Answer / 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.

Can anybody please refer the best and simple book for about Protection relays and in detail of all its protection ?

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