Abnormally high current during transformer turns ratio test 1

[Rwelagedara]

Dear All,
We are performing a transformer ratio test on a 1 MVA,33kV/400V transformer using a three phase transformer ration meter from a reputed manufacturer.The ratio meter is also showing the current (mA) in each phase in addition to the turns ratio value. During the testing, we found out that the ratio values are in acceptable range in all three phases. But current in one phase(phase A) is abnormally high. This high current is around 120 mA, whereas the current on other two phases is just around 12 mA. Is this an indication of an issue . We have tested many transformers before but this is the first time we see this kind of a very high current in one phase. Please assist.

 

[waross]

Thanks Pete. I guess I am on the right track but going the wrong way. grin


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[prc]

1) electricpete, thanks for your confidence in me for transformers. But I can also go wrong!

2) You have already listed the three typical defects that can be detected by this sensitive test. ie turn shorts, core defects, and connection mistakes. One more category that can be detected is defects in tap-changer.

3) When the shorting between turns is only spot touching due to damage in turn insulation, it will not come out in resistance measurement and TTR but will come in excitation current. When a turn to turn short happens in an open-circuited transformer, shorted turn loop carries a heavy current as resistance is too small. Then to compensate for this AT, a current is drawn from the line ie the winding becomes an auto-transformer.

4) Difference in phase currents due to core asymmetry will never be 10 times, but only 5-10 %. Current can increase due to core saturation from winding resistance test (using DC current) But it will be seen in all phases.

5) Today most sensitive and widely used diagnostic tests for transformers are two- DGA and single-phase LV excitation test. This LV current test was developed and formulated by Doble during 1960s.