reverse use of standard delta Y three phase transformer. 208 in for 480 out

[kwired]

I don't have a 2014 as we are on the 2011. Isn't the option still there to either bond and ground a corner of the delta OR provide Ground Detectors?

None of that has changed, if you have a secondary with no neutral that is still necessary, but it is saying you can't just take any transformer and supply it from whichever side is convenient for you, it must be intended to be "back fed" if that is what you are going to do.

What I have in quotations in post 6 is the entire 450.11(B), and it is a new section in 2014.

 

[GoldDigger]

The problem I still have is that the determination of which is the forward and which is the back direction is 100% clear when the supply is on one side and a load is on the other.
But for a PV situation, is forward and back defined by power flow in normal operation or by upstream/downstream or by which winding power is first applied to (energization), or even which winding has voltage adjusting taps?
In a PV environment the directions will differ depending on which definition(s) you use.

 

[winnie]

GoldDigger raises a point that has come up in several threads recently. Which side of the transformer should be considered the 'primary' when power is being provided by a line interactive inverter.

My take: The _primary_ of the transformer is the one that gets energized first, the one with the voltage taps that you adjust to get proper core excitation.

Transformers, by their physics, are bidirectional. In normal use, reactive power is circulating back and forth. So once the device is installed and energized there is no difference between primary and secondary.

Where you have to distinguish between primary and secondary is when you consider things such as taps (how does the tap setting change core saturation) and on startup (which coil will see 'inrush' current).

Even though power is flowing from the PV inverter to the transformer and then to the grid, IMHO the primary of the transformer is on the grid side of things. The grid is setting the voltage of the system, and inrush will be from the grid side. The PV inverter won't even supply power until it sees voltage from the grid, and the PV output voltage is set by the grid (via the transformer).

An analogous situation would be with a transformer connected induction generator; until the transformer is energized _from the grid side_, and until the grid is providing exictation, no power flows from the generator.

-Jon

 

[GoldDigger]

I agree with you on that analysis. The one "directional" issue remaining, IMHO, is the situation if the transformer uses compensated windings and does not have a tap range sufficient to get the voltage under load on the GTI side back to normal. And that is a non-issue if power type dry transformers do not use compensated windings.

 

[jim dungar]

And that is a non-issue if power type dry transformers do not use compensated windings.

The smaller the transformer the more likely compensated windings have been employed.
From my experience:
above 30kVA compensated windings are a non issue
below 15kVA compensated windings are likely to be of concern
below 3kVA compensated windings are all extremely common, especially because these units rarely have 'taps'.

 

[GoldDigger]

I agree with you on that analysis. The one "directional" issue remaining, IMHO, is the situation if the transformer uses compensated windings and does not have a tap range sufficient to get the voltage under load on the GTI side back to normal. And that is a non-issue if power type dry transformers do not use compensated windings.

And just to bring it out explicitly, I would expect that if the wye versus delta primary winding usage is questioned, the answer is that since the grid is very stiff and the GTI voltage will simply follow it, the transformer should either be delta-delta (if the GTIs use a line to line connection and can tolerate corner grounding) or delta-wye with the delta side facing POCO.