What is the function of restraining coils in differential relay?

Restraining coils are also called as bias coils. Due to the difference in the magnetizing currents of the upper and lower current Transformers the current through the operating coil will not be zero even under normal loading conditions or external fault conditions. therefore to provide stability on external faults bias coils are provided. To obtain the required amount of biasing a suitable ratio of the biasing coils with restraining coils to be provided.