One arrangement that is extensively used is the differential relay with a
variable-percentage characteristic, alternatively described as being percentage
biased, which has an additional unit, the restraining coil, in addition to the operating
coil as shown in Figure 8.2. Since this type of relay is most common in
differential relaying schemes, all references to differential relays in this chapter will
be on the basis of using this type of relay. The current in the operating coil is
proportional to (I1 – I2). If N is equal to the number of turns of the restraining coil
with the operating coil connected to the mid-point of the restraining coil, then the
total ampere-turns are equal to I1(N/2) þ I2(N/2), which is the same as if (I1 þ I2)/2
flowed through all of the restraining coil. The operating characteristic of the relay
with this form of restraint is shown in Figure 8.3.