E. Effect on protective relays
In this section, behavior of protective relays, which are
exposed to Ferroresonance, is assessed in Silver station.
1) Distance relay with OSB element
Distance relay includes two protective zones, which first
zone is set to line impedance and second zone acts as starting
zone. It is set according to load impedance with longer trip
time delay. In order to prevent false operation of the relay in
power swing, it is surrounded by mho characteristic scheme.
The scheme includes outer and inner elements with specific
maximum blocking time of 0.01 s. Occurring power swing in
effect of Ferroresonance with slip frequency of about 10 Hz
and then 16 Hz (Grand Rapids in generator mode) causes
operation of OSB element and blocks distance relay. As it is
shown in Fig. 7, impedance loci oscillates in a limited area
between OSB characteristics. Current is increased with a high
magnitude however, because of presenting fundamental value
with a low magnitude due to existing harmonics; in addition,
increasing the voltage, impedance loci does not enter to any
zone of distance relay.