The next significant step was the introduction of the concept of voltage supervision.1 It may
be expected that during the inrush conditions, the transformer voltage would be close to normal,
while during faults, the voltage would be much less. Thus, an undervoltage relay may be used to
supervise the differential relay. If the undervoltage relay can be set to distinguish between a normal
transformer and a faulted transformer, then it could be used to block the differential relay when it
detects a voltage above its setting. In general, this type of voltage supervision is not preferred, as
the undervoltage relay tends to be slow, and consequently the entire protection becomes slower.
More importantly, this type of protection requires a voltage source for the transformer relay, which
is an added expense, and may not be justifiable in many cases.