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Thread: Recycling Power Through a Wye-Wye Transformer

  1. #1
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    Recycling Power Through a Wye-Wye Transformer

    Good afternoon,

    I work in a company that specializes in 3-phase power converters. Our assembly line electrical test stations need to be able to test our 480V and 208V units, both at 100 A. However, the current reduction at the end of the circuit is only about 5 A. In order to try to save money and electricity, we reroute the remaining 95A back into the test circuit. This means that our power usage for one of these test stations should average maybe just above 5 A multiplied by the test voltage.

    Okay. So, the main voltage we run throughout our facility for tests is 480V. We really do not want to run an additional line in order to test our 208V units, or have to run even more lines to test 380V or 400V units as they are developed. Recycling the 480V power is very easy because it can junction directly into the main line. But, we need a transformer to test our 208V units, and as a result we must develop a custom solution to recycle this power while still achieving isolation of the unit under test. That is something I forgot to mention - we need a transformer at each test station in order to ensure all units being tested are isolated from the rest of the building.

    Anyway, we have developed a plan to recycle the 208V power in a very unconventional way. We desire to have a custom transformer produced with 480V primary and 480V secondary, as well as 208V taps on both the primary and secondary windings. What we plan on doing for our 208V tests is having 480V power initially enter with relatively high current into the transformer. It would then be transformed into 208V, 100A, and pass through our test unit and test equipment. Once it has passed through all of these, it returns to the input side of the transformer enters the primary winding via the 208V tap. Once the current is steady at 95A entering the primary winding (recall 5A current loss in system), then only a few Amps will be required to sustain the system, entering in the 480V input.

    So basically, at t=0:
    I_480_in=43A
    I_480_out=0A
    I_208_in=0A
    I_208_out=100A

    Then, at t=inf:
    I_480_in=3A
    I_480_out=0A
    I_208_in=95A
    I_208_out=100A

    Now, the only way this could work is by using Wye connection types on both the primary and the secondary. This would permit two inputs for the transformer, because current and voltage vectors align. This would not be possible for a delta-wye or delta-delta transformer.

    So, specs look something like this:

    kVA: 75
    Qty: 7
    Primary: 480V Wye
    Secondary: 480V Wye
    Taps: 208V Wye on both primary and secondary
    Material: Aluminum
    Temp Rise: 150

    The main issue that we have is that no one is willing to make it for us. We have tried many custom transformer companies, and received answers ranging from, "it's not safe," to, "you don't have enough volume."

    Is this in fact an unsafe design? If it is unsafe, why? If it's not unsafe, why won't anyone produce it?

    Thanks for the help in advance!!!

  2. #2
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    What is the conversion?
    ac/dc?
    ac/ac voltage level, freq?

    so you put power into the converter
    100 A and you lose 5 A of power in the process?
    and you take the converter output back into the supply grid?



  3. #3
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    Single ph equivilent
    is this what you want to do?
    do you not see a few issues?



  4. #4
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    Ingenieur,

    We have a number of different converters that we test from DC/DC to AC/AC to grid tied inverters. These don't matter so much because our test filtering equipment converts the electricity back to 3-phase AC 60 Hz 208V for return to the grid or the transformer in this case.

    I do not see an issue with your diagram, although the block you have drawn and labeled "power converter" might be better named simply as "load". Power converter misleads one to think there is DC or something else coming out of the test apparatus system. The electricity exiting the test system and returning to the transformer is of the same phase, voltage, and frequency as that entering the system (60 Hz 208V).

  5. #5
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    Assuming you can keep the 480 and 208 in phase
    if the primary has 1000 turns total
    how many volt/turn above the 208 tap?
    and how many below?
    how many amps in the winding above the 208 tap?
    how many below?
    translated to the secondary with a fixed turns ratio?

    have you shown this to your plant power ee?
    how many xfmr mfgs said no go due to safety?
    Last edited by Ingenieur; 03-08-17 at 07:33 PM.



  6. #6
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    Quote Originally Posted by JohnSmith1 View Post
    Good afternoon,The main issue that we have is that no one is willing to make it for us. We have tried many custom transformer companies, and received answers ranging from, "it's not safe," to, "you don't have enough volume."ce!!!
    Call and tlak to Tierney in Seattle, they have made anything I've ever asked them to make, and were low bidder besides on some 150each qty builds of 120/480 delta wye xfmrs.

  7. #7
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    Quote Originally Posted by JohnSmith1 View Post

    Recycling the 480V power is very easy because it can junction directly into the main line.
    Can I stop you right there? When you say it needs to draw 100 amps and return 95 amps, I assume you mean it needs to draw about 50KW, and return 47500 watts to the source. (Even more power if we are talking about 3 phase.)

    It is possible to get that with electronics. But it doesn't usually just happen - it takes a lot of design. Power needs a reason to flow. In general, it's not going to flow from a source to a load and back to the source. For the converter to draw 100 amps (and 50KW), it needs to see a low impedance on its output.

    In general, by tying the output back to the input, you are going to make the converter see a high impedance on the output. That's going to reduce the current it draws on the input.

    So can you be a little more specific about how you plan to recycle the 480V power before we get into the various voltages and transformers?

  8. #8
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    I do not understand what you mean by "recycle" power. Power (energy, actually) is consumed by loads and converted to heat, light, mechanical energy, whatever, and you aren't getting it back. Sure, you could use electrical energy to drive a motor that spins a generator that powers an inverter that returns power to the grid, but it's a less than zero sum game; you'll return less to the grid than you used in the motor. The Laws of Thermodynamics have not been repealed.

  9. #9
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    I think what he is proposing
    power up an unloaded 'converter' (undefined) with 5% inherent losses
    feed the output back into the supply
    but what in the supply will make the inverter produce full load?
    the power flow does not make sense
    no 'load'?
    on top of that he wants 2 xfmr inputs on a common winding/core of different voltages
    Last edited by Ingenieur; 03-10-17 at 02:38 PM.



  10. #10
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    Quote Originally Posted by steve66 View Post
    Can I stop you right there? When you say it needs to draw 100 amps and return 95 amps, I assume you mean it needs to draw about 50KW, and return 47500 watts to the source. (Even more power if we are talking about 3 phase.)

    It is possible to get that with electronics. But it doesn't usually just happen - it takes a lot of design. Power needs a reason to flow. In general, it's not going to flow from a source to a load and back to the source. For the converter to draw 100 amps (and 50KW), it needs to see a low impedance on its output.

    In general, by tying the output back to the input, you are going to make the converter see a high impedance on the output. That's going to reduce the current it draws on the input.

    So can you be a little more specific about how you plan to recycle the 480V power before we get into the various voltages and transformers?
    I think he means it is used to test the converter (of whatever type) at its rating and sends the converter output back to the supply. This is not altogether uncommon on test set ups, particularly if there is a lot of power involved that would otherwise be wasted.
    Si hoc legere scis nimium eruditionis habes.

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