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

  1. #111
    Join Date
    Jun 2018
    Location
    Arcata, CA, USA
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    49
    gar, thanks for all your continued thoughts, experiments, and comments on my issue. My apologies for being unresponsive as I've been working away with limited internet access for the last several days. I'm going to have to read through your posts several times to get my arms around it all but will take it under advisement and incorporate where I can.

    Our budget on this particular project is somewhat constrained and right now we are still waiting for the tenant with the UPS to get their protocols down so that they are 100% confident they can safely execute a shutdown test in the afternoon during high solar production conditions; they've tried at night twice and ended up having to reboot servers both times, which would be unacceptable to them during business hours (they are a medical organization with highly critical IT). We think they've got it figured out now but they still need to execute a successful dry run before we can move forward. We have our live test tentatively scheduled for next Monday afternoon. I wish we could move faster but there are a lot of players involved.

    At this point since there is a significant probability the UPS is involved, we have elected to put all other elements of the investigation on hold until we have the results of that test. If it's the UPS, then we either get the tenant to install filtering devices or else replace the UPS (hopefully under warranty since it's less than 2 years old and obviously causing issues in a pre-existing PV system).

    If it's not the UPS, then at that point we will resume experiments to try to identify the root cause of the fields causing the vibration. Along they way we will also be performing several minor repairs that are more likely to be symptoms rather than the root cause, and we are also recommending they reduce the EMT support span from 12 feet to 6 feet throughout its entire 250' run through the building.

    I'll try to post to this thread when I can however I'll be working remotely most of next week with little or no internet so it might be July 9 before I'm able to comment much more. But trust that I will definitely not let this discussion die before the investigation is complete.

    Thanks again everybody!

    Quote Originally Posted by gar View Post
    180623-1123 EDT

    Continuing my vibration experiments.

    With the conduit (basically a thinwall steel tube) I am seeing a vibration at about 8, but not exactly, times the fundamental. Most of my experiments have been with the tube supported at the ends, but only one is rigid. The fundamental vibration is with the midpoint moving up and down.

    With the 10 ft thinwall (both 1/2 and 3/4) I saw a fundamental below 10 Hz, then another major spike around 8 times this, but not an integer multiple. I don't know the origin of this higher frequency component.

    I have only a limited supply of different items. My next experiment was with a 1/2" diameter solid round bar stock 6 ft long of 6061 aluminum. Here I measured 9.5 Hz for fundamental, and harmonics at 19.25, 28.0, 37.6, and 49.75. These are clearly 2 nd, 3 rd, 4 th, and 5 th. Then a larger spike at 85.75 or about 9 times the fundamental. On thinwall I did not see 2 nd thru 5 th harmonics,

    Where does this 8 or 9 times vibration come from?
    .

    Redwood Infrared it would be interesting to know what resonance points exist for your $" conduit.

    .

  2. #112
    Join Date
    Oct 2009
    Location
    Austin, TX, USA
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    9,945
    Quote Originally Posted by Redwood Infrared View Post
    ... we are also recommending they reduce the EMT support span from 12 feet to 6 feet throughout its entire 250' run through the building.
    It occurs to me that you might also suggest that they vary the spacing between supports if you think mechanical resonance could be a contributor.

  3. #113
    Join Date
    Jun 2018
    Location
    Arcata, CA, USA
    Posts
    49

    beanland, welcome to the thread and thanks very much for your comment.

    While I haven't done a very sophisticated audio analysis, just trying a variety of smartphone app spectrum analyzers, we get pretty consistent results pointing to 120Hz as the fundamental audio frequency. That frequency does not appear to change; it is only the volume or amplitude that varies. It seems pretty constant during clear skies but occupants point to it being intermittently somewhat louder on windy partly cloudy days, ramping up and down in a manner that correlates pretty strongly to observable fluctuations in current. I can't say I've observed this myself, but they seem to think it will be loudest as current is ramping up but then quiet down somewhat at peak current once that current stabilizes under clear sky. To me it seems possible they erroneously perceive change in volume as peak volume but frankly it's difficult to measure and impossible to recreate unless weather conditions cooperate and so far we just haven't had a lot of luck in that regard throughout my half dozen visits to the building.

    Other than the solar inverters and the one tenant UPS, there are no transformers nor any VFDs that I know of inside the building. There are several tenants with security concerns so I have not yet completed a survey of every subpanel/server room, etc. It is possible I might find a small buck boost or VFD in one of those suites so I've not eliminated that possibility but there was nothing highly evident in power quality monitoring at the building's main service panel pointing to anything like that. The one 10kVA UPS that we have identified remains a variable that we are actively pursuing and we hope to have its affect on the hum fully characterized by this coming Monday afternoon.

    There is a utility pad transformer located just outside the building approximately 5 feet away from the building MSP closet, and that transformer does also emit a substantial 120Hz audible hum. The utility has conducted PQM at three panels within the building including the MSP and they seem confident the problem does not originate with them but I have put in a request to have a lineman open up their transformer so we can verify the integrity of the ground and neutral bonds in that unit.

    I will state that my observations point to the aluminum conductors as the main mover in the vibration equation. Clearly the sound is that of the conduit but we think it's the conductor vibration being amplified by the conduit, not the conduit vibrating and causing the conductors to move. This just seems intuitively correct based on the tactile magnitude of the conductor vibration at the terminal end compared to what you can feel in the conduit at any point along its path.

    Quote Originally Posted by beanland View Post
    I just stumbled on this thread and have been involved in electrical acoustic noise for 30+ years, off an on. In general, it is iron that vibrates due to magnetostriction. A step-up/down or buck/boost transformer core will "hum" at 120Hz with strong 240Hz and 480Hz tones. However, magnetistriction is due to the magnetic flux created by voltage, not the current, so it should be constant. Are there any transformers in the building that could be the source of the noise?

    If the current harmonics from the inverters cause a strong change in voltage, due to a "weak" utility source, this harmonic voltage will cause every transformer, iron core ballast, and motor to be noisy. A harmonic filter can lessen the current harmonics, thus lowering the voltage harmonics, thus reducing the acoustic noise.

    Also, are there any VFDs in the building driving motors? A VFD creates a messy waveform which causes the iron in a motor to vibrate noisily.

  4. #114
    Join Date
    Apr 2008
    Location
    Ann Arbor, Michigan
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    6,978
    180629-2154 EDT

    Redwood Infrared:

    Based on experiments I ran and theory the following points apply:

    1. A conductive loop AC current will cause a mechanical vibration of the loop at twice the AC frequency.

    2. Experimentally I could see 120 Hz mechanical vibration of the current loop with a 60 Hz sine wave current.

    3. A 10 ft long thinwall steel conduit has a fundamental mechanical resonance in the 6 Hz range, and another around 50 Hz. I don't known the origin of the 50 Hz. Seems to be a 7 to 8 to 1 ratio of the fundamental.

    4. The mechanical resonance was obtained with an accelerometer and mechanical tapping of the conduit. You can see and feel the fundamental.

    I have previously suggested doing bench testing. This gets you away from the actual installation, and allows doing controlled experiments. This is simple to do and much less costly than playing around onsite.

    Get 12 ft of your conduit, suspend on non-restraining supports at the ends (strings or vee like soft under supports), tap the center, and see where there are resonance points.

    I am suspecting there is a resonance near 120 Hz. If this is so, then insert a loop of #12 or larger wire, and run a large 60 Hz current thru the loop from a transformer with a low output voltage. This should excite vibration at 120 Hz. Even though the mechanical resonance is not at 120 Hz you may still get substantial sound. What I saw as an about 8 times higher resonance point on the 1/2" conduit, may be at a different ratio on a larger pipe.

    Rigid termination of the conduit instead of the soft termination should raise the fundamental resonant frequency. Also conduit added to each end will have an affect on resonance.

    You really need to do offsite experiments to get a handle on how various parameter changes vary sound levels or resonance points.

    .

  5. #115
    Join Date
    Apr 2008
    Location
    Ann Arbor, Michigan
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    6,978
    180630-2418 EST

    The following link does not directly relate to this thread, but is quite interesting on the subject of sound in rooms, reflections, resonances, and etc.

    https://www.crutchfield.com/S-tpOB2a...acoustics.html

    Also see

    https://digitalcommons.unl.edu/cgi/v...xt=physicskatz

    .
    Last edited by gar; 07-01-18 at 12:33 AM.

  6. #116
    Join Date
    Apr 2008
    Location
    Ann Arbor, Michigan
    Posts
    6,978
    180630-2453 EDT

    Another one. See comments by bmh1944

    http://www.oocities.org/teeley2/tubing.html

    .

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