Earth Currents with Submersible Pumps 3

[Marke]

I have an interesting problem as follows:
Submersible pump 185KW.
400V 50Hz controlled by a soft starter.
MEN supply. Star connected transformer secondary dedicated to this application with the neutral earthed.

12 inch bore, 10 inch pump, 125 Meters deep.

Earth cable from pump is attached to the well casing at the top of the well.

If the earth cable is not attached to the well casing, there is 15Amp AC between the well casing and the earth cable to the pump motor. If the earth is connected, 90 Amps flows between the motor earth and the well casing.
If the motor stops, no voltage and no current.

Any thoughts?

Best regards,



Mark Empson
L M Photonics Ltd

 

[electricpete]

Here is data file for when the the ground wire is open-circuited (not connected in a parallel loop with the pipe)

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[waross]

Nice slide show Pete. Another suggestion; There is not much flux getting to the pipe. How about close up the spacing between the wires, they are generally twisted tight together. With .6 inch wires, the centers may be as little as .7inches.
Ignore the casing. The cable twist will cancel most of the induction and as I mentioned, not that much flux gets to the casing compared to flux influencing the ground wire.
Can you easily plug these numbers into your equation?
Thanks.
Yours
Bill

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

 

[thewellguy]

In all of the sebmersible pumps that I have seen. the wire is attached to the outside of the drop pipe and held in place with banning and/or wire.

 

[electricpete]

To clarify / correct my previous discussion of results of the two previous simulations:

With ground conductor is perfectly shorted to the pipe at each end, the flux solution is on slide 2, circulating current in groundwire and pipe is approx 18A (slides 3 and 4) and induced voltage is approx 0.3 vac (slide 5)

With ground conductor open (not connected at each end) as per slide 6, there is no circulating current and induced voltage is 0.5vac in the groundwire and 0.1 vac in the pipe.

Should have been:

With ground conductor is perfectly shorted to the pipe at each end, the flux solution is on slide 2, circulating current in groundwire and pipe is approx 18A (slides 3 and 4) induced voltage shown on slide 5 is end-to-end voltage... this voltage does not have much physical significance since it is difficult to measure... voltage measured by connecting test leads would depend on positions of the leads

With ground conductor open (not connected at each end) as per slide 6, there is no circulating current and induced voltage is 0.5vac in the groundwire and 0.1 vac in the pipe. The voltage measured at one end would be the vector difference of these two induced voltages (which is approx 0.5 vac)

How about close up the spacing between the wires, they are generally twisted tight together. With .6 inch wires, the centers may be as little as .7inches.

Attached is another powerpoint with results of center-to-center distance reduced to 0.75” (that leaves 0.075” for insulation).

The short circuit result (slide 1) is |I| = |15.273 + j*7.339| = 16.94A.
(contrast to previous open circuit result: |I| = |115.506 + j* 9.26| = 18.06A).
The end-to-end voltage in short-circuited configuration on slide 2 as previously discussed... not much significance.

The open circuit results:
Slide 3: |Vgroundwire| = | | = 0.488608+0.2396i = 0.544 vac
Slide 4: |Vpipe| = -3.42E-003+1.95E-002i = 0.019 vac
Total Open Circuit Voltage = |Vgroundwire – Vpipe| = |0.492028+0.2201i| = 0.539 vac
(contrast to previous open cricuit result |V| = 0.49 vac

So moving the wires closer together did not significantly change the results, and in fact slightly reduced the open circuit voltage and short circuit current.... opposite the direction of effect needed to try to recreate the measured results of approx 90A short circuit and 15vac open circuit.

I think the next logical step would be to try moving the bundle of 4 conductors close to the edge and repeating the calculation. Since twist is not modeled, the resulting induced Voc and Isc will clearly be higher than if the twist were included. But it is still a useful excercize.... if the result is that the calculated Voc and Isc with twist neglected remain far less than measured, we might begin to conclude that induced voltage of this magnitude is not credible. If on the other hand, we exceed the measured values, then we need to sharpen our pencil on modeling the twist.

I just saw thewellguy's post... I can change the model to put the wires outside. Mark - do you know the configuration.

In either case, I think waross hit on an important subtle point that it certainly took me awhile to figure out... the location of the groundwire within the 4-wire twist is not symmetric with respect to all three phases and therefore there is an induced voltage. LPS for you.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Here is the attachment that I meant to include in my last post (23 Mar 10 8:55)

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[waross]

As I understand the installation of submersibles, there are two pipes to consider;
The well casing of 10 inches in diameter and the pump discharge piping. The pump discharge piping may or may not be ferrous and if conductive may or may not have an insulating coupling installed.
Mark;
Are you able to check the current on the three phase conductors with the grounding conductor connected and disconnected from the well casing?
A four trace scope check will be interesting if possible.
I wonder if there is a bad batch of submersible cable out there. It should show up on a megger check though.
Are there any large users or producers of electricity nearby?

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

 

[waross]

A slip of the mind, the well casing is 12 inches not 10 inches.

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

 

[electricpete]

I think my choice of current phases, positions and plotting options in the previous powerpoints was unfortunate. I have an idea for a slightly different plot that I think will do a better job of graphically presenting the mechanism for induced voltage in ground wire. It is simply to swap the A and B phases. In that case we should see some flux lines tending to encircle the ground conductor even in the open-circuited condition (which provides a better picture of the mechanism for the induced voltage). Here is the details associated with the plotting and phase considerations...

The magnetic vector potential solution for the sinusoidal time varying system is a complex number containing magnitude and phase information at each point in 2-D space. We have to choose real or imaginary component of vector magnetic potential for plotting the flux lines. The component I plotted was the real portion. That represents flux lines for the component of flux that is in-phase with the real component of current which happens to be phase A here. (You can confirm in my previous plots that the plotted flux lines tend to encircle A phase conductor which is the upper left of the four conductors.). But the next time I do that plot, I am going to swap A and B phase locations so that A is in the upper right hand corner (diagonally opposite ground conductor located on the lower left). Then when flux lines are plotted as above (the real portion of magnetic vector potential), the flux lines will still encircle A... but then I’ll bet those flux lines will also tend to encircle the ground wire even in the open-circuit condition. These [time-varying] flux lines encircling the ground wire will be the flux that induces the voltage in the ground wire. I’ll post that plot tonight.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[Marke]

The well casing is ferrous and 12 inch diameter.
The riser (from the pump) is also ferrous and would be around 8 inch diameter.
The cables are not twisted as there is not room, but are spaced around the casing as much as is possible.

The gap between the outer casing and the riser will vary considerably over the length, but there are periodic spacers between the riser and casing. - these things are never perfectly straight.

It is possible that the pump and or riser are in contact with the casing at one or more places on the way up.

Pete, your calcs and simulations confirm my expectation that there is more than just coupling between the cables as the induced power is much higher than your calcs suggest.

One idea that I have, is that if the motor is hard up against the outer casing then there will be a magnetic field conducted to the casing which will be predominantly from a single phase. this field will then be carried by the casing to the top or part way up where there is physical contact between the riser and the casing providing a return path for the flux.
This can act as the core of a transformer. If there is any rotation of the conductors around the riser on the way up, there will be a voltage induced.
Is this a valid theory?
How to fix?

Best regards,

Mark Empson
L M Photonics Ltd

 

[waross]

The motor flux should be mostly contained in the stator iron. I am concerned about the spacing of the wires. It intruduces a lot of maybes into the problem. How far is it from the well casing to the control panel?
Is there one or more ground electrode at the panel?
A couple of easy tests;
Ram a rod or large screw driver into the earth 5 or 10 feet away from the casing. Check the open circuit voltage between the screwdriver and the casing and the ground conductor. This may indicate whether the voltage is being induced in the ground casing, the grounding conductor or both.
It may also just confuse us. Grin.

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

 

[electricpete]

So we have the 4 conductors located in an annulus between steel riser (roughly 8” diameter) and steel casing (roughly 12” diameter). Conductors spaced apart but exact position unknown.

If I were goint to try to arrange a setup that would create a large voltage between Groundwire and outer casing, I would want a lot of flux flowing circumferentially between groundwire and outer casing. So I would try putting conductors in the following locations:
A – 6:00 position adjacent to outer cylinder (casing)
G - 6:00 position adjacent to inner cylinder (riser)
B – 4:00 position adjacent to inner cylinder
C – 8:00 position adjacent to inner cylinder.

I will try simulating that configuration tonight to see how high voltage it creates. I’m making an assumption that the inner/outer rings and groundwire are connected at bottom but not at the top in the open circuit configuration.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[thewellguy]

electricpete I freely admit that the math and formulas that you are discussing are about my head. I would suggest that the pump/pump installation is not the problem. I say this because I have worked on hundreds of well pumps from 1/2 hp to 450hp subs and I have only seen this once. This was at a nuke plant. The well was located in the middle of an anoid field (I forget why) and there was current flowing in the ground and because the well was the best conduit around the current was going to the well casing. It was so bad that if you let a machine parked in the area overnight you had to ground the machine to discharge it before you got in to avoid being shocked.

This is why I asked earlier if anything was ground to the well casing from the building or other source.

 

[Marke]

Hello thewellguy

The current is confined to within the well system.
There is a very small current (<1A) flowing between the well casing and the switchboard earth, so it is definitely the circuit down the hole.

Best regards,

Mark Empson
L M Photonics Ltd

 

[waross]

Let's see how Pete's numbers come out.

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

 

[electricpete]

thewellguy - I can guarantee you know boatloads more about submersibles than me. I have never seen one. We didn't have any at all until just recently we replaced our above ground well water pumps with submersibles... I was not involved in that job. I am just talking through some possibilities. By the way, maybe the voltage you mentioned was dc cathodic protection system? We have that at our nuke plant.

Attached is results of several trials in the order I tried them. The bottom line is that the results shown in last 3 slides show 10vac induced with 200A, so I guess we could get 15vac with 300A.

The "worst case" locations for conductors was nothing like I thought it would be (my first attempt shown in first few slides). The results are a little beyond my understanding or intuition and I got to the final geometry more or less by trial and error.....there may certainly be worse geometries I didn't find. One thing I noticed is the flux pattern within iron looks strange when iron is not laminated. I tried out laminated iron a few places and flux pattern is more what I expect.... also the induced voltages higher but certainly laminations are not representative of pipe. (The final results were not laminated.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

By the way Mark I think I agree with your scenario. The results of interaction with a single conductor are shown on slide 12 where phases B and C currents were simply set to 0 (even though their circular shapes still appear). The result was somewhere up around 80vac on groundwire. So if irregular geometry creates contact that short circuits out the flux contribution from those other two phases, it may begin to resemble the single conductor scenario with those very high induced voltages.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Sorry, I was a little off on the single conductor scenario. The outerpipe voltage did not remain negligible during the scenario of only one conductor (it was negligible in all other scenario's). Also that was a laminated scenario. So that 80 volts laminated number I mentioned above is definitely not a realistic number. But I still suspect that scenario described above where contact shorts out flux from the other two phases can help get toward higher induced voltages.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Last comment I wanted to make (for tonight), I called the flux pattern in iron "erratic" and "unexpected". What I meant is that it is different than I am used to looking at because I have only looked at laminated devices. The pattern shown is expected as a result of eddy currents which limit the penetration of flux into the iron... which in turn also affect flux pattern in the air gap area as you can see.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[waross]

Thanks for the good work, Pete. Your simulations confirm my suspicions and the effect may be even worse than I expected.
The stupid answer is to just change the casing to a larger casing so that there is room to twist the conductors.
This is not an option for an existing installation, but time will tell.
Based on this experience, there may be justifications for demanding sufficient space for twisted conductors in future installations.
My thoughts and recommendations, subject to other suggestions;
1> The excess current results from voltages induced in the grounding conductor. These voltages are unacceptably high due to the unsymmetrical spacing of the individual conductors.
2> I am concerned that these currents may cause accelerated corrosion of the pump and casing. To that end I propose abandoning the grounding conductor.
3> Grounding; Grounding is primarily for personal safety and secondarily for the proper operation of protective devices. The Canadian Code allows protection by physical separation such as barricades or elevation in leu of grounding. submersible pump is certainly not a hazard to persons, nor is it a fire hazard. It does not need a ground conductor for safety.
4> More grounding. I would ground the casing to the panel and ground the discharge piping to the casing. The point is that any thing and everything that may be touched by a person standing on the ground or any two things that may be touched simultaneously MUST be bonded to ground.
5> If bonding everything at the surface after the abandonment of the downhole grounding conductor results in any objectional currents we may investigate the option of protection by barriers. An insulating coupling in the discharge piping near the surface may be indicated.
6> Should the pump fail due to corrosion, the conductors MUST be twisted in the replacement installation.
7> You mentioned other pump problems in the area, Mark. Are these similar installations with random spaced conductors?
Yours
Bill

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

 

[jonr12]

Does the soft starter switch over to DOL after start up?