Common terms used in power system protection

In our earlier blogs we have tackled power system components. Now we need to understand the common terms used in power system protection. We will define each terms simply to better understand each term.

1. Pick-up value(relay) - it is defined as the change of status. Let say when a relay changes it state from un-energized status to energized condition. When a relay is triggered means the status of the output contact will change from normally open (NO) to normally close (NC) and vice versa status.

2. Drop of value(relay) - it is a value of a relay is to change from energized status to de-energized status. The drop-off relay will be determined from the change.

3. Sensitivity - it is a characteristic of a relay that is the minimum pick-up value of the relay. For overcurrent relay the minimum operating current is termed as sensitivity of the relay.

4. Through fault -  this refers to the fault current flowing through the protective zone for a fault outside the protective zone.

5. Stability of Protection - it is defined as stable when the protection system remains in-operative for all system conditions except for which the protection is designed. 

6. Operating Time - the operating time of a relay is the time elapsed from the initiation of the relay to close the output contact.

7. Protected Zone - section or area of the power system covered by a protection scheme.


8. Blind Zone - area of a power system not covered by the protection scheme.