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During charging of Transformer breaker when2ndary sideof Trafo outgoing breaker is open condition, then only primary side is charged to magnetise the core to produce induction effect of Trafo.Now as we know primary side current I1, primary side CT2ndary current i1 & Trafo 2ndary side current I2 &2ndary side CT2ndary current i2.
As per Differential theory in normal condition I1= I2, i1=i2.
But during charging of Transformer from primary side I1>>>>I2 & i1>>>>i2.Differential will act, if differential realy bias setting is not proper or Differential relay trip time is less than that of Transformer Magnetising time. tripping time should be slitely above live side charging current &/or trafo charging time.
As per2nd harmonics is conceren it is due to magnetising Inrush current.In case of tripping of transformer in other order harmonics, it is due Thirisror Triggering in other industrial load connected to the system in normal running condition.
first of all we need to see the loop drawing then we can trace the faulty componants so we can fix it
If transformer trips during its energizing , it is quite likely to be because of inrush or harmonic.
