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Thread: NGR and Wye-Wye transformer

  1. #1

    NGR and Wye-Wye transformer

    I am running a power system study on a rather interesting case.

    Utility transformer is a 7.5 MVA 138kV delta to 4160V wye transformer. The customer is engineering the new construction, but asked us to recommend an NGR while I'm performing the short circuit and arc flash studies. That was easy enough-- we went with a 6 ohm resistor to limit the fault current to 400A. This will help with ground fault protection for the (3) 4160 motors. The part where it seems to be unusual (to me at least) is the downstream transformer: 300 kVA 4160V wye to 480V wye, with H0 and X0 linked and grounded.

    As far as I can tell from the customer's cable schedule, they are only running three conductors between the utility transformer wye secondary and the downstream wye primary. Won't that allow the downstream wye primary to wander? Or is that really the way it's supposed to be? It seams to me that they need to run a neutral conductor between the two transformers, and leave the H0 and X0 connected on the downstream transformer, but disconnect them from ground. This would cause all faults on the 4160V and the 480V systems to return through the NGR, right? Would that limit the 480V ground fault current to 277V/6ohm = 46A? Or is it correct as originally designed, and we have two separately derived systems: a low resistance ground 4160V system, and a solidly grounded 480V system? Or am I just all out of line?

    I have been on the phone with SKM technical support (they make the software I am using for this study), and they are working on figuring out how exactly to model it in their software. They told me this was quite unusual. I have bounced it off of several people in my office, but no one's seen a system quite like this before... Surely someone on this board has, and can give me some tips!

    Thank you in advance!
    Nathaniel

  2. #2
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    Quote Originally Posted by diakonos1984 View Post
    I am running a power system study on a rather interesting case.

    Utility transformer is a 7.5 MVA 138kV delta to 4160V wye transformer. The customer is engineering the new construction, but asked us to recommend an NGR while I'm performing the short circuit and arc flash studies. That was easy enough-- we went with a 6 ohm resistor to limit the fault current to 400A. This will help with ground fault protection for the (3) 4160 motors. The part where it seems to be unusual (to me at least) is the downstream transformer: 300 kVA 4160V wye to 480V wye, with H0 and X0 linked and grounded.

    As far as I can tell from the customer's cable schedule, they are only running three conductors between the utility transformer wye secondary and the downstream wye primary. Won't that allow the downstream wye primary to wander? Or is that really the way it's supposed to be? It seams to me that they need to run a neutral conductor between the two transformers, and leave the H0 and X0 connected on the downstream transformer, but disconnect them from ground. This would cause all faults on the 4160V and the 480V systems to return through the NGR, right? Would that limit the 480V ground fault current to 277V/6ohm = 46A? Or is it correct as originally designed, and we have two separately derived systems: a low resistance ground 4160V system, and a solidly grounded 480V system? Or am I just all out of line?

    I have been on the phone with SKM technical support (they make the software I am using for this study), and they are working on figuring out how exactly to model it in their software. They told me this was quite unusual. I have bounced it off of several people in my office, but no one's seen a system quite like this before... Surely someone on this board has, and can give me some tips!

    Thank you in advance!
    If you have a grounding resistor, you can't connect the 'grounded' conductor to anything else. The primary wye will not 'as the grounding resistor will 'anchor' it. Wye/wye transformers are not used:

    1. Triplen harmonics can pass through the transformer. This will cause problems with interference.
    2. Relay coordination/selectivity becomes more difficult. As the zero sequence current goes right through the transformer, the ground fault on the secondary appears as ground fault on the primary.
    3. Utility ditribution uses wye-wye transformers see cost benefits of lower initial capital investment. They also are concerned about the potential ferro-resonance that would appear if one phase is lost on the primary. In industry or commercial applications this would not be a major concern.

  3. #3
    Thank you Laszlo,

    So the best solution would be to replace the wye-wye downstream transformer with a delta-wye transformer. The secondary wye would be solidly grounded, making our low resistance ground 4160V system and our solidly grounded 480V system separately derived, providing for better coordination and stopping the triplen harmonics.

    Since they have already purchased the transformer (though it isn't set yet), they aren't going to appreciate me telling them this... If they break the H0 to X0 connection on the downstream wye-wye, and ground the secondary, would that be an acceptable solution? Or have we introduced problems by not anchoring our 4160V primary wye to anything?

    Thank you very much!
    Last edited by diakonos1984; 01-27-09 at 01:28 PM. Reason: misspelling of Laszlo's name
    Nathaniel

  4. #4
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    Quote Originally Posted by diakonos1984 View Post
    Thank you Laszlo,

    So the best solution would be to replace the wye-wye downstream transformer with a delta-wye transformer. The secondary wye would be solidly grounded, making our low resistance ground 4160V system and our solidly grounded 480V system separately derived, providing for better coordination and stopping the triplen harmonics.

    Since they have already purchased the transformer (though it isn't set yet), they aren't going to appreciate me telling them this... If they break the H0 to X0 connection on the downstream wye-wye, and ground the secondary, would that be an acceptable solution? Or have we introduced problems by not anchoring our 4160V primary wye to anything?

    Thank you very much!
    Of course the best solution would be to replace the transformer with a D/Y.

    I think the secondary solution may work, but I honestly don't know without studying it in detail or maybe modeling it in SKM. (Although the software may not even have that transformer model in file.) If your conditions allow, consdier installing a resistor on the Xo as well, just make sure that your HRG detection has harmonic restraint components. Be careful when you installing 480V components with electronic controls such as ASD's and UPS's that your Vendor is aware that you have an HRG system. In some cases your equipment may already have the installation features to install it either on SG or HRG systems but we ran into UPS's when it has to be specified ahead otherwise a field modification kit is supplied. (I don't like field fixes.)

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