Protection Engineering Basics and Schemes Protection Engineering | Page 198
Restraint current is introduced here in protection relay to prohibit differential relay to operates when fault occur outside differential protected zone, when external fault occurs the direction of both currents( secondary side) will be same at fault, but there will be impact of this external fault on transformer, to prevent this impact we used restraint current factor in protection relay so our relay will not trip except on fault in selected differential protected unit.
I restraint = I 1 −( −I 2) = I 1 + I 2
You can see both current enter or exist is in same direction which
When fault occurs at under differential protected equipment, then∆ > 0 or user defined, this happened because initially when system is in normal condition. I 1 and I 2 Are in same direction, i 1 and i 2( secondary input to differential relay) are in opposite direction and at differential relay they added to give zero amperes. When fault incept