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Thread: Mysterious vibration in long conduit between solar inverters and main service panel

  1. #21
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    Quote Originally Posted by junkhound View Post
    Ultrasound indicated only audible frequencies

    So, what are those frequencies ? all 60 Hz harmonics or ? Any current waveforms - elg harmonics?

    What is the PWM operating frequency of the inverters?

    Conduit span between clamps?

    For instance, if the objectionable noise is at say 180 or 540 Hz, one could infer in phse harmonics on the core wires leading to the magnetic forces already mentioned. A 540 Hz whine sure could drive folks out of an office.
    I should concede I merely inferred the hum is audible only because I could not pick up anything highly directional in the ultrasound frequencies. I assume if I could have heard an ultrasound squeal it would have revealed a point source of the problem but there just wasn't anything there at all. I also tried using a simple sound meter with a frequency analyzer in a phone app but even in the office environment the background ambient noise levels made it impossible to distinguish anything on a day with only 75% solar output. We might try that again if we get a good full sun day any time soon, and if so I'll try to gain access to the conduit above the ceiling so I'm closer to the source of sound and removed from other artifacts.

    The inverters are Fronius 12.0-3 with frequency range of 45-65 Hz.

    We do have voltage and current waveforms as well as voltage and current THD all monitored at the inverter combiner box, the AC Solar Disconnect, and the building's MSP over a period of about 3 days. Nothing was flagged as particularly unusual by the utility or the electrician. I and V waveforms looked normal to me, other than the 8A on the ground but that's been explained here.

    I'm not terribly familiar with interpreting THD waveforms but will ask for permission to post them here if that would be helpful.

    I'm not sure of the span between conduit clamps but it's on my list of things to check on our next site visit.

    I would have guessed it was a higher pitch than 540 hz but I just listened to that on a function generator and yeah, that's probably a pretty fair guess.

  2. #22
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    Quote Originally Posted by ATSman View Post
    Based on 2 jobs we have done several years ago, My gut feeling on this is that the root cause of the vibration and humming noise is due to net currents (as others have mentioned); currents that return to the source thru system grounding paths (conduits, building steel, metal enclosures, metal raceways, metal roofs, etc) instead of current carrying conductors. In both cases the symptoms were the same: screen jitter on the old CRT monitors of all the work stations. One we found using a Gauss Meter to measure the unusually high EMI in the affected areas. Also very high ground currents measured on the ground cables terminated on the ground bus of the main power panel. This one turned out to be loose bolt connections on the neutral bus (4000A service) bars at the outside utility pad mounted transformer. The other cause turned out to be the miss-match of the connectors between work station old and new partitions. During expansion, the new type partition sections caused the 120v hot conductors to connect to the ground conductors of the adjoining partitions resulting in net currents. If these currents are high enough then it could be the source of the humming since steel is not designed to carry current.
    Just a thought....hope this helps.
    This is not entirely inconsistent with my initial (though perhaps poorly characterized) hunch regarding ground loops. We've got multiple ground paths including redundant ECG, the parallel EMT conduits, and the steel building frame. The question is what magnitude of stray current could produce such a highly audible hum?

    I'm on the hunt for a Guass meter this morning, may end up just buying a cheap one to get through this job.

    So far our investigation has been limited to internal components described above. I've not cracked open the building's MSP and don't know whether or not the electrician ever got into the X-former outside but perhaps it's time to broaden the footprint of the investigation to eliminate those components.

    When you found the loose bolt connections on the Neutral bus bars in the utility pad transformer, were they thermally hot? If so had that condition propagated elsewhere?

  3. #23
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    Quote Originally Posted by Ingenieur View Post
    what size conductor?
    what size conduit? what material? how as it supported?

    the fact that it proportional to load is telling
    I suppose a lot could be read into your last statement here. Could you please elucidate with respect to your thinking? What exactly does the proportionality tell you?

  4. #24
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    Quote Originally Posted by Fulthrotl View Post
    just addressing the sound, and not the cause, i've heard rattling of conductors in conduit twice
    in my life. the loudest was at anaheim convention center, with a bolted fault on an 800 amp
    trade show power panel in the overhead. it almost sounded like someone using a chipping hammer
    against the conduit. it tripped a 3000 amp section.

    i don't see an 8 amp current flow causing enough flux to make noice sufficient to drive people from
    their offices nearby, no matter what type of leakage or flow it is.

    i'm thinking harmonics, and a standing wave built up, resonating in the conduit. i'd get half a dozen
    sandbags, fill them partly, so they can be tossed over the conduit(s) in question, and see if that
    changes the frequency or intensity of the sound. think frets on a guitar string.

    it's fast and easy. cheap too. see if they will dampen the sound.
    It's worth a try if only to zero in on the point source of the sound, if in fact it's not resonating along the entire length of the conduit. As solutions go though I'm afraid it might not be much more than a bandaid on a wound that continues to bleed. We've already got components out of spec thermally so one way or the other we need to get to the root cause of the issue.

  5. #25
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    Quote Originally Posted by Fulthrotl View Post
    if you have enough length on the aluminum conductors, my suggestion is to remove the setscrew lugs,
    dress the ends of the cables, and use hypress lugs, with the appropriate dies. then seal the ferrules
    with either 3M cold shrink, or Panduit heavy walled UL listed 600 volt heat shrink.

    use grade 8 fine threaded bolts, washers, and nylock nuts on the connections.

    that will elminiate any termination heat, and not require PM's other than checking the torque
    on the nuts periodically.
    Thank you very much for this, I will take it under advisement and pass it along.

  6. #26
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    At the risk of being as redundant as a parallel ground path, THANKS AGAIN!

    I am blown away by the attention so quickly paid to this thread and all the very high value questions and suggestions. I've participated in a lot of online fora over the years and this one is definitely aces.

  7. #27
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    Redwood, I just sent you a PM to check.

    Bill

  8. #28
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    Quote Originally Posted by Redwood Infrared View Post
    I suppose a lot could be read into your last statement here. Could you please elucidate with respect to your thinking? What exactly does the proportionality tell you?
    a short only depends on v, not i, and v is constant
    if it were a short the buzz would be the same regardless of i

    the more i the stronger the magnetic field
    how close are the parallel conduits run together?

    my wag why it is gradually getting louder is due to the vibration loosening the fasteners over time...it can oscillate more
    The difference between genius and stupidity is that genius has its limits.

  9. #29
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    Quote Originally Posted by Redwood Infrared View Post
    I am blown away by the attention so quickly paid to this thread and all the very high value questions and suggestions. I've participated in a lot of online fora over the years and this one is definitely aces.
    most of us here like a puzzle to solve.
    many of us here are in fact god's gift to the electron.
    honest. just ask us.

    if the conductors in each pipe were all of the same phase,
    they would be so hot you couldn't touch them with a good
    load on them. induction heating is pretty simple to diagnose.

    i don't think the lugs used are the problem, i just mentioned
    hypress and ul listed heat or cold shrink 'cause it bulletproofs
    the installation, and makes it permanent and durable. it won't
    solve this problem, imho.

    have you stuck an o scope on any of these feeders yet?
    ~New signature under construction.~
    ~~~~Please excuse the mess.~~~~

  10. #30
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    Quote Originally Posted by Ingenieur View Post
    a short only depends on v, not i, and v is constant
    if it were a short the buzz would be the same regardless of i

    the more i the stronger the magnetic field
    how close are the parallel conduits run together?

    my wag why it is gradually getting louder is due to the vibration loosening the fasteners over time...it can oscillate more
    Yes, I agree with all of this. I'm still reluctant to guess which came first, the oscillation or the loose fasteners, but there is little doubt we've got a feedback loop going on where the more it vibrates the looser the more it loosens up, and the looser it gets the more it vibrates.

    So far I've only observed the conduits right where they enter the panels so they're only an inch or two apart. They're probably not separated much more than that across their run but I don't know that yet and will try to verify tomorrow along with a bunch of other data points I'll be collecting thanks to all the help offered here.

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