Some of the relays used for generator protection require voltage as one of their inputs. An
example is the distance backup scheme discussed in the previous section. Should the voltage
transformer supplying these relays lose its potential supply, the relays lose their restraint and
the impedance seen from the relay appears to be zero. This means that the loss of a fuse, which
causes no damage whatever, will result in the trip of the generating unit. Even the loss of
one fuse in a three-phase voltage supply will change the magnitude and phase of the supplied
voltages to cause the undesired trip. One solution to this problem is to supervise the voltage
sensitive relays with an voltage balance relay, which compares the magnitude of voltage from
two sources and energizes an alarm when the two sources are not nearly equal. This and other
schemes are described in [2].