Differential Relay

Biasing: There could be differential current in the system even when there is no genuine internal fault in the protected zone; causes could be:

Tap Changer Action in Transformers

CT Mismatch

Unequal CT Secondary Lead Length, etc.

One wouldn't want the differential relay to operate for such differential currents. So, one tends to keep a small setting of say 10% or 15% in the differential relay, so that upto this 10 or 15% differential current is conceived by the relay to be "healthy condition differential current" and NOT fault condition; and, thus the relay wouldn't operate upto this set bias percentage. Any differential current above this setting is conceived to be fault condition and the relay reacts.

Restraint: Even when biased, at certain instances, like magnetsisng in-rush conditions during transformer switch-on or sudden load throw off conditions, there could be differential currents much beyond the maximum possible bias setting in the relay, causing relay opeartion during such situations. Special algorithms like second harmonic restraint or fifth harmonic restraint are used to prvenet the differential relay operation during such conditions.