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Thread: zig zag transformer

  1. #21
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    Quote Originally Posted by wbdvt View Post
    Why? A 25MVA, 13.2kV Transformer would be about 1100A full load. The system is being supplied by National Grid and this may be a large manufacturing facility supplied off of a 115kV transmission line.
    This is,a small rural county public safety building. The supply is 13. 2 kv. I am,suprised they take,service at MV
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  2. #22
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    ok, I am surprised also but can't see the rest of the dwg that shows more of the NG side of things. That may shed some light but all this is off topic from OP's original question. More of curiosity now.

  3. #23
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    The zig-zag transformer is used to reduce current flowing in the neutral due to harmonics.

  4. #24
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    Quote Originally Posted by david luchini View Post
    The zig-zag transformer is used to reduce current flowing in the neutral due to harmonics.
    David,
    Are harmonics an issue with grid interactive inverters? I have never had a zig zag spec'ed on a PV system before.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  5. #25
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    Effective Grounding

    National Grid generally requires any PV plant >500kW, and sometimes smaller ones, to have what they call "effective grounding". This can take the form of either a grounding transformer/Zig-Zag transformer or a Neutral Ground Reactor. I believe both would fall under the general category of impedance grounding. I generally see Zig-Zag transformers used with Wye-G:Wye-G and NGRs used with Wye-Gelta, although I'm not sure of the exact reasons behind that.

    The best way I understand it is the utility is concerned with the impact of the PV system during a nearby ground fault on the transmission system. With a solidly grounded Wye-Wye system the fault current has a return path through the PV system secondary, which could desensitize the utility ground fault detection and cause high fault currents to flow. It makes sense to me that adding impedance between the PV system neutral and ground could reduce these fault currents.

    I believe there are also concerns with temporary overvoltage under some of these fault scenarios, which NGRs/Zig-Zags can mitigate. I'm less clear on how this works.

    I admit I don't truly understand some of the fundamentals here, this is just what I've picked up in dealing with NGRID. It also often appears that the utilities themselves don't understand what they mean by "effective grounding". We just suck it up and buy a zig-zag to make them happy.

    Solectria has published some good documentation on this issue that may be helpful.

  6. #26
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    Quote Originally Posted by pcanning87 View Post
    National Grid generally requires any PV plant >500kW, and sometimes smaller ones, to have what they call "effective grounding". This can take the form of either a grounding transformer/Zig-Zag transformer or a Neutral Ground Reactor. I believe both would fall under the general category of impedance grounding. I generally see Zig-Zag transformers used with Wye-G:Wye-G and NGRs used with Wye-Gelta, although I'm not sure of the exact reasons behind that.

    The best way I understand it is the utility is concerned with the impact of the PV system during a nearby ground fault on the transmission system. With a solidly grounded Wye-Wye system the fault current has a return path through the PV system secondary, which could desensitize the utility ground fault detection and cause high fault currents to flow. It makes sense to me that adding impedance between the PV system neutral and ground could reduce these fault currents.

    I believe there are also concerns with temporary overvoltage under some of these fault scenarios, which NGRs/Zig-Zags can mitigate. I'm less clear on how this works.

    I admit I don't truly understand some of the fundamentals here, this is just what I've picked up in dealing with NGRID. It also often appears that the utilities themselves don't understand what they mean by "effective grounding". We just suck it up and buy a zig-zag to make them happy.

    Solectria has published some good documentation on this issue that may be helpful.
    Thank you. That sheds some light on where the requirement came from. I was more leaning toward that it was the designer's decision not a utility requirement because I have worked on a dozen other large systems in NGRID territory, but they were all just under 500KW, so these requirements likely didnt kick in.

    So there is a chance this is a silly requirement accomplishing little/nothing, however I am also willing to accept that perhaps this is just above my knowledge level FWIW, I have worked on a number of ther 1-2 meg systems in other utilities' territory, and the interconnect was solely throught a Wye:Wye transformer.

    Thanks for the solectria reference, I will look into it.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  7. #27
    Quote Originally Posted by zbang View Post
    1200A at 13.2KV ??? That can't be right.

    (Is this when an RFI becomes a WTF?)
    1200A is the smallest practical size switchgear breaker available. Not much market for a lower ampacity breaker. Most of the time with MV switchgear you're going to have a programmable relay anyway. Just set it as needed and take advantage of mass produced 1200A breakers. Actually, I'm not aware of a switchgear manufacturer that offers a MV breaker less than 1200A.

    Sent from my SM-N950U using Tapatalk

  8. #28
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    Quote Originally Posted by pcanning87 View Post
    National Grid generally requires any PV plant >500kW, and sometimes smaller ones, to have what they call "effective grounding". This can take the form of either a grounding transformer/Zig-Zag transformer or a Neutral Ground Reactor. I believe both would fall under the general category of impedance grounding. I generally see Zig-Zag transformers used with Wye-G:Wye-G and NGRs used with Wye-Gelta, although I'm not sure of the exact reasons behind that.

    The best way I understand it is the utility is concerned with the impact of the PV system during a nearby ground fault on the transmission system. With a solidly grounded Wye-Wye system the fault current has a return path through the PV system secondary, which could desensitize the utility ground fault detection and cause high fault currents to flow. It makes sense to me that adding impedance between the PV system neutral and ground could reduce these fault currents.

    I believe there are also concerns with temporary overvoltage under some of these fault scenarios, which NGRs/Zig-Zags can mitigate. I'm less clear on how this works.

    I admit I don't truly understand some of the fundamentals here, this is just what I've picked up in dealing with NGRID. It also often appears that the utilities themselves don't understand what they mean by "effective grounding". We just suck it up and buy a zig-zag to make them happy.

    Solectria has published some good documentation on this issue that may be helpful.
    Exactly correct here. For even more interesting information on the subject you can read IEEE 142 the Green Book on grounding and IEEE 1547 interconnection requirements for PV systems. Effective grounding is a method used for decades on rotating generators, it's questionable that it does anything for static inverters used in PV systems but utilities are used to requiring them on generators so they thought, "why not just keep going with that."

    Effective grounding usually calls for the neutral to be grounded through an impedance and not solidly, so that's a little unusual here. I would not be surprised if this effective grounding system was not improperly designed for the application.

    Effective grounding is supposed to limit ground fault overvoltage and load rejection overvoltage after the local substation isolates a distribution line with PV attached and the PV is slow to disconnect. The alternative is direct transfer trip where the local substation directly commands the PV systems to disconnect at the same time it isolates the line. That is usually much more expensive so people were happy to have the effective grounding option.

    The remote trip on the zig-zag breaker is to make sure the PV system is disconnected from the distribution system if it has a fault or if it's intentionally disconnected, but usually that only trips the PV system interconnection and not the whole site.
    Last edited by pv_n00b; 09-30-17 at 03:18 PM.

  9. #29
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    Quote Originally Posted by pv_n00b View Post
    Exactly correct here. For even more interesting information on the subject you can read IEEE 142 the Green Book on grounding and IEEE 1547 interconnection requirements for PV systems. Effective grounding is a method used for decades on rotating generators, it's questionable that it does anything for static inverters used in PV systems but utilities are used to requiring them on generators so they thought, "why not just keep going with that."

    Effective grounding usually calls for the neutral to be grounded through an impedance and not solidly, so that's a little unusual here. I would not be surprised if this effective grounding system was not improperly designed for the application.

    Effective grounding is supposed to limit ground fault overvoltage and load rejection overvoltage after the local substation isolates a distribution line with PV attached and the PV is slow to disconnect. The alternative is direct transfer trip where the local substation directly commands the PV systems to disconnect at the same time it isolates the line. That is usually much more expensive so people were happy to have the effective grounding option.

    The remote trip on the zig-zag breaker is to make sure the PV system is disconnected from the distribution system if it has a fault or if it's intentionally disconnected, but usually that only trips the PV system interconnection and not the whole site.
    Ok this is starting to make a littler more sense. It sounds like the transformer was supposed to be delta on the PV side and the system grounding comes from the zig zag. The designer misunderstood the intent of the utility requirement. Do others agree with this hypothesis?
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  10. #30
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    Quote Originally Posted by electrofelon View Post
    The inverters are SMA sunny tripower, 30KW and do have a neutral. But I do agree that the motivation is likely to do with some (irrational) fear that the inverters will lose their reference to ground and float. Not only do I see that is very imnprobably, but even if it did I dont see that as that big a deal really.
    If the inverter is star connected with a neutral, the phase voltages could become unbalanced, should connection to the ground of the neutral is lost. To avoid it happening, the zigzag transformer is provided as an additional neutral grounding measure.

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