EMI/RFI under ground?

[Jraef]

Just checking to see if anyone else has run into this. I have a customer who has a 200HP VFD on a submersible pump. 1/4 mile away is another station that has pressure transducers. Whenever the VFD is running, the pressure transducers go haywire. It's been thoroughly vetted, the link is definitely there. No VFD, no interference; VFD running, squirrely transducers.

First course of action will be ferrite cores on the transducer wires, then EMI/RFI filters on the line side, but the only connection is the utility; two separate services / transformers 1/4 mile apart. there are no other wires, pipes or conduits connecting the two sites (according to them). To me that seems implausible, but that is the cheaper filter. Final action will be to suggest Sine Wave Filters on the output of the VFD, but those are going to be expensive.

What I want to know is if anyone has experienced something similar due to the fact that the pump cables for submersibles are almost always unshielded. I've had all sorts of issues with unshielded cables topside, but have always assumed that with the sub cables being under ground, no harm/ no foul. But this has me rethinking that. I've been taught that radio waves don't penetrate the earth; too dense. But someone once told me that FM does pass through hills and mountains if the distance is short. I have no confirmation of that, but this situation has me wondering if that might be true.

 

[rlundsrud]

Can you get the transducer part numbers?

 

[GoldDigger]

As far as I know broadcast FM signals do not pass through the dirt, but if the crest of a hill is fairly sharp the radio waves can diffract off the edge and travel down the slope on the back side.

Sent from my XT1585 using Tapatalk

 

[Open Neutral]

Just checking to see if anyone else has run into this. I have a customer who has a 200HP VFD on a submersible pump. 1/4 mile away is another station that has pressure transducers. Whenever the VFD is running, the pressure transducers go haywire. It's been thoroughly vetted, the link is definitely there. No VFD, no interference; VFD running, squirrely transducers.

RF goes where it wants to.

Have you filtered not just the transducer wires, but the other connections to whatever signal processor they feed? Not just the power but other connections. Can you test-run it off an inverter to isolate it?

(You may have a striped snipe hunt here; keep us advised...)

 

[Phil Corso]

Jraef...

Are both stations fed from the same source? And is it overhead?

Rrgards, Phil Corso

 

[gar]

171104-1200 EDT

I would take one transducer and disconnect it from whatever it is connected to. This will completely electrically isolate the transducer. Use a battery to power (excite) the transducer. Use a battery powered scope, or possibly a Fluke 87, to look at the transducer output. You may want to use a preamp so you can see 1 microvolt increments.

A Fluke 87 or equivalent resolves 10 microvolts.

A typical transducer might have an output of 2 mV/V at full scale.

If you get substantial noise, then you need to evaluate the cable to the transducer. Is it twisted pair for the signal output, and is it Beldfoil shielded?

If you have noise and a good cable, then can low pass filtering solve the problem? If not, then you need a preamp near the transducer.

If there is no noise with the transducer isolated, then you need to find out how the noise enters the systerm.

.

 

[Jraef]

I don't have all of the details yet, the site is two states away and I'm not going there until the end of the month. But I do know that the only physical connection is the HV utility feed going to both site service transformers, and I presume a water pipe because the submersible is a well pump, the other site is a pressure reducing valve station. I don't know for sure if the HV is overhead, but this is in Utah, so I seriously doubt their utility would bury the HV lines.

Excellent idea on powering the transducer via an isolated power source, I hadn't thought of that. It would narrow the possibilities for sure. I'm hoping it's a 24VDC powered transducer, but if not, I have a 12VDC to 120VAC power supply unit I'll loan them for this. Thanks!

 

[Besoeker]

Just checking to see if anyone else has run into this. I have a customer who has a 200HP VFD on a submersible pump. 1/4 mile away is another station that has pressure transducers. Whenever the VFD is running, the pressure transducers go haywire. It's been thoroughly vetted, the link is definitely there. No VFD, no interference; VFD running, squirrely transducers.

First course of action will be ferrite cores on the transducer wires, then EMI/RFI filters on the line side, but the only connection is the utility; two separate services / transformers 1/4 mile apart. there are no other wires, pipes or conduits connecting the two sites (according to them). To me that seems implausible, but that is the cheaper filter. Final action will be to suggest Sine Wave Filters on the output of the VFD, but those are going to be expensive.

What I want to know is if anyone has experienced something similar due to the fact that the pump cables for submersibles are almost always unshielded. I've had all sorts of issues with unshielded cables topside, but have always assumed that with the sub cables being under ground, no harm/ no foul. But this has me rethinking that. I've been taught that radio waves don't penetrate the earth; too dense. But someone once told me that FM does pass through hills and mountains if the distance is short. I have no confirmation of that, but this situation has me wondering if that might be true.

Are the transducers in a metal box? are their cables shielded?

 

[zbang]

If the well casing is metal, it shouldn't matter that the pump leads aren't shielded themselves. Are they in grounded metal all the way from the VFD to the well head? In this case, a cheap AM radio can be your friend for troubleshooting.

A second on trying isolated power for the transducers.

Is there a metallic connection between well and valve station? Everything bonded & earthed?

As a very long shot, a corroded pipe joint can form a detector...
> PN junctions make envelope detectors, but only if improperly biased
> (or not biased at all). I'm wondering if there is something funky with
> the phantom powering.
--leanstotheleft
Oh, horsefeathers. Given enough RF, an unclean thought can be a detector.
--John Higdon

(that quote is too good not to use , IIRC it's from rec.radio.broadcasting)

 

[Phil Corso]

Gentlemen,

Many years ago, in the early VFD days, I investigated a similar incident! Two plants, one, a Ref'y, the other, a Chemical plant! They were about 1/2 mile apart and connected to the same OVH (say, 200kV)! At the start of a VFD (say 200-400Hp) in the Chem plant, the Ref'y tripped off-line! Of course, neither knew of the other!

The Ref'y restarted, and operated for a long of time, say weeks, until the Chem plant's VFD was started again!

Eventually, the cause was traced to RFI produced by the Chem plant's starting VFD, thru it's utility Xfmr, then the OVH... causing the Refinery's protective HV relaying to trip!

Hope it helps!

Phil

 

[junkhound]

RF wiil easily penetrate dry low conductivity soil, whereas not too well on damp acidic soil.


Scope or spectrum analyzer on the transducer ac power lines (small 600V 1000 pF cap in series to scope input helps to reduce the 60 Hz levels so you can see the HF coupling levels) will tell you if the VFD noise is coupled thru the power lines. RF filter caps on power line may do the trick, the ferrite beads will likely do little.

 

[Ingenieur]

does the pump pump to the valve pit?
with a metallic pipe?
are the xfcr's mntd to the pipe?

a submersible pump typically has a flexible cord
this is run parallel to the pump dsch pipe

 

[Open Neutral]

Oh, horsefeathers. Given enough RF, an unclean thought can be a detector.
--John Higdon

(that quote is too good not to use , IIRC it's from rec.radio.broadcasting)

That's John all over. A great character from many sides.
He owned ATI.com (Answering Technologies Inc.) and resisted ATI's offers in Canada [video board mfgr] for years; I think he's sold it now.

 

[zbang]

He owned ATI.com (Answering Technologies Inc.) and resisted ATI's offers in Canada [video board mfgr] for years; I think he's sold it now.

Yep, he sold that domain a while ago. I think he's (semi) retired now, too.

 

[winnie]

What switching frequency is the VFD running at? I wonder if the distance between the VFD and the other station is such that you are seeing 1/4 wavelength of the VFD frequency or some harmonic.

I think that EMI through the soil is unlikely, but EMI via the power lines is plausible.

-Jon