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    Transformer Issue

    I went to a job today to check a ground current issue on a new UPS, they were measuring 15.6 amps on a ground conductor. The UPS is 480/480 3-wire with a factory supplied 480 delta to 208/120 wye 150 kva shielded isolation transformer. So I am told I could not verify the transformer type.

    There is a ground jumper from the frame of the transformer the bottom of a ground bar, from the ground bar there is a bond jumper to a Ground bar in the UPS and then from the ground bar in the UPS there is a bond jumper back to the top of the ground bar in the transformer cabinet. Current seems to be circulating on the from the transformer frame to the UPS ground bar and back to the transformer.

    No load on the secondary, Primary current 15/11/8. With 70% load on the transformer and the ground current went to 22 amps. Utilized a resistive load.

    Results were the same with the UPS on and with the UPS in bypass and the UPS off.

    Removed the top bond jumper in the transformer to the UPS ground bar and current was still on the bottom jumper in the transformer.

    The ground bars are mounted to the metallic frame of the transformer and UPS, so the effect is the same with or without the top ground jumper.

    No measurable current on the grounding electrode conductor, Neutral-Ground Bond or any EGCs
    Any thoughts?
    Brian John
    Leesburg, VA

    #2
    Is the UPS connected to supply or checked during isolation? What are voltage readings, phase-phase and phase-ground? What currents do you get with the transformer de-energized?
    BB+/BB=?

    Comment


      #3
      I noticed you said no EGC currents so that may eliminate common mode signals.

      Check the voltages to see if you might have a partial fault in the transformer. You might meg it also.
      BB+/BB=?

      Comment


        #4
        This was brought up on another forum.

        I think the transformer may be an autotransformer


        Not sure the practical reason for an autotransformer for a voltage change of 480 to 208/120? Seems the copper and steel required would be same as a Delta Wye or close.

        Have to think about the XO would that be ground on the primary
        Brian John
        Leesburg, VA

        Comment


          #5
          Originally posted by brian john View Post
          I went to a job today to check a ground current issue on a new UPS, they were measuring 15.6 amps on a ground conductor. The UPS is 480/480 3-wire with a factory supplied 480 delta to 208/120 wye 150 kva shielded isolation transformer. So I am told I could not verify the transformer type.
          What is the 480/208 transformer for?
          Si hoc legere scis nimium eruditionis habes.

          Comment


            #6
            The UPS is 480/480 the transformer feeds the critical loads at 208/120 VAC
            Brian John
            Leesburg, VA

            Comment


              #7
              what are voltage readings. a core may be shorted to a winding

              Comment


                #8
                Primary A-B 480, B-C 480, C-A 480
                Secondary A-B 208, B-C 208, C-A 208, A, B, C to N 120
                Brian John
                Leesburg, VA

                Comment


                  #9
                  How is the static bypass fed? Is it 480V and fed to its SS on the primary side of the XFMR or 208V onto the secondary?

                  Since you said it was a factory transformer, that why the bypass voltage is important, because as you know there is a N-G bond up at that source that is providing the needed grounding.
                  Ron

                  Comment


                    #10
                    Originally posted by ron View Post
                    How is the static bypass fed? Is it 480V and fed to its SS on the primary side of the XFMR or 208V onto the secondary?

                    Since you said it was a factory transformer, that why the bypass voltage is important, because as you know there is a N-G bond up at that source that is providing the needed grounding.

                    The Bypass is on the primary and is 3-wire as well. Isolated the transformer from the UPS, with no connection to the UPS Bypass the issue with the transformer secondary
                    still existed
                    Brian John
                    Leesburg, VA

                    Comment


                      #11
                      We had a similar situation with the UPS powered down. Almost 40 amps in our case.
                      We found main electrical system neutral connected to ground in 7 different locations.

                      Once they were removed this current disappeared.

                      Good Luck
                      May the force be with you

                      Comment


                        #12
                        Grounds were checked throughout the distribution system and minimal ground current 2-3 amps. The levels exterior to the transformer remain the same with the transformer on or off.
                        Brian John
                        Leesburg, VA

                        Comment


                          #13
                          Update to this ground current issue than a question


                          The transformer was meggered and TTR'ed (Turns Ratio Tested) no problems there.




                          The transformer is a 480 to 208/120 transformer Delta Wye.


                          The distribution system is solidly grounded.


                          The excitation current is A-12, B-8, C-7 Amps, Generally, I believe we see A and C phase higher than B phase.


                          At no load, the current on the frame ground is 15.9 amps. This current is only measured on the frame ground conductor installed by the factory


                          No one including the manufacturer can give me an answer.


                          There is a sticker I noticed today that says due to high leakage current this transformer must be grounded.


                          Now my question


                          The manufacturer says to ground the UPS one way if the distribution system is high impedance grounded and ground another way if low impedance grounded. THE UPS IS NOT THE SOURCE OF THE GROUND CURRENT, this has been ruled out by isolating the transformer from the UPS. But I still have a question.


                          I believe that a low impedance ground is the same as solidly grounded the engineer says no it is similar to a high impedance grounded system just a different value impedance, the manufacturer says that is a design issue and not their concern.


                          Thoughts?
                          Brian John
                          Leesburg, VA

                          Comment


                            #14
                            Originally posted by brian john View Post
                            The manufacturer says to ground the UPS one way if the distribution system is high impedance grounded and ground another way if low impedance grounded. THE UPS IS NOT THE SOURCE OF THE GROUND CURRENT, this has been ruled out by isolating the transformer from the UPS. But I still have a question.


                            I believe that a low impedance ground is the same as solidly grounded the engineer says no it is similar to a high impedance grounded system just a different value impedance, the manufacturer says that is a design issue and not their concern.


                            Thoughts?
                            For generator protection, a low impedance ground is similar to a high impedance ground. Low impedance uses a series resistor in the ground path. A high impedance ground uses a grounding transformer with a resistor on the secondary side that reflects impedance to the primary/ground path side (this is what I am used to). Both will clamp ground faults to levels much lower than a solid ground so are similar in that respect. Think of solid as hundreds to thousands of amps, low as tens to hundreds, and high as tens of amps or less.

                            It sure sounds like a shield drain issue since the other data does not indicate a fault.
                            Not sure what is up with the transformer.
                            Do you have a transformer manufacturer contact? I usually don't use shielded units but could ask our rep and he might could hunt down an applications engineer at the plant.
                            BB+/BB=?

                            Comment


                              #15
                              Originally posted by mivey View Post

                              It sure sounds like a shield drain issue since the other data does not indicate a fault.
                              Not sure what is up with the transformer.
                              Do you have a transformer manufacturer contact? I usually don't use shielded units but could ask our rep and he might could hunt down an applications engineer at the plant.

                              Tamura Elcomponics Technologies Pvy. Ltd
                              Pandithanahalli, Karnataka 572104, India

                              The weird thing about the nameplate, It states Primary 480 Secondary 208, Transformer is a Delta Wye 480 to 208/120. Generally, I would expect the name plate to state 208/120. on a Delta Wye.


                              Brian John
                              Leesburg, VA

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