Transformer sizing for step up and step down transformers

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iwire

Moderator
Staff member
Location
Massachusetts
He mentioned instead of using five 120/240V 1ph inverters, he's using two 480V 3ph.
So if he meant 5 of the 11400w 1ph models, or 57kW, maybe then he means 2 of the 33kW to replace them?
Or five 8kW 1ph models with 2 of the 20kW models? Still no idea.

I would have to use 5 single phase se inverters when I can use the 2 480 inverters less everything just a fancy transformer.

Who cares what a DIY homeowner thinks?
 
Who cares what a DIY homeowner thinks?

With all due respect, it seems like you do, as you keep questioning my thoughts.
Re: the comment on the other thread- yes, people here have been helping me, so attempting to help others would seem like a reasonable thing to do.

As far as this thread, is there something I've said which is incorrect?
There seems to be some question as to whether the OP is on the right track with his PV system, and a few people in this thread with "proper credentials" that think he isn't.

If the OP has 400A of 120/240V service, that's a 50kVA service transformer.
So if he's trying to interconnect to it with 40kVA of 3ph PV inverter output, it seems like that might be possible, while not ideal, but others say it is not possible.
If he's trying to connect 2 of the 33KVA inverters to the grid through a 3ph 75kVA step down xfmr and then a 1 ph 50kVA...that would seem like a very bad idea.

If you look back at comment #17, it *was* mentioned that "I (PVFarmer) might be onto something"...
And if I happen to be the owner of the smallest PV installation company in America, isn't that, well...my business?
Thanks. Have a nice day.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
There seems to be some question as to whether the OP is on the right track with his PV system, and a few people in this thread with "proper credentials" that think he isn't.

FWIW, my credentials are not in quotes. I stand by what I have said, but I also stand ready to change my position in the presence of factual information that proves my position to be incorrect. It is a necessary skill for an engineer to strive to be correct rather than to simply "win" an argument.
 
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FWIW, my credentials are not in quotes. I stand by what I have said, but I also stand ready to change my position in the presence of factual information that proves my position to be incorrect. It is a necessary skill for an engineer to strive to be correct rather than to simply "win" an argument.

Sorry, my apologies- I didn't mean that as in yours aren't proper at all- more as in my credentials are that I believe I have a grasp of the facts.
What facts there are at least.
I agree with you about the 3ph PV to 1ph service being "off" somehow, but we haven't even gotten the details down yet here.
If his 75kVA xfmr is 3ph to 3ph with a center tap, he's only using 1/3 of it for the 120/240V split phase output- maybe the SE guy was saying it's OK to do that with 20kVA of 3ph inverters?

It certainly would be interesting if that was the case. You'd still be paying about 3x the $$ for the transformer if so...
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
...
If his 75kVA xfmr is 3ph to 3ph with a center tap, he's only using 1/3 of it for the 120/240V split phase output- maybe the SE guy was saying it's OK to do that with 20kVA of 3ph inverters?...

I still don't think it will work. If the third leg of the inverter is not connected to a terminal with a voltage reference then the inverter will still not turn on.

If I had to place a bet I'd guess that the 'SE guy' thought the question was about an actual 3-phase high leg delta and using a wye-delta transformer, but that's me speculating somewhat.

Also, unbalanced outputs on three-phase transformers can result in dangerous overcurrents. Not sure if it would in this case, but maybe one of the engineers here can say.
 

zman990

Member
Location
US
Wow this thread has gotten a lot of attention.
So instead of 5 single phase inverters I want to use 2 three phase inverters.
In data sheet it give max continues amperage you multiply that number times 1.25 giving you to 50 amp 3p breakers equals 100 amps continuous.
I am going to make my long run with 480/277 instead of 240/120.
Then next to service I am going to step down with a transformer.
The do a supply side conductor tap and fused disconnect.
So technically I can tap upwards of 330 amps of solar with out changing anything on the service.
This is a very common practice as a solar integrator.
I do have a more concerning issue and that is that inverters are 1000volt dc inverters and 690.7 says no good to install that high of Voltage on single family dwelling even though no dc voltage will be anywhere near home it is a ground mount.
It does appear I should be able to do this as long as I am able to keep any unqualified person from having access to the dc side of system.
To all the people who feel this is impossible and wrong you simply are not right it is like I said a common practice on larger single phase jobs.


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So instead of 5 single phase inverters I want to use 2 three phase inverters.
In data sheet it give max continues amperage you multiply that number times 1.25 giving you to 50 amp 3p breakers equals 100 amps continuous.
I am going to make my long run with 480/277 instead of 240/120.
Then next to service I am going to step down with a transformer.
The do a supply side conductor tap and fused disconnect.
So technically I can tap upwards of 330 amps of solar with out changing anything on the service.
This is a very common practice as a solar integrator.

There does appear to be a big miscalculation here!
If I'm understanding you, you're saying you are using two of the 33kW 3ph inverters, which put out 40A each, or 80A total. You're using a 100A breaker for that combined output.

Here's the problem- that 66kW and 80A of combined 480V output is also = to 66kVA.
66,000 / 120V = 550 amps at 120V!

Like you said, if you can tap 330A of 120V, that's only 40kVA, which makes sense because your 400A of service is most probably a single 50kVA xfmr. You could possibly tap up to 375A.
But you are actually putting out 550A of PV, right?
You'd have to have a 75 or 100VA service xfmr to handle that, so why not get a new 3ph service instead of a larger 1ph service?

I still don't think it will work. If the third leg of the inverter is not connected to a terminal with a voltage reference then the inverter will still not turn on.

If I had to place a bet I'd guess that the 'SE guy' thought the question was about an actual 3-phase high leg delta and using a wye-delta transformer, but that's me speculating somewhat.

Also, unbalanced outputs on three-phase transformers can result in dangerous overcurrents. Not sure if it would in this case, but maybe one of the engineers here can say.

I agree about the voltage reference part, as far as the details we have, and about the overcurrent thing.
It seems like even if there was some way around the voltage reference issue, the xfmr would still be only using 1/3 of the windings on the low voltage side, so the one winding would have to handle 3x as much overcurrent as normal.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Wow this thread has gotten a lot of attention.
...
I am going to make my long run with 480/277 instead of 240/120.
Then next to service I am going to step down with a transformer.
The do a supply side conductor tap and fused disconnect.
So technically I can tap upwards of 330 amps of solar with out changing anything on the service.
This is a very common practice as a solar integrator.
...

It is a common practice to use step down/up transformers, and to do supply side taps, but it is not a common practice to connect 3-phase inverters to a single phase service. In fact it's a non-existent practice.

You've been told a few times. I suggest you go back to the manufacturer with a very clear question about phases (not voltage) before you lose a lot of money. Just my advice.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Wow this thread has gotten a lot of attention.
So instead of 5 single phase inverters I want to use 2 three phase inverters.
In data sheet it give max continues amperage you multiply that number times 1.25 giving you to 50 amp 3p breakers equals 100 amps continuous.
I am going to make my long run with 480/277 instead of 240/120.
Then next to service I am going to step down with a transformer.
The do a supply side conductor tap and fused disconnect.
So technically I can tap upwards of 330 amps of solar with out changing anything on the service.
This is a very common practice as a solar integrator.
I do have a more concerning issue and that is that inverters are 1000volt dc inverters and 690.7 says no good to install that high of Voltage on single family dwelling even though no dc voltage will be anywhere near home it is a ground mount.
It does appear I should be able to do this as long as I am able to keep any unqualified person from having access to the dc side of system.
To all the people who feel this is impossible and wrong you simply are not right it is like I said a common practice on larger single phase jobs.


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If you think that I will accept the word of an anonymous voice on an internet forum as any sort of authority, you have another think coming. Please furnish the name and phone number of the SolarEdge applications engineer who told you that you can interconnect a three phase inverter with a single phase service and I will call and ask him myself. If he confirms it and answers my questions to my satisfaction I will stand corrected and post my retraction in here. Heck, I will start doing it myself.

I have been designing commercial and residential PV systems for six years, and I have well over a hundred projects under my belt from 5kW to over 800kW. I have an electrical engineering degree, my Professional Engineer's license, and I am a NABCEP certified Solar Professional. I don't know everything, but I know a lot. If what you claim is possible, I have never heard of it, and if it were a "common practice", I feel certain that someone in the four solar companies I have worked for or the numerous clients I have worked with as a freelance engineer would have at least mentioned it. No one has.

Someone here has misunderstood something and I do not think it is I. Prove me wrong and I will admit it, but just saying so doesn't cut the mustard.
 
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zman990

Member
Location
US
https://www.google.com/search?q=Tra...q=Transformer,dri+type++480V+Input,+240V+Outp
Alright they start at pg 38 scroll down to ge dri type transformer pdf they exist my solar friends I may may just be thinking out side box but idk why u would want to do it any other way for larger ground mount.
Main issue is 1000volt dc on property zoned at residential.
The solar edge inverter is 33kw 480/277
Customer service link
http://www.solaredge.us/groups/contact
My original ? How do we size the transformer for solar application? I went with 112.5 kva!!!!! Fyi
https://www.google.com/search?q=sol...ndroid-att-us&sourceid=chrome-mobile&ie=UTF-8
The data sheet for 33kw solar edge. The max continuous output is 40 amp@220 so 40a*1.25 for breakers so 50 amp breakers would be used to combine before landing transformer



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iwire

Moderator
Staff member
Location
Massachusetts
https://www.google.com/search?q=Tra...q=Transformer,dri+type++480V+Input,+240V+Outp
Alright they start at pg 38 scroll down to ge dri type transformer pdf they exist my solar friends

Direct link to the pdf, go to page 10-38 https://www.geindustrial.com/catalog/buylog/10_BuyLog2013_DryTypeTransformers.pdf


Dry Type Transformers
Midtapped
Three-Phase TP-1

Product Description

GE Type QL midtapped transformer enables the user to transform
three-phase power from 480 Volts primary to 240 Volts secondary
and have 120 Volt, reduced capacity tap (RCT) single-phase
capability as well. This is because a single-phase midtap is
brought out of one coil of the unit’s three-phase secondary winding.
These transformers are UL listed, File E-79145.

Application
The Type QL midtapped design can be used wherever there is 480
Volt, three-phase supply available and the load is primarily 240
Volt three-phase with a nominal amount of 120 Volt, single-phase
power required. Normally, in this instance, a small single-phase
as well as a three-phase transformer would be required to
provide the necessary transformation.

Caution: When utilizing the 120 Volt midtap for single-phase
applications, the single-phase load should not exceed 5 percent
of the three-phase kVA rating. The three-phase kVA load must be
reduced by the same percentage as that added by the singlephase
load. Additional loading beyond 5 percent may cause the
transformer to overheat and fail. If the single-phase load is in
excess of 5 percent, it is recommended that


zman, its not that we don't think the transformer exists, it is that it will not do the job you think it will.
 

kwired

Electron manager
Location
NE Nebraska
Looks like a three phase to three phase transformer to me.

Can you operate single phase loads from the secondary - yes.

Can you supply it with single phase and get single phase out - yes but not at full nameplate KVA

Can you supply it with three phase and supply a one single phase load at full nameplate KVA - no

You could supply it with three phase and supply three single phase loads that total nameplate kVA, but need to balance them across all three phases.

There is no simple winding type of converter to go from single phase to three phase. To convert full nameplate kVA from three phase to single phase you need some other conversion device like an inverter, a motor-generator, rotary or static phase converters, even certain zig - zag connected transformers, but none of those work in reverse to convert single phase to three phase. An inverter could be designed to do this though, you just don't have one that does and they apparently are not that commonly made in a single unit in the kVA rating you are looking at but three phase is.
 
1. why u would want to do it any other way for larger ground mount.

2. Main issue is 1000volt dc on property zoned at residential.

3. The solar edge inverter is 33kw 480/277 .... I went with 112.5 kva

4. I am going to make my long run with 480/277 instead of 240/120.

5. https://www.google.com/search?q=sol...ndroid-att-us&sourceid=chrome-mobile&ie=UTF-8
The data sheet for 33kw solar edge. The max continuous output is 40 amp@220 so 40a*1.25 for breakers so 50 amp breakers would be used to combine before landing transformer

5. Dude- seriously. From your link. It doesn't say it supports 120/240V grids anywhere.
Data sheet-
SE33.3K
(REQUIRES MEDIUM VOLTAGE TRANSFORMER) // Grids Supported - Three Phase 3 / N / PE (WYE with Neutral)

Plus, 480V to 240/120V is *not* a MV xfmr!

1. I am doing it another way in one case- getting 480/277V service for the 480/277V inverters. Not SE inverters, SMA.
So there are no xfmr losses on customer side going from PV to grid. The farm's loads are 120/240 split phase, but almost all the motors say 208V on their plates, so 45kVA of xfmr with 208/120V 3ph secondary for the 25kVA max of loads.

2. Check your local zoning- a real farm can be in a residential zone, but there are agricultural "variances". I mean as in, if the farm is incorporated as an agricultural business, the farm buildings aren't dwellings.
Aren't you inverters going outside, so all of the 600+V wires will be outside?
Also, look at page 2 here, about Section 101.31.
http://solarenergy.advanced-energy.com/upload/File/Application Notes/ENG-600vor1000V-260-02.pdf

3. A 112.5kVA xfmr for TWO of the 33kW inverters? No!
As iwire pointed out-
the single-phase load should not exceed 5 percent... Additional loading beyond 5 percent may cause the transformer to overheat and fail.

Your 112.5kVA xfmr can only handle 5.625kVA of 120/240V!
You'd want a 1500kVA xfmr- 5% of that is 75kVA, enough for your 66kVA of PV.
The largest 480V to 240/120V xfmr I can find is 750kVA.
BUT- you have three or four engineers (and myself) telling you that what you want to do isn't possible. Even if it was, that would be WAY too much $$!

4. I have to say that doing the long run at 480/277V and then stepping *down* the voltage sort of defeats the purpose of doing the run at higher voltage- you'd want to step UP to the grid at that point (especially with a line/supply side connection!) and step down for load only.

5. 66kVA of 480 3ph PV output = 550 amps at 120V. You can't land that with 400A service, which is a 50kVA xfmr.
You have to get a 75kVA 480/277V service minimum. My engineer recommended getting 150kVA of 480/277V service for about the same amount of PV as you are installing, so the PV will be easily "expandable".
The SE inverter specs say "MV transformer required" - I read that as you can't even use a 480/277V to 208/120V step down xfmr for the 480/277V inverters and a 208/120V service..
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
... I may may just be thinking out side box but idk why u would want to do it any other way for larger ground mount. ...

Because...physics. And because the point of the project is to get the inverters to turn on and produce power. And because buying $17,000 of equipment you can't use is a mistake.

Also this is interesting, from the catalog and iwire's quote...

When utilizing the 120 Volt midtap for single-phase applications, the single-phase load should not exceed 5 percent of the three-phase kVA rating. ... Additional loading beyond 5 percent may cause thetransformer to overheat and fail.

Since you're planning to use the 240/120 coil as your output, that would be your single-phase load. Setting aside that the inverter will never turn on (because: inverter design and physics), if it did turn on your entire output is supposed to be less than 5 percent of the kVA rating of your transformer. You'd need a 1330kVA transformer. Does that at least give you pause? [EDIT: I see even PVfarmer noticed this. Good job PVfarmer. Don't take the actual number too seriously though. ;)]

You stand to lose (or lose for someone) upwards of $20,000 here, what with labor costs to redo everything if you go ahead and install it wrong. Maybe you should contact an electrical engineer with some solar experience and ask their fee for a consultation, and do a little risk vs. cost estimation. Say he charges $500, that's not bad to ensure you're not blowing tens of thousands.
 
Alright they start at pg 38 scroll down to ge dri type transformer pdf

zman-
The SE inverters are the same as SMA 480/277V *WYE* inverters, in that the output has nothing to do with 480V- L1, L2, and L3 are all 277V L-N. You can't drop the N.
Your 112.5kVA xfmr is 480V *DELTA* - you can't use that. It says so in the installation manual!
The 5 wires on pg. 25 being L1, L2, L3, *N*, and PE.
You have to use the neutral. You can't connect the N to the delta 480V connection.
With SMA inverters, you're "recommended" to step the 3 Ls of 277V *up* to MV delta. (Or MV grounded wye, but it is a fact- you can't connect SMA Tripowers straight to a delta connection on the inverter side of any xfmr(s)...at all. Seems the same for SE inverters to me.)

Page 25
For installation on a delta grid without neutral, multiple single phase inverters can be used.

Connecting the AC Grid to the Inverter
Use a three-wire cable for a single phase connection or a five-wire cable for three phase connection. The maximum wire size for the input terminal blocks is 16mm².


Page 27
NOTE
If power control is enabled, it is important to respect the order of grid lines connection to the inverter. A 120deg phase difference should be kept between L1 to L2 and between L2 to L3 (L1-L2-L3 and not, for example L1-L3-L2). If the grid lines are not in this order, an error is displayed on the LCD and the inverter will not produce power.
http://www.solaredge.com/files/pdfs/products/inverters/guides/se-inverter-installation-guide.pdf
 
I have been on this forum for less than 10 days

Well then here's something you might not know- the fact that me and jben have agreed on something is nothing short of a small holiday miracle!
You might want to take note of that fact.
Again- why would you want to step all that PV output DOWN, when most of it is probably going to the grid?

The only way you'd want a 112.5kVA xfmr would be to step UP, so why not make that step up be = to the grid voltage- then it's the POCO's transformer.
No idea what your loads are, but if your 120/240V 400A main breaker is fed by a 50kVA 120/240V service xfmr now, you could worry about the xfmr(s) for the loads and let the POCO worry about the 480V PV to grid voltage xfmr(s).
You wouldn't need 112.5kVA for the PV output to loads xfmr(s)- 75 or even 50kVA, possibly even a 45kVA bank (3 x 15kVA 1ph) might work- you have to figure the existing 120/240V load pretty closely to decide on that of course.
Around here, 480/277V service options are either 75 or 150kVA of xfmr(s), either a bank of 3 (25 or 50kVA each) overhead, or a single pad mounted.

jaggenben
I see even PVfarmer noticed this. Good job PVfarmer. Don't take the actual number too seriously though.

Thanks, and why would I after seeing the PRICE!!! :huh: ---> http://www.temcoindustrialpower.com/products/Transformers_-_General/HT2586.html

Ok, so IF the OP had one 480V delta output inverter...and it happened to be 33kW...
http://attachments.temcoindustrialpower.com/Data_sheet/Hammond-NMK750KD.pdf

It would be "connectable" to one of those ^^, wiring diagram below...

http://attachments.temcoindustrialpower.com/Wiring_diagram/transformer_wiring_HT_diagram22.pdf

but what would be happening with H2 from the 480V side and X2 on the 120/240V side?
Don't yell at me!
But I can sort of see how, if current has to go from X2 to X1 and X3, (if that's physically possible) it would cause some serious issues.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Yet another question that comes up is whether POCO will be happy about that large a PV contribution on a single phase service in the first place. :)
Oh, wait, it must be OK because it is a three phase inverter..... :angel:
 
You could supply it with three phase and supply three single phase loads that total nameplate kVA, but need to balance them across all three phases.

Could you elaborate on that some?
"Supply three single phase loads"... but not with a GTI alone, right?

If you had a 15kW 480V 3ph grid tied inverter.
That's 18A at 480V and 62.5A at 240V.

But you can't just attach a grid tied inverter to three 480V to 120/240V 1 ph xfmrs and then 1 motor each to those that draw 5000w / 20.83A each, without the grid being involved, correct?
The motors wouldn't give a "reference voltage" to the inverters...at all?
There has to be a MV grid and step up transformer attached to any brand of GTI for reference AND protection I believe.

Yet another question that comes up is whether POCO will be happy about that large a PV contribution on a single phase service in the first place. :)
Oh, wait, it must be OK because it is a three phase inverter..... :angel:

Right! Like I said before, over 50kVA of 1 phase PV to grid around here would get a "why?" from the POCO, and it is 100% their call on allowing it.
 
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