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Thread: Connecting 3-phase inverters to corner-grounded delta transformer

  1. #1
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    Connecting 3-phase inverters to corner-grounded delta transformer

    We are designing a storage system with a 3-phase inverter, to be connected to the grid via a transformer. I just got the schematics from the utility company, and they said that one of the phases of the transformer is grounded on the primary side (I guess it's a case of a corner-grounded delta transformation). My understanding is that normal inverters would not work in such a setup, am I right? Could someone point me to some reference where I can read about issues that might occur? Tried to google but couldnt find anything of use.

    Pic of schematic:

    https://imgur.com/PGBjZrA

    Thank you all for your time

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  2. #2
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    It's the configuration on the inverter side that counts. If they are corner grounding their distribution system on the HV side that will not affect the inverter.

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    Thank you, so if I understand your reply properly, what needs to be done is to also corner ground the transformer close to the inverter at the HV side (that transformer comes from us, so we get to pick its configuration). It would only be an issue if there would be no transformer in the middle

    Thank you again

  4. #4
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    Quote Originally Posted by lyapunov00 View Post
    Thank you, so if I understand your reply properly, what needs to be done is to also corner ground the transformer close to the inverter at the HV side (that transformer comes from us, so we get to pick its configuration). It would only be an issue if there would be no transformer in the middle

    Thank you again
    No. The utility side of the transformer doesnt get grounded again. The transformer doesnt care that it is fed with one phase that is grounded. The inverter side would be grounded because it is a separately derived system and the grounding configuration would have to match what the inverters want, which for 99% of installs these days would be grounded Wye.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  5. #5
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    It's impressive how much production value there is in that utility schematic.

  6. #6
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    Sorry about that, the real schematic might be confidential so I drew something quick.

    There's just one thing that I didn't quite get, I understand how the transformer should be set on the inverter side (grounded Wye or whatever the inverter manufacturer requires), but not sure what to do on the utility side.

    Thank you for your help, as you can see I'm a newbie on this

  7. #7
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    Quote Originally Posted by lyapunov00 View Post
    Sorry about that, the real schematic might be confidential so I drew something quick.

    There's just one thing that I didn't quite get, I understand how the transformer should be set on the inverter side (grounded Wye or whatever the inverter manufacturer requires), but not sure what to do on the utility side.

    Thank you for your help, as you can see I'm a newbie on this
    On the utility side, the transformer can either be
    1. a delta winding, with the corner ground not causing any difference to actual operation unless the higher voltage from tap to ground is a specification issue, or
    2. A wye winding, with the wye point left unconnected, since there is nothing to connect it to. Again, the windings need to be rated for the maximum voltage to ground that they will see.
    Just be careful to get the right transformer winding turns ratio for the configuration (wye or delta) that is used.

  8. #8
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    Quote Originally Posted by lyapunov00 View Post
    Sorry about that, the real schematic might be confidential so I drew something quick.

    There's just one thing that I didn't quite get, I understand how the transformer should be set on the inverter side (grounded Wye or whatever the inverter manufacturer requires), but not sure what to do on the utility side.

    Thank you for your help, as you can see I'm a newbie on this
    Basically, in the low voltage world, the utility side of a transformer is fed just like it is any other device, with the the exception that if there is a neutral terminal, it is not used. The primary and secondary are isolated, so the transformer erases the grounding configuration of the utility and you start over at the secondary.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  9. #9
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    Thank you all, I got it now. As you can imagine I am not responsible for this but wanted to understand what the customer would be telling me

  10. #10
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    Quote Originally Posted by lyapunov00 View Post
    Thank you, so if I understand your reply properly, what needs to be done is to also corner ground the transformer close to the inverter at the HV side (that transformer comes from us, so we get to pick its configuration). It would only be an issue if there would be no transformer in the middle

    Thank you again

    Your schematic might be leaving out some important points, such as where the service disconnect is. Your schematic shows what looks like a utility distribution transformer directly feeding your PV transformer with no intervening disconnect or protection. Assuming a usual service there would be a main service entrance switchboard where one leg of the 3 phases is grounded to give a corner grounded delta service. That feeds the delta side of your PV transformer and as with any load on the load side of a grounded service entrance, it does not get grounded again.

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