Nitrogen Purging of Conductors

[zog]

Yes, there is a jacket on multi conductor cables.

The conductor that I have referenced in this thread is single conductor.

Sorry, I rarely deal with LV cable. But it does use XLPE for the insulation correct? Either way XLPE has problems with waier trees cause by internal moisture at any voltage, the fact that my example was a MV cable changes nothing about the problems with water in the cables, causing water trees, and causing low insulation resistance values.

 

[iwire]

Either way XLPE has problems with waier trees cause by internal moisture at any voltage,

So your saying that an insulation that is the same throughout and is rated to run submerged in water will be damaged by water on the inside?

I am having a real hard time believing it is only water proof in one direction.

 

[zog]

So your saying that an insulation that is the same throughout and is rated to run submerged in water will be damaged by water on the inside?

I am having a real hard time believing it is only water proof in one direction.

If it uses XPLE, yes over time it may. Wasnt designed that way, but we know know that it happens.

 

[LarryFine]

I guess that means that water, like electricity, seeks all pathways back to the source.

 

[drbond24]

There is no PVC jacket on XHHW, the insulation appears to be homogeneous.

This is correct. We don't put jackets on single conductor XHHW.

I've seen it with mylar (?) tape around the conductor before the actual XHHW insulation is applied. I'm not sure it has any dielectric properties, however.

The mylar does nothing but keep the insulation from sticking to the copper so badly that you can't strip it off easily. It has no electrical properties whatsoever.

I am having a real hard time believing it is only water proof in one direction.

The insulation is the same on the inside as it is on the outside, and it will hold water in as well as it holds water out.

The only time I was ever involved with nitrogen purging conductors it was as temporary remediation while a shutdown could be scheduled when a conductor or conductor set meggered scary-bad. This was at a military facility. The Nitrogen was added through a T-tap rubber fitting designed for that use while the conductors were still in service. This business of nitrogen purging XHHW to pass a megger test seems like total nonsense. It will get your numbers up (probably), until the water returns. There's still a fundamental insulation compromise someplace. PMDMS dielectric injection (much like is done on MV cables) seems like it would be a more permanent repair. Nitrogen purging is a temp repair (when the bottles are left in place) or a way to "fake" a good megger test. The problem is not "fixed".

It depends on how the water got there. IMO, it is far more probable that the water entered the cable through the ends at some point before it was installed. If this is the case, removing it will solve the problem.

Of course if there is a hole in the insulation somewhere then the water is going to return, and you are right about the Nitrogen being a temporary fix. I just see this as a much less likely situation.

 

[don_resqcapt19]

It depends on how the water got there. IMO, it is far more probable that the water entered the cable through the ends at some point before it was installed. If this is the case, removing it will solve the problem.

This is where I have the problem. Just looking at the short term, I don't understand how adding additional conductor (the water) under the insultation changes the megger reading. Are you saying that the megger reading of the conductor with the water in it installed in a dry metal raceway would be less than that of a dry conductor installed in a water filled metal raceway?

 

[drbond24]

Are you saying that the megger reading of the conductor with the water in it installed in a dry metal raceway would be less than that of a dry conductor installed in a water filled metal raceway?

All I was trying to say in that particular instance was that the presence of water does not automatically indicate gaping holes in the insulation. I have already voiced my own opinions about the water in previous posts. What I know for a fact is that every how-to manual I can find regarding insulation resistance tests say repeatedly that everything must be clean and dry.

My position, were I in iwire's place, would be this (again this is my own opinion and nothing more):

The insulation tested poorly. We can

1) purge with Nitrogen and try the test again. If it passes now, the water was the problem and the problem is solved. I have saved the time and trouble of replacing the cable when it was unnecessary. If it still doesn't pass then we can still replace it and will have only lost the time and money it took to perform the purging. Proceed to step two.

2) Send the wire back to the manufacturer. This will take some amount of time that in any case will be significantly greater than purging the cable with Nitrogen. Time is money, so even if the manufacturer replaces it at no cost I will have lost the time it takes to drive a truck from the plant to me, assuming there is more cable like mine sitting around ready to be sent out. If not, I also have to wait for it to be manufactured.

I don't see what would be lost by trying the Nitrogen purge.

The only time I was ever involved with nitrogen purging conductors it was as temporary remediation while a shutdown could be scheduled when a conductor or conductor set meggered scary-bad. This was at a military facility. The Nitrogen was added through a T-tap rubber fitting designed for that use while the conductors were still in service. This business of nitrogen purging XHHW to pass a megger test seems like total nonsense. It will get your numbers up (probably), until the water returns. There's still a fundamental insulation compromise someplace. PMDMS dielectric injection (much like is done on MV cables) seems like it would be a more permanent repair. Nitrogen purging is a temp repair (when the bottles are left in place) or a way to "fake" a good megger test. The problem is not "fixed".

After reading this again, I don't understand. Is the water a problem or not? If purging the water out fixes the problem then the water WAS the problem. If there is a terrible problem with the insulation, it will test bad full of water or not, right? Seems like you're arguing both points here. If the water is the problem then the solution is to find where the water is getting in and stop it. The entry point cannot be poor insulation or blowing out the water wouldn't have fixed the megger reading, right? What you are admitting is that the water in the cable is the reason it megged bad, and you are therefore agreeing with me. Its nice to have someone on my side here. :wink:

 

[ashtrak]

Going no where!!!!

Going no where!!!!

I'm obviously suffering from a lack of field experience on this subject. All I have is an equation that tells me what the insulation resistance should be and some pdf files about how to measure it. IMHO, going on with this debate will not accomplish anything constructive. iwire is going to do what is best for him and I would do the same thing in his place. No hard feelings.

Seems very constructive to me. I've been well educated on some things, this is a learning experience for me.
I've seen phone lines and communication cables hooked up to nitrogen bottles in my life time, but I've never seen power, cable 60 cycle or for that matter any cycle (excuse me,Hertz) purged with nitrogen. I'm hanging on every word.
Where I work there are relics of nitrogen manifolds that used to go to duct banks that are long abandoned and old bottles laying around. There was even a "bottle gang" that did nothing but change bottles every day till they retired.
Ahh.. the good old days. I only have to walk down to the basement for an experience. Oh well..only slighty off the subject.

 

[zog]

All I was trying to say in that particular instance was that the presence of water does not automatically indicate gaping holes in the insulation. I have already voiced my own opinions about the water in previous posts. What I know for a fact is that every how-to manual I can find regarding insulation resistance tests say repeatedly that everything must be clean and dry.

My position, were I in iwire's place, would be this (again this is my own opinion and nothing more):

The insulation tested poorly. We can

1) purge with Nitrogen and try the test again. If it passes now, the water was the problem and the problem is solved. I have saved the time and trouble of replacing the cable when it was unnecessary. If it still doesn't pass then we can still replace it and will have only lost the time and money it took to perform the purging. Proceed to step two.

2) Send the wire back to the manufacturer. This will take some amount of time that in any case will be significantly greater than purging the cable with Nitrogen. Time is money, so even if the manufacturer replaces it at no cost I will have lost the time it takes to drive a truck from the plant to me, assuming there is more cable like mine sitting around ready to be sent out. If not, I also have to wait for it to be manufactured.

I don't see what would be lost by trying the Nitrogen purge.



After reading this again, I don't understand. Is the water a problem or not? If purging the water out fixes the problem then the water WAS the problem. If there is a terrible problem with the insulation, it will test bad full of water or not, right? Seems like you're arguing both points here. If the water is the problem then the solution is to find where the water is getting in and stop it. The entry point cannot be poor insulation or blowing out the water wouldn't have fixed the megger reading, right? What you are admitting is that the water in the cable is the reason it megged bad, and you are therefore agreeing with me. Its nice to have someone on my side here. :wink:

You nailed it, I agree 100% with your recomendations.

 

[don_resqcapt19]

1) purge with Nitrogen and try the test again. If it passes now, the water was the problem and the problem is solved. I have saved the time and trouble of replacing the cable when it was unnecessary. If it still doesn't pass then we can still replace it and will have only lost the time and money it took to perform the purging. Proceed to step two.

Unless you know how the water got in, you can't say that the cable is good just because it tested good after you drove the water out.

After reading this again, I don't understand. Is the water a problem or not? If purging the water out fixes the problem then the water WAS the problem.

I don't agree with that either. You can have bare spots on the conductor and have it test good with a megger if there is no water in the conduit.

 

[don_resqcapt19]

Zog,
Would you expect a conductor with water under the insulation to have a lower insulation resistance reading than the same conductor that only has water on the exterior surface of the insulation? If so why?
As a testing engineer, if you had this cable that tested bad with water in it and then it tested good after you dryed it out, would you accept it? I would not. I can see no way that the water inside (short time) will change the insulation resistance any more than water on the outside.

 

[LarryFine]

After reading this again, I don't understand. Is the water a problem or not? If purging the water out fixes the problem then the water WAS the problem. If there is a terrible problem with the insulation, it will test bad full of water or not, right? Seems like you're arguing both points here. If the water is the problem then the solution is to find where the water is getting in and stop it. The entry point cannot be poor insulation or blowing out the water wouldn't have fixed the megger reading, right?

My thoughts at this point:

If purging improves the megger readings, then the water must have been an issue.

If the purging is a permanent fix, the water shouldn't have been the issue.

If the purging is a temporary fix, the insulation is the issue, damaged or defective.

If the cable is new (which it is, re the OP), why the reading is poor doesn't matter.*


*Well, maybe a little: if the insulation was damaged during installation, the purging will be a very temporary fix.

 

[HighWirey]

Has anyone done it or know about it?
I was asked to find out about it.
Apparently a new conductor Meged out poorly. The conductor manufacturer has suggested the conductor is wet and has recommended that the conductor have a 15 to 20 PSI supply of Nitrogen or Dry air applied at one end then let run 8 hours after all signs of moisture are gone.
Thanks in advance, Bob

Does your OP refer to conductors in cable tray, in above ground conduit or in underground duct?

We always had an unlimited quantity of dry GN2 available, have used that free resource, and never had success recovering a bad run. You may have a chance to recover your conductor if it is installed above ground. In water saturated conduit or duct banks, give it up. An insulation void is an insulation void. They never heal up.

Best Wishes Everyone

 

[drbond24]

You nailed it, I agree 100% with your recomendations.

If purging improves the megger readings, then the water must have been an issue.

I'm turning this around. My head is back above the water. :grin: :grin:

You can have bare spots on the conductor and have it test good with a megger if there is no water in the conduit.

So you are not only saying that water isn't a problem, you're saying water is REQUIRED to megger the insulation?

Would you expect a conductor with water under the insulation to have a lower insulation resistance reading than the same conductor that only has water on the exterior surface of the insulation? If so why?

A very good question for which I do not have an answer at present. I'll be doing some more research if I can find more stuff to read.

 

[weressl]

Water trees are usually found in XLPE type MV cables (Can be in any type) and lead to eletrical trees that quickly cause failures. There are several companies with this injection process with impressive data to back up thier claims of effectiveness (Some of which I have a lot of respect for) but some other "cable guys" I know say otherwise, I guess the jury is out and this type of process may be considered "life extension" process

Yep, and if you have rheumatoid arthritis I have some snake oil , fix you right up!

 

[mdshunk]

After reading this again, I don't understand. Is the water a problem or not? If purging the water out fixes the problem then the water WAS the problem. If there is a terrible problem with the insulation, it will test bad full of water or not, right? Seems like you're arguing both points here. If the water is the problem then the solution is to find where the water is getting in and stop it. The entry point cannot be poor insulation or blowing out the water wouldn't have fixed the megger reading, right? What you are admitting is that the water in the cable is the reason it megged bad, and you are therefore agreeing with me. Its nice to have someone on my side here. :wink:

In the case I referenced, the insulation was pretty bad. A continuous nitrogen purge removed the conductive short circut path (the water) for the time being, until the conductors could be replaced.

 

[zog]

Yep, and if you have rheumatoid arthritis I have some snake oil , fix you right up!

Thats my gut feeling too but these guys had some pretty solid test data on this at the EPRi cable conference.

http://www.cabtl.com/life.htm

I am sure they will send you a copy of thier findings, Dr. Bogdan Fryszczyn was the guy I talked to about the effectiveness of the process. (rejuvination with injections)

 

[mdshunk]

It depends on how the water got there. IMO, it is far more probable that the water entered the cable through the ends at some point before it was installed. If this is the case, removing it will solve the problem.

Are you just nuts, or a hard head in general? Good conductor insulation could be completely full of water inside and still meg out just fine. Removing the water will get you an artificially good megger reading because the conductive material (the water) is no longer bridging the gap through the "holes" in the insulation. Removing the water didn't actually "fix" anything. The bad insulation remains. Like it or not, the water will return and the conductors will meg at the same low levels in the future since the conductors obviously had insulation issues to even meg bad in the first place.

 

[winnie]

My thoughts on this (and I am guessing here):

No insulation is perfect. There is always _some_ current leakage through the insulation, and thus a finite resistance.

In use, a small amount of current leakage must be tolerated between an energized conductor and its surroundings.

The quality of modern insulating plastics is so high that the leakage expected is far, far lower than one could actually tolerate.

The result that a resistance test which indicates what would be and _acceptable_ level of leakage (if evenly distributed) is _taken to mean_ that there is a defect in the insulation. The conductor might function just fine with 1 megohm resistance to surrounding material per 1000 feet of conductor...but insulating plastic is so good that if you saw a resistance so low (1 megohm per 1000 feet) you would rightly conclude that there is a hole or other defect in the insulation.

Because there is always going to be a small amount of current leakage, your insulation resistance test has to have some standard for the minimum permissible resistance.

It seems plausible to me that insulation would have _different_ current leakage values wet versus dry. Wet the insulation might cause enough leakage to _fail_ the test...but be good enough insulation to function acceptably. In this case, you might have insulation with no defect at all, and which would function acceptably when wet, but it would fail the insulation resistance test because of water.

On the other side of the coil, dry air is a rather good insulator. If you have an insulated conductor with holes in it, and the holes don't bring the conductor into contact with surrounding conductive material, then that hole would not cause a low resistance reading. This argues that if you actually want to detect holes in the insulation, you actually need to surround the insulation with a conductive medium, and perform your resistance test between the conductor and the 'conductive medium', eg. water.

-Jon

 

[iwire]

I will add this, the conduit is flooded, it was installed below the water table, it was flooded from day one and will remain flooded indefinitely.

There are five conductors in each raceway, the conductors that test good have readings above 500 mega ohms, the ones that test bad are below one (1) mega ohm.

I don't think those reading suggest a slight leakage of the insulation in general but a compromised insulation.