What are the causes of unbalance current during no load test of transformer?

[za1820]

What are the causes of unbalance current during no load test of transformer?

 

[za1820]

You give the primary resistance, have you also checked the secondary?

Edit: Beaten to it by @Julie.

Yes, Secondary winding resistance is: 2.83K-Ohm, 2.82K-Ohm, 2.83K-Ohm

 

[za1820]

OK

Since you see the current between two phases on the delta winding, this means the one winding is either at fault, or (more likely) the secondary has a fault - I would guess one high voltage winding has a fault somewhere along its winding to earth (the star point being earthed) or has an inter-turn winding fault.

This phase will be the one corresponding to the branch of the delta connecting both current taking phases (~8A).

Can you flash test the hv windings - ~27-35kV as it may be an insulation breakdown issue, so won't be shown via an insulation resistance test?

If its an inter-turn winding fault it is a little more difficult to detect with an assembled tx.

(The above assumes there is absolutely nothing connected on the hv side - obviously if there is a single phase load/fault in that part of the system it will reflect as a phase-phase load/fault on the primary )

The star point is not earthed, All the transformer test results are satisfactory, Test we conducted are following;
IR (Both Windings), TTR, Open Circuit Test, Winding resistance

 

[Julie.]

The star point is not earthed, All the transformer test results are satisfactory, Test we conducted are following;
IR (Both Windings), TTR, Open Circuit Test, Winding resistance

What voltage did you apply to the primary side?

 

[za1820]

What voltage did you apply to the primary side?

400V and no load current was; 7.79A, 0.14A, 8.15A

 

[Julie.]

400V and no load current was; 7.79A, 0.14A, 8.15A

Test voltages

As stated you either have a breakdown at high voltage, or you have inter-turn failure probably on the single hv winding.

Have you flash tested it or not?

If so what voltages hv & lv?