getting the proper transformer for a 40kw system

[alfiesauce]

I'm installing a 40kw 10k solar array that is going to, when combined output 120/208 3phase, on a building we intend to connect to the 277/480v service. There are no existing transformers close to where I need to convert to the 277/480 that I am able to tie into, so I need to supply a transformer.
I was given the transformer "everybody uses for this" and am concerned about connecting it up.
The transformer is a 480v delta primary with a 120/208 wye secondary. Originally I thought this was perfect until I read that I cannot connect the x0 point on the transformer if using it to backfeed, which I am.
the 10k solar inverters do not actually output a neutral, they do not establish a ground point on the inverters like the old enphase microinverters, however it would seem I still need one as when connected in the 3 phase system it won't be balanced perfectly. They output 208 single phase on one inverter, I have 6 inverters that I will be connecting to a 3 phase combiner panel that will then produce a 208 3 phase system.
I am wondering if, because there is no other reference to ground on the secondary side of the transformer, if applying a GEC to the x0 and creating a grounded conductor reference at that point, if that should be OK, of if I need to find another solution. Is it possible they are actually producing 208 delta and I don't need to connect a neutral at all to the system?
any thoughts would be greatly appreciated.

 

[GoldDigger]

If the inverters do not use any neutral connection and have basically floating 208 outputs, I see no reason not to ground X0.
There will be no current through it and it will simply serve to limit the phase to ground voltage.
If there were any loads in addition to the GTIs, I would want to think it through more. But that should not be the case.

 

[electrofelon]

Side question relating to this topic: do the new requirements that backfed transformers be labeled/approved for such use apply to pv grid interactive installs?

 

[GoldDigger]

Side question relating to this topic: do the new requirements that backfed transformers be labeled/approved for such use apply to pv grid interactive installs?

Backfed in terms of reversal of the role of primary and secondary or passing of power in "backwards" direction regardless of ratio? It almost makes a difference which for GTI PV.

 

[iwire]

If the inverters do not use any neutral connection and have basically floating 208 outputs, I see no reason not to ground X0.
There will be no current through it and it will simply serve to limit the phase to ground voltage.
If there were any loads in addition to the GTIs, I would want to think it through more. But that should not be the case.

You should never ground XO when back feeding a delta / wye.

If you lose a supply phase the jumper to ground will carry current and can melt. I know of two instances of this.

 

[iwire]

This is from Square Ds FAQ regarding reverse wiring transformers

3. If a Delta-Wye transformer is to be backfed so that the Wye side is the input, do not connect the neutral terminal to the primary system neutral, nor should the neutral terminal be connected to ground.

http://www.schneider-electric.us/si...country=US&lang=EN&locale=en_US&redirect=true

 

[Smart $]

You should never ground XO when back feeding a delta / wye.

If you lose a supply phase the jumper to ground will carry current and can melt. I know of two instances of this.

Were those two instances backfed with GTI only? ...or POCO only? ...or both?

When you say lose a supply phase, are you referring to POCO supply or GTI supply? ...and do you literally mean 'phase' or 'line'?

What melts, the jumper?

I'm trying to comprehend how this happens.

 

[Smart $]

This is from Square Ds FAQ regarding reverse wiring transformers



http://www.schneider-electric.us/si...country=US&lang=EN&locale=en_US&redirect=true

Your link does not go directly to the FAQ.

Try this one:

http://www.schneider-electric.us/su...n_US&searchid=1407420637087&answerid=16777217

 

[iwire]

Customer owed transformer, inside the building, reverse wired.

Building had a single phase event from power company, they lost a phase.

The bonding jumper someone put in from XO to ground melted the terminal or hardware during this event.

This is the best of my recollection, I did not go on the call, my coworker did.

The other time it was a forum thread.

During that thread it was pointed out that current from the remaining two connected phases was trying to get back to the source via this X0 grounding connection.

 

[iwire]

Your link does not go directly to the FAQ.

I know, it never does.

 

[Smart $]

I know, it never does.

Right click on the link that takes you to the FAQ page and choose "Copy Link Address" (Chrome) or "Copy Shortcut" (IE), rather than copying the address bar when viewing the page.

 

[Smart $]

Customer owed transformer, inside the building, reverse wired.

Building had a single phase event from power company, they lost a phase.

The bonding jumper someone put in from XO to ground melted the terminal or hardware during this event.

This is the best of my recollection, I did not go on the call, my coworker did.

The other time it was a forum thread.

During that thread it was pointed out that current from the remaining two connected phases was trying to get back to the source via this X0 grounding connection.

Sounds like in both cases it was a 'line' lost rather than a 'phase' and the source of low voltage side was POCO...???

 

[iwire]

Sounds like in both cases it was a 'line' lost rather than a 'phase'

:?:?

Interchangeable slang to me. I don't understand your distinction.


The power company had a dead 'phase' or dead 'line' coming to the building. You know, phases A&C have 120 to ground and 208 between them but any readings to phase B are either 0 or very low.

and the source of low voltage side was POCO...???

Not low, absent.

 

[iwire]

Right click on the link that takes you to the FAQ page and choose "Copy Link Address" (Chrome) or "Copy Shortcut" (IE), rather than copying the address bar when viewing the page.

Have you tried your link?

All I get is a Square D error page.

 

[Smart $]

Have you tried your link?

All I get is a Square D error page.

Yes, I have tried it. Takes me directly to the FAQ page in Chrome. Just tried in IE and it gives error.

:?

Edit: Just retried in IE and it goes directly to FAQ page. :?:?:?

 

[iwire]

Edit: Just retried in IE and it goes directly to FAQ page. :?:?:?

No good in Firefox.:huh::?

No big deal, go to the main FAQ and type in reverse wired transformer and it comes up.

 

[Smart $]

:?:?

Interchangeable slang to me. I don't understand your distinction.

Line is the conductor and only one end of the 'phase'. 'Phase' refers to line [-to-line] voltage and angle.

If you lose a line in a wye supply, you lost a leg, but technically you lost two phases. If you lose a line in a delta supply, you lose two phases. Single phasing refers to what's left, not what was lost (two phases).

The power company had a dead 'phase' or dead 'line' coming to the building. You know, phases A&C have 120 to ground and 208 between them but any readings to phase B are either 0 or very low.

That would be 'line' or 'leg' B was lost... but I understand many (most?) refer to this as losing ?B, but that is technically incorrect as both ?B (AB) and ?C (CB) were lost.

Not low, absent.

I was referring to the low voltage side being backfed by POCO source... not the then current condition...

:rotflmao:

 

[Smart $]

No good in Firefox.:huh::?

No big deal, go to the main FAQ and type in reverse wired transformer and it comes up.

Yep... it's a problem with how their site scripts/codes the links.

I think it may work once you have a Schneider page open or in active cache, i.e. their site interprets it as an external link on first visit.

 

[ggunn]

I'm installing a 40kw 10k solar array that is going to, when combined output 120/208 3phase, on a building we intend to connect to the 277/480v service. There are no existing transformers close to where I need to convert to the 277/480 that I am able to tie into, so I need to supply a transformer.
I was given the transformer "everybody uses for this" and am concerned about connecting it up.
The transformer is a 480v delta primary with a 120/208 wye secondary. Originally I thought this was perfect until I read that I cannot connect the x0 point on the transformer if using it to backfeed, which I am.
the 10k solar inverters do not actually output a neutral, they do not establish a ground point on the inverters like the old enphase microinverters, however it would seem I still need one as when connected in the 3 phase system it won't be balanced perfectly. They output 208 single phase on one inverter, I have 6 inverters that I will be connecting to a 3 phase combiner panel that will then produce a 208 3 phase system.
I am wondering if, because there is no other reference to ground on the secondary side of the transformer, if applying a GEC to the x0 and creating a grounded conductor reference at that point, if that should be OK, of if I need to find another solution. Is it possible they are actually producing 208 delta and I don't need to connect a neutral at all to the system?
any thoughts would be greatly appreciated.

Why don't you use 480V native inverters and avoid the issue and the expense of a transformer? For 40kW AC you could use a pair of Solectria or Sunny Tripower 20kW inverters.

Also, I was told by a transformer company when I had the opposite problem (480V inverters and a 208V interconnection) that transformers (theirs, at least) are not step-up to step-down interchangeable. With the same terminals they had different part numbers for up and down conversion.

 

[SolarPro]

Why don't you use 480V native inverters and avoid the issue and the expense of a transformer?

Agreed. There are so many 3-phase transformerless string inverters on the market right now with a native 480 Vac output that the premise behind the OP seems flawed. Keep it simple. Spec an inverter that does everything you need it to do. Bob's your uncle.

You can down load an excel spreadsheet with current inverter specs here and sort and search the data for an appropriate product.