Transformer

Dear Sir,
I am facing the problem in 300 kva transformer, Input voltage is given 220 Volt 3 ph. Output voltage available at terminal only 355 Volt 3ph. Originally transformer is input 230 X 3ph and Output is 400 X 3ph, transformer is not auto transformer it is step up transformer
Please suggest me how to currect the output

Based on input of 230 and output volt of 400 I get ratio of 1.73. So on a 220 line you should get 382. Are you sure your voltage readings are correct ie 355 A B and A C and B C
and you have 220 across all phases?
and you still have this connected wye wye.
Do I really see factory dwg showing both star points tied together and that is how you have it?
Are you taking these readings with no load or with unit loaded?
What is the load?

Dan Bentler

I agree with Dan and I suggest that you also do,
Checks that all 3 phases on your input are drawing some current.
As it may be possible that if you have a blown supply fuse, voltages may appear to be present at the inputs, but may be the reason for a lot lower than expected output voltage, because of vector phase shifts.

Note that the nameplate says output voltage is 400 at no load. It may well be 380 at full load.

At that rate, your measured output voltage is not that far off.

I can't read the impedances but if they are higher than normal, the voltage will sag even more on heavy load.

There may be nothing wrong here (other than maybe selecting the wrong transformer).