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Harmonics on 3 phase 3 wire system

  1. Apr 20, 2015 #1
    Lets say I have a switchboard that requires ONLY 3 Phase loads, transformer, UPS, Motors, ETC. ( no neutral is required at all)
    Now lets say I am feeding this from a generator where I have 4 wires from the alternator ABCN connected to the aforementioned switchboard.
    the N of the generator alternator is Bonded to ground inside of the aforementioned switchboard but NOT at the generator location.

    The 3 phase loads that are feeding from the switchboard are non-linear and have a fairly "dirty" harmonic profile.

    The question is will i see any harmonic loads on the neutral conductor between the main service and the generator?

    can someone point me to a white paper or other documentation that clarifies this point either way?

    thank you

    -Deepro
     
  2. jcsd
  3. Apr 20, 2015 #2

    jim hardy

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    Bonding is irrelevant

    If your loads are wye connected with their neutral tied to neutral of generator, then all their third harmonic current will return through that neutral. You'll see in your neutral the sum of their 3rd harmonic currents. That's why for computer systems the neutral conductor is sized same as lines.
    If your loads are delta connected then 3rd harmonic current is constrained to the phase windings.
    Place a clamp-on current probe around neutral and observe current. If it's 180 (150?) hz, is its amplitude objectionable?

    We once corrected objectionable 3rd harmonic current by opening the neutral connection . The downside was the system would not continue to operate when a single line fuse opened.

    I wish i knew of a white paper - would've saved us a lot of midnight oil.
    In recent years the rise of variable speed drives has prompted a lot of articles on harmonics. Try a search on "third harmonic neutral "

    below is hardly a white paper but it describes our experience in mid 1970's .

    http://www.designnews.com/author.asp?section_id=1368&doc_id=273075 [Broken]
     
    Last edited by a moderator: May 7, 2017
  4. Apr 20, 2015 #3

    jim hardy

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  5. Apr 20, 2015 #4
    Thank you very much but you missed the point of the question. Perhaps this is my fault that I did not explain it well. My English is not perfect.

    Generator is Wye,

    5 wires extend from the generator to the main switchboard. A, B , C , N , G

    (here is the important part ) At the main switchboard there is NO loads connected to the neutral.

    Copied from the original question “I have a switchboard that requires ONLY 3 Phase loads, transformer, UPS, Motors, ETC. ( no neutral is required at all)"
     
  6. Apr 20, 2015 #5

    jim hardy

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    indeed i didn't get from your post that the neutral in the switchboard (if indeed there is one) is not connected.
    So your loads might be all delta,
    or wye with neutral left floating.
    If the latter there will be interesting voltage between switchboard neutral and the generator's neutral, known as "neutral instability",


    I hope you are able to connect some meters and oscilloscopes .
    Harmonic problems can be hideous to figure out. We had shaft currents eating bearings in rotating machines, and industry knowledge of the phenomenon was sparse until VFD's pushed the problem down into home appliance sized motors.

    Don't apologize for your English, it's quite good. I suppose a picture is worth a thousand words in any language.

    old jim
     
  7. Apr 20, 2015 #6
    Thanks Jim, there is not floating neutral. The N and G and bonded at the main service. The N feeds back to the N in the alternator and the G feed to the casing of the generator the N and G are not bonded at the generator. I cannot scope this as it is not installed this is a theatrical question.
     
  8. Apr 20, 2015 #7

    jim hardy

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    that's interesting. Our code has requirements for "separately derived service" which a generator would be.

    http://www.mjobee.com/projects&news/delta xfmr1.pdf page 25 of 146

    so it looks at first glance like you're okay bonding at the panel's service entrance. I'm accustomed to bonding right at the source.
     
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