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Thread: zig zag transformer

  1. #61
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    Quote Originally Posted by Sahib View Post
    David & electrofelon: Would you mind giving your comments on the paper in post#45. That may clarify the matter a bit.
    Sahib, that paper really isn't applicable. These inverters take a neutral and it is grounded. That's the way we do things here in the us. There are 2 options: 1. The zig zag is for harmonics. 2. The zig zag is there to satisfy a utility requirement (and possible misapplied).

    I guess a 3rd option is that the designed put the zig zag in for a redundant system groundING means in case the MBJ was lifted, neutral failed, etc.... IMO that would be ridiculous.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  2. #62
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    Sketch the 0 seq network diagram
    the Yg:Yg xfmr makes it interesting (seems like a bad option imo)
    phase shift?
    both sources util & pv will supply sec gnd fault i
    Looks like they are trying to give each source a rtn path?
    The difference between genius and stupidity is that genius has its limits.

  3. #63
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    Quote Originally Posted by Ingenieur View Post
    both sources util & pv will supply sec gnd fault
    Looks like they are trying to give each source a rtn path?
    The issue for the POCO is a solidly grounded PV system could make the protection coordination ineffective by its contribution to fault current. So the POCO (in US, of course)usually demands an effective grounding. In some cases, however, POCO may allow solid grounding (as in OP case with solidly grounded zigzag transformer) if the PV system trips fast in case of POCO distribution line fault.

  4. #64
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    Was the MV-LV transformer delta at any point in time?
    I'm in over my head...

  5. #65
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    Quote Originally Posted by zbang View Post
    1200A at 13.2KV ??? That can't be right.

    (Is this when an RFI becomes a WTF?)
    1,200 is a very common standard for 15-35kv breakers.
    I'm in over my head...

  6. #66
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    It is still a lot of watts (or maybe just VA).

    Sent from my XT1585 using Tapatalk

  7. #67
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    The drawing says the “purpose” is for grounding .


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  8. #68
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    Quote Originally Posted by GoldDigger View Post
    It is still a lot of watts (or maybe just VA).

    Sent from my XT1585 using Tapatalk


    Yup, though this post may be on to something:

    http://forums.mikeholt.com/showthrea...99#post1861899


    I was not aware of the customer generation- but that could be the reason considering it radically changes what typical fuses, reclosers and feeder breakers see during fault conditions (usually reduced fault current from the in-feed). The zig-zag might be there just for that reason, to increase short circuit currents as seen by protective devices, but if that was the case it would make much more sense on the MV side.

    Heres my issue with this: there is nothing that indicates selective coordination or what currents will pass through the zig-zag during fault conditions. Short circuits out on the utility feeder could trip the 1,200amp zig-zag, 3000 amp fused main, or devices on the 13.2 kv side. Unless every detail is known that zig-zag could be major headaches.


    This is one of those things that could be either a very brilliant solution to a unique problem, or very bad idea for a none issue depending on what the engineer(s) are trying to accomplish.

    Electrofellon: Please keep us updated.
    I'm in over my head...

  9. #69
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    looks live the pv sources are delta?
    the legend/note shows 4w + G, dwg shows 3 plus egc

    remove the zig-zag
    3000 fs opens
    a single phase from the pv side of the switch (not the xfmr/util side) goes to gnd
    no fault rtn path, frame is elevated
    if the zig-zig is in the ckt a single gnd fault will induce a ph-ph fault thru the zig-zag

    if the fs is closed (or fuses good) the ph gnd fault has 2 paths
    the zig-zag ph-ph
    the xfmr X0 thru a winding to a ph
    The difference between genius and stupidity is that genius has its limits.

  10. #70
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    Quote Originally Posted by electrofelon View Post
    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.
    Wye-Wye is the utilities choice 99% of the time. In fact some POCOs forbid delta on any distribution transformer. Wye-Wye completes the circuit, has no phase shift, far less susceptible to ferroresonance, and inductive tank heating is solved simply with a "give me a 5 limb instead of a 3 limb"

    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.
    I'm confused. Reactors and NGRs limit zero sequence current, while wye grounded-delta and zig-zag provide a zero sequence source. Yg-delta and zig-zag would not be effective (imedpance) grounding in my book unless the N terminal to ground of the zig-zag or the secondary delta (brocken delta, acorss the terminals) had a reactor or resistor across it.



    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.
    Transmission to distribution, or sub transmission to distribution transformers are almost always delta-wye (wye ungrounded-delta-wye grounded or delta zig-zag when 0* phase shift is needed). Faults on those systems would have no effect on the PV system or the ground step distance protection of the tranmission system. In fact one major reason behind the delta wye selction is so ground faults on the distrbution system do not pass to the transmission system.

    As for increased fault current on the 13.8kv system, a grounding transfomer would make it far worse, a grounding transfomer with an impedance would have no effect in reducing fault current.



    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.
    Can you elborate more on how this is typically done? And how the equipment is sized? The way I see it is that zig zag will either prvide a zero sequnce source or not- unless as you mentiuon hjas the ability to limit certain transients.


    Quote Originally Posted by electrofelon View Post
    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?


    That could be it- and thats what I would have done if I wanted a major zero sequence source out on the feeder. But the question lays as to why if that was the case.



    Quote Originally Posted by Ingenieur View Post
    Sketch the 0 seq network diagram
    the Yg:Yg xfmr makes it interesting (seems like a bad option imo)
    phase shift?
    both sources util & pv will supply sec gnd fault i
    Looks like they are trying to give each source a rtn path?
    Honestly, unless a see A LOT of detailed numbers, and some proof based reasoning, bad option would be correct.
    I'm in over my head...

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