Over Current and Earth Fault Protection for Generator Backup

Over Current and Earth Fault Protection for Generator Backup :

➡️It is true that the protection scheme provided for the generator should not operate for through fault (faults beyond protective zone) conditions. But generator is the source by which any fault on the system is going to feed by it only, protection against such abnormal conditions should be provided.

➡️For generators above 1 MW where primary protection to stator winding is provided by differential protection and the over current and earth fault protection gives back up protection for external phase to phase faults and earth fault.

➡️Since the faults in stator winding are fed by the stator winding itself, their influence on current in the outgoing terminals of generator depends upon fault level of the main bus.

➡️Hence over current and earth fault relays do not provide satisfactory protection against internal faults.

➡️However the over current and earth fault relays provide back up protection to generator against external faults (faults in bus zone, transmission zone) The setting of over current and earth fault protection is selected such that relay does not operates for through faults.

Sequence of operation

➡️However if fault F continues for a long time due to failure of line protection (1) the fault will be fed by the generator. Hence the over current and earth fault protection of generator (3) may be set to operate with due time lag for higher values of external fault currents.

➡️Hence high set, definite minimum time, induction type inverse over current, earth fault relays are recommended for generator back up.