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Thread: Grounding a Wye Wye Transformer

  1. #1
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    Grounding a Wye Wye Transformer

    The application is a 25kv/480 Pad Mounted Transformer feeding a MCC (w/ Main Breaker) approximately 150 ft away. There is no Secondary OCD at the Transformer. The XO is connected to a High Resistance Ground (Post Glover) in the MCC. The Transformer HO is connected to both the POCO ground and the Plant grounding grid.

    The question is: should the POCO grounding conductor be connected to the Plant grounding grid or should the two be isolated from each other? I've studied for months to find the answer and still don't have a clear understanding. Thanks

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    Quote Originally Posted by stickelec View Post
    The application is a 25kv/480 Pad Mounted Transformer feeding a MCC (w/ Main Breaker) approximately 150 ft away. There is no Secondary OCD at the Transformer. The XO is connected to a High Resistance Ground (Post Glover) in the MCC. The Transformer HO is connected to both the POCO ground and the Plant grounding grid.

    The question is: should the POCO grounding conductor be connected to the Plant grounding grid or should the two be isolated from each other? I've studied for months to find the answer and still don't have a clear understanding. Thanks
    Normally pad mounted transformers have "bay-o-net" fuses installed from the manufacturer on the secondary side. think of the ground connections if this were an overhead application.

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    HO and XO should not be bonded 'before' (i.e. in the transformer tank) the neutral grounding resistor. Other than that I see nothing overtly wrong with HO being bonded to the plant grounding grid.
    Just because you can, doesn't mean you should.

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    Quote Originally Posted by jim dungar View Post
    HO and XO should not be bonded 'before' (i.e. in the transformer tank) the neutral grounding resistor. Other than that I see nothing overtly wrong with HO being bonded to the plant grounding grid.
    Normally pad mount transformers have a common XO/HO bushing. (I hate stating "normally" all the time, but there are always an exception or two...)

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    Quote Originally Posted by jim dungar View Post
    HO and XO should not be bonded 'before' (i.e. in the transformer tank) the neutral grounding resistor. Other than that I see nothing overtly wrong with HO being bonded to the plant grounding grid.
    Jim, does that mean its okay either way? Apparently its not a code issue. I'm trying to weigh out the advantages/disadvantages.

    In this thread Hv&Lv mentions a "common HO/XO Bushing". In that instance, what happens if the POCO uses individual cutout-fuses and only one is closed? Doesn't the Primary voltage cross-over to the Secondary Wye?

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    Quote Originally Posted by stickelec View Post
    Jim, does that mean its okay either way? Apparently its not a code issue. I'm trying to weigh out the advantages/disadvantages.

    In this thread Hv&Lv mentions a "common HO/XO Bushing". In that instance, what happens if the POCO uses individual cutout-fuses and only one is closed? Doesn't the Primary voltage cross-over to the Secondary Wye?
    You may want to check to see if there is an externally operated HO disconnect on this XF. SOME manufacturers offer this option as standard.
    You will have to ask the POCO, because by externally I mean it accessible in the primary side of the XF.(POCO only)
    Last edited by Hv&Lv; 10-22-11 at 11:00 AM.

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    If there is a H0-X0 connection, then your grounding resistor will effectively try to change the utility primary grid into a high resistance ground system - you will have all sorts of problems. The last install we had like this, required the transformer tank to be opened and the H0-X0 bond removed (the utility supplied the wrong transformer)

    Opening fuses one-at-a-time causes problems with ferro-ressonance. The primary voltage does not "cross over to the secondary wye".
    Just because you can, doesn't mean you should.

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    Quote Originally Posted by jim dungar View Post
    If there is a H0-X0 connection, then your grounding resistor will effectively try to change the utility primary grid into a high resistance ground system - you will have all sorts of problems. The last install we had like this, required the transformer tank to be opened and the H0-X0 bond removed (the utility supplied the wrong transformer)

    Opening fuses one-at-a-time causes problems with ferro-ressonance. The primary voltage does not "cross over to the secondary wye".
    If HO and XO are connected via the common POCO and Plant grounding systems - couldn't that result in the issue you mentioned? I was on one job where the POCO did supply a Transformer with an internal bond, and when they closed the first cutout it blew-up the MCC 2000 amp Molded Case CB, blew-out the 600v cabling in the Cable Tray, and blew-holes in the sides of the Cable Tray. There engineer admitted it was due to the internal bond - they removed it during the Transformer repair.

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    Quote Originally Posted by stickelec View Post
    If HO and XO are connected via the common POCO and Plant grounding systems...
    I would guess that close to 100% of our local POCOs supplied transformers do have a H0-X0 connection.
    This is why putting HRG systems on POCO service entrances is not something I usually recommend.
    Just because you can, doesn't mean you should.

  10. #10
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    Quote Originally Posted by jim dungar View Post
    I would guess that close to 100% of our local POCOs supplied transformers do have a H0-X0 connection.
    This is why putting HRG systems on POCO service entrances is not something I usually recommend.
    The HRG was put on the Secondary to lower the arc-flash hazard (we don't use the 277v). After the experience I mentioned, I'm a bit gun-shy when I see HO and XO bonded for any reason. It may be fine as long as the entire grounding system integrity is maintained - but as soon as a ditcher cuts it, all bets are off. I guess my question comes down to: is it safe to not bond HO and XO in an industrial setting? I really appreciate the help. Thanks

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