XLPE cable puncture

[varri79]

Dear Gentlemen,
We have noticed a small puncture (approx 0.5 sq.mm )on R-core (near termination, both R-core and Y-core are touching each other)of XLPE 11kV cable at feeder end. And also noticed white precipitation around the puncture. But puncture is not upto the conductor level, perhaps pin hole upto the conductor. Anyone please explain about formation of white precipitation around puncture.
Thanking you

 

[PWR]

I am not familiar with "R and Y core" terminology. But, I believe you are describing that two phase conductors are in contact with each other a short distance from their ending terminations. If the touching occurs in the region where the shield has been removed, the voltage stress is uneven and able to create corona or partial discharge. That phenomenon essentially attacks or breaks down the insulation and the white residue is a by product of the process. If you can separate the conductors and clean the area, you may need to do nothing further. In that region of the termination, the insulation is intended to primarily provide creepage distance or tracking resistance between the actual termination of the conductor and ground. If it is cleaned and there is still substantial insulation at the thinnest point, you will probably be ok. If the damage is severe, you will need an experienced individual to undertake repairs.

 

[ScottyUK]

varri,

I agree with PWR - it sounds like PD activity on the XLPE insulation. Not something you want on your cable. Lots of info on the net if you Google 'partial discharge'. If you can oversheath the offending area with a Raychem joint andf clean the damage, and then increase the separation you should be ok.

PWR,

The terminology is from the UK's phase identification system: Red, Yellow, and Blue being the phase colours and Black being the neutral if present. On LV installations our old system has been abandoned to fall in line with the Eurocracy which demands Brown, Black, and Grey for phase colours and Blue for the neutral. The HV system is unaffected by this madness.


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Your body might be a temple. Mine is an amusement park...

 

[davidbeach]

How do you distinguish brown from black under bad lighting conditions? Or from a dirty gray for that matter?

I'm sure it is all a matter of personal preference, but I much prefer the US conventional (no color system in the NEC) system of Black/Red/Blue for 208/120 and Brown/Orange/Yellow for 480/277 with neutral being white or gray and ground being bare or green. For MV cable, the cable is banded with colored electrical tap, either using Black/Red/Blue or using white tap with one band for phase A, two for B, and three for C.

 

[RalphChristie]

How far is the puncture from the crutch?

 

[benlanz]

Is the outer semicon layer punctured or just the jacket? What are the R-core and Y-core? Are these parts of the termination?

Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

 

[ausphil]

Is the screen of the cable aluminium?

If it is, then the "precipitation" is probably an aluminium oxide. It is formed when you get water in contact with the aluminium, then, as the oxide takes up a larger volume than the aluminium, the oxide "flows" along the cable to where it can escape.

We have had a similar problem on stranded aluminium LV 4 core cables. During installation or from damage in subsequent years, small nicks in the oversheath have let water into the cable. Over time, the insulation (being plastic) has become brittle and has split with constant heating and cooling. This has allowed the water to get into contact with the conductors, starting the oxidation process.

We have noticed it in the run of the cable, as well as oozing out the end of the lugs at the substation. I've got a great photo, but I'm buggered if I can get it to display on the post - sorry. It basically looks like limestone formations in caves - flowing white streams down the cable from the termination.

 

[varri79]

Hi Guys,
Thanks for your replies, i am sorry that i was out of office for last few days.

After this cable problem, I had checked all our 11kv cables / terminations and found similiar punctures and precipatations in 5 more cables.

These are occuring just 8 to 15 centimeters from the crunches.

The puncture is not up to the core (i mean the conductor), and it is the jacket that is failing and these are part of termination.

As ScottyUK, PWR and ausphil suggest, we are going to clean & reterminate some of the cables.