Transformer sizing for step up and step down transformers

[PVfarmer]

Forget the PV for a moment, there is no such transformer that will take 120/240 single phase in and output 480/277 three phase, there are phase converters, VFD's or other units that use a combination of other basic components that can do this - but none of them can be "reverse fed" to convert three phase to single phase.

I hear you -forget the PV, except for one thing...
zman keeps asking about 1000V DC systems- so that means there is no doubt he's trying to use the 480/277V wye SE inverters.
So he'd be using the transformer he says he ordered (if he ordered the one he posted a slip for way back in post # 7 or 8) to convert the 480/277 wye inverters (which are necessary for 1000V DC) *down* to 120/240 1ph.
However, it seemed the xfmr in his pic was 480 delta!

Forgetting PV, if you had a 480/277V wye source of power (N included), you would *not* get a 480V delta transformer, no matter what phase or voltage the other side of it was, step up or down.

Back to PV, the neutral is required by the 480/277V PV inverters for voltage reference- where's the N going to go if you've got a 480V delta xfmr? Into the ground?
That sounds like a pretty poor place for a voltage reference! :huh:

kwired- Can you answer this?
Seems like 480V primary xfmrs are quite common, however 480/277 *wye* primary xfmrs aren't standard at all.
480/277 wye *secondary* xfmrs are the common ones. They all say "reverse connectable".
If you wanted to supply loads from a 480/277V wye source, are there any obvious contrasts between using:

a bank of three single phase 277V to 120V xfmrs, wired in wye on both sides, so you've got 208/120V wye for loads.
vs.
a single 208/120V pri / 480/277 secondary (both wye of course) xfmr "reverse connected".

Not asking about price or available space here. For instance, 45kVA of xfmr(s), so a bank of three 1ph 15kVA 277V -> 120V vs. a single 3ph 45kVA 208/120V -> 480/277V in reverse.

I can't seem to come up with any difference beyond a few hundred $$ or less.

Anyhoo, this is how I see the current situation in this thread.
zman appears to have a 480V delta to 120/240V 1ph xfmr on the way.
He wants to use SE inverters, either 1ph or 3ph, not suite sure, but he wants to install 66kW or more of AC PV output.
He only has a 50kVA (or maybe even 25kVA) 120/240V service xfmr!
STOP RIGHT THERE!!

There's no way you can go ahead and install 66+kW of PV without talking to your POCO first. Do that, zman.

Beyond that, there are 480V delta output inverters made- pretty sure Fronius and KACO for two. These don't have an N output circuit, and are 480V L-L.
I'd assume they make those inverters because...wait for it...
people still have 480V delta services.
Unfortunately, zman, you don't.
The only way that I can comprehend the 75kVA 480V delta -> 120/240V xfmr you ordered working for you is if you had 480V delta service, and wanted to supply 7500w of 120/240V loads (10% of the 75kVA and limited to 3250w on each line of 120V) from a breaker off of a main 480V delta panel to that new xfmr.

If you are going to go with 1 ph SE 120/240V inverters, you...wait for it...
don't need to buy an xfmr at all! Cancel the order! Why would you step it up to 480V when 480V isn't involved anywhere in the system? :huh:

At least I wouldn't think the POCO would charge you for changing a 50kVA to a 75kVA service can.

Please, if there's something else zman's new xfmr is intended for besides a 480V delta source or power (or reverse fed by 120/240V for some totally weird reason), somebody let me know?

 

[jaggedben]

Solar edge asked told me I need a 120/240 primary and 480/277 secondary so that is what is being quoted by a manufacturer that told me they can make anything.
So price is number one concern will it be cheaper if so I may be on to something if not it was fun and has a chance of being the best way of installing larger ground mounts on single phase services idk we see

To repeat, this makes perfect sense if they think you have a three-phase high-leg delta service, and it makes zero sense if you have a single-phase service. If all you told them was that you have a 240/120 service and are using three-phase inverters, they probably assumed high-leg delta because they wouldn't think that someone would use three phase inverters on a single phase service.

Easy confusion to have happen. Big consequences.

If it talking about me I attached a picture of the service

I remember. Sure looks single phase to me. Unless that panel is only connected to one phase. Is there another three-phase panel? I've seen one service that one panel for the single-phase and an different one for the three-phase. Weird old thing.

Ok so 6*11.4 se inverters is 6*60=360 amps single phase so tap conductors line side fuse at 400 all good not my problem if utility has transformer issues not associated with cost of installing.

It may or may not be your problem. A lot of times the rules change for systems over 10kW, or 30kW. It depends on the laws in your state.

Ladies and gentlemen it can someone explain nominal operating voltage in data sheet for solar edge

I agree they wrote the datasheet in a weird way. They should have just said "AC Output: 480/277 Wye, three-phase."

The nominal voltages are the middle numbers. For purposes of transformers and system design, you mostly ignore the other numbers. Actual utility voltage can go up and down, and the inverter will shut off if it goes below the min or above the max. You need to consider voltage/rise with respect to the max.

I think most of us are asking (for your best interest) that you confirm that it is indeed 120/240 single phase that they are going to send you, as most are betting there is a misunderstanding and they intend to send you a 120/240 delta three phase unit.

This is a basic misapplication of transformers in general before you even get into some of the more technical reasons why it might not work.

Forget the PV for a moment, there is no such transformer that will take 120/240 single phase in and output 480/277 three phase, there are phase converters, VFD's or other units that use a combination of other basic components that can do this - but none of them can be "reverse fed" to convert three phase to single phase.

Hey you're pretty smart for a guy who doesn't know solar! Seriously, thanks for chiming in with that.

 

[kwired]

kwired- Can you answer this?
Seems like 480V primary xfmrs are quite common, however 480/277 *wye* primary xfmrs aren't standard at all.
480/277 wye *secondary* xfmrs are the common ones. They all say "reverse connectable".
If you wanted to supply loads from a 480/277V wye source, are there any obvious contrasts between using:

a bank of three single phase 277V to 120V xfmrs, wired in wye on both sides, so you've got 208/120V wye for loads.
vs.
a single 208/120V pri / 480/277 secondary (both wye of course) xfmr "reverse connected".

Not asking about price or available space here. For instance, 45kVA of xfmr(s), so a bank of three 1ph 15kVA 277V -> 120V vs. a single 3ph 45kVA 208/120V -> 480/277V in reverse.

I can't seem to come up with any difference beyond a few hundred $$ or less.

Most common thing probably found in a three phase unit is 480 delta connected primary and 208/120 wye secondary.

There seems to be a lot of these back fed when 480 is needed for a specific load but 208 is service voltge, but still not nearly as many as what are used for step down.

Most 240 volt delta systems are utility supplied - but you occasionally find a 480x240 three phase unit - usually a specific reason for preferring 240 over 208 though.

But is rare to find a Wye primary on any of them AFAIK. You generally don't want to connect the neutral on a wye primary when it is a single core unit as it will often result in some circulating currents that will cause damage. Multi-core units or banks don't have this problem. I don't have all the reasons why on top of my head as I don't deal with this problem on a daily basis. Several stories on this site where someone back fed a 480x208/120 and burned it up, or at least experienced high current when they connected the neutral in the primary to the source (low side when backfeeding), but if they leave that neutral "float" there isn't that problem.

 

[GoldDigger]

The circulating current problem when connecting the neutral on the primary side happens when you have a delta connected secondary. It does not matter whether it is a single core or individual transformers. And even a wye-wye transformer can be a problem if it contains a "buried" delta tertiary.
The problem occurs *only* when there is a voltage imbalance on the primary side. Engineering and operational attention to voltage balance is one reason that utilities can safely use grounded wye to delta configurations.

 

[PVfarmer]

But is rare to find a Wye primary on any of them AFAIK. You generally don't want to connect the neutral on a wye primary when it is a single core unit as it will often result in some circulating currents that will cause damage. Multi-core units or banks don't have this problem. I don't have all the reasons why on top of my head as I don't deal with this problem on a daily basis. Several stories on this site where someone back fed a 480x208/120 and burned it up, or at least experienced high current when they connected the neutral in the primary to the source (low side when backfeeding), but if they leave that neutral "float" there isn't that problem.

GoldDigger The circulating current problem when connecting the neutral on the primary side happens when you have a delta connected secondary. It does not matter whether it is a single core or individual transformers. And even a wye-wye transformer can be a problem if it contains a "buried" delta tertiary.

The problem occurs *only* when there is a voltage imbalance on the primary side. Engineering and operational attention to voltage balance is one reason that utilities can safely use grounded wye to delta configurations.

Sweet answers, people!
kwired-
That's what I keep trying to tell zman- he can't "float" the neutral with a 480/277V WYE inverter- the N is part of the whole shebang.
So his 480V delta xfmr isn't going to work. At all.

GoldD- you were totally right back in post #68 when you mentioned flux! Why you are right, I have no clue.

Could you clarify the "voltage imbalance on primary side thing"?
Say you wanted to use the xfmr linked below, reverse fed, with the 480/277V PV inverters connected to grid line side, so the PV is going into a main panel and out to the (secondary side/480/277V) of the xfmr thru a breaker in the main panel.

In that scenario, would the PV inverters (and grid) be taking care of balancing the voltage on the "primary side", "primary side" wires being connected to the xfmr "secondary side"?
So the 480V load would be supplied by the 480V delta "primary" of the xfmr?
There's a 208/120 pri and 480/277 sec model of the same type of xfmr.

Just checking- so you're saying a 208/120 wye primary / 480/277 secondary run in reverse is inherently better than a 208 delta pri / 480/277V run in reverse?


My POCO says-
If the Customer is permitted to interconnect through an un-grounded source,
a “zero-sequence” voltage or “3V0”” scheme will be required on the primary
side of the approved delta primary wound transformer supplying the DG
system. In cases where the Company’s EPS is an ungrounded circuit, the
3V0 scheme may be waived at the Company’s discretion on a case by case
basis. See Figure 4 for additional information.

Harmonic Mitigating Transformers with zero sequence flux cancellation technology are specifically designed to treat the harmonics generated by computer equipment and other non-linear, power electronic loads.
They treat zero sequence harmonics (3rd, 9th and 15th) within the secondary windings and 5th and 7th harmonics upstream with an appropriate phase shift.
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0664.html

 

[GoldDigger]

PVfarmer,
If the wye voltages on a POCO 208Y/120 service are 118, 120, and 122, then the vector circle of voltages on the secondary delta will not form a closed triangle. To correct this currents must flow in both primary and secondary until the IR voltages drops make the triangle close.
The current required could be greater than the full load voltage of the transformer, since the resistances involved are so low.

As for the designation of primary, the most widely accepted is that it is the side which gets energized first and therefore supplies the inrush and magnetizing current. It is also usually where any voltage adjusting taps are located. It is also where the phase and voltage relationships are established.
In a PV interconnection the primary goes on the POCO side.
And that must be delta or center-floating wye.
The fact that power flows from PV to POCO does not affect the primary designation. If the POCO side would be the preferred output side of the transformer it would be wired backwards, and the transformer would have to be approved for reverse operation by the manufacturer.

 

[PVfarmer]

In a PV interconnection the primary goes on the POCO side.
And that must be delta or center-floating wye.
The fact that power flows from PV to POCO does not affect the primary designation. If the POCO side would be the preferred output side of the transformer it would be wired backwards, and the transformer would have to be approved for reverse operation by the manufacturer.

Another great answer!
I'm not saying using that linked transformer for a POCO connection- just for local loads.
With a 480/277V inverter and 480/277V service, and the inverter connected line side via junction box...then from the JB to a main breaker/panel- then say a 70A breaker in the panel for 45kVA of xfmr.

There's a 208/120 pri and 480/277 sec model of the same type of xfmr....

So both PV and grid power would be 480/277V stepped down by being reverse fed, into the 480/277 secondary and out to loads from the 208/120 primary.

So when the xfmr is reversed, this that you said-
"The circulating current problem when connecting the neutral on the primary side happens when you have a delta connected secondary."

could also mean, "circulating currrent problem when connecting the primary (supply) wire neutral to the wye xfmrs *listed* secondary happens when you have a delta primary *listed* side supplying loads?

 

[GoldDigger]

It does not matter whether there are loads on the delta or not.
You can only use a transformer in reverse (e.g. step up used for step down) the manufacturer must specify that it is reversible.
And it does not matter whether it is the listed primary or not, the side facing the supply should not be wye with neutral connected if the load side is delta.

 

[ggunn]

Solar edge asked told me I need a 120/240 primary and 480/277 secondary so that is what is being quoted by a manufacturer that told me they can make anything.

Understand that when a transformer manufacturer says that they can make "anything" they mean that they can wind a transformer with any voltage ratio and kVA capacity you like. They do NOT mean that they can make a transformer that will allow you to connect a three phase inverter to a single phase system.

 

[PVfarmer]

Originally Posted by zman990

Solar edge asked told me I need a 120/240 primary and 480/277 secondary so that is what is being quoted by a manufacturer that told me they can make anything.


zman!
We're on page 13 here and we're still missing 2 very important facts:
1. Which model of SE inverters you are using.
Is it-
A. 480/277V 3 phase
or
B. 120/240V 1 phase

If we don't know that, we have no idea which way you are running your xfmr.
But this-
I need a 120/240 primary and 480/277 secondary

is NOT the same thing as the pic you posted- your pic didn't say 277V, it just said 480 to 240/120. That sounds like delta 480V.

So!

2. What xfmr did you order?

The 3 phase SE inverters (wye) connected to a 480V delta xfmr...
1. won't work.
2. If the xfmr was 480/277V 3ph wye to 120/240 1ph (which doesn't exist), but if it did, you'd only get 25kVA of 120/240 out of it.

If you are connecting 120/240V inverters to the low side of the 480V xfmr you bought...WHY?
You don't have 480V anything!
You'd connect them straight to your 120/240V service.

Understand that when a transformer manufacturer says that they can make "anything" they mean that they can wind a transformer with any voltage ratio and kVA capacity you like. They do NOT mean that they can make a transformer that will allow you to connect a three phase inverter to a single phase system.

Solar edge asked told me I need a 120/240 primary and 480/277 secondary

There has to be a miscommunication here for zman- ^^ that would be a xfmr that changed 1ph into 3ph.
The are 1071 choices for 120/240V pri here- ALL single phase.The choices under "input" (primary) are 110/220, 115/230, and 120 x 240 - searching for "120/240V pri" shows that there is no such thing is what I'm getting at.
http://www.temcoindustrialpower.com...h_bar=1&t1=120/240V pri&submit_x=0&submit_y=0

 

[zman990]

They actually can't make a transformer so to all you that laughed and said I'm cra,y pretty much I agree it would be a good way to reduce cost but not possible.
I give up it is 6 inverters


Sent from my HTC One_M8 using Tapatalk

 

[electrofelon]

If you have a long run and want to reduce wire losses/wire costs, you may want to investigate running the PV source or output circuits across the long run rather than the inverter output circuit. That gets you in the 400's for 600 volt systems.

 

[ggunn]

They actually can't make a transformer so to all you that laughed and said I'm cra,y pretty much I agree it would be a good way to reduce cost but not possible.
I give up it is 6 inverters

I never laughed at you or said you were crazy, only that you were mistaken and not very knowledgeable in the fundamentals of AC power. Forge on and keep learning; that's what we all do.

 

[jaggedben]

They actually can't make a transformer so to all you that laughed and said I'm cra,y pretty much I agree it would be a good way to reduce cost but not possible.
I give up it is 6 inverters

Just tryin' to save you money, bro.

If you have a long run and want to reduce wire losses/wire costs, you may want to investigate running the PV source or output circuits across the long run rather than the inverter output circuit. That gets you in the 400's for 600 volt systems.

Notably, with Solaredge the DC nominal voltage varies according to the grid connection. For 240V single phase it is 350V. I'm not totally sure, but I believe the system will compensate for voltage drop to maintain the input voltage, as long as your optimizer strings are long enough to do it.

 

[PVfarmer]

If you have a long run and want to reduce wire losses/wire costs, you may want to investigate running the PV source or output circuits across the long run rather than the inverter output circuit. That gets you in the 400's for 600 volt systems.

zman- He's right about that. You can do your 350 foot run with DC instead of the 480VAC that you wanted- it'll *probably* output almost nearly as much PV, like within a few kWH a year. (See ++ at end of comment)
I did about 2 dozen simulations for exactly that situation- try this program called Helioscope (it's free for the 1st month)- you'll see what I mean.
You can plug in the SE inverters and whatever panels you want to use, and move all the combiners around and change wire sizes- it's pretty great.

Then you can use the NREL/SAM simulation program to figure out the $$ side of things. (Cost/electric bill avoided/incentives,etc.)

And about this-
I give up it is 6 inverters
Don't forget- THE POCO!!! They are very important here!
That 400A main that you posted a pic of doesn't necessarily mean that there's a 400A xfmr feeding it.
You've got 300A with those 6 inverters. Your POCO won't be happy about 300A through what might be a 200A xfmr. --300A = (12kVA/240V)*6)

I had the same sort of thing here- the farm had a 400A main switch, I thought it was a 50kVA/400A xfmr/service- then I talked to the POCO connections dept, and they said it was *25kVA*. Whoops- glad I checked!

Max PV with 50kVA of transformer wasn't really enough either for the farm's load (almost but not quite)- but 150kVA of 480/277V 3ph sure is!! :happyyes:
You'll never know until you talk to them-can't hurt to ask, if they'll give ya a low or no-cost upgrade to 480/277V service...well then you wouldn't need a xfmr for those 480/277V SE inverters.
(In my opinion, 3ph PV is better for the grid than 1ph PV, so they shouldn't charge for that type of thing.)

++Just a warning- if you try Helioscope and compare the 480/277V inverters at ~800VDC to the 120/240V models at ~400VDC with all other things being equal...well, you might like it, a lot!

 

[PVfarmer]

Forge on and keep learning; that's what we all do.

That goes for me too, right man?

Help me out here- something is still weird here.
zman has to have a 400A breaker for his 300A of PV- where's it going to go?
Can't go line side of a 400A main.
He could just get a 600A main panel then to fix that? But those cost as much as an inverter...

But besides that- he can't put 72kVA of 1ph xfmrs through what is probably a 50kVA 1ph xfmr. A 75kVA might be allowed, but a 100kVA sounds more like the right thing.

And, one might think 300A of 240V PV could go through a 50kVA xfmr, as 50kVA is a "400 amp service", but 50kVA @ 240V is only 208A. Correct?

 

[electrofelon]

Help me out here- something is still weird here.
zman has to have a 400A breaker for his 300A of PV- where's it going to go?
Can't go line side of a 400A main.

I am not going to read back and refresh myself on what the numbers were, but if the 300A of PV is the sum of the inverter outputs, then you would need 300 X 1.25 = 375 minimum conductors and 400 A OCPD. As long as the service entrance conductors were rated 375 or higher, why do you think you couldn't line side tap it?

 

[jaggedben]

...
Can't go line side of a 400A main.
...

Sure it could. Might be easier said than done from a practical point of view, but not a code violation.

 

[PVfarmer]

I am not going to read back and refresh myself on what the numbers were, but if the 300A of PV is the sum of the inverter outputs, then you would need 300 X 1.25 = 375 minimum conductors and 400 A OCPD. As long as the service entrance conductors were rated 375 or higher, why do you think you couldn't line side tap it?

Sorry- unfinished sentence there.
He can't line side tap a 400A breaker if he has a 200A service.
We don't know what the service is or rating of service conductors.

So I meant-
Can't go line side with a 400A of PV..IF it's a 200A service.

This is what I'm more concerned with- if his service xfmr is 208A/50kVA...then 300A/72kVA of PV won't work.

he can't put 72kVA of 1ph xfmrs through what is probably a 50kVA 1ph xfmr.

 

[PVfarmer]

This is what I'm trying to say- the farm here had the 2nd service on this list- 200A. And a 25kVA xfmr.
But the loads were actually more like 150A of 240V 1ph motors, up to 200A at times, *plus* all the 120V loads.

You can't just throw 300A of PV on something like that.
So zman should really talk to his POCO!