Connecting 480 3ph inverters to 480 Delta--208/120 transformer

So Cal

Member
Location
Los Angeles, Ca
I'm using 2-STP20000TL inverters that have 480/277 terminals. The 50 Kva, 480 Delta, 208/120 transformer will not (I assume) have a neutral terminal. How would one wire this?
 

Carultch

Senior Member
Location
Massachusetts
I'm using 2-STP20000TL inverters that have 480/277 terminals. The 50 Kva, 480 Delta, 208/120 transformer will not (I assume) have a neutral terminal. How would one wire this?
50 kVA is not a standard 3-phase size. 45 kVA is the closest.


You'll either need a WYE on the 480V side, or a zig-zag transformer to generate the neutral from a 480V delta.
 
I'm using 2-STP20000TL inverters that have 480/277 terminals. The 50 Kva, 480 Delta, 208/120 transformer will not (I assume) have a neutral terminal. How would one wire this?
This situation seems to come up quite often and I always wonder why the choice is made to deal with transformers rather than spec inverters that match the electrical service? Just out of curiousity, what are your reasons, or did you not design the system?
 

Carultch

Senior Member
Location
Massachusetts
This situation seems to come up quite often and I always wonder why the choice is made to deal with transformers rather than spec inverters that match the electrical service? Just out of curiousity, what are your reasons, or did you not design the system?
I would usually spec inverters that match the service voltage.

It could be a value engineering decision. You generally get more economical inverters in the 480V output, both in terms of cost and efficiency. Plus you have the opportunity for 1000V systems on the DC side, that are seldom possible with 208V systems. If that is enough to justify the tradeoff of the extra transformer, with its losses and its cost, I can understand why someone would specify a system this way. Granted it does make the system much more complicated, adding another lead time intensive piece of equipment, and another piece of floor space you have to reserve.
 

jaggedben

Senior Member
This situation seems to come up quite often and I always wonder why the choice is made to deal with transformers rather than spec inverters that match the electrical service? Just out of curiousity, what are your reasons, or did you not design the system?
Somebody sold the system with those inverters because they have a deal on them or already have them in stock or something like that. And it's either sunk cost or someone is too timid to request a change order.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
This situation seems to come up quite often and I always wonder why the choice is made to deal with transformers rather than spec inverters that match the electrical service? Just out of curiousity, what are your reasons, or did you not design the system?
Partly it's because there is little overlap between native 480 and 208 inverter sizes. That's getting better, though. I ran into it once because we got a contract to build 11 systems for a single customer and only one of his sites had a 208V service, and we got a good deal on 480V inverters because we bought a boatload of them.
 
50 kVA is not a standard 3-phase size. 45 kVA is the closest.

You might be able to use a 45kVA- SMA says the inverter kVA has to be 90% of the xfmr size, so 40kVA plus 10% = 44kVA?

You'll either need a WYE on the 480V side,

You have to have a "grounded neutral leading out" on the 480V side, going by the manual. Whatever that may mean, I am not precisely sure.

I always wonder why the choice is made to deal with transformers

Like he said, there's the 1000V DC thing, a 24000TL is maybe $600-1200 less than 3 8000TLs, so that helps pay for the xfmr (or pays for it completely) and if you are running any amount of AC wire from inverters to PCC it makes a pretty big difference in impedance losses.
If a 20000TL puts out as much or more than three 7000Tls...which they seem to do where I am- I think the higher DC voltage ends up with less losses when it's overcast often maybe.
COuld be the opposite elsewhere, but an extra MW a year is nothing to sneeze at!

I would usually spec inverters that match the service voltage.

Yes, if you can, sure, but there are sometimes "rules"... over 50kW of PV where I am *might* have to be a 3 phase service.
I can see how that makes sense- if you wanted to put in 50kW of 120V (/208 or /240V) inverters, the POCO would have to put in a 75kVA xfmr- but 75kVA also happens to be 200A of 208/120.
They might say "could you do six 8000w or 10000w inverters instead and we'll go with three phase". It is a lot better for the grid I suppose.
Then of course the customer would need a 208V to 240V buck boost or something...
 
50 Kva, 480 Delta, 208/120 transformer will not (I assume) have a neutral terminal. How would one wire this?
I wouldn't use a 45 kVA- I'd use 3 of these, wired in Y on both sides.
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0025.html

The xfmr(s) are the only way the PV power is getting anywhere- I'd probably even use the better 25kVA model of that one above that costs $1000 more.
With 45kVA you're running the xfmr(s) at 80% much of the time- if you go with 75kVA of xfmr(s), the 40kVA of inverters will be 53.3% of the xfmr(s)- much more efficient.

If I'm reading this right, you'd have to go from 480/277V inverters thru a xfmr to 480V delta and then another xfmr, 480 delta to 208/120Y, and they'd both have to be 2x the inverter kVA.
This is for medium voltage connections, but wouldn't the step down connection have to be the same setup?

edit:whoops!
forgot the link.
http://files.sma.de/dl/7418/STP24-US_MV_Trafo-TI-en-10.pdf
 

iwire

Moderator
Staff member
Location
Massachusetts
50 kVA is not a standard 3-phase size. 45 kVA is the closest.

You might be able to use a 45kVA- SMA says the inverter kVA has to be 90% of the xfmr size, so 40kVA plus 10% = 44kVA?

You'll either need a WYE on the 480V side,

You have to have a "grounded neutral leading out" on the 480V side, going by the manual. Whatever that may mean, I am not precisely sure.

I always wonder why the choice is made to deal with transformers

Like he said, there's the 1000V DC thing, a 24000TL is maybe $600-1200 less than 3 8000TLs, so that helps pay for the xfmr (or pays for it completely) and if you are running any amount of AC wire from inverters to PCC it makes a pretty big difference in impedance losses.
If a 20000TL puts out as much or more than three 7000Tls...which they seem to do where I am- I think the higher DC voltage ends up with less losses when it's overcast often maybe.
COuld be the opposite elsewhere, but an extra MW a year is nothing to sneeze at!

I would usually spec inverters that match the service voltage.

Yes, if you can, sure, but there are sometimes "rules"... over 50kW of PV where I am *might* have to be a 3 phase service.
I can see how that makes sense- if you wanted to put in 50kW of 120V (/208 or /240V) inverters, the POCO would have to put in a 75kVA xfmr- but 75kVA also happens to be 200A of 208/120.
They might say "could you do six 8000w or 10000w inverters instead and we'll go with three phase". It is a lot better for the grid I suppose.
Then of course the customer would need a 208V to 240V buck boost or something...
I wouldn't use a 45 kVA- I'd use 3 of these, wired in Y on both sides.
http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0025.html

The xfmr(s) are the only way the PV power is getting anywhere- I'd probably even use the better 25kVA model of that one above that costs $1000 more.
With 45kVA you're running the xfmr(s) at 80% much of the time- if you go with 75kVA of xfmr(s), the 40kVA of inverters will be 53.3% of the xfmr(s)- much more efficient.

If I'm reading this right, you'd have to go from 480/277V inverters thru a xfmr to 480V delta and then another xfmr, 480 delta to 208/120Y, and they'd both have to be 2x the inverter kVA.
This is for medium voltage connections, but wouldn't the step down connection have to be the same setup?

edit:whoops!
forgot the link.
http://files.sma.de/dl/7418/STP24-US_MV_Trafo-TI-en-10.pdf
I am confused, you are not an electrician, you are not an engineer. Just what are your qualifications to give any of this advice?
 
I am confused, you are not an electrician, you are not an engineer. Just what are your qualifications to give any of this advice?
I'm really just going by the owner's manual.
Pardon- I'm not giving it as advice written in stone- just saying this is how I understand it, and wondering if others see it the same way.
Going by that previous SMA link, the only way you'd want to connect a Tripower to a 480 delta >> 208/120 Y xfmr is by using a 480/277 Y to 480 delta xfmr first, and then another one for 480 delta >> 208/120, and both of those would have to be sized 2x of the inverter output.
That seems overcomplicated- if I'm wrong about it, great! That would simplify things.
You could go from inverters to 208 delta, but the service isn't delta.
However- that SMA pdf is for connecting to medium voltage- can't find one about stepping DOWN the inverters, but I don't know why it would be different for step down.

But then again, according to this link below, delta-delta does have some benefits.

I understand tripLETS, from music.
TripLENS are new to me.


In summary, removal of the neutral conductor leads to a “hot” center-point on the load “Y”, and also to harmonic load phase voltages of equal magnitude, all comprised of triplen frequencies. In the previous simulation where we had a 4-wire, Y-connected system, the undesirable effect from harmonics was excessive neutral current, but at least each phase of the load received voltage nearly free of harmonics.
Since removing the neutral wire didn’t seem to work in eliminating the problems caused by harmonics, perhaps switching to a Δ configuration will. Let’s try a Δ source instead of a Y, keeping the load in its present Y configuration, and see what happens. The measured parameters will be line current (voltage across R[SUB]line[/SUB], nodes 0 and 8), load phase voltage (nodes 8 and 7), and source phase current (voltage across R[SUB]source[/SUB], nodes 1 and 2). (Figure below)
http://www.allaboutcircuits.com/textbook/alternating-current/chpt-10/harmonics-polyphase-power-systems/
 
Where did you get this information?
Pages 3,4,5 and 7 of this pdf, and the 90% part appears in one of the installation/owner's manuals also.
http://files.sma.de/dl/7418/STP24-US_MV_Trafo-TI-en-10.pdf

Page 3 and 4 say 90%, 5 says 10%, and 7 says 50%.

Page 3-
The maximum apparent power of all inverter (S PV = ΣS PVi ) connected to the low voltage side is:
• Less than or equal to 90% of the rated power of the transformer (S XMFR ) S PV ≤ 0,9 · S XMFR.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
If I'm reading this right, you'd have to go from 480/277V inverters thru a xfmr to 480V delta and then another xfmr, 480 delta to 208/120Y, and they'd both have to be 2x the inverter kVA.
There is no reason one would have to go through two transformers to get from 480/277 to 208/120 and no reason for the transformers' kVA rating to have to be twice the inverter's kW rating. I have done this; have you?
 
Welcome to the Internet. :D
Didn't you mean InterNETS? :blink:
And ain't that what the internets are for...reading stuff?

But seriously- please correct me if I'm reading it wrong!

Ok, about using 480/277 instead of 208/120 inverters... for instance...

Say you have a panel to inverter DC run that is fairly long, 200 feet or so.
Then you have a fairly large building in a direct line between panels and PCC/service point.
You can go around the building OR straight through it.

Other option is:
Single phase 208/120 inverters, wired for 3 phase vs. 480/277 3 phase inverters.

Going around the building, with 208/120 inverters, you are using considerably more DC wire, at lower voltage- the AC wires may be shorter, but...the DC losses add up. Possibly not the best option overall.
Going thru the building with 208/120 (say 100 feet or so). the *AC losses* add up.
Going around the building with 480/277, same amount of wire, less DC losses.
Same going thru the building- less AC losses over 100 feet at 480/277.

That's just using 100 feet as an "example", but isn't there a certain number there where it does matter?
 
Originally Posted by PVfarmer

If I'm reading this right, you'd have to go from 480/277V inverters thru a xfmr to 480V delta and then another xfmr, 480 delta to 208/120Y, and they'd both have to be 2x the inverter kVA.


There is no reason one would have to go through two transformers to get from 480/277 to 208/120 and no reason for the transformers' kVA rating to have to be twice the inverter's kW rating. I have done this; have you?
I didn't say one HAD to- in fact, it's the NOT recommended way, on pages 6 and 7. I meant: you'd have to go from 480/277V TRIPOWER inverters to a 480/277 >> 480 delta xfmr.
Are you saying you did that 380/277 to 208/120 thing with one three phase xfmr? Because I can't seem to find one- a single 3 phase 208/120 step up to 480/277, sure, but...

3 Incompatible transformers with STP TLUS-10

Sunny Tripower TLUS needs a neutral conductor, so transformers with delta topology in the low voltage side are not
compatible for direct connection of the inverter (see figure below e.g. YNd1).

For this topology, it is necessary to install a second LV/LV transformer in order to adapt the grid topology according to
the Sunny Tripower TLUS needs. This transformer has to be a galvanic separation transformer due to the different earthing
topologies in both side of the LV/LV transformer.

See below two examples of this solution:
Both transformers shall have following ration of installed power
S PV ≤ 0,5 · S XMFR_LV/LV
S PV ≤ 0,5 · S XMFR_MV/LV
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
And ain't that what the internets are for...reading stuff?
Reading things is not the same as being taught them.

The reality is, many manufacturers have no idea how 'electrical power systems' actually function and interact. Their instruction examples cannot possibly be able to address all of the world's electrical sources.
 
The reality is, many manufacturers have no idea how 'electrical power systems' actually function and interact. Their instruction examples cannot possibly be able to address all of the world's electrical sources.
Point taken, thanks.
If the OP's inverter maker says it's OK to use 480V delta, great.
I'm just worried at the very least, about that pdf I keep posting, which does come from the SMA page, and I'm planning on using SMAs.
Even if connecting them to a 480V delta xfmr would work in some way, I'd think that even if an inverter went kaput, even for some other reason, the 480V delta thing would void the warranty.
 
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