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    Wye-Delta Grounding Transformer

    Hello, good day! I just want to ask about the ground of the wye-delta grounding transformer. Do we have to connect it to the ground bus of the main switchboard and/or ground it directly through ground rods? Thank you in advance for your help.

    #2
    First you need to differentiate between system grounding and equipment grounding. Are you grounding the secondary system?
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

    Comment


      #3
      IMHO,
      It should be connected to ground in 3 different places.
      The grounding conductor of the Primary circuit, building steel, and the supply side bonding jumper.
      See article 250.

      Comment


        #4
        wye-delta is a rare duck in the low voltage world, except for when someone is using a delta-wye in reverse because its laying around and just too temping to not use. May we ask what is the application? Most important thing is to keep the wye point floating - dont connect anything to it.
        Ethan Brush - East West Electric. NY, WA. MA

        "You can't generalize"

        Comment


          #5
          Also you might see a transformer with a wye primary and a 'floating' delta uses as a 'grounding transformer' to derive a neutral in an otherwise ungrounded system. So yes please to the original poster please describe in more detail what you are doing.

          -Jon

          Comment


            #6
            Originally posted by winnie View Post
            Also you might see a transformer with a wye primary and a 'floating' delta uses as a 'grounding transformer' to derive a neutral in an otherwise ungrounded system. So yes please to the original poster please describe in more detail what you are doing.

            -Jon

            Yeah after reading the OP a few times, the wording does correspond with that use.
            Ethan Brush - East West Electric. NY, WA. MA

            "You can't generalize"

            Comment


              #7
              Originally posted by jojo View Post
              Hello, good day! I just want to ask about the ground of the wye-delta grounding transformer. Do we have to connect it to the ground bus of the main switchboard and/or ground it directly through ground rods? Thank you in advance for your help.
              Please clarify what you have for a transformer and what it is being used to do.

              Is it a transformer with Wye primary and delta secondary with primary purpose to change voltage, or is is some sort of autotransformer used to create a neutral conductor in a system that has no neutral? makes a big difference in how to answer the question.
              I live for today, I'm just a day behind.

              Comment


                #8
                Hi all, thank you for the quick replies. The scenario is this: the source to an industrial facility is 2.4kV-460V delta secondary (2000kVA pad-mounted transformer). This goes to the main switchboard in the electrical building. And the main switchboard supplies the (7) seven buildings in the facility. At first I suggested 400Y/230V so there's no problem for the grounding issues. But there's no choice since the motors to be used for the facility are ANSI rated, meaning 460V 3-phase motors. So for the three-phase, it will have 460V, 3-wire plus eqpt ground. For single phase, there will be a 460V-230V step down transformer (we use 230V single phase, not 120V) plus eqpt ground. My concern here is during a ground fault especially for the motors. Since the system is delta, the breakers won't trip. And there might be other issues since the system is ungrounded like prone to transient overvoltages as stated in IEEE Green Book and from other elec references (Beeman, etc). That's why there's a need for a grounding transformer, also that the zero sequence current will have the return path to source. The grounding transformer will be wye-delta, and rated 200kVA or 225kVA 460V three phase (at least 10% from the 2000kVA source). My concern also is about the ground from the grounding transformer, if it should be connected to the main switchboard ground bus and/or connect it directly to the ground through the ground rods.
                Last edited by jojo; 02-05-19, 05:29 PM.

                Comment


                  #9
                  Perhaps others will chime in who have more knowledge on the subject, but have you considered a zig-zag transformer? I wonder if that is a better option.
                  Ethan Brush - East West Electric. NY, WA. MA

                  "You can't generalize"

                  Comment


                    #10
                    Hi Ethan, thank you. I think zig-zag transformer is much costly than the standard wye-delta transformer or even wye)delta banked transformer? Not sure if there's readily available zig-zag transformer here.

                    Comment


                      #11
                      Originally posted by jojo View Post
                      Since the system is delta, the breakers won't trip. And there might be other issues since the system is ungrounded like prone to transient overvoltages as stated in IEEE Green Book and from other elec references (Beeman, etc).
                      You can corner-ground the delta, or use a ground-detection set-up.
                      Master Electrician
                      Electrical Contractor
                      Richmond, VA

                      Comment


                        #12
                        Ungrounded delta systems are commonly used where continuity of service is desired. The specific purpose is to prevent breakers from tripping in the event of a single ground fault.

                        If you ground your delta system, you are specifically giving up this continuity of service feature. However you are correct about the risk of transient over-voltage in ungrounded systems.

                        My understanding is that if you use a 'grounding transformer', you must then ground the derived neutral just as you would ground the neutral of a normal wye secondary.

                        Your existing system is supposed to have ground fault detection, to inform you of a fault so you can fix it before a second fault occurs.

                        Given the size and voltage of your system, if grounded I believe that ground fault protection is also required to prevent arcing faults from causing serious damage without tripping breakers. Basically in large 480/277V systems you can get arc faults which are sustained and dissipate lots of energy but low enough current to not trip the breakers.

                        You might consider a resistance grounded system. In the event of a ground fault, only low current flows, so breakers don't trip. However enough current flows that ground fault detection can quickly identify the location of the fault, and for some loads you can use ground fault relays to shut them down in the event of a ground fault. Resistance grounded systems avoid the transient overvoltage problem of totally ungrounded systems.

                        Both zig-zag transformers and wye:delta grounding transformers derive a good low impedance neutral. A simple wye transformer circuit does not have a low impedance neutral, meaning that the line to neutral voltage is not stable and only very limited line to neutral current will flow. Something external is needed to stabilize the line-neutral voltage and allow sufficient ground fault current to flow to trip a breaker. In a wye:delta grounding transformer that 'something external' is current circulating on the delta side.

                        I don't have the background to properly size any of these options.

                        -Jon

                        Comment


                          #13
                          Originally posted by winnie View Post
                          In a wye:delta grounding transformer that 'something external' is current circulating on the delta side.


                          -Jon
                          Jon,

                          Is a "Wye-delta grounding transformer" a specific thing for the purpose or can one just use any "regular" (appropriately sized) wye-delta transformer?

                          I hear a zig zag has a lower impedance neutral, but not sure if/when that matters
                          Ethan Brush - East West Electric. NY, WA. MA

                          "You can't generalize"

                          Comment


                            #14
                            Originally posted by electrofelon View Post
                            Jon,

                            Is a "Wye-delta grounding transformer" a specific thing for the purpose or can one just use any "regular" (appropriately sized) wye-delta transformer?

                            I hear a zig zag has a lower impedance neutral, but not sure if/when that matters
                            The issue with neutral impedance is what happens when current tries to flow from line to neutral.

                            If the neutral impedance is high, then when line to neutral current flows, then the voltage across that phase coil drops quickly, limiting the current flow. If the goal is lots of short circuit current flow to trip a breaker, then a high impedance neutral won't provide this.

                            I don't know if there are any real differences between a wye delta grounding transformer and regular transformer. Perhaps there is a difference of which coils are wound as the primary.

                            -Jon

                            Comment


                              #15
                              Jon, yes the purpose is to trip the breaker during line-to-ground fault. It would be better to give up that feature of delta ungrounded system because the in the event that the 2nd fault occurs, the voltage during fault might be 6 to 8 times its normal voltage. Putting too much stress on the cable insulation (usually 600V for low voltage systems) might lead to another fault which is a disaster to the system.
                              If we should treat the grounding transformer like the normal wye, then i supposed that from the main switchboard ground bus, we will connect a wire to the center of the wye side of the grounding transformer. Please advise if my understanding is correct.
                              Normally for 600V below, we are using solidly grounding system.
                              Thank you for your comments/advises, I really appreciate it.

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