Percentage restraint slope is a concept that is universally used in both EM and digital relays to provide security against false operations during through faults.
It is recognized that the higher the through-fault current, the greater the possibility that the mismatch in CT performance will cause a false differential error current.
In percentage restraint differential relays, the higher the through-fault current, the greater the value of differential current it takes to operate the relay.
Figure9 illustrates this concept for a digital relay.
The operating current (Io) is the vector sum of the primary and secondary p.u. currents.
The p.u. current in differential relays is the CT current on the primary and secondary divided by the relay tap setting for that winding.
The differential relay pickup must be set above the steady-state transformer magnetizing current and generally is set in the 0.2–0.3 p.u. range.