This design improved inrush restraint for low-loss steel core transformers. Figure 10 shows inrush current for a typical transformer and the relative level of second- and
fourth-harmonic content.
Programming a digital relay with too low of an inrush restraint setting risks the relay
restraining for an internal fault because CT saturation can also produce significant levels of even harmonics.
Transformer relays are equipped with a nonharmonically restrained high-set differential element (87H) that provides protection for high-current magnitude internal
faults where CT saturation can occur.
This element is set above the expected inrush current.