Hi Trumpy,
Please note that this thread is almost 2 years old....
In any case, there are two different and important aspects of 'reverse feeding' a transformer. One is which way power is flowing, the other is which side of the transformer is 'primary'.
In this application, the 'primary' of the transformer is the 208V delta side, connected to the utility service. The 208V side gets energized first, the 208V side sees the magnetizing inrush, and the 208V side will have the necessary 'taps' to adjust for variations in supply voltage versus the nominal 208V value, and the 208V side will be setting the magnetic flux level that the transformer is operating at (thus the need for taps on the 208V side).
The 208V side remains the primary even when the PV system is producing and power is flowing from the 480V side to the 208V side.
The PV inverters are not setting the voltage; they are responding to the voltage from the transformer.
So IMHO the normal 208V delta to 480/277V wye transformer is not being 'reverse fed' in this application. This transformer is a step up transformer with its utility supply connected on the 208V side. The fact that power is flowing from the 480V to the 208V side is no different than having a load with a power factor on the 480V side, where power flows from the 480V to 208V side for part of the AC cycle.
Now: if the PV system were operating in 'island mode' producing 480V 60Hz even with the transformer disconnected, and you were planning to connect the transformer after the 480V side was 'hot' then this would be reverse feeding.
There might be other reasons for using some sort of special PV rated transformer, eg. harmonic control, voltage regulation with reverse power flow, warm fuzzy feelings, 'effective grounding', etc. But with respect to the original question, a step up transformer is still being used as a step up transformer.
-Jon
