2) Overcurrent relay
Oscillated waveform of current consists of all orders of
harmonic up to 10-60 % of fundamental. The value of total
current is about 5 times greater than fundamental value,
nevertheless; value of fundamental (1.72 Arms sec
) exceeds from
overcurrent pickup value (1.25 Arms sec
); therefore, relay picks
up and then drops off subsequently according to swing
frequency. The magnitude of current is decreased in the time
of generator mode. As a result, the magnitude of fundamental
value becomes low proportionally. Although fundamental
value of current is increased up to overcurrent pickup value
for a short time, it does not meet trip time value; therefore,
relay does not operate. Fig. 8 shows the rms value of
fundamental component of current and operation of relay
before and after generator mode.
3) Differential relay
Setting parameters of differential relay are modified and
relay is tested to certify stability in case of any through fault
current in no-Ferroresonant state. Secondary current of noload transformer is not increased as well as primary current in
Ferroresonance, therefore; differential current is increased.
Hence, operating point inters to tripping area and relay
operates despite the fact that there is no internal fault in
protective zone. Fig. 9 shows operation of differential relay.