Transformer sizing for step up and step down transformers

[PVfarmer]

You might be on to something before even considering conversion between three phase and single phase, based on what OP said with :"100amps of 480/277 from solar", that alone is about 83 kVA, yet he says he already has a 75 kVA transformer. Something don't seem right already and I am not even experienced with PV applications.

This is getting way off the beam. Unless we are shown otherwise, the OP is talking about a 240/120V split (single) phase service. Talking about 240V high leg services just muddies the water; connection to such a service with a three phase inverter may or may not work, depending on whether or not the three phase inverter needs a neutral (if it does it cannot use the center tap as such and therefore would require an isolation transformer) and whether the transformer driving the high leg of the service can take the output of the inverter (it's typically a lot smaller than the transformer(s) driving the other two phases).

If he has a 240/120V split phase service, no transformer will allow him to interconnect a three phase inverter to it.

kwired-
Yep, like I said, not sure how to get 100A of any voltage from those inverter models, which are 12A, 24A, and 40A @ 480V.
A 75kVA step up (or down) xfmr would be the right size for two of the 33kW inverters or so, or 3 of the 20kW models.

ggunn-
Farms are the most common place for 240D center tapped service, that's why I mentioned it.
I personally think it would help if everyone called 240D center tapped "240/120V" and called split phase L-L-N "120/240V". But that doesn't seem to be a standard thing.
The POCO calls 3ph 480/277V and calls 1ph 277/480V in the documents, but then in emails 3ph gets referred to as 277/480V so you have to double check...it's weird.

Anyway, wouldn't using something like this for just one of the 12 amp/10kW/480V inverters sort of...work, but not be worth doing?
10kW is = to 10kVA, so if you wanted to get to 120V from 10kVA of 480V 3ph, going with that 10% thing, you'd have to use at least 100kVA of transformer- which would be silly, paying more for the xfmr than the inverter.
Distribution Transformer 112.5 kVA 480V Pri x 240 D/120 CT V Sec

If there was no PV involved, and you had 10kW max of 120V loads, you'd need 100kVA worth of service xfmr(s)- doesn't seem like there would be any difference when instead it's PV output to the grid, despite the grid being an "infinite" input.
Why pay a lot of $$ for a xfmr and only use 2 windings and 10% of its capacity?

 

[ggunn]

ggunn-
Farms are the most common place for 240D center tapped service, that's why I mentioned it.
I personally think it would help if everyone called 240D center tapped "240/120V" and called split phase L-L-N "120/240V". But that doesn't seem to be a standard thing.
The POCO calls 3ph 480/277V and calls 1ph 277/480V in the documents, but then in emails 3ph gets referred to as 277/480V so you have to double check...it's weird.

Nevertheless, the OP didn't say the service to which he was referring was 240/120 three phase high leg, he just said that what he was contemplating would work because large farms do it all the time.

My initial comment was that if the service he is talking about is 240/120 split (single) phase, nothing he can do with transformers will make a three phase inverter work with it. If the service in question is indeed split phase, which he has never contradicted, then it's not at all the same thing as what the farms have.

 

[PVfarmer]

You might be on to something before even considering conversion between three phase and single phase, based on what OP said with :"100amps of 480/277 from solar", that alone is about 83 kVA, yet he says he already has a 75 kVA transformer. Something don't seem right already and I am not even experienced with PV applications.

If the service in question is indeed split phase, which he has never contradicted,

kwired-
Hold on, let me rephrase what I said-
A 75kVA step up (or down) xfmr would be the right size for two of the 33kW inverters or so, or 3 of the 20kW models.


No, not exactly the right size. From what I know, 75kVA would work for two of the 33kW/33kVA models, because 66kVA total +10% of 66 = 72.2kVA, so under 75kVA of xfmr.
Three of the 20kW/20kVA models would work, because that's 60kVA. plus 10% is 66kVA.

*But*, if you're getting the xfmrs for the purpose of converting PV output, you'd want them to run at their highest efficiency, so say if you knew the xfmr(s) ran best between 35-65% of their total kVA rating, you'd really want to put two of the 20kW inverters with 75kVA of xfmr(s), because the inverters will be putting out say 25-40kW "most" of the time, and 25-40kw is almost within the 35-65% "sweet spot" of the xfmr(s).
(25-40kW =33% - 53% of xmfr kVA)
---
ggunn-
I'm just saying, I haven't seen a 83+kVA service which is 120/240V split phase.
Around here, over 50kVA of 120/240V, which I'd call a 400 amp/120V service, even for "commercial", is a "non-simple" connection (or less simple), because :
"above 400A (non-self contained, metering transformers are used)"

And, wouldn't the output be limited along the lines of the demand?:
Single-phase, 120/240 volt service is limited to 50 kVA maximum where utilization equipment
includes individual motors not over 6-1/2 HP. Self-contained meter socket applications are limited
to 72kW demand.

edit: Those limits are "new" I suppose, so maybe over 50kVA of 120/240V split phase is common in older services, or not around here?

 

[jaggedben]

... There isn't as much need for converting three phase to single phase until you bring renewable energy sources into the picture. ...

Bringing renewable energy sources into the picture creates no such need. Any such need would be independent of that.

As has been said, there is no possible way to connect 3-phase inverters to single-phase services under UL 1741. Unlike step-down/up transformers, phase converters do not work in both directions. That is required for a UL1741 inverter to get a proper reference from the grid.

If you can show otherwise we'd all be very interested. :lol:

 

[jaggedben]

Every one is saying it won't work well I need more than a feeling I need math and proof because this is how people do this on large farms in Midwest

I could write a multi-paragraph response, or refer you to other threads on this forum. But I think ggunn has the right short answer: call the inverter manufacturer support and ask if it will work. Give them the exact model of equipment that is converting single-phase service voltage to 3-phase.

 

[PVfarmer]

1. I need to have a better understanding of sizing the kva rating

2.Also which type is best for solar dry or liquid filled transformers.

3. I have an example for step down
100amps of 480/277 from solar and need to tie in to customers service.
480v X 100 amp X 1.73 3phase power factor = 83 KVA X 1.25 percent rule 103.8 KVA transformer

4. Do I really need to add another 1.25% multiplier?

1. What is the kVA of the POCO to customer xfmr? Is it really a single 100kVA / 833 amp 120/240V split phase service?

2. Dry in your case...arguably?
Arguably, when choosing transformers, the changeover point between dry-types and wet-types is between 500kVA to about 2.5MVA, with dry-types used for the lower ratings and wet-types for the higher ratings.
http://electrical-engineering-portal.com/right-choice-of-dry-type-or-liquid-filled-transformer

3. If you mean 4 of the 20kW inverters and therefore 96 amps- not sure why you're using 1.25% there for the xfmr.
You'd have 96A * 1.25 = 120 amps of breaker (or a 30A breaker for each inverter and a 120A breaker for their combiner output?)

But for the xfmr it would be 96A * 480V * 1.73 = 79.72kVA of PV output (that's with the 4 x 20kVA inverters).
And for one type of inverter (not the one you posted), the PV output has to be <=90% the interconnection xfmr(s) kVA rating, so that'd be 88.6kVA, and a 90kVA xfmr would work.

But!- if you really have 100A of 480V 3ph PV output- you'd possibly want at least (100A * 480V * 1.73) / .9 = 92.3kVA of transformer, but the next size up of 3ph 480V to 208/120V xfmr is 112.5kVA I believe.

If you really do have 80kW/80kVA and 96 amps of PV output, seems like a 75kVA xfmr isn't enough.

4. Not sure if you even need to use the first 125%.

 

[zman990]

75 kva isn't enough but the reason why I would want to do this is cost 480 solar inverters are cheaper it is less amperage for my my 350+ run to the service.
I would have to use 5 single phase se inverters when I can use the 2 480 inverters less everything just a fancy transformer.
I will call them at se

Sent from my HTC One_M8 using Tapatalk

 

[kwired]

75 kva isn't enough but the reason why I would want to do this is cost 480 solar inverters are cheaper it is less amperage for my my 350+ run to the service.
I would have to use 5 single phase se inverters when I can use the 2 480 inverters less everything just a fancy transformer.
I will call them at se

Sent from my HTC One_M8 using Tapatalk

If 75 kVA isn't enough then how are you going to limit production so you don't overload it (assuming it will work otherwise)?

Does cost of transformer offset cost of the other units you don't want to purchase (again assuming it does work for the application)?

 

[PVfarmer]

75 kva isn't enough but the reason why I would want to do this is cost 480 solar inverters are cheaper it is less amperage for my my 350+ run to the service.
I would have to use 5 single phase se inverters when I can use the 2 480 inverters less everything just a fancy transformer.
I will call them at se

I hear you. Not sure which state you are in, but here in my state the POCO encourages you to go 3ph with any PV over 50kVA, because too much 1ph causes "issues" for the grid. It seems to be something you have to negotiate with the POCO, if you want to do over 50kVA of 1ph PV.
But, pretty sure (knock on wood) that they don't charge the customer for the 3h xfmr(s) upgrade (here).

So your two 33kW inverters are 66kW/kVA and 80 amps at 480 3ph...I'll ask you the same question my engineer asks me- are you ever going to upgrade, either the loads or PV output?
If so, you might want to look into getting 150kVA of 3ph service in this case, either "400 amps" of 208/120V or "200 amps" of 480/277V. (Really 180A at 480V and 416A at 208V if you have 150kVA of service xfmrs).

Then the POCO will be happy with the 3ph connection, and you'd either use a 480V to 208/120V xfmr for the loads only, or to step down to service voltage.
And if the loads overall are smaller than the PV output, maybe the smaller xfmr just for 208/120V loads would save you some $ as far as "less everything"?
These are of course suggestions, hope it works out for you.

 

[zman990]

For all you non believers this is a very common practice for large single phase systems inverter manufacturer chuckled and said of course u can that is how you it.
Nothing needs to be changed it is a 400 amp service and it will be line side tap so I'm good.
Hope everyone involved learned something

Sent from my HTC One_M8 using Tapatalk

 

[PVfarmer]

For all you non believers this is a very common practice for large single phase systems inverter manufacturer chuckled and said of course u can that is how you it.
Nothing needs to be changed it is a 400 amp service and it will be line side tap so I'm good.
Hope everyone involved learned something

I'm still a bit confused.
When you said "100 amps of PV output", did you mean a 100A breaker? As in two of the 33kW 480 3ph inverters, so 80A total of PV output and a 100A breaker for both of them?
And then do you mean 400A service as in @240V, and therefore a 100kVA service xfmr?

If so, 75kVA is enough xfmr for 66kVA of PV output *if* it is 3ph, but...
right after you said this in italics about 208V, you posted a pic of a slip for a 480V to 240/120V xfmr..? Which when listed that way sounds like 480V 3ph to 240D 3ph/120V center tapped.
So I'm wondering why you said 208V.

If it is a 120V center tapped xfmr, you're only using 2 windings of a CT'd xfmr for the 120V...you might want to check this link out.
http://www.temcotransformer.com/centertaptransformer.html

Between me taking 480 to 208 then 208 to single phase I am wondering why if it is not possible ge makes a transformer that does it.

 

[jaggedben]

View attachment 13926
Hope u can read it

For all you non believers this is a very common practice for large single phase systems inverter manufacturer chuckled and said of course u can that is how you it.
Nothing needs to be changed it is a 400 amp service and it will be line side tap so I'm good.
Hope everyone involved learned something

I'm gonna need more than a screenshot I can't read to learn something here. :roll:

 

[PVfarmer]

Nothing needs to be changed it is a 400 amp service and it will be line side tap so I'm good.

Your post #7 was the 400A main breaker, which is 120/240V split phase.
Your post #8 is a pic of the 3 phase transformer slip.
If the #8 xfmr is the same as they are talking about in italics from the previous Temco link, you'd want a 225kVA xfmr to step the 66kVA of 480V PV down to 120V.
225kVA / 3 / 2 = two output circuits of max 37.5kVA @ 120V for the two 33kVA inverters.
But...225kVA of xfmr is A LOT of $$!

If the transformer has a three phase rating of 30kva, each phase winding has a capacity of 10kva. Being that the center tapped winding is only used on one of the three output windings, the capacity of each of the two low voltage circuits is one half each of one third of the transformer total capacity. In the scenario above, each low voltage circuit will be capable of a maximum of 5kva.

jaggedben
I'm gonna need more than a screenshot I can't read to learn something here.

That is pretty hard to read!
But, it does seem like he's trying to do what you guys (+ggunn) have previously talked me out of, in a way?

 

[GoldDigger]

It seems to come down, except for the transformer sizing issue, to the following:

1. The OP says that there exists a transformer that converts between a three phase grid interactive inverter output *operating with three equally spaced phase vectors* and single phase service. This contradicts the laws of physics, and I cannot say any more than that.
2. If the inverter manufacturer says that it can be done and is common, maybe we should consider that the manufacturer is just saying that the inverter will operate properly if all three outputs are connected to a single reference phase. I.E. that particular three phase inverter really is just three single phase inverters in one package and no phase offsets between the outputs is required by the control mechanism.
If this turns out to be true, the whole controversy arises from an incorrect use of terminology.
However, in the latter case, a single core three phase transformer is NOT the way to make that interconnection.
And if a single phase to single phase transformer is used the current in the PV side winding will be simply the numeric sum of the three output currents with no vectors or sqrt(3) factor involved.

 

[GoldDigger]

P.S.: As for that hard to read transformer data picture, I wonder whether it is a delta to high-leg delta transformer (three phase output), or a delta input and delta output with two secondary windings on each phase so that it can be wired for either 120 or 240 volts?

 

[PVfarmer]

P.S.: As for that hard to read transformer data picture, I wonder whether it is a delta to high-leg delta transformer (three phase output), or a delta input and delta output with two secondary windings on each phase so that it can be wired for either 120 or 240 volts?

From what I can read of post #8, he seems to have this kinda thing, but a GE model/75kVA.
His post #8 says 240/120V 3ph output, as does this xfmr, and the first example you mentioned. Except the diagram doesn't seem to say anything about "high leg". Not sure how you get 240V from this if you're using the 120V tap.
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0315.html
wiring:
http://attachments.temcoindustrialpower.com/Wiring_diagram/transformer_wiring_HT_diagram21.pdf

Now for your 2nd example, do you mean as in a bank of three of these wired in delta input? I don't understand what you mean by getting both 120 and 240V from delta output in this case.
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT2319.html

Thanks.

 

[ggunn]

For all you non believers this is a very common practice for large single phase systems inverter manufacturer chuckled and said of course u can that is how you it.
Nothing needs to be changed it is a 400 amp service and it will be line side tap so I'm good.
Hope everyone involved learned something

Sent from my HTC One_M8 using Tapatalk

Let me get this straight. Someone at SolarEdge told you that you can connect an SE10KUS 10kW three phase inverter to a 400A 240/120V split phase service (NOT a 240V three phase service with a high leg, supplying 240/120V from a center tap between two phases)? If that is so, please PM me his name and phone number, or post it here if you think he wouldn't mind. I am perfectly willing to "learn something", but not from an anonymous voice on the internet.

BTW, SE also makes a SE10000A-US 10kW single phase inverter. My guess is that this is the one he was talking about.

 

[GoldDigger]

From what I can read of post #8, he seems to have this kinda thing, but a GE model/75kVA.
His post #8 says 240/120V 3ph output, as does this xfmr, and the first example you mentioned. Except the diagram doesn't seem to say anything about "high leg". Not sure how you get 240V from this if you're using the 120V tap.
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0315.html
wiring:
http://attachments.temcoindustrialpower.com/Wiring_diagram/transformer_wiring_HT_diagram21.pdf

Now for your 2nd example, do you mean as in a bank of three of these wired in delta input? I don't understand what you mean by getting both 120 and 240V from delta output in this case.
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT2319.html

Thanks.

I was not saying you get both at once. I meant you could wire for one or wire for the other.

 

[jaggedben]

It seems to come down, except for the transformer sizing issue, to the following:

1. The OP says that there exists a transformer that converts between a three phase grid interactive inverter output *operating with three equally spaced phase vectors* and single phase service. This contradicts the laws of physics, and I cannot say any more than that.

I'm sure you could say more about the physics, but kudos to you for stopping there.

2. If the inverter manufacturer says that it can be done and is common, maybe we should consider that the manufacturer is just saying that the inverter will operate properly if all three outputs are connected to a single reference phase. ....

I would wager that the manufacturer was asked an unclear question and they focused on the voltage aspect. Perhaps we shall find out.

Let me get this straight. Someone at SolarEdge told you that you can connect an SE 27.7kW three phase inverter to a 400A 240/120V split phase service (NOT a 240V three phase service with a high leg, supplying 240/120V from a center tap between two phases)? If that is so, please PM me his name and phone number, or post it here if you think he wouldn't mind. I am perfectly willing to "learn something", but not from an anonymous voice on the internet.

+100

BTW, SE also makes a SE10000 single phase inverter.

They even make an 11400.

 

[PVfarmer]

Let me get this straight. Someone at SolarEdge told you that you can connect an SE10KUS 10kW three phase inverter

BTW, SE also makes a SE10000A-US 10kW single phase inverter. My guess is that this is the one he was talking about.

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 where 100 amps of PV comes from...

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

I was not saying you get both at once. I meant you could wire for one or wire for the other.

Righto, gotcha, that makes perfect sense now.