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Thread: Typical Transformer %Z Values

  1. #1
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    Typical Transformer %Z Values

    Anyone have a chart of typical impedance values (%z) for both dry-type and oil filled transformers? Here is what I have on file, but I don't know whether it is purely for dry-type or how accurate they are...

    Code:
    240/1
    25 KVA = 1.6
    37.5 KVA = 1.6
    50 KVA = 1.7
    75 KVA =1.6
    100 KVA = 1.6
    
    208/3
    112.5 KVA = 2
    150 KVA = 4
    225 KVA = 4.5
    300 KVA = 5
    500 KVA = 5
    750 KVA = 5.75
    1000 KVA = 5.75
    1500 KVA = 5.75
    2000 KVA = 5.75
    
    480/3
    112.5 KVA = 1
    150 KVA = 1.2
    225 KVA = 1.2
    300 KVA = 1.2
    500 KVA = 1.3
    750 KVA = 5
    1000 KVA = 5
    1500 KVA = 5
    2000 KVA = 5
    2500 KVA = 5
    Thanks,
    Keri

  2. #2
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    Those values look like ones quoted by most power companies for oil-filled units.

    Energy efficiency laws have forced many transformers to be redesigned so it is hard to predict the %Z of small, <300kVA, size dry-type units.
    Just because you can, doesn't mean you should.

  3. #3
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    Thanks Jim. I've noticed that Eaton gives a range for their dry types, so I will just go with the lowest for my worst case scenario.

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    If what I recall from that parameter of the transformers is right, the insulation type, whether oil or dry, has nothing to do with the value of %Z. That value is related to the impedance across the entire winding. So the winding does not care if he is submerged in OIL or AIR.

    those values that you post seem pretty fine.
    "I am the son and heir... Of nothing in particular"

  5. #5
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    Quote Originally Posted by Mayimbe View Post
    If So the winding does not care if he is submerged in OIL or AIR.
    That is true only for identical windings.
    However, inductance is the predominant impedance in a transformer, so the configuration and size of the core and coil assembly will impact the %Z. When looking only at KVA ratings, liquid filled transformers usually have relatively smaller and more compact windings and therefore lower %Z than do dry type units.
    Just because you can, doesn't mean you should.

  6. #6
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    Quote Originally Posted by jim dungar View Post
    That is true only for identical windings.
    However, inductance is the predominant impedance in a transformer, so the configuration and size of the core and coil assembly will impact the %Z. When looking only at KVA ratings, liquid filled transformers usually have relatively smaller and more compact windings and therefore lower %Z than do dry type units.

    %Z is related to the windings, not to the insulation, thats what I said. Or thats what I was trying to say.
    "I am the son and heir... Of nothing in particular"

  7. #7
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    Quote Originally Posted by Mayimbe View Post
    %Z is related to the windings, not to the insulation,
    Yes, but the winding construction depends on the insulation.
    You can't really consider them to be independent parameters.

  8. #8
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    impediance levels

    From what I have discovered over the last year with the introduction of EE transformers, and the different DOE CSL levels (2,3,5) that benchmarks the efficiency above TP-1. Is that it is forcing manufacturers to re-design their transformers (round / mitered corners) and even core materials etc. the result is lower impedances and this varies from manufacturer to manufacturer, weather aluminum or copper temperature rise, etc. the other issue it affects is the inrush on the transformer, where before assumed was a typical 6-8x fla maufacturers are publishing data in excess of 20xfla. so what does that do to your breaker TCC and nuisance tripping. from what I have been able to discover there are multiple types of inrush the biggest ones are a "cold" start (primary closed / secondary OCPD open no load) and "hot" restrike (momentary loss of voltage xfmr. is re-energized with secondary load applied)

    When this occurs there are multiple things that add up to higher levels of inrush and possible nuisance tripping.
    One last item, don't forget as you impedance falls your secondary short circuit values will rise significantly. Just my 2 cents.

  9. #9
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    Quote Originally Posted by Besoeker View Post
    Yes, but the winding construction depends on the insulation.
    You can't really consider them to be independent parameters.
    Agree with you. The winding construction depends on the insulation. But once it have been constructed... well, I believe they are independent parameters, the %Z of the insulation.

    Say I give you a piece of wire (copper) of length L, and a magnetic core with NxN dimension. Then I ask you to please roll that wire on the core in a way that you get something like a "winding". Then submerge the winding in air and oil. Get the values of %Z in both cases. I think they will be pretty much the same.

    If I give you another piece of wire of lenght equal to L-dl, where dl is a lenght diferential. And you got the same core that I gave you before. The winding will have two diferent values of %Z now. If I had gave you a diferent core, the values of %Z will change as well. whether you have submerged the winding in OIL or AIR.

    I have consider them to be independent parameters in this scenario.
    "I am the son and heir... Of nothing in particular"

  10. #10
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    Quote Originally Posted by Mayimbe View Post
    %Z is related to the windings, not to the insulation, thats what I said. Or thats what I was trying to say.
    Actually, liquid vs air is, most commonly, a discussion about cooling mediums not conductor insulation materials. Liquid filled transformers can be physically smaller than air cooled transformers even if the actual conductor insulation is the same.
    Just because you can, doesn't mean you should.

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