EMI/RFI under ground? 2

[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. 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.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Are you able to check everything in sight with a scope and watch for similar noise patterns?
The neutral may be acting as an antenna.
Are there any metallic pipelines that could be acting as a signal conductor?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

You skipped over the other electrical connection - the wet soil that you're pumping the water out of. Two well separated paths makes a good loop.

 

[ScottyUK]

It may be easier to treat the symptoms at the pressure transmitters than the cause at the VFD. Not the purist's way of doing things but perhaps the more economical.

Understanding if the noise is capacitively coupled (voltage) or magnetically coupled (current) would be a starting point. Capacitively-coupled noise can often be handled by electrostatic shielding, magnetically-coupled noise tends to be more awkward mitigate especially if it is in the earthing system.

 

[jraef]

I won't get a chance to visit the site until the end of the month, so for now this is all remote troubleshooting. My plan as of today is to have them power up the transducers with batteries or a stand-alone power supply to see if we can isolate the source of the noise.

They said there are no other connections, but the submersible is a well pump and the other site is a pressure reducing valve station, so I'm pretty sure there is a pipe between them and now I'm wondering if someone used the pipe as the grounding electrode...

Hadn't thought about the wet soil as a transmission medium either, thanks for the ideas.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ScottyUK]

Even if they didn't intentionally use the pipe as the earth electrode, it's pretty much inevitable that it will become one unless there's something to isolate the pump from the pipe.

An isolation flange of the type usually used in cathodic protection systems to separate above-ground equipment from underground structures would probably suffice, and they're not insanely expensive if you wanted to try. There would be some level of capacitive coupling across the joint, but probably not enough for a strong interfering signal to be imposed on the pipe. If you're not familiar with them, they usually look like this:

 

[Marke]

This is not uncommon here. The issue is conducted emissions where there is capacitive switching currents carried into the frame of the motor from the PWM voltage waveform on the stator.
These switching currents need to return to the source of the current which is the DC Bus of the VFD.
If the riser pipe from the pump and motor is steel, then it can be used as part of the return path.
A good high frequency path from the headworks connected to the pump back to the DC bus will usually overcome the problem.
The return path conductor needs to have a surface area significantly larger than the surface area of the phase conductors between the VFD output and the motor. A 50 : 1 ratio works well and is the order of ratio achieved by the use of a screened cable.
Running a good braided conductor, aluminium bar, steel tube, or any other conductor with a good surface area from the motor frame to the VFD frame will provide such a path, but you still need a path from the VFD frame to the DC Bus.
Many European VFDs include coupling capacitors between the DC Bus and the frame for this purpose. They are commonly described as first level EMC filtering. If these capacitors are not fitted, they can be added. Typically, I would use capacitors rated at 1KV DC of 100 nF to 470nF capacitance with as short leads as possible.
If the direct path to the DC bus is not a very low impedance path, then the currents will take every other path possible and usually find their way back to the DC bus via the AC input and rectifiers. This means that these currents will flow through all other equipment connected to the three phases on the input of the VFD.
These currents will commonly affect everything connected on the same transformer secondary, but not so commonly affect equipment on an separate transformer.

I have also experienced considerable problems in rural networks emminating from the active front end inverter. This is much harder to mitigate and at times, the only successful mitigation is a low impedance, high power snubber network across the three phase whith literally KWs being dissipated.
The input LCL filters on active front end drives are designed for a very low impedance voltage source. If the impedance at higher frequencies is significant, there can be significant common mode voltages impressed on the supply. I have measured 128V of 8KHz on a 230V 50Hz supply. (fried two smart meters and a power quality meter)

Quick fixes that can yield improvements are : common mode choke ("Ferrite ring") on VFD output can take the edge off the switching waveform and 2uF capacitors across the 4-20mA input that is connected to the pressure transducers.

Best regards,
Mark

Mark Empson
Advanced Motor Control Ltd

 

[itsmoked]

Great Info Mark!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Valvecrazy]

A submersible well pump usually works against a high static head anyway, so a VFD is not very useful. If this is the case simply removing the VFD would be an easy solution to the problem. Then if the submersible is not exactly sized to the application, a simple pressure reducing type valve might be even more useful than a VFD. One thing for sure is that the pressure reducing valve will not cause any electrical interference.

 

[jraef]

Mark,
As always, you are a treasure trove of great info.

Thanks!


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden