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What is Closed Delta?

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    #31
    Originally posted by pv_n00b View Post
    Think of it as a black box experiment. Suppose I have two 3 phase, 480/240V, 3W, 100kW transformers, one open and one closed delta. I connect 3 480V conductors to the primary and three 240V single phase loads on the secondary, one on each phase. Is there any voltage or current measurement that I can take that will allow me to tell which is which? The answer is no, they both act exactly the same outside the box. If I unbalance the loads and take measurements can I tell which is which? Nope, outside the box it looks the same.

    Internally they are very different, different number of windings, different winding rating. If you want to look at the internal current flow you apply vector math and it all works out. It's actually a fun exercise in excel.
    Actually, if you measure the internal resistance by measuring the line to line voltage drop under load you will see a definite difference between the "open" leg of the triangle and the two closed legs.
    If you apply too large a nominally balanced motor load the difference in voltage drop can cause excess current in two of the three phases.

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      #32
      Originally posted by GoldDigger View Post
      Actually, if you measure the internal resistance by measuring the line to line voltage drop under load you will see a definite difference between the "open" leg of the triangle and the two closed legs.
      If you apply too large a nominally balanced motor load the difference in voltage drop can cause excess current in two of the three phases.
      You are correct in your two points but I was looking at how the transformer looks to a load under normal working conditions. That's why I said by making voltage and current measurements only, not measurements that show the internal design of the transformer. Obviously if you start probing the internal makeup of the transformer you will see differences. Overloading the transformer to get measurements outside of normal working conditions also would not apply.

      I was pointing out that under normal usage measuring what the load sees you can't tell the difference between a closed and open delta and this is one of the things that makes them useful.

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        #33
        Originally posted by pv_n00b View Post
        You are correct in your two points but I was looking at how the transformer looks to a load under normal working conditions. That's why I said by making voltage and current measurements only, not measurements that show the internal design of the transformer. Obviously if you start probing the internal makeup of the transformer you will see differences. Overloading the transformer to get measurements outside of normal working conditions also would not apply.

        I was pointing out that under normal usage measuring what the load sees you can't tell the difference between a closed and open delta and this is one of the things that makes them useful.
        But it is exactly under normal usage that the difference in internal impedance (reflected in differing terminal voltages) causes problems with voltage regulation. This is, among other things, the reason that single phase loads across the open side of the delta are discouraged, and even fully balanced three phase loads should not be too large relative to the transformers' nominal capacity.

        I do not consider looking at the voltage drop at the black box terminals under load "looking at the internal design of the transformer". I consider it looking at the transformer characteristics, whatever their source, that affects the performance of the loads.
        A three transformer closed delta with one of the pots having a higher %Z relative to the others or a smaller size can have the same performance problems as an open delta.

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          #34
          Originally posted by ActionDave View Post
          Perhaps, but within the context of here in the States, you are only going to see delta power derived from pole mounted transformers. There may be some exceptions, but they are rare.
          That must be a regional thing....wye connected pole transformers are common around here.
          Don, Illinois
          (All code citations are 2017 unless otherwise noted)

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            #35
            Originally posted by GoldDigger View Post
            But it is exactly under normal usage that the difference in internal impedance (reflected in differing terminal voltages) causes problems with voltage regulation. This is, among other things, the reason that single phase loads across the open side of the delta are discouraged, and even fully balanced three phase loads should not be too large relative to the transformers' nominal capacity.

            ....
            It seems to me that open deltas therefore ought to be high-leg so that I (the electrician) know which leg is only supposed to be used for three phase loads. So to speak to the OP's question, open deltas are (hopefully) a subset of high-leg deltas. (Closed deltas are another story).

            I'd also note that, without taking any electrical measurements, one way to tell if a service is open delta (albeit maybe not 100% reliably) is to look outside and count the cans hanging from the pole.

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              #36
              Originally posted by don_resqcapt19 View Post
              That must be a regional thing....wye connected pole transformers are common around here.
              Wye connected pole transformers are common here too, pad mounted deltas are not.
              If Billy Idol is on your playlist go reevaluate your life.

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                #37
                Originally posted by jaggedben View Post
                It seems to me that open deltas therefore ought to be high-leg so that I (the electrician) know which leg is only supposed to be used for three phase loads. So to speak to the OP's question, open deltas are (hopefully) a subset of high-leg deltas. (Closed deltas are another story).

                I'd also note that, without taking any electrical measurements, one way to tell if a service is open delta (albeit maybe not 100% reliably) is to look outside and count the cans hanging from the pole.
                Open or closed delta doesn't matter, if the midpoint of one phase is grounded, you will have a high leg on open or closed delta. If midpoint is not grounded - then you need to either corner ground the system or set it up as an ungrounded system and provide a ground fault monitoring system, again can be open or closed delta.
                I live for today, I'm just a day behind.

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                  #38
                  Originally posted by jaggedben View Post
                  It seems to me that open deltas therefore ought to be high-leg so that I (the electrician) know which leg is only supposed to be used for three phase loads....
                  If it is an existing installation it's the leg with all the breaker blanks.
                  If Billy Idol is on your playlist go reevaluate your life.

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                    #39
                    Originally posted by GoldDigger View Post
                    But it is exactly under normal usage that the difference in internal impedance (reflected in differing terminal voltages) causes problems with voltage regulation. This is, among other things, the reason that single phase loads across the open side of the delta are discouraged, and even fully balanced three phase loads should not be too large relative to the transformers' nominal capacity.

                    I do not consider looking at the voltage drop at the black box terminals under load "looking at the internal design of the transformer". I consider it looking at the transformer characteristics, whatever their source, that affects the performance of the loads.
                    A three transformer closed delta with one of the pots having a higher %Z relative to the others or a smaller size can have the same performance problems as an open delta.
                    I've never read anything saying that connecting a load across the open phase is discouraged. I'd like to see a good source for that.

                    There will be a small variation in the terminal voltage under load because the 2 transformers are operating under different power factors. But that's not due to a variation in the internal impedance, even though it might look like it. It's due to the transformers having different apparent currents with corresponding different voltage drops. But it's not a large difference, the difference is only 30° and the internal impedance of a good transformer is pretty low. In a lab with a good test setup, no noise, balanced load and sensitive measurement equipment you can tell it's an open delta. Someone with a DMM making measurements in the service entrance panel in a commercial building with unbalanced loading trying to tell if it is an open delta or closed delta? I would say they are not going to be able to tease out that difference in voltage regulation. It's going to look pretty much the same as a closed delta in the field.

                    This is a good analysis of the math behind open deltas:

                    http://www.aast.edu/pheed/staffadmin...e=staffcourses

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                      #40
                      Originally posted by pv_n00b View Post
                      I've never read anything saying that connecting a load across the open phase is discouraged. I'd like to see a good source for that.
                      Even a three phase load will be connected across the open phase.

                      One must consider the fact that the source doesn't have the same capacity as it would if it were a full delta with all three coils the same capacity.

                      Such applications of this kind of system are usually where there is limited three phase load, a rather fixed load, or in remote locations where it is deemed more practical then bringing the third phase to the location, over sizing the open leg transformer or even both transformers from what they would be if a closed delta may be necessary if there is going to be significant load on it, but other then the remote location application one is probably better off just building a closed delta in the significant load applications.
                      I live for today, I'm just a day behind.

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                        #41
                        So the question for a PV engineer is how do you know how much backfeed the open phase could take? I'm guessing there's probably no shortcut to sending an interconnection application to the utility and waiting for their engineering department.

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