in order to detect ground faults on a given winding more sensitively than overall differential protection able to do.
On which principle they based ?????
The principle of comparison of measured variables by comparing the residual current of the phase CT of the given winding with the current associated with ground star point.
Why residual current occur????? or why CT saturated???
Saturation Saturation is the sudden variation of material permeability from high value to low value at the point known as the saturation bend.
In case of saturation induction then increases only slowly and Ie ( excitation current ) deforms to form peak.
Why stabilizing resistor inserted in the differential path for REF relay ???
If any CT ( phase CT or neutral CT ) get saturation (due to DC component of current during trafo energization (Inrush current)) .
So CT 's high magnetizing inductance breaks down to low value determined by permeability of air.
Differential current or residual current caused by CT saturation does not flow completely through differential path (due to high Impedance in path) but flows through low magnetizing inductance of saturated CT.
Hence insertion of resistance has a restraining effect.
Voltage across stabilizing resistance at Max. through fault = Max. through fault current ( CT resistance + 2 * Cable resistance )
BTW we are never allow to saturate our CT's by selecting proper knee point voltage.
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