How do you test power transformer stability?

The stability and sensitivity test shall be performed for unit protection to prove

the stability for through fault and sensitivity for the internal faults. The principle

of the test is as followed.

The protected zone of unit protection is restricted by location of the CTs. For

stability test the primary current shall be injected through one set of two CTs

such a way one of current inwards and other outwards. At relay terminal no

operating current shall be observed for proper connection. Sensitivity test shall

be conducted with same set-up with either reversed CT polarity or injecting

primary current on only one CT of the set. At relay terminal, operating current

shall be observed for proper function.