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Thread: Transformer sizing for PV system

  1. #11
    Join Date
    Jun 2015
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    Newport County, Rhode Island, USA
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    Quote Originally Posted by jaggedben View Post
    1 if you have a manufacturer requirement and a code requirement that aren't the same you just have to keep track of which is greater.
    2 It would be helpful if SMA was crystal clear about what their documentation applies to,
    3 but it could apply to places where the NEC isn't code and you can't expect them to always be 100% on top of what the various local codes are.
    4 I could be wrong but I believe that a transformer may not be required to be rating 125% of the inverter output rating. Unlike conductors and OCPDs.
    Quote Originally Posted by electrofelon View Post
    5 Transformers have no 125% rule/continuous load fudge factor. In fact, 110.3(B) is the only thing that indirectly specifies the sizing/loading of transformers.
    6 I dislike these silly manufacturer requirements like this one saying the transformer must be sized at 90%. They should just stick to making inverters. That said, if they state that in the instructions, 110.3 compels us to follow it.
    1- Agreed. However, I find this discussion at this link a bit... alarming.
    ( https://www.solarpaneltalk.com/forum...ith-lost-phase )
    The installation consists of 4-SMA Tripower 20000TL inverters with a load side connection via a 480/208 Y-Y 100kW transformer to the MDP. The inverters are 3 phase 277/480 and the grid is 3 phase 120/208.

    2- Right, the pdf is about connecting to MV, but it doesn't explain the why re: 90% at all, and I can't find a pdf about stepping down.

    3- Delta and wye are the same in Germany....but the grid isn't...so...who knows?

    4- Pretty sure there's a post on this forum about that somewhere.

    5- Do you think the loads or the inverters are more important, if picking one of A,B,C?
    Inverters aren't technically computers....or?
    The grid doesn't like harmonics, so get a harmonic mitigating model makes sense to me.

    A Energy efficient k-factor transformer designed to tolerate heating due to harmonics associated with nonlinear loads.
    -
    B Harmonic mitigating transformers are superior to K-Rated and conventional transformers in reducing voltage distortion (flat-topping) and power losses due to current harmonics created by single-phase, nonlinear loads such as computer equipment.
    -
    C These energy efficient general purpose transformers are designed for linear loads and are most frequently used for applications such as commercial buildings which will supply a variety of general loads.
    http://www.temcoindustrialpower.com/...05160&n=ACAg.p

    6- I really don't know- maybe the impedance increases with a xfmr running at 98%, and it bothers the inverters somehow?

  2. #12
    Join Date
    Dec 2012
    Location
    Placerville, CA, USA
    Posts
    17,258
    Re point #6:
    The lower the rating of the transformer, the higher the voltage drop (voltage rise) across the transformer. And the harder it is to properly detect the low impedance of the grid for anti-islanding purposes.
    The difference between 80% and 125% is not enormous, but you have to draw the line somewhere.

    Sent from my XT1585 using Tapatalk

  3. #13
    Join Date
    Jun 2015
    Location
    Newport County, Rhode Island, USA
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    Quote Originally Posted by GoldDigger View Post
    Re point #6:
    The lower the rating of the transformer, the higher the voltage drop (voltage rise) across the transformer. And the harder it is to properly detect the low impedance of the grid for anti-islanding purposes.
    This is intriguing, pardon what may be clueless questions and/or layman's terms...
    (point #6 might have something to do with that link I posted in point #1 re:islanding then? Hmm.)

    Example equipment is:
    1. 3x 24.1kW (29A) inverters, 480Y/277V output, so the 3 inverters = 87A / 72.3kVA.
    2. 75kVA xfmr = 480Y/277V to MV (or 208V)
    3. (Side note: That'd be a 200A 208Y120V or 100A 480Y/277V "POCO service", but you'd be outputting 87A through a 90.2A/75kVA xfmr, sounds kinda-a-bit-too-close for me. (And 72.3KVA = 200.9A or 200.6A when converted to 208Y/120V...)


    With the 72.3kVA/87A of PV the xfmr is running at 96.4% of its 75kVA/100% rating.

    So when you said "lower the rating" you meant the 100% xfmr as compared to the PV output?

    This lower rating could be said as "higher the (PV:xfmr rating) ratio", or that 96.4% (ratio is 0.964:1).

    The xfrmr runs less efficiently at 125% (1.25:1) than it does at 80% (0.8:1), and less efficiently at 96.4% than at 90% or 80% or 65%.
    This is because of higher voltage drop (voltage rise).

    The inverter(s) have to detect the low impedance of the grid.

    This low impedance is actually "approaching infinite" on grid side.
    When you are going ---> inverter 480Y277V - connected to LV side / air gap/ then grid side /MV side of POCO's xfmr, the impedance of POCO xfmr is a factor.
    (Side note: Now, if there is a step-down from inverter-to 208Y/120V xfmr, then another step-up-to-POCO MV xfmr involved, the impedance is MUCH different than straight 480-MV?)

    But what I want to know is-
    With this voltage drop/rise "across" the xfmr...which is caused by the xfmr being closer to its 100% rating and less efficient/running hotter...
    do the inverters "have to"...or...do they "end up"... decreasing their voltage?

    The inverters could have more DC power than they can handle and therefore be clipping, ok.
    When so, they still want to put their max (72.3kVA for instance) output into a grid with as little impedance as possible, at as close to 480Y/277V as possible.

    When say a 75kVA step-down xfmr is "holding them back" so to speak, it is because...

    the xfmr is putting out <208V, and therefore "asking for" <480V from the inverters?

    Could one say the 208V impedance is "clogging up" the xfmr on the inverters' side?

    Are amps lost to heat because of lower voltage?
    -or-
    Do I have it backwards and the inverters "have to" or "end up" INCREASING their AC voltage to compensate for lower 208V voltage?

    Or!
    Does the xfmr just heat up due to physics because 72.3KVA of PV is 96.4% of its 100%, and the windings are more efficient at 90%, and voltage drops on both sides?

    Thanks again, if this makes sense...

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