Backup impedance Protection Working Principle (21G)

Backup impedance Protection Working Principle (21G)

The Backup Impedance Protection is used to protect the generator from supplying the over loaded or faulty system and it is a definite time graded protection, for a short circuit phase fault in Generator, Generator Transformer, Bus duct or as back up of uncleaned external network fault. It is backup protection of the generator over current protection. In measures ratio of the voltage from the potential transformer and current from current transformer supplied by the generator and initiates trip signal when the measured impedance is less than the present value.


Backup impedance Protection consist of Two stage operations such as

Stage I initiates the grid circuit breaker tripping

Stage II initiates the Generator circuit breaker tripping

Relays acted :

  • 21 G Flag operation at Protection panel.
  • Acting of Master relay for grid at stage I and if the fault is uncontrolled, then the generator breaker trips on stage II
  • Indication at Annunciation Panel.

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