External Fault Backup Protection for Generators:

Overcurrent and earth fault protection is provided for backup protection of large sized (above 1 MVA) generators protected by differential protection against external phase-to-phase faults and earth faults. The schematic arrangement is illustrated in Fig. 11.24 (a). Induction type IDMT relays may be employed for this purpose.

Since the faults in stator winding are fed by the stator winding itself, their influence on current in the outgoing terminals of generator is governed by the fault level of the main bus. Thus overcurrent and earth fault relays do not provide satisfactory protection against internal faults.

Sequence of operation is illustrated in Fig. 11.24 (b). The relays setting is selected that the generator overcurrent and earth fault protection does not normally respond to external faults such as at F.

However, overcurrent and earth fault protection of generator (3) may be set to operate with due time lag for larger values of external fault currents so that the fault F is not supplied by the generator in case it persists for a long time duration.

Hence high set, definite minimum time, induction type, inverse overcurrent, earth fault relays are required for backup protection of generators.