measuring the yellow phase of distribution transform the voltage dropped drastically off load, whiles all the other phases are normal. I tried to increase the tapping still no change,please help?:

Abdul,

Something is grossly wrong. I would start with a noncontact thermometer and start scanning all the connections on the yellow phase, I suspect that you will find a loose connection that has developed into a high resistance point with excessive voltage drop that is now overheating. Let us know what you find.

Alan

Okay I'll try and will let you know.

May a poor connection in the Tap Changer. Suggest taking an oil sample and get some dissolved gas analysis done. This will give an indication of any arcing within the transformer. I am assuming that the transformer is oil filled.

I agree with Reg. It can be a loose contact in the tap switch (guess it is off-circuit tap switch).
The off-circuit tap switches are known to give contact problems if not operated for a long time.

I will be opening the transformer today to check all connections and also oul sampling will be done. will give you full updates

Can you provide the following,
1) Either measuring voltage or current, the voltage drops.
2) T/F vector group.
3) Turn ratio.
4) Measure individual winding resistance value.

On-load circulating voltage offload significant drop. This suggest. that the Transformer winding is open circuited or defective tap changer.


Kind regards