Nitrogen Purging of Conductors

[don_resqcapt19]

Unless it is MV cable, that always is tested on the spool and after installation before being energized.

Bob said the cable in question is 600 volt cable. My comment is based on non-shielded cable.

 

[don_resqcapt19]

What I'm saying is that with water inside the insulation, there is a path for the current that shouldn't be there. It doesn't have to go through the insulation anymore, it can go around.

Unless the open end of the cable is laying in the water the current still has to go through the dielectric to cause the megger test to fail.

When you connect the leads of the megger to the conductor and the insulation, the water is a short between them.

I have never seen one of the test leads connected to the insulation. One is connected to the conductor and the other to the EGC or the metallic raceway.

You aren't testing the insulation at all, you're measuring the insulation resistance of the water which is going to be waaaaaaaaaaaay lower than the insulation. There is water in the copper and there is water on the inside wall of the insulation and it is the same water.

Yes the water and the copper are both inside the insulation and for the megger test to fail the insulation has to be faulty or be damaged. The water inside the insulation is nothing more than more conductor inside the insulation. That in itself should not cause the the conductor to megger bad.

 

[don_resqcapt19]

T
Seriously though, and with all due respect, you are wrong. I am not saying that there is no possibility that the insulation is damaged. It has happened before. However, water in the cable ruins the insulation test, period. How can a bunch of electricians all of a sudden have no problem with water being all over the electrical equipment? Isn't that something that you would usually avoid?

I have personally worked on 5kV cable that was full of water and it caused no problems because there was no damage to the dielectric. It surprised the heck out of me when I untaped the conductor to bus connection in the 5kV switchgear and water started coming out of the conductor. The run was about 100' long and we got about a quart of water out of each of the conductors. These conductors had been in service until a few hours before I took the tape off. The water got into the conductor over many years as the result of an improper outdoor termination. A "acorn" bolt type mechanical connection was used outside with the bare end of the conductor pointing up. Over the years the water filled the spaces between the strands and acted as part of the conductor. It caused no problems and will never cause problems as long as there is not an issue with the dielectric insulation.
I can see no way where water inside the insulation will cause the megger to show a low reading unless there is some problem with the insulation.

 

[zog]

Bob said the cable in question is 600 volt cable. My comment is based on non-shielded cable.

Right, just making a clarification for others reading the thread, no intentions to say you were wrong in your post there Don.

 

[iwire]

Right, just making a clarification for others reading the thread, .

Back to the Med Volt cable, I have never worked with that. I assume the test you do is between the shield and the conductor?

 

[zog]

I have personally worked on 5kV cable that was full of water and it caused no problems because there was no damage to the dielectric. It surprised the heck out of me when I untaped the conductor to bus connection in the 5kV switchgear and water started coming out of the conductor. The run was about 100' long and we got about a quart of water out of each of the conductors. These conductors had been in service until a few hours before I took the tape off. The water got into the conductor over many years as the result of an improper outdoor termination. A "acorn" bolt type mechanical connection was used outside with the bare end of the conductor pointing up. Over the years the water filled the spaces between the strands and acted as part of the conductor. It caused no problems and will never cause problems as long as there is not an issue with the dielectric insulation.
I can see no way where water inside the insulation will cause the megger to show a low reading unless there is some problem with the insulation.

I am with Don on this one, dosent seem right at forst until you really put some thought into it, I have also seen water pour out of a just taken out of service MV cable and still test good, however, some types of insulation can withstand water intrusion better than others, water tress can form in all insulations (Specifically XLPE) over time causing you insulation resistance of the insulation to grandually decrease (With a tan delta test) but just having water in a cable alone dosent cause low megger readings.

 

[cschmid]

I am with Don on this one, dosent seem right at forst until you really put some thought into it, I have also seen water pour out of a just taken out of service MV cable and still test good, however, some types of insulation can withstand water intrusion better than others, water tress can form in all insulations (Specifically XLPE) over time causing you insulation resistance of the insulation to grandually decrease (With a tan delta test) but just having water in a cable alone dosent cause low megger readings.

Just for the record this is a first for me I have never heard of nitrogen purging..I am kind of curious of how nitrogen (N) with a boiling point of -320.42 ?F can purge a wire of moisture by being forced through the wire inside of the insulation..Am I understanding you correctly on the procedure and the element..I would think this must be expensive wire..because this is not a cheap process that we are talking about..Please enlighten me as I am learning on this one..

 

[iwire]

Nitrogen is basically dirt cheep, the plumbers use it all the time for line testing. Much faster then an air compressor and required if it is refrigeration lines.

The instructions also say we could use dry compressed air, that would be more difficult then just getting nitrogen.

I believe it works the same way a fan blowing on a wet carpet drys it, you keep the gasses around what you want to dry moving and that allows the liquid to evaporate.

 

[zog]

Back to the Med Volt cable, I have never worked with that. I assume the test you do is between the shield and the conductor?

You should do some sort of AC overpotential test (Tan delta, VLF, or 60hZ AC) with corona supression on the other end of the cable uner test. The test is done from conductor to the sheild which is grounded. The other 2 phase should also be grounded so you are testing all parts of the insulation.

 

[cschmid]

I am still curious of how you do it force it through the conduit or through the conductor..

I am trying to wrap my head around how much you are going to need to flush this for a 24 hour period of time..

600 CU XHHW-2 VW-1 600 volt conductors in conduit, it is not like these are in a small conduit.. and the amount of air flow needed to accomplish task is rather large..I would thik the amount of nitrogen needed would be rather large..

Zog can you explain some of the tests you are talking about and the cost of the gear required to perform them..

Please help to educate us..

 

[zog]

Zog can you explain some of the tests you are talking about and the cost of the gear required to perform them..

Please help to educate us..

That is a huge topic, just finished a 3 day meeting with EPRi discussing MV cable testing procedures and methods. I hate to go there but you really need to read IEEE-400 (I know everyone hates when I reference answers) but the basics are.

DC hipot, not recommended by IEEE of ICEA (Or NETA) for service aged MV cables (Considered a dstructive test), dosent tell you much anyways, test equipment is cheap ($5,000 or so)

AC Hipot, better test, problem is for cable testing of a decent length, you need a huge power supply (Due to Xc of MV Cables), not practical.

VLF - (Very low Freq) uses a .1-.01Hz signal to conduct overpotential test, tells you if there are any serious problems with your cable or accessories, $20k gets you a decent VLF test set

Tan Delta, many say is the best condition assessment test, uses same basic principal as a power factor test, looking at charging VA and Watts loss to determine capacitive and resistive properties of the insulation, can find water trees and installation errors, the standards today for MV acceptance tests. Costs, $5,000 or so for an attachment for your VLF test set.

PD - Partial discharge - test is done on line, uses a test signal and reflection signals to measure PD activity in a void in the insulation, condition assessment, cost $100,000 in equipment, assessmenet fees for data if you dont have your own database of PD test results. Very popular because it is done on line, no need to do a S/D. And before someone asks, no this is not live work, there are no exposed live parts for this test, perfectly safe.

 

[zog]

I just ballparked those costs, but you get the idea, maybe better deals can be found, we just like our fancy toys

 

[cschmid]

yeppers doubt I will performing some of them tests..Thank you Zog Now If I have more questions I will ask..thanks and good night..

 

[zog]

yeppers doubt I will performing some of them tests..Thank you Zog Now If I have more questions I will ask..thanks and good night..

Skol Vikings!

 

[iwire]

I am still curious of how you do it force it through the conduit or through the conductor..

It was suggested to do it this way

 

[cschmid]

we are talking forcing it through the conductors..

I did see the picture of the device..That is why I am curious..because nitrogens boiling point is -320.42 ?F..that means the first several feet of the wire is going to experience extreme temperature changes..When you flash freeze the insulation at temperatures outside of any of it ratings, would that not lead to a premature break down of the insulation..

I just don't know how freeze drying can be helpful to the wire..imagine the temp change effects on the conduit I would almost think it could cause a premature break down of the conduit if used on the conduit..not so much with steel but PVC should turn white and become brittle..

 

[drbond24]

Kidding aside did you bother to take a look at the link I posted?

In the testing method I pointed out can you explain how water inside the insulltion change the result?

Yes, I did. I've seen that document before. Probably have it saved somewhere deep in the bowels of my hard drive.

I understand what you are saying, but I still have a problem with the water. I can't offer anything new today that I didn't say yesterday. I'll keep thinking about it.

 

[zog]

we are talking forcing it through the conductors..

I did see the picture of the device..That is why I am curious..because nitrogens boiling point is -320.42 ?F..that means the first several feet of the wire is going to experience extreme temperature changes..When you flash freeze the insulation at temperatures outside of any of it ratings, would that not lead to a premature break down of the insulation..

I just don't know how freeze drying can be helpful to the wire..imagine the temp change effects on the conduit I would almost think it could cause a premature break down of the conduit if used on the conduit..not so much with steel but PVC should turn white and become brittle..

It is not liquid nitrogen, just compressed dry air or nitrogen.

 

[zog]

It was suggested to do it this way

That is the excat rig we used at my last job when we did this.

 

[cschmid]

ahh there the term liquid nitrogen Vs Compressed dry air..Now I can better wrap my head around it..I associated nitrogen with liquid nitrogen solely..got it..