Dyn11 no load current unbalance

[Power0020]

I tested some newly manufactured 1600 kVA 22/0.4 kV Dyn11 transformers, at no load energinzinig the LV side with the MV side open,I have noticed that one of the transformers shows abnormally low no load current but with similar no load power loss. Also, all the transformers showed unbalance with lower reading on center phase (B), the technician highlighted that this is due to the summation of magnitization on center phase!...I couldn't figure that out but all other load tests and resistance measurements were ok.

Any clue?

 

[krisys]

It is not rare to have unbalance in the no load current as the no load magnetizing current is rich in second harmonic components. (About 20%). It could be a metering error as well.

The magnetic balance test, which is supposed to be a commissioning test which shall identify, if there is any issues of magnetic imbalance.

As the no load current is very small (about 1%) of the rated current, in my view the situation should be acceptable. This is true as long as the other side voltage is fine. Nevertheless, check for any abnormal sound, heating etc. for a while and then leave with the transformer.

 

[prc]

When we apply voltage to a star connected winding, with 3 phase core type construction, the middle phase will draw nearly half the current compared to outerlimb windings. But with delta connected windings, less current will be in one of the outer phases, with Yd1 (Dy11)connection in U phase and with Yd11(Dy1) it will be in W phase.This difference seems to be due to difference in length of magnetic paths.

Difference in excitaion current between units is common.This is due to the difference in core building.If there is more gaps at the lamination joints, excitation current will be more.

Magnetic balance test is not a proper test and relevant only for distribution trfs where you can apply nearly rated voltage at site.More appropriate and correct test is to measure single phase low volatge excitaion current with other windings kept open(Doble test as they popularised it since 1964)