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    Ungrounded Wye

    In reviewing some field notes and pictures from a plant where we are doing some unrelated control system work I noted that the site's existing 1200A service (480V) appears to be ungrounded wye. The plant operator had noted to me in a visit that the electrical dist. was ungrounded delta, and I had not looked closely at the connections on the outdoor transformers when on-site. I have seen ungrounded delta systems with Ground detection systems but I don't think I've seen ungrounded wye - is this a common installation?

    Site is an industrial utility plant, some large motor loads, probably 50 yrs old. Fed from (3) 1 ph. transformers connected delta primary. They do have indicating lights installed on each incoming phase for monitoring loss of phase but don't have any alarming or any of the markings identified in 250.21(C). At least not from what I can see in pictures.

    #2
    Originally posted by nhee2 View Post
    In reviewing some field notes and pictures from a plant where we are doing some unrelated control system work I noted that the site's existing 1200A service (480V) appears to be ungrounded wye. The plant operator had noted to me in a visit that the electrical dist. was ungrounded delta, and I had not looked closely at the connections on the outdoor transformers when on-site. I have seen ungrounded delta systems with Ground detection systems but I don't think I've seen ungrounded wye - is this a common installation?

    Site is an industrial utility plant, some large motor loads, probably 50 yrs old. Fed from (3) 1 ph. transformers connected delta primary. They do have indicating lights installed on each incoming phase for monitoring loss of phase but don't have any alarming or any of the markings identified in 250.21(C). At least not from what I can see in pictures.
    Ungrouded wye and delta are the same thing. The only thing different is the transformer secondary is wye and left floating rather than delta; everything else is exactly like an ungrounded delta. Very common. This allows for one transformer stock to cover both 277/480 grounded wye customers as well as 480 volt customers seeking an ungrounded system. The bond strap is simply removed from the XO bushing, rather than having to order a delta secondary unit which works the same as a wye with the strap removed.


    Just to point out, where the secondary is left floating or is anything other than grounded wye, having a delta primary connection is a good idea. All else is the same.
    Our comedian shamelessly joked about a blackout. Talk about dark humor.

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      #3
      OK, thanks. Like I said, most installation I have seen are 4W grounded wye or and if ungrounded are 3W delta. Note this was (3) 1 ph. and not a 3ph transformer however I understand your explanation. I guess common to everyone else, new to me. Or maybe just not recognized by me previously.

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        #4
        Is there a neutral conductor with the service conductors? May have originally been delta but got changed to a wye transformer for whatever reason. Hopefully the left the neutral floating at the transformer or you do have a grounded system with no grounded conductor to the service - not a good idea, as ground faults will leave you with up to 277 volts to ground on EGC's and anything bonded to them but will not operate any overcurrent devices unless you are lucky enough to have a very low impedance ground path back to the source somehow
        I live for today, I'm just a day behind.

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          #5
          A delta-delta and delta-wye transformer will necessarily belong to different vector groups, if anything unusual about the wiring and loads makes that significant.
          And if POCO brings a neutral (but ungrounded) conductor to the service, the NEC makes it more restrictive to go ungrounded.
          Last edited by GoldDigger; 07-14-14, 05:17 PM.

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            #6
            There is no neutral supplied from the utility transformers. We are only upgrading some controls and not doing anything with the power distribution, so I did not look closely at the service configuration while on site. However I had taken a picture of the service transformers and in filing the photos, had taken a closer look.

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              #7
              Originally posted by GoldDigger View Post
              A delta-delta and delta-wye transformer will necessarily belong to different vector groups, if anything unusual about the wiring and loads makes that significant.
              And if POCO brings a neutral (but ungrounded) conductor to the service, the NEC makes it more restrictive to go ungrounded.

              You do have a 30 degree displacement on a standard delta wye but in a radial feed system with nothing paralleled it makes no difference.
              Our comedian shamelessly joked about a blackout. Talk about dark humor.

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                #8
                Originally posted by nhee2 View Post
                OK, thanks. Like I said, most installation I have seen are 4W grounded wye or and if ungrounded are 3W delta. Note this was (3) 1 ph. and not a 3ph transformer however I understand your explanation. I guess common to everyone else, new to me. Or maybe just not recognized by me previously.
                The concept still applies to single phase banks since stocking just 277 volt cans is cheaper than 277 and 240/480 volt cans. You may not have looked at the connections before. When ever a person walks into a facility fed by only 3 wires and is ungrounded the actual transformer secondary connection may be delta, open delta, wye or Tee. But to keep it simple people just call it "ungrounded delta"
                Last edited by mbrooke; 07-14-14, 06:20 PM.
                Our comedian shamelessly joked about a blackout. Talk about dark humor.

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                  #9
                  Originally posted by nhee2 View Post
                  There is no neutral supplied from the utility transformers. We are only upgrading some controls and not doing anything with the power distribution, so I did not look closely at the service configuration while on site. However I had taken a picture of the service transformers and in filing the photos, had taken a closer look.
                  If you can post the pics I can confirm whats going on. Others do bring up a good concern, if your wye is grounded (and it might be as simple as a 10 guage wire crimped to the neutral wire briding the pots together going over to a ground rod) you will need the neutral run to the building and connected to the grounding system. A ground fault will energize the grounding system creating both a shock and fire hazard.
                  Our comedian shamelessly joked about a blackout. Talk about dark humor.

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                    #10
                    Here's a pic of the secondary side of the transformer installation. Don't see any ground connection to the neutral.
                    Attached Files

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                      #11
                      Originally posted by nhee2 View Post
                      Here's a pic of the secondary side of the transformer installation. Don't see any ground connection to the neutral.
                      Is that POCO or customer's equipment in the photo's?

                      I don't see one either, wouldn't know if it is possibly bonded to the transformer cases or not, but still not seeing any GEC even to the case - maybe on the primary side - that one is a little harder to see just what is there.
                      I live for today, I'm just a day behind.

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                        #12
                        My understanding is that the POCO owns those transformers.

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                          #13
                          Originally posted by nhee2 View Post
                          My understanding is that the POCO owns those transformers.
                          Is it the sole supply to the facility?

                          Where is metering equipment located?

                          Is there any grounding done elsewhere?

                          If it is POCO you would expect it to be grounded at the transformer(s) if it is intended to be a grounded system, but without knowing some history you could have an unusual situation there. Though it may be a good idea to address the unusual situation and make changes to make it more clear just what the system is - could save someone's life someday.
                          I live for today, I'm just a day behind.

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                            #14
                            Looks like Delta ungrounded wye. Typical when poco wants to keep 1 stock of transformers or has no 240/480 volt units. BTW, the stress cones and the bare concentric termination on those cables isn't done right.
                            Last edited by mbrooke; 07-16-14, 10:29 PM.
                            Our comedian shamelessly joked about a blackout. Talk about dark humor.

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                              #15
                              Is it the sole supply to the facility?

                              Where is metering equipment located?

                              Is there any grounding done elsewhere?

                              If it is POCO you would expect it to be grounded at the transformer(s) if it is intended to be a grounded system, but without knowing some history you could have an unusual situation there. Though it may be a good idea to address the unusual situation and make changes to make it more clear just what the system is - could save someone's life someday.
                              This is the only utility service - there is also a standby generator on site. The service feeds an MCC with 1000A mains, and metering is located at the MCC. I did not explore the grounding while on site, as I was there for some control system work and we are not doing anything to the existing electrical dist.

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