8.5 Supervised differential relays
The inrush current phenomenon in large power transformers presented an insurmountable obstacle
to the percentage differential relay design when it was introduced. Measures were introduced for
counteracting the tendency of the relay to trip for inrush conditions. One of the earliest ideas was
to desensitize the differential relay when the transformer is energized. Thus, the less sensitive relay
would not see the inrush current, thus avoiding a false trip. However, desensitizing (or disabling)
the differential relay during energization is a poor practice, as it is precisely during the initial