What could cause a 15kV MC cable to be very hot to the touch?

[lookingtolearn]

This situation has 3 parallel 3/0 15kV MC cables feeding a productin facility. One of the three cables is very hot to the touch (I couldn't keep my hand on it for more than 10 sec.) while the other two are not even warm. I'm not sure of the age but probably 30+ years.

My first thoughts are bad terminations, but 2 0f 3 conductors of one phase would have to be bad to cause only one cable to get hot. that seems unlikely.

Does anyone know of any other cause for a cable to be hot the entire length. If ther were insulation issues would that cause the cable to get hot?

Any help would be much appreciated.

 

[lookingtolearn]

This plant is in Davenport Iowa. I work out of the corporate office in Madison WI. The incoming 13.8kV is 3 wire and we step it down to 480V 4 wire at the delta/wye transformers.

I'm guessing earth ground is being used as the return ground path for the utility but I am working on finding out for sure.

Hopefully the IR scanning will tell all and I will be able to get this issue resolved.

 

[lookingtolearn]

I just looked at some thermal images of the termination of the MC cable conductors to the overhead utility conductors.
I'll try to describe this properly so it makes sense.
I call the utility conductors A, B and C phase layed out from left to right.
I call the three parallel conductors 1, 2, and 3 layed out from closest to me to farthest away from me. so this forms a 3 X 3 matrics.

The IR scan found three hot spots, one on each phase.
At connection A2 the differential compared to the other A phase connections is about 65 deg F, with a temperature of 135 deg F.
At connection B1 there is a 430 deg F differential with a temp. of 515 deg F. (this is bad)
At connection C2 the differential is about 70 degF with a temp. of 146 deg F.

So there are deffinitely some issues here and they will be addressing these as soon a possible.

One thing they told me is that all of the connections to the utility from the cable conductors are pigtailed using split bolts. Is this a common practis?

One other thing I just found out is that there is evidence of arcing from the armor on the MC cable and the cable tray. I'm thnking Maremite's comment about a high impedence ground fault might account for this. Any thoughts?

Again thanks for all the feedback. Long term we will be replacing all this cable I just hope that nobody or nothing gets hurt before we get that done.

 

[dpc]

515 deg F??

I'd be making immediate plans to de-energize this feeder. This is not something you can wait to deal with. You really have a responsibility to get this taken care of ASAP - like today. Or stand a 24x7 fire watch.

 

[lookingtolearn]

They will be repairing these ASAP and keeping an eye on them until then.

One question I have has to do with the cable that is running hot. I don't think fixing these terminatins will fix the problem with the entire cable being hot, I think there are some issue with the cable still, I also found out that these cables are at least 40 years old.

When they reenergize this cable, is there a big risk that the insulation will fail (I'm assuming at this point that the insulation isn't in the best of shape on that cable that has been running hot)?

 

[dpc]

Well, the heat should keep it dry, but a 40 year old 15 kV cable is probably on borrowed time even under good conditions. The less you mess with it (until you replace it) the better off you will be.

If the insulation is as hot as it sounds, it may have already basically failed.

I'd budget to replace the whole thing.

 

[alehman]

Maybe the cable impedances are not matched, causing the bulk of the A-phase current to flow on A2. This could be caused by significantly different conductor lengths (all parallel conductors must be the same length so impedances are equal), or there are one or more bad connections causing unequal current division.

Another possibility is that for some reason you have high shield current, causing a cable shield to overheat. This could be caused by improper grounding.

And what dpc said about shutting it down... Now, before something bad happens.

 

[lookingtolearn]

I appreciate all the input and am wondering if any of you have the name of any experts out there that I could bring in as a an advisor. I'm looking for some one who has a lot of experience with this type of situation.

Thanks, Hopefully it won't be long and this will be resolved with out incident.

 

[waross]

If this was a design problem I would expect all three cables to be hot.
I suspect that you have bad terminations diverting current away from one phase in the hot cable. The unbalanced current could explain both the heating and the marking on the cable tray. The diverted current will be causing an unbalance in the other two cables also but the unbalance will be divided between two cables and the effects will not be as severe.
The first step is to have the crew clean and redo the terminations at both ends.
If you suggest to the tradesmen to take the time to do a good job cleaning the terminations, and when in doubt, take the time to do it right you will probably be succsessfull.
A couple of tips;
Avoid bending the old cables. For overhead connections use a rope to suppoprt the cable close to its normal position rather than letting it hang down.
If you are connecting copper to aluminum outside, the aluminum must be above the copper. If the copper is above or beside the aluminum, rainwater will react with the copper and cause corrosion of the aluminum.
Use anti-corrosion compond such as Penetrox or No-alox.
respectfully

 

[sslobodan]

You haven't said one important thing. Is the cable oil insulated? The old cables have paper/oil insulation so the common problem in them is for oil to evaporate or leak if you have leaking problem somewhere. I run on few such problems. The solution is simple, you need to pour oil again in it. If it is oil insulated cable than I can bet 9:1 that it is the problem because it is normal with such cable when it losses oil. You need to check this fast because paper isolation is will break down very fast.

 

[lookingtolearn]

This is not oil insulated cable. How the heck would you refill 300 ft of cable if it was oil filled? That just sounds funny to me.
Not sure what you comment, "you haven't said one important thing" means.
Any way we reterminated the connections from the MC cabel to the overhead wire and things seem to be fine for now. We will replace all of this cable in the near future but for now I think the imediate concerns have been addressed.

Thanks to all of you for your input.

 

[DanDel]

The cable armor heat has disappeared? There's no unusual heat from an IR scan now?

 

[lookingtolearn]

When we put loads back on the system the temperature of the suspect cable was reading about the same as the others. I kow when it was reading hot they had about 400A at on the primary and when I took the readings after we fixed the terminations they were only at about 300A. So they certainly were not up to full load or even normal load yet. They were also only running for a few hours when I took the last scans so I'm not sure how long it takes for the temperature to stabalize.
Keeping my fingers crossed that we don't see the same temperture difference in the cable as we did before. They are suppose to take readings again this morning and then on Thursday. We will keep an eye on it until we actually replace all of it in the fall.