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Thread: Ideal xfmr(s) for connecting 480/277Y inverters (60kW total) to 12470/7200 grid

  1. #1
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    Ideal xfmr(s) for connecting 480/277Y inverters (60kW total) to 12470/7200 grid

    Hello, this is my first question on this forum. I've found so many good answers here I'm a little confused!

    I have a rough draft (very rough some would probably say) of a one line drawing if anyone wants to see it.

    About the inverters: 60kW total (which is also 60kVA...?, because the PF of the inverters is =1. Or...1 / 0.8 leading…0.8 lagging)
    Consisting of four 15000TL inverters:
    Output phases / line connections 3 / 3-N-PE
    Nominal AC voltage 480 / 277 V WYE

    The grid is 12470Y/7200.
    We have single phase, 7200V to 120/240 service now, 200 amps.
    It goes through one 25kVA xfmr, which is 208 amps, 4% over the service level. Not quite enough.
    So...60kW of inverter power, when the current single phase (7200V) inverter on the pole is 25kVA, isn't going to sit well, as in, if we turned that PV on, BOOM, not good at all!

    The lowest amperage 480/277 service offered is 100A.
    The 12.47 to 480/277 xfmrs seem to be VERY expensive.
    It seems like it would be less $$ if the POCO went with three 7200 --> 480/277 transformers, but I don't know their prices yet on everything.
    BUT...60kW doesn't match up easily with common sizes of xfmrs.

    20kVA or 60kVA xfmrs aren't common. 25kVA, 30, and 75 are.
    Going by things I've seen here, there are different ways to do this. I'm looking for simplest.
    The POCO is OK with a line side connection, so....
    how important to both the grid people AND the overall balance of both system and grid is it to match the xfmrs and inverters as closely as possible?

    SO...which one here? If they are OK with not going for a whole 100A of xfmrs on the pole, A seems "low", C seems "high" or "hot", B seems like it may be best match overall and for 100A service, but B is another 100+ panels and a lot more paperwork.

    A. Four 15000TLs - 60kW - 72A ---> three 25kVA xfmrs?
    B. Four 20000TLs - 80kW - 96A ---> 3 30kVA xfmrs? (96A being that 4% under thing again)
    C. Five 15000TLs - 75kW - 90A ---> 3 25kVA xfmrs?
    D. Three 24000TLs - 72kW - 87A ---> 3 30kVA?
    E. Totally over the top- TWO 24000TLs with the same amount of panels as A, for a 1.66 ratio of panel watts/inverter output. ---> One 50kVA transformer. 4% under again. File that under "so crazy it might work", I have to try a simulation with that one. Hmm.
    F. other?

    They aren't kidding when they say these inverters are "flexible"!
    There was a comment in this forum about using two larger and one smaller transformers for the grid connection, not sure how one would "balance" that to the grid's benefit.
    I also saw something about having a Delta between the two wyes to...trap harmonics?
    Also something about "wild leg" wiring for 120/240.

    The going from 480/277 Y to 120/240 part seems straightforward enough, any thoughts there would be appreciated too.
    This, for instance?

    http://www.temcoindustrialpower.com/...al/HT0213.html
    or...this?

    http://www.temcoindustrialpower.com/...al/HT0193.html

    We're getting another xfmr to supply the current 120/240 wiring off of the PV output- the PV will be going in line side. If we do this right, with some batteries, there won't be a drop of power used all year.

    But then...too much inverter will sit there wasting power at night, which adds up eventually.
    Too little is of course also a bad idea, but...
    whew, it really gets complicated!

    Thanks, if anyone can help out here.
    These inverters seem like you could program them to put out a few kWs while you sleep and THEN turn on the coffee maker at 7AM!


    http://www.wseas.org/multimedia/jour...035702-208.pdf
    2. Step-up transformers for conventional PV plants
    The cost of the step-up transformer for a PV plant can be evaluated as a composition of four contributions, namely:
    • initial cost
    • cost of the energy wasted due to transformer
    overloads
    • cost of energy wasted due to transformer
    efficiency
    • cost of energy wasted due grid instability

    2.3 Energy losses due to transformer efficiency
    The power wasted due to no load and copper losses
    during the k-th interval can be computed through the
    following equation, assuming constant the
    amplitude of the inverter output voltage:
    (equation won't paste correctly)

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    a HA!

    Never give up! It isn't the 4% I mentioned before, it's 10%.

    One question semi-answered. 60kW of this particular inverter must go thru a 66kVA transformer minimum.
    The maximum apparent power of all inverter (SPV = ?SPVi) connected to the low voltage side is:
    • Less than or equal to 90% of the rated power of the transformer (SXMFR) SPV ? 0,9 · SXMFR.

    http://files.sma.de/dl/7418/STP24-US...o-TI-en-10.pdf

    But I'm still not sure about whether "less than" is enough of an effect to worry about.
    60kW thru a 75kVA xfrmr would be 80%.
    Hmm.

    Hey! Wrong voltage here, but they do make them.
    Would this be a "special order" type thing? I don't want to want 2 months for it...
    http://www.lcmagnetics.com/transform...-kva-pn-19063/

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    I'm really not qualified enough to talk about transformers but I have a couple comments and questions.

    1) Seems like you are not adding the NEC required 125% factor on your required amperages. So, for example, your four 20000W inverters at 96A would really require a 120A or larger service.

    2) If the POCO is going to have to upgrade the service or install an entirely new one, then of course you want to plan on the PV being connected line side. That detail should really should not require their permission, although I understand some jurisdictions have trouble with the idea of just following the NEC.

    3) Why not just tell the POCO that you want a 480/277 service, and you will backfeed X amps, and see what they tell you is necessary? The good part of this is that they are totally responsible for the transformer and connections being specified correctly, right? For that matter, why not let them do the work? (Is there even an option?) I guess they may gouge you, and I don't know how it works in your area with respect to who contracts for installing services. But personally, I would rather just give specs to the POCO and let them engineer it, than be responsible for figuring it out.

    4) Related to number 3... Where would the service point be? And how will it be metered?

    5)
    If we do this right, with some batteries, there won't be a drop of power used all year.

    But then...too much inverter will sit there wasting power at night, which adds up eventually.
    ...
    ...
    These inverters seem like you could program them to put out a few kWs while you sleep and THEN turn on the coffee maker at 7AM!
    Whose inverters are you talking about here?

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    Quote Originally Posted by jaggedben View Post

    1) Seems like you are not adding the NEC required 125% factor on your required amperages. So, for example, your four 20000W inverters at 96A would really require a 120A or larger service.

    2) If the POCO is going to have to upgrade the service or install an entirely new one, then of course you want to plan on the PV being connected line side. That detail should really should not require their permission, although I understand some jurisdictions have trouble with the idea of just following the NEC.

    3) Why not just tell the POCO that you want a 480/277 service, and you will backfeed X amps, and see what they tell you is necessary? The good part of this is that they are totally responsible for the transformer and connections being specified correctly, right? For that matter, why not let them do the work? (Is there even an option?) I guess they may gouge you, and I don't know how it works in your area with respect to who contracts for installing services. But personally, I would rather just give specs to the POCO and let them engineer it, than be responsible for figuring it out.

    4) Related to number 3... Where would the service point be? And how will it be metered?

    5) Whose inverters are you talking about here?
    Howdy, thanks in advance.
    1. Could you elaborate on that 125% factor? I tried looking that up, and it seemed to be about fuses. Sorry if I was confusing, I'm not getting into the fuses yet.
    Just kind of wondering...having a transformer twice the size of your output would be wasting money, but also less efficient electrically.
    But you can't go 1:1, 75kW into 75kVA of xfrmrs, that's too hot. I guess that would even depend on the type of xfrmr. I'd like to get rid of the old oil-filled can and put something more efficient, perhaps on the ground. More on that.

    2. I was wondering about the line side thing originally, but if it is better...they should of course allow it! Seems like they are OK with it. Maybe the guy already knew how snazzy these inverters are (SMA Tripower US models)

    3. "see what they tell you is necessary?"
    That's sorta what I'm worried about- necessary doesn't...necessarily always mean "best".
    Plus, I'm sort of trying to impress them, in order to speed things up- it could take them 2 or 3 weeks to design it, but if I show them something, we might be able to skip that whole part.

    4. The service point is...weird. It's 250+ feet from the road, so we have 250+ feet of wires straight from 1 of the overheads, two poles, to the 25kVA xfmr, then overhead wires from there to a barn with the meter outside a shed and main panel inside the shed. We might be able to sneak it underground from the bottom of the pole instead. The PV power is coming from the other side of the barn and going into the grid, and will feed the barn through a 480/227 to 120/240 single phase or maybe 240V transformer. The 60kW of PV is sized to cover the usage in December/January, which ends up, because of the seasonality of the load, being 4x the usage in June. Which = $$. Which is nice for us...BUT we end up putting out a ton in summer when the grid is strained, so good for them too.

    I suppose most residential xfmrs are on public property. I did impress the guy a little by figuring out the feed wires are 7200V. They sent me a pre-application info sheet that said "12.47kV radial feeder".
    Side note of historical trivia- rural feeders used to be single phase 7200V, fed by 3 phase Delta 7200V. But as demand grew, they changed them all (or most) to 3 phase 4 four wire 12.47kV by adding another wire for neutral/ground.
    Kind of a neat trick!
    The early rural systems
    were 7200-volt delta, three-phase in most cases, with
    7200-volt single-phase branch lines. These systems
    have now given way to 7200/12,470Y-volt three-phase
    four wire systems. This change became necessary due
    to the fact that existing 7200-volt delta, three-phase
    lines became more heavily loaded and also due to the
    fact that rural lines have been extended to much greater
    length than formerly.

    http://www.cooperindustries.com/cont...rs/R201902.PDF


    5. These puppies. Germany does have a pretty solid rep in the engineering things department.
    http://www.sma-america.com/products/...wnloads-137610

    Thanks again!

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    usually only systems large enough to justify the added cost are interconnected at medium voltage. you should consider some single phase inverters with a single transformer. I would recommend Schneider electric inverters which are sized to even output current increments 50/75/100a.

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    Quote Originally Posted by jtopham26 View Post
    usually only systems large enough to justify the added cost are interconnected at medium voltage. you should consider some single phase inverters with a single transformer. I would recommend Schneider electric inverters which are sized to even output current increments 50/75/100a.
    Howdy- I'm not quite awake yet, but I don't get what you mean by the first sentence there. The grid voltage is 12470Y/7200, so any type of inverter has to be stepped up to either/or 12470 / 7200V (3 phase / 1 phase).
    With the transformer we have now, I guess we could step the 480/277 3ph output down to 2 legs of 120V and put it through like that, but that doesn't make sense, and limits us to 25kW.

    We could get 120/240 output inverters, but that's sort of a lot less efficient than 480/277 3 phase output, and I don't think you can go over 600V DC with the Schneiders?
    The SMAs, you can run them at ~789V DC or half that, and it doesn't make too much difference. But compare the SMA output to other companies output at below 600V DC/120 or 240AC, and there is a difference.
    That's using a simulation like SAM or PVSyst, or SMA has a good one on their site.

    I think I see what you're saying-
    If we got one 45kVA 7200V - 120V xfmr, it would be 624A @ 120V.
    We could put 6 100 amp Schneiders into it.
    But why not just get 3 25 or 30kVA xfmrs and go with 480V 3ph inverters?

    It's the 12.47kV to either 480 or 240 or 120V xfmrs that are big $$.
    Along the lines of $20G for a single 12.47 >> any of those (I think even $22G for the 12.47 >> 120 model.)
    Whereas 7200V >> the same are around $5G each.
    I could be off there, but I guess that makes sense due to amount or size of wire?
    Of course the grid people will have real wholesale prices, but still...

    To me, it seems "not enough", if we're getting upgraded service for the purpose of cranking out PV power, to stay with single phase.
    They're going to be running 250+ feet of wire to our pole, and it will be all three mains from the grid, so the power is going from inverter level to grid level on our property, coming in 3 phase.
    Thanks for the suggestion, I will try a simulation using your setup and let you know the results.

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    you can go 1kv you can also get 277v inverters... maybe not Schneider but they're around

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    I really don't understand your scenario I guess... if you have an existing service just backfeed that with your inverters. you shouldn't need a transformer. can you post a single line of your existing installation?

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    Quote Originally Posted by jtopham26 View Post
    I really don't understand your scenario I guess... if you have an existing service just backfeed that with your inverters. can you post a single line of your existing installation?
    Yep, here you go- the service now is just 200 amp 120/240 single phase, meaning two lines of 120 to get the 240. It comes from a single 7200V to 120V can on a pole type xfmr which is 25kVA.
    I guess that puts out 2 lines of 120V from the single overhead line from the grid (grid has 3 lines of 7200V and a neutral, so grid setup is 12470Y/7200). We get one of those three and the neutral at our pole now.
    Meaning, I think, the 7200V to 120/240V xfmr simply puts out 2 lines of 120 and you combine them at the panel for 240V.

    That's too small to backfeed with 60kw (which is also 60kVA when the inverter power factor is =1.)
    So we're getting 480Y277V service, 100 amps, which would be ~90kVA worth of xfmrs.

    I suppose we could get a 75kVA single phase 7200V to 12/240V, but the inverters are 3 phase and 480/277, while the load is one phase, and the PV output is = to or 2x the load, so going 3 phase into the grid and then getting a smaller xfmr for the load side seems more efficient than putting the 3 phase PV Power into a transformer to make it one phase and then back into the grid through the correctly sized xfmr.

    90 kVA xfmr is kind of too much for the 60kVA inverter output- I'm wondering how much of a problem that is.
    I'm assuming that we'd get 3 xfmrs (7200V to 480/277) instead of one (12470 to 480/277) when we go to 480/277 3ph, just because one is seemingly more $$.

    Hopefully, three 25kVA xfmrs to make 75kVA total will be..."not too much" size wise. 80% PV output to xfmr size seems like a good number if 90% is the max.

    Thanks for your help.

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    Hey man- this is a typo- i meant too much TRANSFORMER will sit there wasting power at night. The inverters don't waste any.
    Whoopsie.

    What I meant was:
    You've got a xfmr sized to the PV output.
    So at night, when the output=zero, and the load is zero or close to it, the transformers are wasting watts, like say .3 kWh or something.
    So that's an issue for the grid, because it's on their side of the meter- they don't want to oversize the xfmr too much.
    Say the load at night is a few refrigerators and clocks and just small stuff- when the refridges aren't running, the load is almost nothing.
    If you have a battery bank it might be =zero.
    So when load =zero and output is also zero the xfmr is sitting there on the pole like a lump, wasting more and more the more oversized it is.

    Quote Originally Posted by jaggedben View Post

    5) me- "But then...too much inverter will sit there wasting power at night, which adds up eventually."

    you- Whose inverters are you talking about here?

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