There are two types of backup protection that might be applied to a generator: backup of relays protecting the generator protection zone and backup of relays protecting external zones. Some types of backup protection may be graded to coordinate with both internal and external protective devices.

The negative-sequence relay might be considered a form of backup protection since most faults should be cleared by the stator differential protection with the negative-sequence relay acting as backup.
Balanced faults that are not cleared promptly can also cause considerable damage to a generator and backup protection is warranted. One type of such protection is to provide a distance relay that is supplied with current from a CT in the generator neutral and voltage from the generator terminals. Such a relay can recognize balanced faults both internal and external to the generator.

The connection makes the relay directional from the neutral but gives it reach in both directions from the voltage transformer location, thereby sensing both generator and GSU transformer faults. This type of relay can also be set to reach through the transformer, making it operable for phase faults on the high-voltage side of the transformer. Coordination is achieved through time delay.

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