[danar]

I energize a Y-Yn transformer 13.8kv/380/220. At no load
all line to line voltage at the secondary were 380v but L1-N=215, L2-N=209 and L3-N=218 any one can explain me what is the cause of these unbalance voltage?? Ty..

[sarnian]

Hello danar

Seems a case where the transformer is not properly balanced, which caused the neutral to move slightly from the nil position. Did you also test the voltages under equal loading? Same result?

[danar]

Yes I connected a three phase load but same result. Somebody advise me to ground the neutral on the primary side but there is no neutral bushing in the primary..

[sarnian]

Hello danar

As you have a Y-Yn transformer there is no need for a primary neutral!
The problem is in the transformer itself.
Do you expect problems from using the available 215V, 209V, and 218V ?
That's all within 2½% of the proper neutral point.

[sathe1963]

Once request to check ratio test of the transformer. If OK, ground neutral point on secondary side directly or through ngr according to the rating of the transformer.