Voltage Neutral to Grnd - Transformer WYE-Grnd 120/208 step up to WYE-Grnd 480/277

[synchro]

Greetings,

I have an installation with a related issue.

Is a PV installation where the System is 208V and im using a Y-Y transformer for getting 480VY for the solar PV inverters.

The issue is that when i connect the 480VY/208VY transformer i get a circulating current from the neutral of this transformer to the neutral of the distributión transformer of the client 12470Y/208Y. Is there a particular reason for this? What could be a solution?

I take it that you're getting this neutral current without the PV inverters operating? If so then the Y-Y transformer may have a tertiary delta winding that in simple terms will try to force the line to H0 voltages on the 208V side to be equal. And so any imbalance in the L-N voltages from the POCO could cause significant neutral current to flow if H0 is bonded. The transformer at the link provided by electron_ranger in post #9 of this thread has a tertiary winding.
If your transformer has a tertiary winding the H0 of the 208V wye should be floating. Then the H0 voltage is free to move to an operating point where the L-H0 voltages on the 208V side are equalized by the tertiary winding, therefore also equalizing the L-N voltages on the 480V side of the Y-Y.

But if the Y-Y doesn't have a tertiary winding, then a floating H0 would allow the L-H0 voltages on the 208V side (and therefore the L-X0 voltages on the 480V side) to drift around and be unstable. The zero sequence impedance seen by the inverters would be high even when X0 is bonded because of the Y-Y configuration, which would not be a good situation since the inverters are effectively current sources not voltage sources.
Basically, floating the center connection to a wye is appropriate if you want to equalize currents, but not for equalizing voltages.

 

[LarryFine]

But if the Y-Y doesn't have a tertiary winding, then a floating H0 would allow the L-H0 voltages on the 208V side (and therefore the L-X0 voltages on the 480V side) to drift around and be unstable.

That what I was/am thinking. A Y-Y, like a D-D, is effectively three individual 1ph transformers, and both ends of each winding needs to be supplied with a fixed voltage. Otherwise, the secondary voltages would vary with load, and be reflected by the primaries.