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    Transformer Neutral required for Inverter Interconnection?

    New to transformers with solar, can you help me out?

    System Specs:
    300 kW of PV with 9 String inverters operating at 480/277 Wye combined with an AC combiner. Combined inverter output power is run through a transformer to step down the combined inverter outputs from 480/277 Wye to a 240 Delta (high leg) and then it will make a supply side connection at the main building service (allowed by AHJ & Utility). The utility service is a 240 Delta High-leg. There are no loads in the inverter combiner, only utility interactive power production sources (solar inverters).

    Question: DO WE NEED TO INSTALL A NEUTRAL CONDUCTOR ON THE 240 DELTA SIDE OF THE TRANSFORMER? (BETWEEN THE TRANSFORMER AND THE UTILITY INTERCONNECTION)

    I have confirmed with the inverter manufacturer that the inverters only use the neutral for instrumentation & voltage referencing (not a current carrying conductor). Note: We will have a neutral installed on the 480/277 side (solar inverter side) of the transformer and this neutral is bonded to ground in the transformer. The inverter manufacturer has stated that this is sufficient for the instrumentation needs of the inverters and the inverters don't need to have a neutral on the 240 Delta side of the transformer.

    The electrical engineer we had review the design is telling us that we need a neutral on the 240 Delta side of the transformer to handle the 120 V (& 208 V High-leg) loads in the building.

    Our master electrician feels that because this is only a utility interactive power production source with a true 240 Delta output from the transformer we don't need the neutral conductor between the transformer and the supply-side connection to the utility. It is purely a power production source that will never run when the utility is down. Therefore we should let any 120 V loads (or high-leg loads) be handled by the utility. Why introduce neutral currents onto our transformer when we don't have to.

    I'm curious to see what you all think. Install the neutral between the transformer and the utility interconnection or leave it out?

    #2
    I'm not an EE, but I'm gonna tell you the answer is No, you do not need a neutral between the service and your transformer.

    My reasoning is as follows...
    1) Believe the inverter manufacturer. They know how their product works. Your EE is obviously less familiar with interactive systems.
    2) The inverters will supply phase-to-phase power to the utility transformer, which will in turn supply current to the neutral on the A-C phase winding, just as it does when powered from the utility side. This is how every single-phase 240V inverter works on a split-phase transformer.
    3) Where would you connect a neutral to a delta transformer?

    Hopefully GoldDigger or another EE will ratify what I'm saying here.

    Caveats:
    1) It would be a good idea to make sure the transformer manufacturer approves of your application just like the inverter manufacturer.
    2) It's tangential to your question, but I've heard that if the service is an open delta, the utility may impose lower limits on how much power you can interconnect.

    Comment


      #3
      Originally posted by SunFish View Post
      The electrical engineer we had review the design is telling us that we need a neutral on the 240 Delta side of the transformer to handle the 120 V (& 208 V High-leg) loads in the building.
      The engineer needs to go back to schooll!
      Ethan Brush - East West Electric. NY, WA. MA

      "You can't generalize"

      Comment


        #4
        If the service is 240/120V 3Ø 4W (delta high leg) it is required to bring the neutral circuit conductor to the service disconnecting means. After the service disconnecting means, a neutral circuit conductor is not required to be run if it is not required for normal operation of the circuit, such as the primary of your delta-wye transformer.
        I will have achieved my life's goal if I die with a smile on my face.

        Comment


          #5
          You have 2 transformers; the utility supply transformer and the solar interconnect transformer.

          The utility supply transformer 240V _secondary_ has a neutral which must be connected to supply your 120V loads.

          The solar interconnect transformer 240V _primary_ is a delta load, and should not have a neutral connection on the primary side.

          If the solar interconnect transformer has a delta primary and a wye secondary, it very likely does not have a neutral tap on the delta side. If it did have a neutral tap, and you connected it, then you would very likely see significant circulating currents between the two transformers.

          -Jon

          Comment


            #6
            Originally posted by SunFish View Post
            New to transformers with solar, can you help me out?

            System Specs:
            300 kW of PV with 9 String inverters operating at 480/277 Wye combined with an AC combiner. Combined inverter output power is run through a transformer to step down the combined inverter outputs from 480/277 Wye to a 240 Delta (high leg) and then it will make a supply side connection at the main building service (allowed by AHJ & Utility). The utility service is a 240 Delta High-leg. There are no loads in the inverter combiner, only utility interactive power production sources (solar inverters).

            Question: DO WE NEED TO INSTALL A NEUTRAL CONDUCTOR ON THE 240 DELTA SIDE OF THE TRANSFORMER? (BETWEEN THE TRANSFORMER AND THE UTILITY INTERCONNECTION)

            I have confirmed with the inverter manufacturer that the inverters only use the neutral for instrumentation & voltage referencing (not a current carrying conductor). Note: We will have a neutral installed on the 480/277 side (solar inverter side) of the transformer and this neutral is bonded to ground in the transformer. The inverter manufacturer has stated that this is sufficient for the instrumentation needs of the inverters and the inverters don't need to have a neutral on the 240 Delta side of the transformer.

            The electrical engineer we had review the design is telling us that we need a neutral on the 240 Delta side of the transformer to handle the 120 V (& 208 V High-leg) loads in the building.

            Our master electrician feels that because this is only a utility interactive power production source with a true 240 Delta output from the transformer we don't need the neutral conductor between the transformer and the supply-side connection to the utility. It is purely a power production source that will never run when the utility is down. Therefore we should let any 120 V loads (or high-leg loads) be handled by the utility. Why introduce neutral currents onto our transformer when we don't have to.

            I'm curious to see what you all think. Install the neutral between the transformer and the utility interconnection or leave it out?
            Leave it out if you want; I don't think it would carry any current, anyway. The inverters certainly don't need it since you have a separately derived system with a new grounded neutral on the 480/277V side.

            One thing that you should check out, though, if you haven't already: Many high leg services have a much smaller transformer driving the B phase and your system will feed all three phases equally. You need to be sure that you don't overload the transformer driving the high leg.

            Comment


              #7
              Originally posted by SunFish View Post
              I have confirmed with the inverter manufacturer

              The electrical engineer we had review the design

              Our master electrician feels that because

              Install the neutral between the transformer and the utility interconnection or leave it out?

              a supply side connection at the main building service (allowed by AHJ & Utility)
              You didn't exactly say you've talked to the people that ^^^ make the decision.
              You are connecting to the POCO's xfmr, they do the calcs to make sure everything is ok.
              They have info that the other folks don't, (DG saturation?) and if they allow it, they have guidelines.
              Of course, the guidelines sometimes say "please contact us"...

              Just a reference example, fig. 4 is on pg. 34 ( https://www9.nationalgridus.com/non_...str_esb756.pdf )

              NatGrid ESB 756:

              If the Customer is permitted to interconnect through an un-grounded source,
              a “zero-sequence” voltage scheme will be required on the primary
              side of the approved delta primary wound transformer supplying the DG
              system. In cases where the Company’s EPS is an ungrounded circuit, the
              scheme may be waived at the Company’s discretion on a case by case
              basis. See Figure4 for additional information

              Where the Company provides secondary service, the Company’s transformer is an
              equipment standard for service delivery voltages offered in M.D.P.U. 1192; see
              Section 3 in ESB 750. Non-standard transformers are not provided by the Company.
              The Company will determine when dedicated services and a dedicated transformer
              are required in order to reduce the impact on other adjacent customers. The need
              for a dedicated transformer(s) may be determined at any point in the generator’s life
              cycle.

              Any DG or aggregate DG below 500kW in a Customer’s facility may be permitted to
              utilize a primary delta - secondary wye grounded or primary wye grounded –
              secondary wye grounded transformer with an ungrounded source. The Company
              reserves the right to require an effectively grounded source for generation 250kW-
              500kW depending on DG saturation and other conditions on individual feeders.

              Comment


                #8
                Originally posted by PVfarmer View Post
                ...

                NatGrid ESB 756:

                If the Customer is permitted to interconnect through an un-grounded source,
                a “zero-sequence” voltage scheme will be required on the primary
                side of the approved delta primary wound transformer supplying the DG
                [FONT=sans-serif]system. ...
                A high leg delta is not an ungrounded source.

                Comment


                  #9
                  Originally posted by jaggedben View Post
                  A high leg delta is not an ungrounded source.
                  There ya go again: telling me something I know quite well, and ignoring my perfectly good advice: talk to the POCO.

                  Are you implying any of this could be done *without* talking to the POCO!
                  Hmm.

                  Comment


                    #10
                    Originally posted by PVfarmer View Post
                    There ya go again: telling me something I know quite well, and ignoring my perfectly good advice: talk to the POCO.
                    Then you brought up something that you knew was off topic, which I'd ask you not to do again.

                    Are you implying any of this could be done *without* talking to the POCO!
                    Hmm.
                    Talking to the POCO isn't always important, but I did give similar advice as you in the first response in this thread. It's still tangential to the OP's question though.

                    Comment


                      #11
                      Originally posted by jaggedben View Post
                      I'm not an EE, but I'm gonna tell you the answer is No, you do not need a neutral between the service and your transformer.

                      3) Where would you connect a neutral to a delta transformer?
                      Thanks Jaggedben, I think I'm definitely leaning towards not using a neutral.

                      Doesn't the neutral connect between the A & C phases on a Delta High-leg? I though the neutral was required in this application if you want to run 120 V loads off the A or C phase and 208 V off the High Leg?

                      Besides being new to transformer applications, the whole Delta High leg thing throws me for another loop being such an outdated type of service...

                      Comment


                        #12
                        Originally posted by ggunn View Post
                        One thing that you should check out, though, if you haven't already: Many high leg services have a much smaller transformer driving the B phase and your system will feed all three phases equally. You need to be sure that you don't overload the transformer driving the high leg.
                        Thanks Ggunn, we will definitely confirm with the utility on the size of the transformer on the B phase.

                        Comment


                          #13
                          Originally posted by SunFish View Post

                          Doesn't the neutral connect between the A & C phases on a Delta High-leg? I though the neutral was required in this application if you want to run 120 V loads off the A or C phase and 208 V off the High Leg?

                          Besides being new to transformer applications, the whole Delta High leg thing throws me for another loop being such an outdated type of service...
                          You are correct that you would need the neutral to get 120V loads (generally, one would not run a 208V load off the high leg), however, thinking of the transformer as a load, it doesnt need the neutral. The neutral on the PV side is derived in the windings on that side and has nothing to do with the neutral on the utility side.

                          Some people come across more high leg delta than others. I see them quite frequently, but have never installed one new.
                          Ethan Brush - East West Electric. NY, WA. MA

                          "You can't generalize"

                          Comment


                            #14
                            Confirmed with our Master on the utility supply, the utility transformers are all the same size (167 kVA) and It is a closed delta config.

                            Comment


                              #15
                              Regardless of whether the service provides 120V to loads as well as 240 line to line three phase, the inverter is not directly supplying those 120V loads.
                              It is perfectly reasonable (although in some cases POCO might not be happy) to have a 240V grid interactive inverter on a single phase 120/240 three wire service and yet put a totally unbalanced set of 120V loads on one side of the service only.
                              The inverter does not care and will source exactly equal current magnitude on L1 and L2, and yet the meter may see no net current on L1 and lots of net current on L2. The POCO transformer (and neutral) will just see a very unbalanced load and will make up the difference.

                              Comment

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