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

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junkhound said:
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.
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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.
 
ATSman said:
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.
Click to expand...

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?
 
Fulthrotl said:
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.
Click to expand...

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.
 
Fulthrotl said:
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.
Click to expand...

Thank you very much for this, I will take it under advisement and pass it along.
 
At the risk of being as redundant as a parallel ground path, THANKS AGAIN!

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.
 
Redwood Infrared said:
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?
Click to expand...

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
 
Redwood Infrared said:
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.
Click to expand...

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.:p

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?
 
Ingenieur said:
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
Click to expand...

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.
 
Fulthrotl said:
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.:p

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?
Click to expand...

I'm a big fan of bulletproof installations, at least getting them to that status after I've identified an anomaly.

To your last question, no, I've not scoped anything and am not tooled up for that myself. I'm meeting with the installing electrician tomorrow to discuss next steps and try to collect answers to as many questions brought up here as possible, i.e., verify all phases present in each conduit, survey conduit structural support infrastructure, etc.

Oh, and I realized smart phones all have a hall effect sensor built in and found a Gauss meter app that appears to work pretty well around the house; we'll see if it's sufficient for characterizing any stray fields that may be propagated along the conduits.
 
zbang said:
Call me, well, something, but perhaps the installing electrician isn't the right person do be looking for the problem. Are they invested in solving the problem?
Click to expand...

Good comment:
example: 10 years ago, flew to Adak and then to a ship that had a 3 MW DG set that refused to operate due to bad controls.
There had been a team of folks from the installing company aboard the week before and they did not find the problem.

Problem was simple, installation had left a few loose wires in a control cabinet that wore thru (due to diesel vibration) and shorted a 5 volt control signal to chassis, somehow initial installers could not find their own initial poor worksmanship.

Tens of thousands of $$ spent correction the omission of a few tie wrap pads.
 
a couple of comments:

over time the vibration loosens fasteners and gets worse

proportional to current

conduit heats up and expands causing the conduit to be under tension/compression, which makes it like a taught string
the closeness of the conduit allows each conduits magnetic field to attract each other, 'strumming' them

the spacing of supports may be too far and they did not allow for expansion or contraction


I'm not sure I'd get in the middle of this mess lol
 
junkhound said:
Good comment:
example: 10 years ago, flew to Adak and then to a ship that had a 3 MW DG set that refused to operate due to bad controls.
There had been a team of folks from the installing company aboard the week before and they did not find the problem.

Problem was simple, installation had left a few loose wires in a control cabinet that wore thru (due to diesel vibration) and shorted a 5 volt control signal to chassis, somehow initial installers could not find their own initial poor worksmanship.

Tens of thousands of $$ spent correction the omission of a few tie wrap pads.
Click to expand...

In what most people would probably consider a 'normal' business client I would absolutely feel that way. But we're a pretty small, insular, and geographically isolated community, and the installer is young, ambitious, and has a generally good reputation that's well deserved. For what it's worth both the contractor and the client have significant presence in the solar space and future collaboration is in both their interests. Regardless of where the fault might lay, I can't imagine he's not highly motivated to make this right no matter what it takes. My perception is nobody involved at this point is that interested in pointing fingers, we just all want to understand it, get it fixed, and move on. I know for my part I will greatly value this as a learning experience and expect that perspective will be shared by all concerned.
 
Redwood Infrared said:
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?
Click to expand...

Not that I can recall......there was no discoloration of the silver plated copper bus to indicate thermal issues there or any other place.
 
Ingenieur said:
a couple of comments:

over time the vibration loosens fasteners and gets worse

proportional to current

conduit heats up and expands causing the conduit to be under tension/compression, which makes it like a taught string
the closeness of the conduit allows each conduits magnetic field to attract each other, 'strumming' them

the spacing of supports may be too far and they did not allow for expansion or contraction


I'm not sure I'd get in the middle of this mess lol
Click to expand...

Another really good point to ponder. I've seen elsewhere on this forum the math on coefficient of expansion for EMT works out to about 3/4" per 100 ft (though not sure how that might vary with pipe diameter). I'm certain there are enough bends in this conduit to relieve any stresses from that but it could very well be the case that there is enough movement to bring them into and out of the proximity where resonance occurs.

I'll know much more about the structural characteristics of the conduit installation this afternoon. And the forecast looks good so hopefully I'll finally observe it under full sun and get to hear the full throated moan that's got everybody so concerned.

As far as getting in the middle of this mess, yeah...I have often found myself in situations where basically there are two parties looking for somebody else to pin blame on and one or the other of them decided I'm that guy. It's never fun when that happens. But I'm really hoping this is a situation where everybody is just looking for a positive outcome and since they've been trying to figure it out for over a year with no progress they figured they'd hand it off to somebody just dumb enough to have no option other than to try harder.
 
ATSman said:
Not that I can recall......there was no discoloration of the silver plated copper bus to indicate thermal issues there or any other place.
Click to expand...

Even without infrared I can usually feel the heat radiating out of the box long before anything gets hot enough to actually start taking the surface finish off conductors, especially in a bus where there's a bit more mass and it's uninsulated. But even with FLIR a lot of folks will miss hot spots on buses since they've got lower emissivity and you won't realize how hot something is if you don't account for that.
 
ATSman said:
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.
Click to expand...

ATSman, forgive me but I keep coming back to this and I'm having trouble parsing your second situation. I'm not sure what you mean by mismatch when you say the 120v hot conductors were connected to the ground conductors - that sounds like a straight up short circuit to me? Can you please clarify?
 
Redwood Infrared said:
ATSman, forgive me but I keep coming back to this and I'm having trouble parsing your second situation. I'm not sure what you mean by mismatch when you say the 120v hot conductors were connected to the ground conductors - that sounds like a straight up short circuit to me? Can you please clarify?
Click to expand...

Its where you have a 120v circuit with no neutral and someone used the ground wire or building ground to complete the circuit.
 
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