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Joined: Aug 2006
Posts: 81
Albert Offline OP
We're installing 167 KW of 480V inverters in a building that has a 120/208V wye service. The inverters need a neutral, so we'll be using a 480/277V wye - 208V delta transformer (225 KVA).

Looking at a similar (almost identical) installation, I saw they used a Hammond HPS Sentinel G transformer marked "Inrush: 15X (Max) / Suitable for PV/Solar Bi-directional use".

Our distributor has proposed the same transformer except that it's marked "Transformer suitable for step up operation only" in place of the markings noted above. It's a stock item, whereas the PV-rated unit has a longer lead time and is more expensive.

A transformer in a PV system like this is an interesting case because the roles of primary and secondary reverse from startup to production operation. When the installation is initially energized, the 208V side will be the primary with little or no secondary load because the inverters will be dormant. But once the inverters begin producing, the 480V side will become the primary, feeding the inverter output to the building through the 208 secondary.

I've learned there are a several factors to consider, as described in these links:

Reverse Feeding Dry-Type Transformers

Can a transformer be back-fed or used in reverse?

Joined: Apr 2002
Posts: 7,389
Likes: 7
I see where this is going, and availability (off the shelf) and 'special order (8-12 weeks) plus high price seems to be the major reasoning for this application question.

I use this solution when a project like what you describe comes in for plan review and release. A email, and/or phone call to the EC or responsible party requiring a signed and sealed letter with info from a NJ Lic. PE/EE that the proposed installation is approved by that PE/EE.

I am not aware of any situations that are related to the dry type transformer 'backfeed' installations. Also, FWIW, to the best of my knowledge, our utility company uses the standard pad mount transformers for direct grid tie solar installs that are utility owned,

Joined: Jul 2002
Posts: 8,443
Likes: 3
Hi Albert,
All this stuff is reasonably new to me with inverters and what-not,
however, I do know power distrbution systems well.

From a personal point of view, I would never reverse-feed any transformer, sure, distribution transformers
you "may" get away with, but you need to realise that the transformer will need to be de-rated,
by a certain factor, I can't off-hand tell you what that is, as they are all application-specific.
It could something as low as 65% or as high as 85%, this de-rating factor needs to be taken into account.

If it were me doing this job, I would wait and get the correct unit for the job.
I don't want to come across as some sort of an (expletive), but all of this should have been worked out
in the original planning stages to procure the right equipment, before the job even started.
I see this time and time again over here, involve so-called "engineers" that couldn't plan their way out of
a wet paper bag.

Joined: Sep 2003
Posts: 650
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.


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