Differential relay required at least a minimum v alue of current flowing through its operating winding for operation.
-They will only operate if the operating current exceeds a certain percentage of the restraint or bias current.
-This ensures the relays current required for operation is always greater than the out of balance current under maximum through fault condition.
The second bias or restraint incorporated into most transfomer differential relays is the bias to prevent relay operation against magnetizing in rush current on energizing the transormer.
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