Answer Posted / amudhavanan

let me explain you with the example of low impedance busbar
protection.

As you Differential protection is based on the Law of
Kirchhoff: in a healthy system, the sum of currents
in a node must be zero.

Let I1, I2, I3 are individual feeder currents.

Then the differential current is sum of all the currents
which result zero under external fault condition. If it
internal fault , sum of all the currents is equal to fault
current.

As you know , the operating current is the sum of
differential and restraining current.

The purpose of using restraining( Bias ) element is to
ensure the relay to operate under internal fault condition.

The restraining current is the vector sum of all the
currents. Hence the restraining element reduces the
operating current of the relay to improve its sensitivity.

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