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

  1. #11
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    Quote Originally Posted by Redwood Infrared View Post
    It will take me a little time to digest all this and respond.

    For what it's worth, the utility analysis was pretty comprehensive and they provided data on current and voltage waveforms at all three of the aforementioned service panels as well as THD on both I and V. I'd have included that info in my original post but ran up against the forum's character limit as it was. In any event nobody seemed to think there was much to go on from all that, although I did notice after we'd measured the odd ground currents that they also saw about 8A on the ground but didn't flag it. They did not, as far as I can tell, try measuring net current by capturing all the phases together to see if there was anything missing that must be running outside the pipe. I think that's a very good suggestion and I'll try to get that accomplished.

    I'll see if I can roust up a Gauss meter too. It's almost too bad we don't still have the occasional CRT under a back table in some hoarder's office but I think we've past the point where any building less than 10 yrs old will not likely have ever housed a CRT to begin with.

    More later, after the wife gets some Sunday chores out of me. Thanks again!
    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.
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  2. #12
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    Quote Originally Posted by Redwood Infrared View Post

    1. Going for the obvious problem first, I suggest they mitigate the heat issues identified with the FLIR survey. Lug connections had already been re-torqued before my arrival but obviously fixed nothing so I suggest removal and visual inspection with renewal of conductor endpoints with fresh application of Noalox if warranted. If upon re-inspection the combiner box main disconnect is still flagged under NEMA Thermal Severity Criteria I’ll recommend replacing that component. I'll be surprised if this eliminates the hum but it needs to be done regardless.


    Or better yet, one of you geniuses who has read this far and hasn’t already decided I’m an idiot and not worth the trouble might already know exactly what’s going on here and can just tell me?

    I thank you in advance for your indulgence and any thoughtful responses this might elicit.
    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.
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  3. #13
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    Wire size

    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
    It's all aluminum conductor.

    Phases are XHHW-2 AL 350 KCMIL.

    Grounds are XHHW-2 AL #3/0 AWG.

    The conduits are 4" EMT that runs above suspended ceiling within the building so I haven't seen most of it but I would assume suspended from unistrut or the like.

  4. #14
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    Did the original installer screw up and mis-wire the paralleled conductors in the conduits? If all three phases, neutral, and ground aren't each present in each conduit, currents will be induced in the conduit(s). This could be the source of the buzzing.


    SceneryDriver

  5. #15
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    Quote Originally Posted by SceneryDriver View Post
    Did the original installer screw up and mis-wire the paralleled conductors in the conduits? If all three phases, neutral, and ground aren't each present in each conduit, currents will be induced in the conduit(s). This could be the source of the buzzing.


    SceneryDriver
    That seems like such an obvious question sitting here now but if I'm honest, I didn't take the time to painstakingly track each and every wire to make sure nothing was routed incorrectly. I just assumed the colored phase labeling tape was applied before it was pulled and nobody would make such an error but I will definitely check this next time I'm there.

    That said, it seems such an error would have showed up in the phasor diagrams produced by the utility's monitoring report, which looked normal failed to identify any such anomaly.

  6. #16
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    Quote Originally Posted by JFletcher View Post
    Welcome to The Forum.

    High currents develop high magnetic fields, enough to make the wires chatter in the conduits. If this noise and vibration was not present initially, I would look for loose hardware contributing to conduit movement and objectionable sound.

    Depending on what and where the vibrations are, you may be able to dampen them with rubber sheeting or wedges.
    The installing electrician claims to have already been through everything and checked torque on all connections. He didn't comment however regarding tightness of structural fasteners along the conduit. They did try the old spray foam in the conduit trick but does that ever work?

  7. #17
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    Quote Originally Posted by jaggedben View Post
    Comments, such as I can make:

    - The 8amps on the solar ground could be from capacitive leakage currents, if the DC side of the solar system(s) are grounded. That is a common feature of such systems, and 8 amps is within reason for a 125kW system.
    - The 2amps on the main premises GEC likely just represents building load having nothing to do with the solar.
    - I'm somewhat doubtful that the parallel ground paths for the solar grounding is inherently contributing to the noise problem. I would more likely suspect insufficient bonding of some metal component to contribute to the problem; that is, trying to unbond certain grounding conductors and components and not others would, to my intuition, possible make the problem worse. It might be interesting to try to unbond and unground all components for a short (minutes) test to see what happens, but it would be pointless to advise on how to do that permanently since it would be non-compliant and significantly unsafe.
    - Has the inverter manufacturer been contacted for any comment or advice?
    - Your item (4) would probably be code compliant but is unlikely to make any difference. Ground rods really don't do much at those voltages.

    Thanks for all these comments, which address many of my questions directly. I just wasn't sure whether the ground currents I was measuring were normal or not.

    The inverter manufacturer was contacted months ago and claimed to have never encountered a situation like this. All the inverters appear to be operating normally and other than the noise issue there have been no indicators they are not performing well.

  8. #18
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    Quote Originally Posted by GoldDigger View Post
    The easiest and most likely productive option would be to pull the wires out of the conduits, twist them tightly and pull them again (hopefully you will have enough extra length to do this, otherwise replace the wire.) This will both reduce extended magnetic fields and keep the wires from vibrating against the conduit and each other.

    Sent from my XT1585 using Tapatalk
    I'm not sure either the building owner or the installing electrician would consider this easy given the run is 250 feet through the middle of a pre-existing building, which required rounding a few corners to accomplish. In a worst case scenario I imagine we could be looking at replacing the wire but there's no way that will happen before we can both positively identify the root cause of the problem and exclude all other feasible mitigations that might be more cost effective.

  9. #19
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    Quote Originally Posted by ActionDave View Post
    Number of times I've found a problem with conductors inside the conduit, 10. Number of times I've found a problem with the way things were wired, 1000.
    This is a profound statement and one I will try to repeat to myself every time another lame brained idea pops into my head.

  10. #20
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    Quote Originally Posted by GoldDigger View Post
    A valid point. Assuming, for the moment, that there is some kind of problem with the wiring, I would probably put a clamp-on ammeter (amp-clamp) around the three phase conductors from each conduit to make sure that the net current is near zero. A relatively small extra termination resistance or other problem on just one wire could lead to very different currents between the two elements of what should be a parallel path. That imbalance could then cause non-zero net current in both conduits. Just one example of a potential problem that would not be seen by simple continuity checks.
    My next step would be amp measurements on all of the individual conductors.
    The utility already did exhaustive monitoring over several days, including current, voltage, and THD on phases and neutral conductors at the inverter combiner box, the AC Solar Disconnect, and the building's MSP. They provided a report with 26 figures displaying their collected data, none of which raised any red flags from those who've reviewed them. I might be able to share that report here but not before I get a release from appropriate stakeholders.

    As far as I can tell they did not try wrapping a single CT around all three phases at the same time to make sure current zeroes out. That seems like a very good suggestion and one I will try to facilitate. One of these days I'm going to spring for a pro quality power monitor but for the time being all I've got is a Fluke T5.

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