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Reverse-feeding transformer in PV system? #219985 03/10/19 09:48 PM
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?

Tools for Electricians:
Re: Reverse-feeding transformer in PV system? [Re: Albert] #219988 03/11/19 07:37 AM
Joined: Apr 2002
Posts: 7,133
HotLine1 Offline
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,



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