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..

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?

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..

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.

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.