Question { Siemens, 43460 }

What is the function of restraining coil in differential
relay? as well as restraining current?

Answer

The concept is simple. In any relay there are two forces namely controlling and deflecting force. Controlling force is responsible for the normal operation of the relay that is during normal current flow (during non faulted conditions). Whenever a fault occurs as we know that the impedance will get reduced and hence the fault current raises above the rated value and flow through the secondary of the CT connected to the relay circuit. Actually both the forces are present during normal operations also. But during fault due to the high fault current the deflecting force raises above the controlling force thus actuating the relay operation. Controlling force is created by the restraining coil and the deflecting force is created by the operating coil. This force production is as simple that it is based on the fact that a current carrying coil experiences a force.