Answer Posted / raghavendran.r

In percentage differential relays, the current from each
current transformer flows through a restraint coil. The
purpose of the restraint coil is to prevent undesired relay
operation due to current transformer errors. The operating
coil current | i1 - i2 | required for tripping is a
percentage of the average current through the restraint
coils.

i1-i2 >=k(i1+i2)/2= k*iavg

where k is the proportion of the operating coil current to
the restraint coil current. For example if k = 0.1, the
operating coil current must be more than 10% of the average
restraint coil current in order for the relay to operate.

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